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/* Alias analysis for trees.
Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009
Free Software Foundation, Inc.
Contributed by Diego Novillo <>
This file is part of GCC.
GCC is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3, or (at your option)
any later version.
GCC is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING3. If not see
<>. */
#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "tm.h"
#include "tree.h"
#include "rtl.h"
#include "tm_p.h"
#include "hard-reg-set.h"
#include "basic-block.h"
#include "timevar.h"
#include "expr.h"
#include "ggc.h"
#include "langhooks.h"
#include "flags.h"
#include "function.h"
#include "diagnostic.h"
#include "tree-dump.h"
#include "gimple.h"
#include "tree-flow.h"
#include "tree-inline.h"
#include "tree-pass.h"
#include "tree-ssa-structalias.h"
#include "convert.h"
#include "params.h"
#include "ipa-type-escape.h"
#include "vec.h"
#include "bitmap.h"
#include "vecprim.h"
#include "pointer-set.h"
#include "alloc-pool.h"
/* Broad overview of how aliasing works:
First we compute points-to sets, which is done in
During points-to set constraint finding, a bunch of little bits of
information is collected.
This is not done because it is necessary for points-to, but because
points-to has to walk every statement anyway. The function performing
this collecting is update_alias_info.
Bits update_alias_info collects include:
1. Directly escaping variables and variables whose value escapes
(using is_escape_site). This is the set of variables and values that
escape prior to transitive closure of the clobbers.
2. The set of variables dereferenced on the LHS (into
3. The set of variables dereferenced on the RHS (into
4. The set of all pointers we saw.
5. The number of loads and stores for each variable
6. The number of statements touching memory
7. The set of address taken variables.
#1 is computed by a combination of is_escape_site, and counting the
number of uses/deref operators. This function properly accounts for
situations like &ptr->field, which is *not* a dereference.
After points-to sets are computed, the sets themselves still
contain points-to specific variables, such as a variable that says
the pointer points to anything, a variable that says the pointer
points to readonly memory, etc.
These are eliminated in a later phase, as we will see.
The rest of the phases are located in tree-ssa-alias.c
The next phase after points-to set computation is called
This pass does 3 main things:
1. All variables that can have TREE_ADDRESSABLE removed safely (IE
non-globals whose address is not taken), have TREE_ADDRESSABLE
2. All variables that may be aliased (which is the set of addressable
variables and globals) at all, are marked for renaming, and have
symbol memory tags created for them.
3. All variables which are stored into have their SMT's added to
written vars.
After this function is run, all variables that will ever have an
SMT, have one, though its aliases are not filled in.
The next phase is to compute flow-insensitive aliasing, which in
our case, is a misnomer. it is really computing aliasing that
requires no transitive closure to be correct. In particular, it
uses stack vs non-stack, TBAA, etc, to determine whether two
symbols could *ever* alias . This phase works by going through all
the pointers we collected during update_alias_info, and for every
addressable variable in the program, seeing if they alias. If so,
the addressable variable is added to the symbol memory tag for the
As part of this, we handle symbol memory tags that conflict but
have no aliases in common, by forcing them to have a symbol in
common (through unioning alias sets or adding one as an alias of
the other), or by adding one as an alias of another. The case of
conflicts with no aliases in common occurs mainly due to aliasing
we cannot see. In particular, it generally means we have a load
through a pointer whose value came from outside the function.
Without an addressable symbol to point to, they would get the wrong
After flow insensitive aliasing is computed, we compute name tags
(called compute_flow_sensitive_info). We walk each pointer we
collected and see if it has a usable points-to set. If so, we
generate a name tag using that pointer, and make an alias bitmap for
it. Name tags are shared between all things with the same alias
bitmap. The alias bitmap will be translated from what points-to
computed. In particular, the "anything" variable in points-to will be
transformed into a pruned set of SMT's and their aliases that
compute_flow_insensitive_aliasing computed.
Note that since 4.3, every pointer that points-to computed a solution for
will get a name tag (whereas before 4.3, only those whose set did
*not* include the anything variable would). At the point where name
tags are all assigned, symbol memory tags are dead, and could be
deleted, *except* on global variables. Global variables still use
symbol memory tags as of right now.
After name tags are computed, the set of clobbered variables is
transitively closed. In particular, we compute the set of clobbered
variables based on the initial set of clobbers, plus the aliases of
pointers which either escape, or have their value escape.
After this, maybe_create_global_var is run, which handles a corner
case where we have no call clobbered variables, but have pure and
non-pure functions.
Staring at this function, I now remember it is a hack for the fact
that we do not mark all globals in the program as call clobbered for a
function unless they are actually used in that function. Instead, we
only mark the set that is actually clobbered. As a result, you can
end up with situations where you have no call clobbered vars set.
After maybe_create_global_var, we set pointers with the REF_ALL flag
to have alias sets that include all clobbered
memory tags and variables.
After this, memory partitioning is computed (by the function
compute_memory_partitions) and alias sets are reworked accordingly.
Lastly, we delete partitions with no symbols, and clean up after
ourselves. */
/* Alias information used by compute_may_aliases and its helpers. */
struct alias_info
/* SSA names visited while collecting points-to information. If bit I
is set, it means that SSA variable with version I has already been
visited. */
sbitmap ssa_names_visited;
/* Array of SSA_NAME pointers processed by the points-to collector. */
VEC(tree,heap) *processed_ptrs;
/* ADDRESSABLE_VARS contains all the global variables and locals that
have had their address taken. */
struct alias_map_d **addressable_vars;
size_t num_addressable_vars;
/* POINTERS contains all the _DECL pointers with unique memory tags
that have been referenced in the program. */
struct alias_map_d **pointers;
size_t num_pointers;
/* Pointers that have been used in an indirect load/store operation. */
struct pointer_set_t *dereferenced_ptrs;
/* Structure to map a variable to its alias set. */
struct alias_map_d
/* Variable and its alias set. */
tree var;
alias_set_type set;
/* Counters used to display statistics on alias analysis. */
struct alias_stats_d
unsigned int alias_queries;
unsigned int alias_mayalias;
unsigned int alias_noalias;
unsigned int simple_queries;
unsigned int simple_resolved;
unsigned int tbaa_queries;
unsigned int tbaa_resolved;
unsigned int structnoaddress_queries;
unsigned int structnoaddress_resolved;
/* Local variables. */
static struct alias_stats_d alias_stats;
static bitmap_obstack alias_bitmap_obstack;
/* Local functions. */
static void compute_flow_insensitive_aliasing (struct alias_info *);
static void dump_alias_stats (FILE *);
static tree create_memory_tag (tree type, bool is_type_tag);
static tree get_smt_for (tree, struct alias_info *);
static tree get_nmt_for (tree);
static void add_may_alias (tree, tree);
static struct alias_info *init_alias_info (void);
static void delete_alias_info (struct alias_info *);
static void compute_flow_sensitive_aliasing (struct alias_info *);
static void setup_pointers_and_addressables (struct alias_info *);
static void update_alias_info (struct alias_info *);
static void create_global_var (void);
static void maybe_create_global_var (void);
static void set_pt_anything (tree);
void debug_mp_info (VEC(mem_sym_stats_t,heap) *);
static alloc_pool mem_sym_stats_pool;
/* Return memory reference stats for symbol VAR. Create a new slot in
cfun->gimple_df->mem_sym_stats if needed. */
static struct mem_sym_stats_d *
get_mem_sym_stats_for (tree var)
void **slot;
struct mem_sym_stats_d *stats;
struct pointer_map_t *map = gimple_mem_ref_stats (cfun)->mem_sym_stats;
gcc_assert (map);
slot = pointer_map_insert (map, var);
if (*slot == NULL)
stats = (struct mem_sym_stats_d *) pool_alloc (mem_sym_stats_pool);
memset (stats, 0, sizeof (*stats));
stats->var = var;
*slot = (void *) stats;
stats = (struct mem_sym_stats_d *) *slot;
return stats;
/* Return memory reference statistics for variable VAR in function FN.
This is computed by alias analysis, but it is not kept
incrementally up-to-date. So, these stats are only accurate if
pass_may_alias has been run recently. If no alias information
exists, this function returns NULL. */
static mem_sym_stats_t
mem_sym_stats (struct function *fn, tree var)
void **slot;
struct pointer_map_t *stats_map = gimple_mem_ref_stats (fn)->mem_sym_stats;
if (stats_map == NULL)
return NULL;
slot = pointer_map_contains (stats_map, var);
if (slot == NULL)
return NULL;
return (mem_sym_stats_t) *slot;
/* Set MPT to be the memory partition associated with symbol SYM. */
static inline void
set_memory_partition (tree sym, tree mpt)
if (mpt)
&& !is_gimple_reg (sym));
var_ann (sym)->mpt = mpt;
if (mpt)
if (MPT_SYMBOLS (mpt) == NULL)
MPT_SYMBOLS (mpt) = BITMAP_ALLOC (&alias_bitmap_obstack);
bitmap_set_bit (MPT_SYMBOLS (mpt), DECL_UID (sym));
/* MPT inherits the call-clobbering attributes from SYM. */
if (is_call_clobbered (sym))
MTAG_GLOBAL (mpt) = 1;
mark_call_clobbered (mpt, ESCAPE_IS_GLOBAL);
/* Mark variable VAR as being non-addressable. */
static void
mark_non_addressable (tree var)
tree mpt;
mpt = memory_partition (var);
clear_call_clobbered (var);
if (mpt)
/* Note that it's possible for a symbol to have an associated
MPT and the MPT have a NULL empty set. During
init_alias_info, all MPTs get their sets cleared out, but the
symbols still point to the old MPTs that used to hold them.
This is done so that compute_memory_partitions can now which
symbols are losing or changing partitions and mark them for
renaming. */
if (MPT_SYMBOLS (mpt))
bitmap_clear_bit (MPT_SYMBOLS (mpt), DECL_UID (var));
set_memory_partition (var, NULL_TREE);
/* qsort comparison function to sort type/name tags by DECL_UID. */
static int
sort_tags_by_id (const void *pa, const void *pb)
const_tree const a = *(const_tree const *)pa;
const_tree const b = *(const_tree const *)pb;
return DECL_UID (a) - DECL_UID (b);
/* Initialize WORKLIST to contain those memory tags that are marked call
clobbered. Initialized WORKLIST2 to contain the reasons these
memory tags escaped. */
static void
init_transitive_clobber_worklist (VEC (tree, heap) **worklist,
VEC (int, heap) **worklist2,
bitmap on_worklist)
referenced_var_iterator rvi;
tree curr;
if (MTAG_P (curr) && is_call_clobbered (curr))
VEC_safe_push (tree, heap, *worklist, curr);
VEC_safe_push (int, heap, *worklist2,
var_ann (curr)->escape_mask);
bitmap_set_bit (on_worklist, DECL_UID (curr));
/* Add ALIAS to WORKLIST (and the reason for escaping REASON to WORKLIST2) if
ALIAS is not already marked call clobbered, and is a memory
tag. */
static void
add_to_worklist (tree alias, VEC (tree, heap) **worklist,
VEC (int, heap) **worklist2, int reason,
bitmap on_worklist)
if (MTAG_P (alias) && !is_call_clobbered (alias)
&& !bitmap_bit_p (on_worklist, DECL_UID (alias)))
VEC_safe_push (tree, heap, *worklist, alias);
VEC_safe_push (int, heap, *worklist2, reason);
bitmap_set_bit (on_worklist, DECL_UID (alias));
/* Mark aliases of TAG as call clobbered, and place any tags on the
alias list that were not already call clobbered on WORKLIST. */
static void
mark_aliases_call_clobbered (tree tag, VEC (tree, heap) **worklist,
VEC (int, heap) **worklist2, bitmap on_worklist)
bitmap aliases;
bitmap_iterator bi;
unsigned int i;
tree entry;
var_ann_t ta = var_ann (tag);
if (!MTAG_P (tag))
aliases = may_aliases (tag);
if (!aliases)
EXECUTE_IF_SET_IN_BITMAP (aliases, 0, i, bi)
entry = referenced_var (i);
/* If you clobber one part of a structure, you
clobber the entire thing. While this does not make
the world a particularly nice place, it is necessary
in order to allow C/C++ tricks that involve
pointer arithmetic to work. */
if (!unmodifiable_var_p (entry))
add_to_worklist (entry, worklist, worklist2, ta->escape_mask,
mark_call_clobbered (entry, ta->escape_mask);
/* Tags containing global vars need to be marked as global.
Tags containing call clobbered vars need to be marked as call
clobbered. */
static void
compute_tag_properties (void)
referenced_var_iterator rvi;
tree tag;
bool changed = true;
VEC (tree, heap) *taglist = NULL;
if (!MTAG_P (tag))
VEC_safe_push (tree, heap, taglist, tag);
/* We sort the taglist by DECL_UID, for two reasons.
1. To get a sequential ordering to make the bitmap accesses
2. Because of the way we compute aliases, it's more likely that
an earlier tag is included in a later tag, and this will reduce
the number of iterations.
If we had a real tag graph, we would just topo-order it and be
done with it. */
qsort (VEC_address (tree, taglist),
VEC_length (tree, taglist),
sizeof (tree),
/* Go through each tag not marked as global, and if it aliases
global vars, mark it global.
If the tag contains call clobbered vars, mark it call
This loop iterates because tags may appear in the may-aliases
list of other tags when we group. */
while (changed)
unsigned int k;
changed = false;
for (k = 0; VEC_iterate (tree, taglist, k, tag); k++)
bitmap ma;
bitmap_iterator bi;
unsigned int i;
tree entry;
bool tagcc = is_call_clobbered (tag);
bool tagglobal = MTAG_GLOBAL (tag);
if (tagcc && tagglobal)
ma = may_aliases (tag);
if (!ma)
entry = referenced_var (i);
/* Call clobbered entries cause the tag to be marked
call clobbered. */
if (!tagcc && is_call_clobbered (entry))
mark_call_clobbered (tag, var_ann (entry)->escape_mask);
tagcc = true;
changed = true;
/* Global vars cause the tag to be marked global. */
if (!tagglobal && is_global_var (entry))
MTAG_GLOBAL (tag) = true;
changed = true;
tagglobal = true;
/* Early exit once both global and cc are set, since the
loop can't do any more than that. */
if (tagcc && tagglobal)
VEC_free (tree, heap, taglist);
/* Set up the initial variable clobbers, call-uses and globalness.
When this function completes, only tags whose aliases need to be
clobbered will be set clobbered. Tags clobbered because they
contain call clobbered vars are handled in compute_tag_properties. */
static void
set_initial_properties (struct alias_info *ai)
unsigned int i;
referenced_var_iterator rvi;
tree var;
tree ptr;
bool any_pt_anything = false;
enum escape_type pt_anything_mask = 0;
if (is_global_var (var))
if (!unmodifiable_var_p (var))
mark_call_clobbered (var, ESCAPE_IS_GLOBAL);
else if (TREE_CODE (var) == PARM_DECL
&& gimple_default_def (cfun, var)
tree def = gimple_default_def (cfun, var);
get_ptr_info (def)->value_escapes_p = 1;
get_ptr_info (def)->escape_mask |= ESCAPE_IS_PARM;
if (!clobber_what_escaped ())
any_pt_anything = true;
pt_anything_mask |= ESCAPE_TO_CALL;
compute_call_used_vars ();
for (i = 0; VEC_iterate (tree, ai->processed_ptrs, i, ptr); i++)
struct ptr_info_def *pi = SSA_NAME_PTR_INFO (ptr);
tree tag = symbol_mem_tag (SSA_NAME_VAR (ptr));
/* A pointer that only escapes via a function return does not
add to the call clobber or call used solution.
To exclude ESCAPE_TO_PURE_CONST we would need to track
call used variables separately or compute those properly
in the operand scanner. */
if (pi->value_escapes_p
&& pi->escape_mask & ~ESCAPE_TO_RETURN)
/* If PTR escapes then its associated memory tags and
pointed-to variables are call-clobbered. */
if (pi->name_mem_tag)
mark_call_clobbered (pi->name_mem_tag, pi->escape_mask);
if (tag)
mark_call_clobbered (tag, pi->escape_mask);
/* If the name tag is call clobbered, so is the symbol tag
associated with the base VAR_DECL. */
if (pi->name_mem_tag
&& tag
&& is_call_clobbered (pi->name_mem_tag))
mark_call_clobbered (tag, pi->escape_mask);
/* Name tags and symbol tags that we don't know where they point
to, might point to global memory, and thus, are clobbered.
FIXME: This is not quite right. They should only be
clobbered if value_escapes_p is true, regardless of whether
they point to global memory or not.
So removing this code and fixing all the bugs would be nice.
It is the cause of a bunch of clobbering. */
if ((pi->pt_global_mem || pi->pt_anything)
&& pi->memory_tag_needed && pi->name_mem_tag)
mark_call_clobbered (pi->name_mem_tag, ESCAPE_IS_GLOBAL);
MTAG_GLOBAL (pi->name_mem_tag) = true;
if ((pi->pt_global_mem || pi->pt_anything)
&& pi->memory_tag_needed
&& tag)
mark_call_clobbered (tag, ESCAPE_IS_GLOBAL);
MTAG_GLOBAL (tag) = true;
/* If a pt_anything pointer escaped we need to mark all addressable
variables call clobbered. */
if (any_pt_anything)
bitmap_iterator bi;
unsigned int j;
EXECUTE_IF_SET_IN_BITMAP (gimple_addressable_vars (cfun), 0, j, bi)
tree var = referenced_var (j);
if (!unmodifiable_var_p (var))
mark_call_clobbered (var, pt_anything_mask);
/* Compute which variables need to be marked call clobbered because
their tag is call clobbered, and which tags need to be marked
global because they contain global variables. */
static void
compute_call_clobbered (struct alias_info *ai)
VEC (tree, heap) *worklist = NULL;
VEC (int,heap) *worklist2 = NULL;
bitmap on_worklist;
timevar_push (TV_CALL_CLOBBER);
on_worklist = BITMAP_ALLOC (NULL);
set_initial_properties (ai);
init_transitive_clobber_worklist (&worklist, &worklist2, on_worklist);
while (VEC_length (tree, worklist) != 0)
tree curr = VEC_pop (tree, worklist);
int reason = VEC_pop (int, worklist2);
bitmap_clear_bit (on_worklist, DECL_UID (curr));
mark_call_clobbered (curr, reason);
mark_aliases_call_clobbered (curr, &worklist, &worklist2, on_worklist);
VEC_free (tree, heap, worklist);
VEC_free (int, heap, worklist2);
BITMAP_FREE (on_worklist);
compute_tag_properties ();
timevar_pop (TV_CALL_CLOBBER);
/* Dump memory partition information to FILE. */
static void
dump_memory_partitions (FILE *file)
unsigned i, npart;
unsigned long nsyms;
tree mpt;
fprintf (file, "\nMemory partitions\n\n");
for (i = 0, npart = 0, nsyms = 0;
VEC_iterate (tree, gimple_ssa_operands (cfun)->mpt_table, i, mpt);
if (mpt)
bitmap syms = MPT_SYMBOLS (mpt);
unsigned long n = (syms) ? bitmap_count_bits (syms) : 0;
fprintf (file, "#%u: ", i);
print_generic_expr (file, mpt, 0);
fprintf (file, ": %lu elements: ", n);
dump_decl_set (file, syms);
nsyms += n;
fprintf (file, "\n%u memory partitions holding %lu symbols\n", npart, nsyms);
/* Dump memory partition information to stderr. */
debug_memory_partitions (void)
dump_memory_partitions (stderr);
/* Return true if memory partitioning is required given the memory
reference estimates in STATS. */
static inline bool
need_to_partition_p (struct mem_ref_stats_d *stats)
long num_vops = stats->num_vuses + stats->num_vdefs;
long avg_vops = CEIL (num_vops, stats->num_mem_stmts);
return (num_vops > (long) MAX_ALIASED_VOPS
&& avg_vops > (long) AVG_ALIASED_VOPS);
/* Count the actual number of virtual operators in CFUN. Note that
this is only meaningful after virtual operands have been populated,
so it should be invoked at the end of compute_may_aliases.
The number of virtual operators are stored in *NUM_VDEFS_P and
*NUM_VUSES_P, the number of partitioned symbols in
*NUM_PARTITIONED_P and the number of unpartitioned symbols in
If any of these pointers is NULL the corresponding count is not
computed. */
static void
count_mem_refs (long *num_vuses_p, long *num_vdefs_p,
long *num_partitioned_p, long *num_unpartitioned_p)
gimple_stmt_iterator gsi;
basic_block bb;
long num_vdefs, num_vuses, num_partitioned, num_unpartitioned;
referenced_var_iterator rvi;
tree sym;
num_vuses = num_vdefs = num_partitioned = num_unpartitioned = 0;
if (num_vuses_p || num_vdefs_p)
for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
gimple stmt = gsi_stmt (gsi);
if (gimple_references_memory_p (stmt))
num_vuses += NUM_SSA_OPERANDS (stmt, SSA_OP_VUSE);
num_vdefs += NUM_SSA_OPERANDS (stmt, SSA_OP_VDEF);
if (num_partitioned_p || num_unpartitioned_p)
if (is_gimple_reg (sym))
if (memory_partition (sym))
if (num_vdefs_p)
*num_vdefs_p = num_vdefs;
if (num_vuses_p)
*num_vuses_p = num_vuses;
if (num_partitioned_p)
*num_partitioned_p = num_partitioned;
if (num_unpartitioned_p)
*num_unpartitioned_p = num_unpartitioned;
/* The list is sorted by increasing partitioning score (PSCORE).
This score is computed such that symbols with high scores are
those that are least likely to be partitioned. Given a symbol
MP->VAR, PSCORE(S) is the result of the following weighted sum
PSCORE(S) = FW * 64 + FR * 32
+ DW * 16 + DR * 8
+ IW * 4 + IR * 2
FW Execution frequency of writes to S
FR Execution frequency of reads from S
DW Number of direct writes to S
DR Number of direct reads from S
IW Number of indirect writes to S
IR Number of indirect reads from S
NO_ALIAS State of the NO_ALIAS* flags
The basic idea here is that symbols that are frequently
written-to in hot paths of the code are the last to be considered
for partitioning. */
static inline long
mem_sym_score (mem_sym_stats_t mp)
return mp->frequency_writes * 64 + mp->frequency_reads * 32
+ mp->num_direct_writes * 16 + mp->num_direct_reads * 8
+ mp->num_indirect_writes * 4 + mp->num_indirect_reads * 2
+ var_ann (mp->var)->noalias_state;
/* Dump memory reference stats for function CFUN to FILE. */
dump_mem_ref_stats (FILE *file)
long actual_num_vuses, actual_num_vdefs;
long num_partitioned, num_unpartitioned;
struct mem_ref_stats_d *stats;
stats = gimple_mem_ref_stats (cfun);
count_mem_refs (&actual_num_vuses, &actual_num_vdefs, &num_partitioned,
fprintf (file, "\nMemory reference statistics for %s\n\n",
lang_hooks.decl_printable_name (current_function_decl, 2));
fprintf (file, "Number of memory statements: %ld\n",
fprintf (file, "Number of call sites: %ld\n",
fprintf (file, "Number of pure/const call sites: %ld\n",
fprintf (file, "Number of asm sites: %ld\n",
fprintf (file, "Estimated number of loads: %ld (%ld/stmt)\n",
? CEIL (stats->num_vuses, stats->num_mem_stmts)
: 0);
fprintf (file, "Actual number of loads: %ld (%ld/stmt)\n",
? CEIL (actual_num_vuses, stats->num_mem_stmts)
: 0);
if (actual_num_vuses > stats->num_vuses + (stats->num_vuses / 25))
fprintf (file, "\t(warning: estimation is lower by more than 25%%)\n");
fprintf (file, "Estimated number of stores: %ld (%ld/stmt)\n",
? CEIL (stats->num_vdefs, stats->num_mem_stmts)
: 0);
fprintf (file, "Actual number of stores: %ld (%ld/stmt)\n",
? CEIL (actual_num_vdefs, stats->num_mem_stmts)
: 0);
if (actual_num_vdefs > stats->num_vdefs + (stats->num_vdefs / 25))
fprintf (file, "\t(warning: estimation is lower by more than 25%%)\n");
fprintf (file, "Partitioning thresholds: MAX = %d AVG = %d "
stats->num_mem_stmts && need_to_partition_p (stats) ? "" : "NO ");
fprintf (file, "Number of partitioned symbols: %ld\n", num_partitioned);
fprintf (file, "Number of unpartitioned symbols: %ld\n", num_unpartitioned);
/* Dump memory reference stats for function FN to stderr. */
debug_mem_ref_stats (void)
dump_mem_ref_stats (stderr);
/* Dump memory reference stats for variable VAR to FILE. */
static void
dump_mem_sym_stats (FILE *file, tree var)
mem_sym_stats_t stats = mem_sym_stats (cfun, var);
if (stats == NULL)
fprintf (file, "read frequency: %6ld, write frequency: %6ld, "
"direct reads: %3ld, direct writes: %3ld, "
"indirect reads: %4ld, indirect writes: %4ld, symbol: ",
stats->frequency_reads, stats->frequency_writes,
stats->num_direct_reads, stats->num_direct_writes,
stats->num_indirect_reads, stats->num_indirect_writes);
print_generic_expr (file, stats->var, 0);
fprintf (file, ", tags: ");
dump_decl_set (file, stats->parent_tags);
/* Dump memory reference stats for variable VAR to stderr. */
debug_mem_sym_stats (tree var)
dump_mem_sym_stats (stderr, var);
/* Dump memory reference stats for variable VAR to FILE. For use
of tree-dfa.c:dump_variable. */
dump_mem_sym_stats_for_var (FILE *file, tree var)
mem_sym_stats_t stats = mem_sym_stats (cfun, var);
if (stats == NULL)
fprintf (file, ", score: %ld", mem_sym_score (stats));
fprintf (file, ", direct reads: %ld", stats->num_direct_reads);
fprintf (file, ", direct writes: %ld", stats->num_direct_writes);
fprintf (file, ", indirect reads: %ld", stats->num_indirect_reads);
fprintf (file, ", indirect writes: %ld", stats->num_indirect_writes);
/* Dump memory reference stats for all memory symbols to FILE. */
static void
dump_all_mem_sym_stats (FILE *file)
referenced_var_iterator rvi;
tree sym;
if (is_gimple_reg (sym))
dump_mem_sym_stats (file, sym);
/* Dump memory reference stats for all memory symbols to stderr. */
debug_all_mem_sym_stats (void)
dump_all_mem_sym_stats (stderr);
/* Dump the MP_INFO array to FILE. */
static void
dump_mp_info (FILE *file, VEC(mem_sym_stats_t,heap) *mp_info)
unsigned i;
mem_sym_stats_t mp_p;
for (i = 0; VEC_iterate (mem_sym_stats_t, mp_info, i, mp_p); i++)
if (!mp_p->partitioned_p)
dump_mem_sym_stats (file, mp_p->var);
/* Dump the MP_INFO array to stderr. */
debug_mp_info (VEC(mem_sym_stats_t,heap) *mp_info)
dump_mp_info (stderr, mp_info);
/* Update memory reference stats for symbol VAR in statement STMT.
NUM_DIRECT_READS and NUM_DIRECT_WRITES specify the number of times
that VAR is read/written in STMT (indirect reads/writes are not
recorded by this function, see compute_memory_partitions). */
update_mem_sym_stats_from_stmt (tree var, gimple stmt, long num_direct_reads,
long num_direct_writes)
mem_sym_stats_t stats;
gcc_assert (num_direct_reads >= 0 && num_direct_writes >= 0);
stats = get_mem_sym_stats_for (var);
stats->num_direct_reads += num_direct_reads;
stats->frequency_reads += ((long) gimple_bb (stmt)->frequency
* num_direct_reads);
stats->num_direct_writes += num_direct_writes;
stats->frequency_writes += ((long) gimple_bb (stmt)->frequency
* num_direct_writes);
/* Given two MP_INFO entries MP1 and MP2, return -1 if MP1->VAR should
be partitioned before MP2->VAR, 0 if they are the same or 1 if
MP1->VAR should be partitioned after MP2->VAR. */
static inline int
compare_mp_info_entries (mem_sym_stats_t mp1, mem_sym_stats_t mp2)
long pscore1 = mem_sym_score (mp1);
long pscore2 = mem_sym_score (mp2);
if (pscore1 < pscore2)
return -1;
else if (pscore1 > pscore2)
return 1;
return DECL_UID (mp1->var) - DECL_UID (mp2->var);
/* Comparison routine for qsort. The list is sorted by increasing
partitioning score (PSCORE). This score is computed such that
symbols with high scores are those that are least likely to be
partitioned. */
static int
mp_info_cmp (const void *p, const void *q)
mem_sym_stats_t e1 = *((const mem_sym_stats_t *) p);
mem_sym_stats_t e2 = *((const mem_sym_stats_t *) q);
return compare_mp_info_entries (e1, e2);
/* Sort the array of reference counts used to compute memory partitions.
Elements are sorted in ascending order of execution frequency and
descending order of virtual operators needed. */
static inline void
sort_mp_info (VEC(mem_sym_stats_t,heap) *list)
unsigned num = VEC_length (mem_sym_stats_t, list);
if (num < 2)
if (num == 2)
if (compare_mp_info_entries (VEC_index (mem_sym_stats_t, list, 0),
VEC_index (mem_sym_stats_t, list, 1)) > 0)
/* Swap elements if they are in the wrong order. */
mem_sym_stats_t tmp = VEC_index (mem_sym_stats_t, list, 0);
VEC_replace (mem_sym_stats_t, list, 0,
VEC_index (mem_sym_stats_t, list, 1));
VEC_replace (mem_sym_stats_t, list, 1, tmp);
/* There are 3 or more elements, call qsort. */
qsort (VEC_address (mem_sym_stats_t, list),
VEC_length (mem_sym_stats_t, list),
sizeof (mem_sym_stats_t),
/* Return the memory partition tag (MPT) associated with memory
symbol SYM. */
static tree
get_mpt_for (tree sym)
tree mpt;
/* Don't create a new tag unnecessarily. */
mpt = memory_partition (sym);
if (mpt == NULL_TREE)
mpt = create_tag_raw (MEMORY_PARTITION_TAG, TREE_TYPE (sym), "MPT");
add_referenced_var (mpt);
VEC_safe_push (tree, heap, gimple_ssa_operands (cfun)->mpt_table, mpt);
gcc_assert (MPT_SYMBOLS (mpt) == NULL);
set_memory_partition (sym, mpt);
return mpt;
/* Add MP_P->VAR to a memory partition and return the partition. */
static tree
find_partition_for (mem_sym_stats_t mp_p)
unsigned i;
VEC(tree,heap) *mpt_table;
tree mpt;
mpt_table = gimple_ssa_operands (cfun)->mpt_table;
mpt = NULL_TREE;
/* Find an existing partition for MP_P->VAR. */
for (i = 0; VEC_iterate (tree, mpt_table, i, mpt); i++)
mem_sym_stats_t mpt_stats;
/* If MPT does not have any symbols yet, use it. */
if (MPT_SYMBOLS (mpt) == NULL)
/* Otherwise, see if MPT has common parent tags with MP_P->VAR,
but avoid grouping clobbered variables with non-clobbered
variables (otherwise, this tends to creates a single memory
partition because other call-clobbered variables may have
common parent tags with non-clobbered ones). */
mpt_stats = get_mem_sym_stats_for (mpt);
if (mp_p->parent_tags
&& mpt_stats->parent_tags
&& is_call_clobbered (mpt) == is_call_clobbered (mp_p->var)
&& bitmap_intersect_p (mpt_stats->parent_tags, mp_p->parent_tags))
/* If no common parent tags are found, see if both MPT and
MP_P->VAR are call-clobbered. */
if (is_call_clobbered (mpt) && is_call_clobbered (mp_p->var))
if (mpt == NULL_TREE)
mpt = get_mpt_for (mp_p->var);
set_memory_partition (mp_p->var, mpt);
mp_p->partitioned_p = true;
mark_sym_for_renaming (mp_p->var);
mark_sym_for_renaming (mpt);
return mpt;
/* Rewrite the alias set for TAG to use the newly created partitions.
If TAG is NULL, rewrite the set of call-clobbered variables.
NEW_ALIASES is a scratch bitmap to build the new set of aliases for
TAG. */
static void
rewrite_alias_set_for (tree tag, bitmap new_aliases)
bitmap_iterator bi;
unsigned i;
tree mpt, sym;
sym = referenced_var (i);
mpt = memory_partition (sym);
if (mpt)
bitmap_set_bit (new_aliases, DECL_UID (mpt));
bitmap_set_bit (new_aliases, DECL_UID (sym));
/* Rebuild the may-alias array for TAG. */
bitmap_copy (MTAG_ALIASES (tag), new_aliases);
/* Determine how many virtual operands can be saved by partitioning
MP_P->VAR into MPT. When a symbol S is thrown inside a partition
P, every virtual operand that used to reference S will now
reference P. Whether it reduces the number of virtual operands
depends on:
1- Direct references to S are never saved. Instead of the virtual
operand to S, we will now have a virtual operand to P.
2- Indirect references to S are reduced only for those memory tags
holding S that already had other symbols partitioned into P.
For instance, if a memory tag T has the alias set { a b S c },
the first time we partition S into P, the alias set will become
{ a b P c }, so no virtual operands will be saved. However, if
we now partition symbol 'c' into P, then the alias set for T
will become { a b P }, so we will be saving one virtual operand
for every indirect reference to 'c'.
3- Is S is call-clobbered, we save as many virtual operands as
call/asm sites exist in the code, but only if other
call-clobbered symbols have been grouped into P. The first
call-clobbered symbol that we group does not produce any
MEM_REF_STATS points to CFUN's memory reference information. */
static void
estimate_vop_reduction (struct mem_ref_stats_d *mem_ref_stats,
mem_sym_stats_t mp_p, tree mpt)
unsigned i;
bitmap_iterator bi;
mem_sym_stats_t mpt_stats;
/* We should only get symbols with indirect references here. */
gcc_assert (mp_p->num_indirect_reads > 0 || mp_p->num_indirect_writes > 0);
/* Note that the only statistics we keep for MPT is the set of
parent tags to know which memory tags have had alias members
partitioned, and the indicator has_call_clobbered_vars.
Reference counts are not important for MPT. */
mpt_stats = get_mem_sym_stats_for (mpt);
/* Traverse all the parent tags for MP_P->VAR. For every tag T, if
partition P is already grouping aliases of T, then reduce the
number of virtual operands by the number of direct references
to T. */
if (mp_p->parent_tags)
if (mpt_stats->parent_tags == NULL)
mpt_stats->parent_tags = BITMAP_ALLOC (&alias_bitmap_obstack);
EXECUTE_IF_SET_IN_BITMAP (mp_p->parent_tags, 0, i, bi)
if (bitmap_bit_p (mpt_stats->parent_tags, i))
/* Partition MPT is already partitioning symbols in the
alias set for TAG. This means that we are now saving
1 virtual operand for every direct reference to TAG. */
tree tag = referenced_var (i);
mem_sym_stats_t tag_stats = mem_sym_stats (cfun, tag);
mem_ref_stats->num_vuses -= tag_stats->num_direct_reads;
mem_ref_stats->num_vdefs -= tag_stats->num_direct_writes;
/* This is the first symbol in tag I's alias set that is
being grouped under MPT. We will not save any
virtual operands this time, but record that MPT is
grouping a symbol from TAG's alias set so that the
next time we get the savings. */
bitmap_set_bit (mpt_stats->parent_tags, i);
/* If MP_P->VAR is call-clobbered, and MPT is already grouping
call-clobbered symbols, then we will save as many virtual
operands as asm/call sites there are. */
if (is_call_clobbered (mp_p->var))
if (mpt_stats->has_call_clobbered_vars)
mem_ref_stats->num_vdefs -= mem_ref_stats->num_call_sites
+ mem_ref_stats->num_asm_sites;
mpt_stats->has_call_clobbered_vars = true;
/* Helper for compute_memory_partitions. Transfer reference counts
from pointers to their pointed-to sets. Counters for pointers were
computed by update_alias_info. MEM_REF_STATS points to CFUN's
memory reference information. */
static void
update_reference_counts (struct mem_ref_stats_d *mem_ref_stats)
unsigned i;
bitmap_iterator bi;
mem_sym_stats_t sym_stats;
for (i = 1; i < num_ssa_names; i++)
tree ptr;
struct ptr_info_def *pi;
ptr = ssa_name (i);
if (ptr
&& (pi = SSA_NAME_PTR_INFO (ptr)) != NULL
&& pi->memory_tag_needed)
unsigned j;
bitmap_iterator bj;
tree tag;
mem_sym_stats_t ptr_stats, tag_stats;
/* If PTR has flow-sensitive points-to information, use
PTR's name tag, otherwise use the symbol tag associated
with PTR's symbol. */
if (pi->name_mem_tag)
tag = pi->name_mem_tag;
tag = symbol_mem_tag (SSA_NAME_VAR (ptr));
ptr_stats = get_mem_sym_stats_for (ptr);
tag_stats = get_mem_sym_stats_for (tag);
/* TAG has as many direct references as dereferences we
found for its parent pointer. */
tag_stats->num_direct_reads += ptr_stats->num_direct_reads;
tag_stats->num_direct_writes += ptr_stats->num_direct_writes;
/* All the dereferences of pointer PTR are considered direct
references to PTR's memory tag (TAG). In turn,
references to TAG will become virtual operands for every
symbol in TAG's alias set. So, for every symbol ALIAS in
TAG's alias set, add as many indirect references to ALIAS
as direct references there are for TAG. */
if (MTAG_ALIASES (tag))
tree alias = referenced_var (j);
sym_stats = get_mem_sym_stats_for (alias);
/* All the direct references to TAG are indirect references
to ALIAS. */
sym_stats->num_indirect_reads += ptr_stats->num_direct_reads;
sym_stats->num_indirect_writes += ptr_stats->num_direct_writes;
sym_stats->frequency_reads += ptr_stats->frequency_reads;
sym_stats->frequency_writes += ptr_stats->frequency_writes;
/* Indicate that TAG is one of ALIAS's parent tags. */
if (sym_stats->parent_tags == NULL)
sym_stats->parent_tags = BITMAP_ALLOC (&alias_bitmap_obstack);
bitmap_set_bit (sym_stats->parent_tags, DECL_UID (tag));
/* Call-clobbered symbols are indirectly written at every
call/asm site. */
EXECUTE_IF_SET_IN_BITMAP (gimple_call_clobbered_vars (cfun), 0, i, bi)
tree sym = referenced_var (i);
sym_stats = get_mem_sym_stats_for (sym);
sym_stats->num_indirect_writes += mem_ref_stats->num_call_sites
+ mem_ref_stats->num_asm_sites;
/* Addressable symbols are indirectly written at some ASM sites.
Since only ASM sites that clobber memory actually affect
addressable symbols, this is an over-estimation. */
EXECUTE_IF_SET_IN_BITMAP (gimple_addressable_vars (cfun), 0, i, bi)
tree sym = referenced_var (i);
sym_stats = get_mem_sym_stats_for (sym);
sym_stats->num_indirect_writes += mem_ref_stats->num_asm_sites;
/* Helper for compute_memory_partitions. Add all memory symbols to
*MP_INFO_P and compute the initial estimate for the total number of
virtual operands needed. MEM_REF_STATS points to CFUN's memory
reference information. On exit, *TAGS_P will contain the list of
memory tags whose alias set need to be rewritten after
partitioning. */
static void
build_mp_info (struct mem_ref_stats_d *mem_ref_stats,
VEC(mem_sym_stats_t,heap) **mp_info_p,
VEC(tree,heap) **tags_p)
tree var;
referenced_var_iterator rvi;
mem_sym_stats_t sym_stats;
tree old_mpt;
/* We are only interested in memory symbols other than MPTs. */
if (is_gimple_reg (var) || TREE_CODE (var) == MEMORY_PARTITION_TAG)
/* Collect memory tags into the TAGS array so that we can
rewrite their alias sets after partitioning. */
if (MTAG_P (var) && MTAG_ALIASES (var))
VEC_safe_push (tree, heap, *tags_p, var);
/* Since we are going to re-compute partitions, any symbols that
used to belong to a partition must be detached from it and
marked for renaming. */
if ((old_mpt = memory_partition (var)) != NULL)
mark_sym_for_renaming (old_mpt);
set_memory_partition (var, NULL_TREE);
mark_sym_for_renaming (var);
sym_stats = get_mem_sym_stats_for (var);
/* Add VAR's reference info to MP_INFO. Note that the only
symbols that make sense to partition are those that have
indirect references. If a symbol S is always directly
referenced, partitioning it will not reduce the number of
virtual operators. The only symbols that are profitable to
partition are those that belong to alias sets and/or are
call-clobbered. */
if (sym_stats->num_indirect_reads > 0
|| sym_stats->num_indirect_writes > 0)
VEC_safe_push (mem_sym_stats_t, heap, *mp_info_p, sym_stats);
/* Update the number of estimated VOPS. Note that direct
references to memory tags are always counted as indirect
references to their alias set members, so if a memory tag has
aliases, do not count its direct references to avoid double
accounting. */
if (!MTAG_P (var) || !MTAG_ALIASES (var))
mem_ref_stats->num_vuses += sym_stats->num_direct_reads;
mem_ref_stats->num_vdefs += sym_stats->num_direct_writes;
mem_ref_stats->num_vuses += sym_stats->num_indirect_reads;
mem_ref_stats->num_vdefs += sym_stats->num_indirect_writes;
/* Compute memory partitions. A memory partition (MPT) is an
arbitrary grouping of memory symbols, such that references to one
member of the group is considered a reference to all the members of
the group.
As opposed to alias sets in memory tags, the grouping into
partitions is completely arbitrary and only done to reduce the
number of virtual operands. The only rule that needs to be
observed when creating memory partitions is that given two memory
partitions MPT.i and MPT.j, they must not contain symbols in
Memory partitions are used when putting the program into Memory-SSA
form. In particular, in Memory-SSA PHI nodes are not computed for
individual memory symbols. They are computed for memory
partitions. This reduces the amount of PHI nodes in the SSA graph
at the expense of precision (i.e., it makes unrelated stores affect
each other).
However, it is possible to increase precision by changing this
partitioning scheme. For instance, if the partitioning scheme is
such that get_mpt_for is the identity function (that is,
get_mpt_for (s) = s), this will result in ultimate precision at the
expense of huge SSA webs.
At the other extreme, a partitioning scheme that groups all the
symbols in the same set results in minimal SSA webs and almost
total loss of precision.
There partitioning heuristic uses three parameters to decide the
order in which symbols are processed. The list of symbols is
sorted so that symbols that are more likely to be partitioned are
near the top of the list:
- Execution frequency. If a memory references is in a frequently
executed code path, grouping it into a partition may block useful
transformations and cause sub-optimal code generation. So, the
partition heuristic tries to avoid grouping symbols with high
execution frequency scores. Execution frequency is taken
directly from the basic blocks where every reference is made (see
update_mem_sym_stats_from_stmt), which in turn uses the
profile guided machinery, so if the program is compiled with PGO
enabled, more accurate partitioning decisions will be made.
- Number of references. Symbols with few references in the code,
are partitioned before symbols with many references.
- NO_ALIAS attributes. Symbols with any of the NO_ALIAS*
attributes are partitioned after symbols marked MAY_ALIAS.
Once the list is sorted, the partitioning proceeds as follows:
1- For every symbol S in MP_INFO, create a new memory partition MP,
if necessary. To avoid memory partitions that contain symbols
from non-conflicting alias sets, memory partitions are
associated to the memory tag that holds S in its alias set. So,
when looking for a memory partition for S, the memory partition
associated with one of the memory tags holding S is chosen. If
none exists, a new one is created.
2- Add S to memory partition MP.
3- Reduce by 1 the number of VOPS for every memory tag holding S.
4- If the total number of VOPS is less than MAX_ALIASED_VOPS or the
average number of VOPS per statement is less than
AVG_ALIASED_VOPS, stop. Otherwise, go to the next symbol in the
list. */
static void
compute_memory_partitions (void)
tree tag;
unsigned i;
mem_sym_stats_t mp_p;
VEC(mem_sym_stats_t,heap) *mp_info;
bitmap new_aliases;
VEC(tree,heap) *tags;
struct mem_ref_stats_d *mem_ref_stats;
int prev_max_aliased_vops;
mem_ref_stats = gimple_mem_ref_stats (cfun);
gcc_assert (mem_ref_stats->num_vuses == 0 && mem_ref_stats->num_vdefs == 0);
if (mem_ref_stats->num_mem_stmts == 0)
mp_info = NULL;
tags = NULL;
prev_max_aliased_vops = MAX_ALIASED_VOPS;
/* Since we clearly cannot lower the number of virtual operators
below the total number of memory statements in the function, we
may need to adjust MAX_ALIASED_VOPS beforehand. */
if (MAX_ALIASED_VOPS < mem_ref_stats->num_mem_stmts)
MAX_ALIASED_VOPS = mem_ref_stats->num_mem_stmts;
/* Update reference stats for all the pointed-to variables and
memory tags. */
update_reference_counts (mem_ref_stats);
/* Add all the memory symbols to MP_INFO. */
build_mp_info (mem_ref_stats, &mp_info, &tags);
/* No partitions required if we are below the threshold. */
if (!need_to_partition_p (mem_ref_stats))
if (dump_file)
fprintf (dump_file, "\nMemory partitioning NOT NEEDED for %s\n",
get_name (current_function_decl));
goto done;
/* Sort the MP_INFO array so that symbols that should be partitioned
first are near the top of the list. */
sort_mp_info (mp_info);
if (dump_file)
fprintf (dump_file, "\nMemory partitioning NEEDED for %s\n\n",
get_name (current_function_decl));
fprintf (dump_file, "Memory symbol references before partitioning:\n");
dump_mp_info (dump_file, mp_info);
/* Create partitions for variables in MP_INFO until we have enough
to lower the total number of VOPS below MAX_ALIASED_VOPS or if
the average number of VOPS per statement is below
for (i = 0; VEC_iterate (mem_sym_stats_t, mp_info, i, mp_p); i++)
tree mpt;
/* If we are below the threshold, stop. */
if (!need_to_partition_p (mem_ref_stats))
mpt = find_partition_for (mp_p);
estimate_vop_reduction (mem_ref_stats, mp_p, mpt);
/* After partitions have been created, rewrite alias sets to use
them instead of the original symbols. This way, if the alias set
was computed as { a b c d e f }, and the subset { b e f } was
grouped into partition MPT.3, then the new alias set for the tag
will be { a c d MPT.3 }.
Note that this is not strictly necessary. The operand scanner
will always check if a symbol belongs to a partition when adding
virtual operands. However, by reducing the size of the alias
sets to be scanned, the work needed inside the operand scanner is
significantly reduced. */
new_aliases = BITMAP_ALLOC (&alias_bitmap_obstack);
for (i = 0; VEC_iterate (tree, tags, i, tag); i++)
rewrite_alias_set_for (tag, new_aliases);
bitmap_clear (new_aliases);
BITMAP_FREE (new_aliases);
if (dump_file)
fprintf (dump_file, "\nMemory symbol references after partitioning:\n");
dump_mp_info (dump_file, mp_info);
/* Free allocated memory. */
VEC_free (mem_sym_stats_t, heap, mp_info);
VEC_free (tree, heap, tags);
MAX_ALIASED_VOPS = prev_max_aliased_vops;
/* Compute may-alias information for every variable referenced in function
Alias analysis proceeds in 3 main phases:
1- Points-to and escape analysis.
This phase walks the use-def chains in the SSA web looking for three
* Assignments of the form P_i = &VAR
* Assignments of the form P_i = malloc()
* Pointers and ADDR_EXPR that escape the current function.
The concept of 'escaping' is the same one used in the Java world. When
a pointer or an ADDR_EXPR escapes, it means that it has been exposed
outside of the current function. So, assignment to global variables,
function arguments and returning a pointer are all escape sites, as are
conversions between pointers and integers.
This is where we are currently limited. Since not everything is renamed
into SSA, we lose track of escape properties when a pointer is stashed
inside a field in a structure, for instance. In those cases, we are
assuming that the pointer does escape.
We use escape analysis to determine whether a variable is
call-clobbered. Simply put, if an ADDR_EXPR escapes, then the variable
is call-clobbered. If a pointer P_i escapes, then all the variables
pointed-to by P_i (and its memory tag) also escape.
2- Compute flow-sensitive aliases
We have two classes of memory tags. Memory tags associated with the
pointed-to data type of the pointers in the program. These tags are
called "symbol memory tag" (SMT). The other class are those associated
with SSA_NAMEs, called "name memory tag" (NMT). The basic idea is that
when adding operands for an INDIRECT_REF *P_i, we will first check
whether P_i has a name tag, if it does we use it, because that will have
more precise aliasing information. Otherwise, we use the standard symbol
In this phase, we go through all the pointers we found in points-to
analysis and create alias sets for the name memory tags associated with
each pointer P_i. If P_i escapes, we mark call-clobbered the variables
it points to and its tag.
3- Compute flow-insensitive aliases
This pass will compare the alias set of every symbol memory tag and
every addressable variable found in the program. Given a symbol
memory tag SMT and an addressable variable V. If the alias sets of
SMT and V conflict (as computed by may_alias_p), then V is marked
as an alias tag and added to the alias set of SMT.
For instance, consider the following function:
foo (int i)
int *p, a, b;
if (i > 10)
p = &a;
p = &b;
*p = 3;
a = b + 2;
return *p;
After aliasing analysis has finished, the symbol memory tag for pointer
'p' will have two aliases, namely variables 'a' and 'b'. Every time
pointer 'p' is dereferenced, we want to mark the operation as a
potential reference to 'a' and 'b'.
foo (int i)
int *p, a, b;
if (i_2 > 10)
p_4 = &a;
p_6 = &b;
# p_1 = PHI <p_4(1), p_6(2)>;
# a_7 = VDEF <a_3>;
# b_8 = VDEF <b_5>;
*p_1 = 3;
# a_9 = VDEF <a_7>
# VUSE <b_8>
a_9 = b_8 + 2;
# VUSE <a_9>;
# VUSE <b_8>;
return *p_1;
In certain cases, the list of may aliases for a pointer may grow too
large. This may cause an explosion in the number of virtual operands
inserted in the code. Resulting in increased memory consumption and
compilation time.
When the number of virtual operands needed to represent aliased
loads and stores grows too large (configurable with option --param
max-aliased-vops and --param avg-aliased-vops), alias sets are
grouped to avoid severe compile-time slow downs and memory
consumption. See compute_memory_partitions. */
unsigned int
compute_may_aliases (void)
struct alias_info *ai;
timevar_push (TV_TREE_MAY_ALIAS);
memset (&alias_stats, 0, sizeof (alias_stats));
/* Initialize aliasing information. */
ai = init_alias_info ();
/* For each pointer P_i, determine the sets of variables that P_i may
point-to. For every addressable variable V, determine whether the
address of V escapes the current function, making V call-clobbered
(i.e., whether &V is stored in a global variable or if its passed as a
function call argument). */
compute_points_to_sets ();
/* Update various related attributes like escaped addresses,
pointer dereferences for loads and stores. This is used
when creating name tags and alias sets. */
update_alias_info (ai);
/* Collect all pointers and addressable variables, compute alias sets,
create memory tags for pointers and promote variables whose address is
not needed anymore. */
setup_pointers_and_addressables (ai);
/* Compute type-based flow-insensitive aliasing for all the type
memory tags. */
compute_flow_insensitive_aliasing (ai);
/* Compute flow-sensitive, points-to based aliasing for all the name
memory tags. */
compute_flow_sensitive_aliasing (ai);
/* Compute call clobbering information. */
compute_call_clobbered (ai);
/* If the program makes no reference to global variables, but it
contains a mixture of pure and non-pure functions, then we need
to create use-def and def-def links between these functions to
avoid invalid transformations on them. */
maybe_create_global_var ();
/* Compute memory partitions for every memory variable. */
compute_memory_partitions ();
/* Remove partitions with no symbols. Partitions may end up with an
empty MPT_SYMBOLS set if a previous round of alias analysis
needed to partition more symbols. Since we don't need those
partitions anymore, remove them to free up the space. */
tree mpt;
unsigned i;
VEC(tree,heap) *mpt_table;
mpt_table = gimple_ssa_operands (cfun)->mpt_table;
i = 0;
while (i < VEC_length (tree, mpt_table))
mpt = VEC_index (tree, mpt_table, i);
if (MPT_SYMBOLS (mpt) == NULL)
VEC_unordered_remove (tree, mpt_table, i);
/* Populate all virtual operands and newly promoted register operands. */
gimple_stmt_iterator gsi;
basic_block bb;
for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
update_stmt_if_modified (gsi_stmt (gsi));
/* Debugging dumps. */
if (dump_file)
dump_mem_ref_stats (dump_file);
dump_alias_info (dump_file);
dump_points_to_info (dump_file);
if (dump_flags & TDF_STATS)
dump_alias_stats (dump_file);
if (dump_flags & TDF_DETAILS)
dump_referenced_vars (dump_file);
/* Deallocate memory used by aliasing data structures. */
delete_alias_info (ai);
if (need_ssa_update_p ())
update_ssa (TODO_update_ssa);
timevar_pop (TV_TREE_MAY_ALIAS);
return 0;
/* Data structure used to count the number of dereferences to PTR
inside an expression. */
struct count_ptr_d
tree ptr;
unsigned num_stores;
unsigned num_loads;
/* Helper for count_uses_and_derefs. Called by walk_tree to look for
(ALIGN/MISALIGNED_)INDIRECT_REF nodes for the pointer passed in DATA. */
static tree
count_ptr_derefs (tree *tp, int *walk_subtrees, void *data)
struct walk_stmt_info *wi_p = (struct walk_stmt_info *) data;
struct count_ptr_d *count_p = (struct count_ptr_d *) wi_p->info;
/* Do not walk inside ADDR_EXPR nodes. In the expression &ptr->fld,
pointer 'ptr' is *not* dereferenced, it is simply used to compute
the address of 'fld' as 'ptr + offsetof(fld)'. */
if (TREE_CODE (*tp) == ADDR_EXPR)
*walk_subtrees = 0;
return NULL_TREE;
if (INDIRECT_REF_P (*tp) && TREE_OPERAND (*tp, 0) == count_p->ptr)
if (wi_p->is_lhs)
return NULL_TREE;
/* Count the number of direct and indirect uses for pointer PTR in
statement STMT. The number of direct uses is stored in
*NUM_USES_P. Indirect references are counted separately depending
on whether they are store or load operations. The counts are
stored in *NUM_STORES_P and *NUM_LOADS_P. */
count_uses_and_derefs (tree ptr, gimple stmt, unsigned *num_uses_p,
unsigned *num_loads_p, unsigned *num_stores_p)
ssa_op_iter i;
tree use;
*num_uses_p = 0;
*num_loads_p = 0;
*num_stores_p = 0;
/* Find out the total number of uses of PTR in STMT. */
if (use == ptr)
/* Now count the number of indirect references to PTR. This is
truly awful, but we don't have much choice. There are no parent
pointers inside INDIRECT_REFs, so an expression like
'*x_1 = foo (x_1, *x_1)' needs to be traversed piece by piece to
find all the indirect and direct uses of x_1 inside. The only
shortcut we can take is the fact that GIMPLE only allows
INDIRECT_REFs inside the expressions below. */
if (is_gimple_assign (stmt)
|| gimple_code (stmt) == GIMPLE_RETURN
|| gimple_code (stmt) == GIMPLE_ASM
|| is_gimple_call (stmt))
struct walk_stmt_info wi;
struct count_ptr_d count;
count.ptr = ptr;
count.num_stores = 0;
count.num_loads = 0;
memset (&wi, 0, sizeof (wi)); = &count;
walk_gimple_op (stmt, count_ptr_derefs, &wi);
*num_stores_p = count.num_stores;
*num_loads_p = count.num_loads;
gcc_assert (*num_uses_p >= *num_loads_p + *num_stores_p);
/* Remove memory references stats for function FN. */
delete_mem_ref_stats (struct function *fn)
if (gimple_mem_ref_stats (fn)->mem_sym_stats)
free_alloc_pool (mem_sym_stats_pool);
pointer_map_destroy (gimple_mem_ref_stats (fn)->mem_sym_stats);
gimple_mem_ref_stats (fn)->mem_sym_stats = NULL;
/* Initialize memory reference stats. */
static void
init_mem_ref_stats (void)
struct mem_ref_stats_d *mem_ref_stats = gimple_mem_ref_stats (cfun);
mem_sym_stats_pool = create_alloc_pool ("Mem sym stats",
sizeof (struct mem_sym_stats_d),
memset (mem_ref_stats, 0, sizeof (struct mem_ref_stats_d));
mem_ref_stats->mem_sym_stats = pointer_map_create ();
/* Helper for init_alias_info. Reset existing aliasing information. */
static void
reset_alias_info (void)
referenced_var_iterator rvi;
tree var;
unsigned i;
bitmap active_nmts, all_nmts;
/* Clear the set of addressable variables. We do not need to clear
the TREE_ADDRESSABLE bit on every symbol because we are going to
re-compute addressability here. */
bitmap_clear (gimple_addressable_vars (cfun));
active_nmts = BITMAP_ALLOC (&alias_bitmap_obstack);
all_nmts = BITMAP_ALLOC (&alias_bitmap_obstack);
/* Clear flow-insensitive alias information from each symbol. */
if (is_gimple_reg (var))
if (MTAG_P (var))
/* Memory partition information will be computed from scratch. */
/* Collect all the name tags to determine if we have any
orphaned that need to be removed from the IL. A name tag
will be orphaned if it is not associated with any active SSA
name. */
bitmap_set_bit (all_nmts, DECL_UID (var));
/* Since we are about to re-discover call-clobbered
variables, clear the call-clobbered flag. */
clear_call_clobbered (var);
/* There should be no call-clobbered variable left. */
gcc_assert (bitmap_empty_p (gimple_call_clobbered_vars (cfun)));
/* Clear the call-used variables. */
bitmap_clear (gimple_call_used_vars (cfun));
/* Clear flow-sensitive points-to information from each SSA name. */
for (i = 1; i < num_ssa_names; i++)
tree name = ssa_name (i);
if (!name || !POINTER_TYPE_P (TREE_TYPE (name)))
if (SSA_NAME_PTR_INFO (name))
struct ptr_info_def *pi = SSA_NAME_PTR_INFO (name);
/* Clear all the flags but keep the name tag to
avoid creating new temporaries unnecessarily. If
this pointer is found to point to a subset or
superset of its former points-to set, then a new
tag will need to be created in create_name_tags. */
pi->pt_anything = 0;
pi->pt_null = 0;
pi->value_escapes_p = 0;
pi->memory_tag_needed = 0;
pi->is_dereferenced = 0;
if (pi->pt_vars)
bitmap_clear (pi->pt_vars);
/* Add NAME's name tag to the set of active tags. */
if (pi->name_mem_tag)
bitmap_set_bit (active_nmts, DECL_UID (pi->name_mem_tag));
/* Name memory tags that are no longer associated with an SSA name
are considered stale and should be removed from the IL. All the
name tags that are in the set ALL_NMTS but not in ACTIVE_NMTS are
considered stale and marked for renaming. */
bitmap_and_compl_into (all_nmts, active_nmts);
mark_set_for_renaming (all_nmts);
BITMAP_FREE (all_nmts);
BITMAP_FREE (active_nmts);
/* Initialize the data structures used for alias analysis. */
static struct alias_info *
init_alias_info (void)
struct alias_info *ai;
referenced_var_iterator rvi;
tree var;
static bool alias_bitmap_obstack_initialized;
ai = XCNEW (struct alias_info);
ai->ssa_names_visited = sbitmap_alloc (num_ssa_names);
sbitmap_zero (ai->ssa_names_visited);
ai->processed_ptrs = VEC_alloc (tree, heap, 50);
ai->dereferenced_ptrs = pointer_set_create ();
/* Clear out all memory reference stats. */
init_mem_ref_stats ();
/* If aliases have been computed before, clear existing information. */
if (gimple_aliases_computed_p (cfun))
reset_alias_info ();
/* If this is the first time we compute aliasing information,
every non-register symbol will need to be put into SSA form
(the initial SSA form only operates on GIMPLE registers). */
if (!is_gimple_reg (var))
mark_sym_for_renaming (var);
/* Next time, we will need to reset alias information. */
cfun->gimple_df->aliases_computed_p = true;
if (alias_bitmap_obstack_initialized)
bitmap_obstack_release (&alias_bitmap_obstack);
bitmap_obstack_initialize (&alias_bitmap_obstack);
alias_bitmap_obstack_initialized = true;
return ai;
/* Deallocate memory used by alias analysis. */
static void
delete_alias_info (struct alias_info *ai)
size_t i;
sbitmap_free (ai->ssa_names_visited);
VEC_free (tree, heap, ai->processed_ptrs);
for (i = 0; i < ai->num_addressable_vars; i++)
free (ai->addressable_vars[i]);
free (ai->addressable_vars);
for (i = 0; i < ai->num_pointers; i++)
free (ai->pointers[i]);
free (ai->pointers);
pointer_set_destroy (ai->dereferenced_ptrs);
free (ai);
delete_mem_ref_stats (cfun);
delete_points_to_sets ();
/* Used for hashing to identify pointer infos with identical
pt_vars bitmaps. */
static int
eq_ptr_info (const void *p1, const void *p2)
const struct ptr_info_def *n1 = (const struct ptr_info_def *) p1;
const struct ptr_info_def *n2 = (const struct ptr_info_def *) p2;
return bitmap_equal_p (n1->pt_vars, n2->pt_vars);
static hashval_t
ptr_info_hash (const void *p)
const struct ptr_info_def *n = (const struct ptr_info_def *) p;
return bitmap_hash (n->pt_vars);
/* Create name tags for all the pointers that have been dereferenced.
We only create a name tag for a pointer P if P is found to point to
a set of variables (so that we can alias them to *P) or if it is
the result of a call to malloc (which means that P cannot point to
anything else nor alias any other variable).
If two pointers P and Q point to the same set of variables, they
are assigned the same name tag. */
static void
create_name_tags (void)
size_t i;
VEC (tree, heap) *with_ptvars = NULL;
tree ptr;
htab_t ptr_hash;
/* Collect the list of pointers with a non-empty points to set. */
for (i = 1; i < num_ssa_names; i++)
tree ptr = ssa_name (i);
struct ptr_info_def *pi;
if (!ptr
|| !SSA_NAME_PTR_INFO (ptr))
pi = SSA_NAME_PTR_INFO (ptr);
if (pi->pt_anything || !pi->memory_tag_needed)
/* No name tags for pointers that have not been
dereferenced or point to an arbitrary location. */
pi->name_mem_tag = NULL_TREE;
/* Set pt_anything on the pointers without pt_vars filled in so
that they are assigned a symbol tag. */
if (pi->pt_vars && !bitmap_empty_p (pi->pt_vars))
VEC_safe_push (tree, heap, with_ptvars, ptr);
set_pt_anything (ptr);
/* If we didn't find any pointers with pt_vars set, we're done. */
if (!with_ptvars)
ptr_hash = htab_create (10, ptr_info_hash, eq_ptr_info, NULL);
/* Now go through the pointers with pt_vars, and find a name tag
with the same pt_vars as this pointer, or create one if one
doesn't exist. */
for (i = 0; VEC_iterate (tree, with_ptvars, i, ptr); i++)
struct ptr_info_def *pi = SSA_NAME_PTR_INFO (ptr);
tree old_name_tag = pi->name_mem_tag;
struct ptr_info_def **slot;
/* If PTR points to a set of variables, check if we don't
have another pointer Q with the same points-to set before
creating a tag. If so, use Q's tag instead of creating a
new one.
This is important for not creating unnecessary symbols
and also for copy propagation. If we ever need to
propagate PTR into Q or vice-versa, we would run into
problems if they both had different name tags because
they would have different SSA version numbers (which
would force us to take the name tags in and out of SSA). */
slot = (struct ptr_info_def **) htab_find_slot (ptr_hash, pi, INSERT);
if (*slot)
pi->name_mem_tag = (*slot)->name_mem_tag;
*slot = pi;
/* If we didn't find a pointer with the same points-to set
as PTR, create a new name tag if needed. */
if (pi->name_mem_tag == NULL_TREE)
pi->name_mem_tag = get_nmt_for (ptr);
/* If the new name tag computed for PTR is different than
the old name tag that it used to have, then the old tag
needs to be removed from the IL, so we mark it for
renaming. */
if (old_name_tag && old_name_tag != pi->name_mem_tag)
mark_sym_for_renaming (old_name_tag);
/* Inherit volatility from the pointed-to type. */
TREE_THIS_VOLATILE (pi->name_mem_tag)
/* Mark the new name tag for renaming. */
mark_sym_for_renaming (pi->name_mem_tag);
htab_delete (ptr_hash);
VEC_free (tree, heap, with_ptvars);
/* Union the alias set SET into the may-aliases for TAG. */
static void
union_alias_set_into (tree tag, bitmap set)
bitmap ma = MTAG_ALIASES (tag);
if (bitmap_empty_p (set))
if (!ma)
ma = MTAG_ALIASES (tag) = BITMAP_ALLOC (&alias_bitmap_obstack);
bitmap_ior_into (ma, set);
/* For every pointer P_i in AI->PROCESSED_PTRS, create may-alias sets for
the name memory tag (NMT) associated with P_i. If P_i escapes, then its
name tag and the variables it points-to are call-clobbered. Finally, if
P_i escapes and we could not determine where it points to, then all the
variables in the same alias set as *P_i are marked call-clobbered. This
is necessary because we must assume that P_i may take the address of any
variable in the same alias set. */
static void
compute_flow_sensitive_aliasing (struct alias_info *ai)
size_t i;
tree ptr;
timevar_push (TV_FLOW_SENSITIVE);
for (i = 0; VEC_iterate (tree, ai->processed_ptrs, i, ptr); i++)
if (!find_what_p_points_to (ptr))
set_pt_anything (ptr);
create_name_tags ();
for (i = 0; VEC_iterate (tree, ai->processed_ptrs, i, ptr); i++)
struct ptr_info_def *pi = SSA_NAME_PTR_INFO (ptr);
/* Set up aliasing information for PTR's name memory tag (if it has
one). Note that only pointers that have been dereferenced will
have a name memory tag. */
if (pi->name_mem_tag && pi->pt_vars)
if (!bitmap_empty_p (pi->pt_vars))
union_alias_set_into (pi->name_mem_tag, pi->pt_vars);
timevar_pop (TV_FLOW_SENSITIVE);
/* Return TRUE if at least one symbol in TAG2's alias set is also
present in TAG1's alias set. */
static bool
have_common_aliases_p (bitmap tag1aliases, bitmap tag2aliases)
/* This is the old behavior of have_common_aliases_p, which is to
return false if both sets are empty, or one set is and the other
isn't. */
if (tag1aliases == NULL || tag2aliases == NULL)
return false;
return bitmap_intersect_p (tag1aliases, tag2aliases);
/* Compute type-based alias sets. Traverse all the pointers and
addressable variables found in setup_pointers_and_addressables.
For every pointer P in AI->POINTERS and addressable variable V in
AI->ADDRESSABLE_VARS, add V to the may-alias sets of P's symbol
memory tag (SMT) if their alias sets conflict. V is then marked as
an aliased symbol so that the operand scanner knows that statements
containing V have aliased operands. */
static void
compute_flow_insensitive_aliasing (struct alias_info *ai)
referenced_var_iterator rvi;
tree var;
size_t i;
timevar_push (TV_FLOW_INSENSITIVE);
/* Since this analysis is based exclusively on symbols, it fails to
handle cases where two pointers P and Q have different memory
tags with conflicting alias set numbers but no aliased symbols in
For example, suppose that we have two memory tags SMT.1 and SMT.2
such that
may-aliases (SMT.1) = { a }
may-aliases (SMT.2) = { b }
and the alias set number of SMT.1 conflicts with that of SMT.2.
Since they don't have symbols in common, loads and stores from
SMT.1 and SMT.2 will seem independent of each other, which will
lead to the optimizers making invalid transformations (see
To avoid this problem, we do a final traversal of AI->POINTERS
looking for pairs of pointers that have no aliased symbols in
common and yet have conflicting alias set numbers.
Note this has to be done first as we only can avoid adding
aliases for common memory tag aliases, not for common symbol
aliases as they might get pruned by the operand scanner later. */
for (i = 0; i < ai->num_pointers; i++)
size_t j;
struct alias_map_d *p_map1 = ai->pointers[i];
tree tag1 = symbol_mem_tag (p_map1->var);
bitmap may_aliases1 = MTAG_ALIASES (tag1);
for (j = 0; j < ai->num_pointers; j++)
struct alias_map_d *p_map2 = ai->pointers[j];
tree tag2 = symbol_mem_tag (p_map2->var);
bitmap may_aliases2 = may_aliases (tag2);
/* By convention tags don't alias themselves. */
if (tag1 == tag2)
/* If the pointers may not point to each other, do nothing. */
if (!may_alias_p (p_map1->var, p_map1->set, tag2, p_map2->set, true))
/* The two pointers may alias each other. If they already have
symbols in common, do nothing. */
if (have_common_aliases_p (may_aliases1, may_aliases2))
add_may_alias (tag1, tag2);
/* For every pointer P, determine which addressable variables may alias
with P's symbol memory tag. */
for (i = 0; i < ai->num_pointers; i++)
size_t j;
struct alias_map_d *p_map = ai->pointers[i];
tree tag = symbol_mem_tag (p_map->var);
tree var;
for (j = 0; j < ai->num_addressable_vars; j++)
struct alias_map_d *v_map;
var_ann_t v_ann;
v_map = ai->addressable_vars[j];
var = v_map->var;
v_ann = var_ann (var);
/* We used to skip variables that have never been written to
if the memory tag has been never written to directly (or
either of them were call clobbered). This is not enough
though, as this misses writes through the tags aliases.
So, for correctness we need to include any aliased
variable here. */
if (may_alias_p (p_map->var, p_map->set, var, v_map->set, false))
/* Add VAR to TAG's may-aliases set. */
add_may_alias (tag, var);
/* We have to add all HEAP variables to all SMTs aliases bitmaps.
As we don't know which effective type the HEAP will have we cannot
do better here and we need the conflicts with obfuscated pointers
(a simple (*(int[n] *)ptr)[i] will do, with ptr from a VLA array
allocation). */
for (i = 0; i < ai->num_pointers; i++)
struct alias_map_d *p_map = ai->pointers[i];
tree tag = symbol_mem_tag (p_map->var);
if (var_ann (var)->is_heapvar)
add_may_alias (tag, var);
timevar_pop (TV_FLOW_INSENSITIVE);
/* Create a new alias set entry for VAR in AI->ADDRESSABLE_VARS. */
static void
create_alias_map_for (tree var, struct alias_info *ai)
struct alias_map_d *alias_map;
alias_map = XCNEW (struct alias_map_d);
alias_map->var = var;
alias_map->set = get_alias_set (var);
ai->addressable_vars[ai->num_addressable_vars++] = alias_map;
/* Update related alias information kept in AI. This is used when
building name tags, alias sets and deciding grouping heuristics.
STMT is the statement to process. This function also updates
static void
update_alias_info_1 (gimple stmt, struct alias_info *ai)
bitmap addr_taken;
use_operand_p use_p;
ssa_op_iter iter;
bool stmt_dereferences_ptr_p;
enum escape_type stmt_escape_type = is_escape_site (stmt);
struct mem_ref_stats_d *mem_ref_stats = gimple_mem_ref_stats (cfun);
stmt_dereferences_ptr_p = false;
if (stmt_escape_type == ESCAPE_TO_CALL
|| stmt_escape_type == ESCAPE_TO_PURE_CONST)
if (stmt_escape_type == ESCAPE_TO_PURE_CONST)
else if (stmt_escape_type == ESCAPE_TO_ASM)
/* Mark all the variables whose address are taken by the statement. */
addr_taken = gimple_addresses_taken (stmt);
if (addr_taken)
bitmap_ior_into (gimple_addressable_vars (cfun), addr_taken);
/* If we have a call or an assignment, see if the lhs contains
a local decl that requires not to be a gimple register. */
if (gimple_code (stmt) == GIMPLE_ASSIGN
|| gimple_code (stmt) == GIMPLE_CALL)
tree lhs = gimple_get_lhs (stmt);
/* A plain decl does not need it set. */
if (lhs && handled_component_p (lhs))
tree var = get_base_address (lhs);
if (DECL_P (var)
/* We are not going to mess with RESULT_DECL anyway. */
&& is_gimple_reg_type (TREE_TYPE (var)))
bitmap_set_bit (gimple_addressable_vars (cfun), DECL_UID (var));
/* Process each operand use. For pointers, determine whether they
are dereferenced by the statement, or whether their value
escapes, etc. */
FOR_EACH_PHI_OR_STMT_USE (use_p, stmt, iter, SSA_OP_USE)
tree op, var;
var_ann_t v_ann;
struct ptr_info_def *pi;
unsigned num_uses, num_loads, num_stores;
op = USE_FROM_PTR (use_p);
/* If STMT is a PHI node, OP may be an ADDR_EXPR. If so, add it
to the set of addressable variables. */
if (TREE_CODE (op) == ADDR_EXPR)
bitmap addressable_vars = gimple_addressable_vars (cfun);
gcc_assert (gimple_code (stmt) == GIMPLE_PHI);
gcc_assert (addressable_vars);
/* PHI nodes don't have annotations for pinning the set
of addresses taken, so we collect them here.
FIXME, should we allow PHI nodes to have annotations
so that they can be treated like regular statements?
Currently, they are treated as second-class
statements. */
add_to_addressable_set (TREE_OPERAND (op, 0), &addressable_vars);
/* Ignore constants (they may occur in PHI node arguments). */
if (TREE_CODE (op) != SSA_NAME)
var = SSA_NAME_VAR (op);
v_ann = var_ann (var);
/* The base variable of an SSA name must be a GIMPLE register, and thus
it cannot be aliased. */
gcc_assert (!may_be_aliased (var));
/* We are only interested in pointers. */
pi = get_ptr_info (op);
/* Add OP to AI->PROCESSED_PTRS, if it's not there already. */
if (!TEST_BIT (ai->ssa_names_visited, SSA_NAME_VERSION (op)))
SET_BIT (ai->ssa_names_visited, SSA_NAME_VERSION (op));
VEC_safe_push (tree, heap, ai->processed_ptrs, op);
/* If STMT is a PHI node, then it will not have pointer
dereferences and it will not be an escape point. */
if (gimple_code (stmt) == GIMPLE_PHI)
/* Determine whether OP is a dereferenced pointer, and if STMT
is an escape point, whether OP escapes. */
count_uses_and_derefs (op, stmt, &num_uses, &num_loads, &num_stores);
/* For directly dereferenced pointers we can apply
TBAA-pruning to their points-to set. We may not count the
implicit dereferences &PTR->FLD here. */
if (num_loads + num_stores > 0)
pi->is_dereferenced = 1;
/* Handle a corner case involving address expressions of the
form '&PTR->FLD'. The problem with these expressions is that
they do not represent a dereference of PTR. However, if some
other transformation propagates them into an INDIRECT_REF
expression, we end up with '*(&PTR->FLD)' which is folded
into 'PTR->FLD'.
So, if the original code had no other dereferences of PTR,
the aliaser will not create memory tags for it, and when
&PTR->FLD gets propagated to INDIRECT_REF expressions, the
memory operations will receive no VDEF/VUSE operands.
One solution would be to have count_uses_and_derefs consider
&PTR->FLD a dereference of PTR. But that is wrong, since it
is not really a dereference but an offset calculation.
What we do here is to recognize these special ADDR_EXPR
nodes. Since these expressions are never GIMPLE values (they
are not GIMPLE invariants), they can only appear on the RHS
of an assignment and their base address is always an
INDIRECT_REF expression. */
if (is_gimple_assign (stmt)
&& gimple_assign_rhs_code (stmt) == ADDR_EXPR
&& !is_gimple_val (gimple_assign_rhs1 (stmt)))
/* If the RHS if of the form &PTR->FLD and PTR == OP, then
this represents a potential dereference of PTR. */
tree rhs = gimple_assign_rhs1 (stmt);
tree base = get_base_address (TREE_OPERAND (rhs, 0));
&& TREE_OPERAND (base, 0) == op)
if (num_loads + num_stores > 0)
/* Mark OP as dereferenced. In a subsequent pass,
dereferenced pointers that point to a set of
variables will be assigned a name tag to alias
all the variables OP points to. */
pi->memory_tag_needed = 1;
/* ??? For always executed direct dereferences we can
apply TBAA-pruning to their escape set. */
/* Mark OP as being dereferenced. */
pointer_set_insert (ai->dereferenced_ptrs, var);
/* Update the frequency estimate for all the dereferences of
pointer OP. */
update_mem_sym_stats_from_stmt (op, stmt, num_loads, num_stores);
/* Indicate that STMT contains pointer dereferences. */
stmt_dereferences_ptr_p = true;
if (stmt_escape_type != NO_ESCAPE && num_loads + num_stores < num_uses)
/* If STMT is an escape point and STMT contains at
least one direct use of OP, then the value of OP
escapes and so the pointed-to variables need to
be marked call-clobbered. */
pi->value_escapes_p = 1;
pi->escape_mask |= stmt_escape_type;
/* If the statement makes a function call, assume
that pointer OP will be dereferenced in a store
operation inside the called function. */
if (is_gimple_call (stmt)
|| stmt_escape_type == ESCAPE_STORED_IN_GLOBAL)
pointer_set_insert (ai->dereferenced_ptrs, var);
pi->memory_tag_needed = 1;
if (gimple_code (stmt) == GIMPLE_PHI)
/* Mark stored variables in STMT as being written to and update the
memory reference stats for all memory symbols referenced by STMT. */
if (gimple_references_memory_p (stmt))
unsigned i;
bitmap_iterator bi;
/* Notice that we only update memory reference stats for symbols
loaded and stored by the statement if the statement does not
contain pointer dereferences and it is not a call/asm site.
This is to avoid double accounting problems when creating
memory partitions. After computing points-to information,
pointer dereference statistics are used to update the
reference stats of the pointed-to variables, so here we
should only update direct references to symbols.
Indirect references are not updated here for two reasons: (1)
The first time we compute alias information, the sets
LOADED/STORED are empty for pointer dereferences, (2) After
partitioning, LOADED/STORED may have references to
partitions, not the original pointed-to variables. So, if we
always counted LOADED/STORED here and during partitioning, we
would count many symbols more than once.
This does cause some imprecision when a statement has a
combination of direct symbol references and pointer
dereferences (e.g., MEMORY_VAR = *PTR) or if a call site has
memory symbols in its argument list, but these cases do not
occur so frequently as to constitute a serious problem. */
if (!stmt_dereferences_ptr_p
&& stmt_escape_type != ESCAPE_TO_CALL
&& stmt_escape_type != ESCAPE_TO_PURE_CONST
&& stmt_escape_type != ESCAPE_TO_ASM)
if (gimple_stored_syms (stmt))
EXECUTE_IF_SET_IN_BITMAP (gimple_stored_syms (stmt), 0, i, bi)
update_mem_sym_stats_from_stmt (referenced_var (i), stmt, 0, 1);
if (gimple_loaded_syms (stmt))
EXECUTE_IF_SET_IN_BITMAP (gimple_loaded_syms (stmt), 0, i, bi)
update_mem_sym_stats_from_stmt (referenced_var (i), stmt, 1, 0);
/* Update various related attributes like escaped addresses,
pointer dereferences for loads and stores. This is used
when creating name tags and alias sets. */
static void
update_alias_info (struct alias_info *ai)
basic_block bb;
gimple_stmt_iterator gsi;
gimple phi;
for (gsi = gsi_start_phis (bb); !gsi_end_p (gsi); gsi_next (&gsi))
phi = gsi_stmt (gsi);
if (is_gimple_reg (PHI_RESULT (phi)))
update_alias_info_1 (phi, ai);
for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
update_alias_info_1 (gsi_stmt (gsi), ai);
/* Create memory tags for all the dereferenced pointers and build the
ADDRESSABLE_VARS and POINTERS arrays used for building the may-alias
sets. Based on the address escape and points-to information collected
earlier, this pass will also clear the TREE_ADDRESSABLE flag from those
variables whose address is not needed anymore. */
static void
setup_pointers_and_addressables (struct alias_info *ai)
size_t num_addressable_vars, num_pointers;
referenced_var_iterator rvi;
tree var;
VEC (tree, heap) *varvec = NULL;
safe_referenced_var_iterator srvi;
/* Size up the arrays ADDRESSABLE_VARS and POINTERS. */
num_addressable_vars = num_pointers = 0;
if (may_be_aliased (var))
/* Since we don't keep track of volatile variables, assume that
these pointers are used in indirect store operations. */
pointer_set_insert (ai->dereferenced_ptrs, var);
/* Create ADDRESSABLE_VARS and POINTERS. Note that these arrays are
always going to be slightly bigger than we actually need them
because some TREE_ADDRESSABLE variables will be marked
non-addressable below and only pointers with unique symbol tags are
going to be added to POINTERS. */
ai->addressable_vars = XCNEWVEC (struct alias_map_d *, num_addressable_vars);
ai->pointers = XCNEWVEC (struct alias_map_d *, num_pointers);
ai->num_addressable_vars = 0;
ai->num_pointers = 0;
FOR_EACH_REFERENCED_VAR_SAFE (var, varvec, srvi)
/* Name memory tags already have flow-sensitive aliasing
information, so they need not be processed by
compute_flow_insensitive_aliasing. Similarly, symbol memory
tags are already accounted for when we process their
associated pointer.
Structure fields, on the other hand, have to have some of this
information processed for them, but it's pointless to mark them
non-addressable (since they are fake variables anyway). */
if (MTAG_P (var))
/* Remove the ADDRESSABLE flag from every addressable variable whose
address is not needed anymore. This is caused by the propagation
of ADDR_EXPR constants into INDIRECT_REF expressions and the
removal of dead pointer assignments done by the early scalar
cleanup passes. */
if (!bitmap_bit_p (gimple_addressable_vars (cfun), DECL_UID (var))
&& !is_global_var (var))
bool okay_to_mark = true;
/* Since VAR is now a regular GIMPLE register, we will need
to rename VAR into SSA afterwards. */
mark_sym_for_renaming (var);
/* The address of VAR is not needed, remove the
addressable bit, so that it can be optimized as a
regular variable. */
if (okay_to_mark)
/* The memory partition holding VAR will no longer
contain VAR, and statements referencing it will need
to be updated. */
if (memory_partition (var))
mark_sym_for_renaming (memory_partition (var));
mark_non_addressable (var);
/* Global variables and addressable locals may be aliased. Create an
entry in ADDRESSABLE_VARS for VAR. */
if (may_be_aliased (var))
create_alias_map_for (var, ai);
mark_sym_for_renaming (var);
/* Add pointer variables that have been dereferenced to the POINTERS
array and create a symbol memory tag for them. */
if (pointer_set_contains (ai->dereferenced_ptrs, var))
tree tag, old_tag;
var_ann_t t_ann;
/* If pointer VAR still doesn't have a memory tag
associated with it, create it now or re-use an
existing one. */
tag = get_smt_for (var, ai);
t_ann = var_ann (tag);
/* The symbol tag will need to be renamed into SSA
afterwards. Note that we cannot do this inside
get_smt_for because aliasing may run multiple times
and we only create symbol tags the first time. */
mark_sym_for_renaming (tag);
/* Similarly, if pointer VAR used to have another type
tag, we will need to process it in the renamer to
remove the stale virtual operands. */
old_tag = symbol_mem_tag (var);
if (old_tag)
mark_sym_for_renaming (old_tag);
/* Associate the tag with pointer VAR. */
set_symbol_mem_tag (var, tag);
/* The pointer has not been dereferenced. If it had a
symbol memory tag, remove it and mark the old tag for
renaming to remove it out of the IL. */
tree tag = symbol_mem_tag (var);
if (tag)
mark_sym_for_renaming (tag);
set_symbol_mem_tag (var, NULL_TREE);
VEC_free (tree, heap, varvec);
/* Determine whether to use .GLOBAL_VAR to model call clobbering
semantics. If the function makes no references to global
variables and contains at least one call to a non-pure function,
then we need to mark the side-effects of the call using .GLOBAL_VAR
to represent all possible global memory referenced by the callee. */
static void
maybe_create_global_var (void)
/* No need to create it, if we have one already. */
if (gimple_global_var (cfun) == NULL_TREE)
struct mem_ref_stats_d *stats = gimple_mem_ref_stats (cfun);
/* Create .GLOBAL_VAR if there are no call-clobbered
variables and the program contains a mixture of pure/const
and regular function calls. This is to avoid the problem
described in PR 20115:
int X;
int func_pure (void) { return X; }
int func_non_pure (int a) { X += a; }
int foo ()
int a = func_pure ();
func_non_pure (a);
a = func_pure ();
return a;
Since foo() has no call-clobbered variables, there is
no relationship between the calls to func_pure and
func_non_pure. Since func_pure has no side-effects, value
numbering optimizations elide the second call to func_pure.
So, if we have some pure/const and some regular calls in the
program we create .GLOBAL_VAR to avoid missing these
relations. */
if (bitmap_empty_p (gimple_call_clobbered_vars (cfun))
&& stats->num_call_sites > 0
&& stats->num_pure_const_call_sites > 0
&& stats->num_call_sites > stats->num_pure_const_call_sites)
create_global_var ();
/* Return TRUE if pointer PTR may point to variable VAR.
MEM_ALIAS_SET is the alias set for the memory location pointed-to by PTR
This is needed because when checking for type conflicts we are
interested in the alias set of the memory location pointed-to by
PTR. The alias set of PTR itself is irrelevant.
VAR_ALIAS_SET is the alias set for VAR. */
may_alias_p (tree ptr, alias_set_type mem_alias_set,
tree var, alias_set_type var_alias_set,
bool alias_set_only)
tree mem;
/* By convention, a variable cannot alias itself. */
mem = symbol_mem_tag (ptr);
if (mem == var)
return false;
/* If -fargument-noalias-global is > 2, pointer arguments may
not point to anything else. */
if (flag_argument_noalias > 2 && TREE_CODE (ptr) == PARM_DECL)
return false;
/* If -fargument-noalias-global is > 1, pointer arguments may
not point to global variables. */
if (flag_argument_noalias > 1 && is_global_var (var)
&& TREE_CODE (ptr) == PARM_DECL)
return false;
/* If the pointed to memory has alias set zero, or the pointer
is ref-all, or the pointer decl is marked that no TBAA is to
be applied, the MEM can alias VAR. */
if (mem_alias_set == 0
|| DECL_NO_TBAA_P (ptr))
return true;
gcc_assert (TREE_CODE (mem) == SYMBOL_MEMORY_TAG);
/* If the alias sets don't conflict then MEM cannot alias VAR. */
if (mem_alias_set != var_alias_set
&& !alias_set_subset_of (mem_alias_set, var_alias_set))
return false;
/* If VAR is a record or union type, PTR cannot point into VAR
unless there is some explicit address operation in the
program that can reference a field of the type pointed-to by
PTR. This also assumes that the types of both VAR and PTR
are contained within the compilation unit, and that there is
no fancy addressing arithmetic associated with any of the
types involved. */
if (mem_alias_set != 0 && var_alias_set != 0)
tree ptr_type = TREE_TYPE (ptr);
tree var_type = TREE_TYPE (var);
/* The star count is -1 if the type at the end of the
pointer_to chain is not a record or union type. */
if (!alias_set_only &&
0 /* FIXME tuples ipa_type_escape_star_count_of_interesting_type (var_type) >= 0*/)
int ptr_star_count = 0;
/* ipa_type_escape_star_count_of_interesting_type is a
little too restrictive for the pointer type, need to
allow pointers to primitive types as long as those
types cannot be pointers to everything. */
while (POINTER_TYPE_P (ptr_type))
/* Strip the *s off. */
ptr_type = TREE_TYPE (ptr_type);
/* There does not appear to be a better test to see if
the pointer type was one of the pointer to everything
types. */
if (ptr_star_count > 0)
if (ipa_type_escape_field_does_not_clobber_p (var_type,
TREE_TYPE (ptr)))
return false;
else if (ptr_star_count == 0)
/* If PTR_TYPE was not really a pointer to type, it cannot
alias. */
return false;
return true;