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//===-- IPConstantPropagation.cpp - Propagate constants through calls -----===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This pass implements an _extremely_ simple interprocedural constant
// propagation pass. It could certainly be improved in many different ways,
// like using a worklist. This pass makes arguments dead, but does not remove
// them. The existing dead argument elimination pass should be run after this
// to clean up the mess.
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/IPO.h"
using namespace llvm;
#define DEBUG_TYPE "ipconstprop"
STATISTIC(NumArgumentsProped, "Number of args turned into constants");
STATISTIC(NumReturnValProped, "Number of return values turned into constants");
namespace {
/// IPCP - The interprocedural constant propagation pass
struct IPCP : public ModulePass {
static char ID; // Pass identification, replacement for typeid
IPCP() : ModulePass(ID) {
bool runOnModule(Module &M) override;
/// PropagateConstantsIntoArguments - Look at all uses of the specified
/// function. If all uses are direct call sites, and all pass a particular
/// constant in for an argument, propagate that constant in as the argument.
static bool PropagateConstantsIntoArguments(Function &F) {
if (F.arg_empty() || F.use_empty()) return false; // No arguments? Early exit.
// For each argument, keep track of its constant value and whether it is a
// constant or not. The bool is driven to true when found to be non-constant.
SmallVector<std::pair<Constant*, bool>, 16> ArgumentConstants;
unsigned NumNonconstant = 0;
for (Use &U : F.uses()) {
User *UR = U.getUser();
// Ignore blockaddress uses.
if (isa<BlockAddress>(UR)) continue;
// Used by a non-instruction, or not the callee of a function, do not
// transform.
if (!isa<CallInst>(UR) && !isa<InvokeInst>(UR))
return false;
CallSite CS(cast<Instruction>(UR));
if (!CS.isCallee(&U))
return false;
// Check out all of the potentially constant arguments. Note that we don't
// inspect varargs here.
CallSite::arg_iterator AI = CS.arg_begin();
Function::arg_iterator Arg = F.arg_begin();
for (unsigned i = 0, e = ArgumentConstants.size(); i != e;
++i, ++AI, ++Arg) {
// If this argument is known non-constant, ignore it.
if (ArgumentConstants[i].second)
Constant *C = dyn_cast<Constant>(*AI);
if (C && ArgumentConstants[i].first == nullptr) {
ArgumentConstants[i].first = C; // First constant seen.
} else if (C && ArgumentConstants[i].first == C) {
// Still the constant value we think it is.
} else if (*AI == &*Arg) {
// Ignore recursive calls passing argument down.
} else {
// Argument became non-constant. If all arguments are non-constant now,
// give up on this function.
if (++NumNonconstant == ArgumentConstants.size())
return false;
ArgumentConstants[i].second = true;
// If we got to this point, there is a constant argument!
assert(NumNonconstant != ArgumentConstants.size());
bool MadeChange = false;
Function::arg_iterator AI = F.arg_begin();
for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) {
// Do we have a constant argument?
if (ArgumentConstants[i].second || AI->use_empty() ||
AI->hasInAllocaAttr() || (AI->hasByValAttr() && !F.onlyReadsMemory()))
Value *V = ArgumentConstants[i].first;
if (!V) V = UndefValue::get(AI->getType());
MadeChange = true;
return MadeChange;
// Check to see if this function returns one or more constants. If so, replace
// all callers that use those return values with the constant value. This will
// leave in the actual return values and instructions, but deadargelim will
// clean that up.
// Additionally if a function always returns one of its arguments directly,
// callers will be updated to use the value they pass in directly instead of
// using the return value.
static bool PropagateConstantReturn(Function &F) {
if (F.getReturnType()->isVoidTy())
return false; // No return value.
// We can infer and propagate the return value only when we know that the
// definition we'll get at link time is *exactly* the definition we see now.
// For more details, see GlobalValue::mayBeDerefined.
if (!F.isDefinitionExact())
return false;
// Don't touch naked functions. The may contain asm returning
// value we don't see, so we may end up interprocedurally propagating
// the return value incorrectly.
if (F.hasFnAttribute(Attribute::Naked))
return false;
// Check to see if this function returns a constant.
SmallVector<Value *,4> RetVals;
StructType *STy = dyn_cast<StructType>(F.getReturnType());
if (STy)
for (unsigned i = 0, e = STy->getNumElements(); i < e; ++i)
unsigned NumNonConstant = 0;
for (BasicBlock &BB : F)
if (ReturnInst *RI = dyn_cast<ReturnInst>(BB.getTerminator())) {
for (unsigned i = 0, e = RetVals.size(); i != e; ++i) {
// Already found conflicting return values?
Value *RV = RetVals[i];
if (!RV)
// Find the returned value
Value *V;
if (!STy)
V = RI->getOperand(0);
V = FindInsertedValue(RI->getOperand(0), i);
if (V) {
// Ignore undefs, we can change them into anything
if (isa<UndefValue>(V))
// Try to see if all the rets return the same constant or argument.
if (isa<Constant>(V) || isa<Argument>(V)) {
if (isa<UndefValue>(RV)) {
// No value found yet? Try the current one.
RetVals[i] = V;
// Returning the same value? Good.
if (RV == V)
// Different or no known return value? Don't propagate this return
// value.
RetVals[i] = nullptr;
// All values non-constant? Stop looking.
if (++NumNonConstant == RetVals.size())
return false;
// If we got here, the function returns at least one constant value. Loop
// over all users, replacing any uses of the return value with the returned
// constant.
bool MadeChange = false;
for (Use &U : F.uses()) {
CallSite CS(U.getUser());
Instruction* Call = CS.getInstruction();
// Not a call instruction or a call instruction that's not calling F
// directly?
if (!Call || !CS.isCallee(&U))
// Call result not used?
if (Call->use_empty())
MadeChange = true;
if (!STy) {
Value* New = RetVals[0];
if (Argument *A = dyn_cast<Argument>(New))
// Was an argument returned? Then find the corresponding argument in
// the call instruction and use that.
New = CS.getArgument(A->getArgNo());
for (auto I = Call->user_begin(), E = Call->user_end(); I != E;) {
Instruction *Ins = cast<Instruction>(*I);
// Increment now, so we can remove the use
// Find the index of the retval to replace with
int index = -1;
if (ExtractValueInst *EV = dyn_cast<ExtractValueInst>(Ins))
if (EV->hasIndices())
index = *EV->idx_begin();
// If this use uses a specific return value, and we have a replacement,
// replace it.
if (index != -1) {
Value *New = RetVals[index];
if (New) {
if (Argument *A = dyn_cast<Argument>(New))
// Was an argument returned? Then find the corresponding argument in
// the call instruction and use that.
New = CS.getArgument(A->getArgNo());
if (MadeChange) ++NumReturnValProped;
return MadeChange;
char IPCP::ID = 0;
INITIALIZE_PASS(IPCP, "ipconstprop",
"Interprocedural constant propagation", false, false)
ModulePass *llvm::createIPConstantPropagationPass() { return new IPCP(); }
bool IPCP::runOnModule(Module &M) {
if (skipModule(M))
return false;
bool Changed = false;
bool LocalChange = true;
// FIXME: instead of using smart algorithms, we just iterate until we stop
// making changes.
while (LocalChange) {
LocalChange = false;
for (Function &F : M)
if (!F.isDeclaration()) {
// Delete any klingons.
if (F.hasLocalLinkage())
LocalChange |= PropagateConstantsIntoArguments(F);
Changed |= PropagateConstantReturn(F);
Changed |= LocalChange;
return Changed;