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//===- SimpleLoopUnswitch.cpp - Hoist loop-invariant control flow ---------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
#include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Sequence.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/LoopAnalysisManager.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GenericDomTree.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <iterator>
#include <numeric>
#include <utility>
#define DEBUG_TYPE "simple-loop-unswitch"
using namespace llvm;
STATISTIC(NumBranches, "Number of branches unswitched");
STATISTIC(NumSwitches, "Number of switches unswitched");
STATISTIC(NumTrivial, "Number of unswitches that are trivial");
static cl::opt<bool> EnableNonTrivialUnswitch(
"enable-nontrivial-unswitch", cl::init(false), cl::Hidden,
cl::desc("Forcibly enables non-trivial loop unswitching rather than "
"following the configuration passed into the pass."));
static cl::opt<int>
UnswitchThreshold("unswitch-threshold", cl::init(50), cl::Hidden,
cl::desc("The cost threshold for unswitching a loop."));
static void replaceLoopUsesWithConstant(Loop &L, Value &LIC,
Constant &Replacement) {
assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
// Replace uses of LIC in the loop with the given constant.
for (auto UI = LIC.use_begin(), UE = LIC.use_end(); UI != UE;) {
// Grab the use and walk past it so we can clobber it in the use list.
Use *U = &*UI++;
Instruction *UserI = dyn_cast<Instruction>(U->getUser());
if (!UserI || !L.contains(UserI))
// Replace this use within the loop body.
*U = &Replacement;
/// Update the IDom for a basic block whose predecessor set has changed.
/// This routine is designed to work when the domtree update is relatively
/// localized by leveraging a known common dominator, often a loop header.
/// FIXME: Should consider hand-rolling a slightly more efficient non-DFS
/// approach here as we can do that easily by persisting the candidate IDom's
/// dominating set between each predecessor.
/// FIXME: Longer term, many uses of this can be replaced by an incremental
/// domtree update strategy that starts from a known dominating block and
/// rebuilds that subtree.
static bool updateIDomWithKnownCommonDominator(BasicBlock *BB,
BasicBlock *KnownDominatingBB,
DominatorTree &DT) {
assert(pred_begin(BB) != pred_end(BB) &&
"This routine does not handle unreachable blocks!");
BasicBlock *OrigIDom = DT[BB]->getIDom()->getBlock();
BasicBlock *IDom = *pred_begin(BB);
assert(DT.dominates(KnownDominatingBB, IDom) &&
"Bad known dominating block!");
// Walk all of the other predecessors finding the nearest common dominator
// until all predecessors are covered or we reach the loop header. The loop
// header necessarily dominates all loop exit blocks in loop simplified form
// so we can early-exit the moment we hit that block.
for (auto PI = std::next(pred_begin(BB)), PE = pred_end(BB);
PI != PE && IDom != KnownDominatingBB; ++PI) {
assert(DT.dominates(KnownDominatingBB, *PI) &&
"Bad known dominating block!");
IDom = DT.findNearestCommonDominator(IDom, *PI);
if (IDom == OrigIDom)
return false;
DT.changeImmediateDominator(BB, IDom);
return true;
// Note that we don't currently use the IDFCalculator here for two reasons:
// 1) It computes dominator tree levels for the entire function on each run
// of 'compute'. While this isn't terrible, given that we expect to update
// relatively small subtrees of the domtree, it isn't necessarily the right
// tradeoff.
// 2) The interface doesn't fit this usage well. It doesn't operate in
// append-only, and builds several sets that we don't need.
// FIXME: Neither of these issues are a big deal and could be addressed with
// some amount of refactoring of IDFCalculator. That would allow us to share
// the core logic here (which is solving the same core problem).
static void appendDomFrontier(DomTreeNode *Node,
SmallSetVector<BasicBlock *, 4> &Worklist,
SmallVectorImpl<DomTreeNode *> &DomNodes,
SmallPtrSetImpl<BasicBlock *> &DomSet) {
assert(DomNodes.empty() && "Must start with no dominator nodes.");
assert(DomSet.empty() && "Must start with an empty dominator set.");
// First flatten this subtree into sequence of nodes by doing a pre-order
// walk.
// We intentionally re-evaluate the size as each node can add new children.
// Because this is a tree walk, this cannot add any duplicates.
for (int i = 0; i < (int)DomNodes.size(); ++i)
DomNodes.insert(DomNodes.end(), DomNodes[i]->begin(), DomNodes[i]->end());
// Now create a set of the basic blocks so we can quickly test for
// dominated successors. We could in theory use the DFS numbers of the
// dominator tree for this, but we want this to remain predictably fast
// even while we mutate the dominator tree in ways that would invalidate
// the DFS numbering.
for (DomTreeNode *InnerN : DomNodes)
// Now re-walk the nodes, appending every successor of every node that isn't
// in the set. Note that we don't append the node itself, even though if it
// is a successor it does not strictly dominate itself and thus it would be
// part of the dominance frontier. The reason we don't append it is that
// the node passed in came *from* the worklist and so it has already been
// processed.
for (DomTreeNode *InnerN : DomNodes)
for (BasicBlock *SuccBB : successors(InnerN->getBlock()))
if (!DomSet.count(SuccBB))
/// Update the dominator tree after unswitching a particular former exit block.
/// This handles the full update of the dominator tree after hoisting a block
/// that previously was an exit block (or split off of an exit block) up to be
/// reached from the new immediate dominator of the preheader.
/// The common case is simple -- we just move the unswitched block to have an
/// immediate dominator of the old preheader. But in complex cases, there may
/// be other blocks reachable from the unswitched block that are immediately
/// dominated by some node between the unswitched one and the old preheader.
/// All of these also need to be hoisted in the dominator tree. We also want to
/// minimize queries to the dominator tree because each step of this
/// invalidates any DFS numbers that would make queries fast.
static void updateDTAfterUnswitch(BasicBlock *UnswitchedBB, BasicBlock *OldPH,
DominatorTree &DT) {
DomTreeNode *OldPHNode = DT[OldPH];
DomTreeNode *UnswitchedNode = DT[UnswitchedBB];
// If the dominator tree has already been updated for this unswitched node,
// we're done. This makes it easier to use this routine if there are multiple
// paths to the same unswitched destination.
if (UnswitchedNode->getIDom() == OldPHNode)
// First collect the domtree nodes that we are hoisting over. These are the
// set of nodes which may have children that need to be hoisted as well.
SmallPtrSet<DomTreeNode *, 4> DomChain;
for (auto *IDom = UnswitchedNode->getIDom(); IDom != OldPHNode;
IDom = IDom->getIDom())
// The unswitched block ends up immediately dominated by the old preheader --
// regardless of whether it is the loop exit block or split off of the loop
// exit block.
DT.changeImmediateDominator(UnswitchedNode, OldPHNode);
// For everything that moves up the dominator tree, we need to examine the
// dominator frontier to see if it additionally should move up the dominator
// tree. This lambda appends the dominator frontier for a node on the
// worklist.
SmallSetVector<BasicBlock *, 4> Worklist;
// Scratch data structures reused by domfrontier finding.
SmallVector<DomTreeNode *, 4> DomNodes;
SmallPtrSet<BasicBlock *, 4> DomSet;
// Append the initial dom frontier nodes.
appendDomFrontier(UnswitchedNode, Worklist, DomNodes, DomSet);
// Walk the worklist. We grow the list in the loop and so must recompute size.
for (int i = 0; i < (int)Worklist.size(); ++i) {
auto *BB = Worklist[i];
DomTreeNode *Node = DT[BB];
assert(!DomChain.count(Node) &&
"Cannot be dominated by a block you can reach!");
// If this block had an immediate dominator somewhere in the chain
// we hoisted over, then its position in the domtree needs to move as it is
// reachable from a node hoisted over this chain.
if (!DomChain.count(Node->getIDom()))
DT.changeImmediateDominator(Node, OldPHNode);
// Now add this node's dominator frontier to the worklist as well.
appendDomFrontier(Node, Worklist, DomNodes, DomSet);
/// Check that all the LCSSA PHI nodes in the loop exit block have trivial
/// incoming values along this edge.
static bool areLoopExitPHIsLoopInvariant(Loop &L, BasicBlock &ExitingBB,
BasicBlock &ExitBB) {
for (Instruction &I : ExitBB) {
auto *PN = dyn_cast<PHINode>(&I);
if (!PN)
// No more PHIs to check.
return true;
// If the incoming value for this edge isn't loop invariant the unswitch
// won't be trivial.
if (!L.isLoopInvariant(PN->getIncomingValueForBlock(&ExitingBB)))
return false;
llvm_unreachable("Basic blocks should never be empty!");
/// Rewrite the PHI nodes in an unswitched loop exit basic block.
/// Requires that the loop exit and unswitched basic block are the same, and
/// that the exiting block was a unique predecessor of that block. Rewrites the
/// PHI nodes in that block such that what were LCSSA PHI nodes become trivial
/// PHI nodes from the old preheader that now contains the unswitched
/// terminator.
static void rewritePHINodesForUnswitchedExitBlock(BasicBlock &UnswitchedBB,
BasicBlock &OldExitingBB,
BasicBlock &OldPH) {
for (PHINode &PN : UnswitchedBB.phis()) {
// When the loop exit is directly unswitched we just need to update the
// incoming basic block. We loop to handle weird cases with repeated
// incoming blocks, but expect to typically only have one operand here.
for (auto i : seq<int>(0, PN.getNumOperands())) {
assert(PN.getIncomingBlock(i) == &OldExitingBB &&
"Found incoming block different from unique predecessor!");
PN.setIncomingBlock(i, &OldPH);
/// Rewrite the PHI nodes in the loop exit basic block and the split off
/// unswitched block.
/// Because the exit block remains an exit from the loop, this rewrites the
/// LCSSA PHI nodes in it to remove the unswitched edge and introduces PHI
/// nodes into the unswitched basic block to select between the value in the
/// old preheader and the loop exit.
static void rewritePHINodesForExitAndUnswitchedBlocks(BasicBlock &ExitBB,
BasicBlock &UnswitchedBB,
BasicBlock &OldExitingBB,
BasicBlock &OldPH) {
assert(&ExitBB != &UnswitchedBB &&
"Must have different loop exit and unswitched blocks!");
Instruction *InsertPt = &*UnswitchedBB.begin();
for (PHINode &PN : ExitBB.phis()) {
auto *NewPN = PHINode::Create(PN.getType(), /*NumReservedValues*/ 2,
PN.getName() + ".split", InsertPt);
// Walk backwards over the old PHI node's inputs to minimize the cost of
// removing each one. We have to do this weird loop manually so that we
// create the same number of new incoming edges in the new PHI as we expect
// each case-based edge to be included in the unswitched switch in some
// cases.
// FIXME: This is really, really gross. It would be much cleaner if LLVM
// allowed us to create a single entry for a predecessor block without
// having separate entries for each "edge" even though these edges are
// required to produce identical results.
for (int i = PN.getNumIncomingValues() - 1; i >= 0; --i) {
if (PN.getIncomingBlock(i) != &OldExitingBB)
Value *Incoming = PN.removeIncomingValue(i);
NewPN->addIncoming(Incoming, &OldPH);
// Now replace the old PHI with the new one and wire the old one in as an
// input to the new one.
NewPN->addIncoming(&PN, &ExitBB);
/// Unswitch a trivial branch if the condition is loop invariant.
/// This routine should only be called when loop code leading to the branch has
/// been validated as trivial (no side effects). This routine checks if the
/// condition is invariant and one of the successors is a loop exit. This
/// allows us to unswitch without duplicating the loop, making it trivial.
/// If this routine fails to unswitch the branch it returns false.
/// If the branch can be unswitched, this routine splits the preheader and
/// hoists the branch above that split. Preserves loop simplified form
/// (splitting the exit block as necessary). It simplifies the branch within
/// the loop to an unconditional branch but doesn't remove it entirely. Further
/// cleanup can be done with some simplify-cfg like pass.
static bool unswitchTrivialBranch(Loop &L, BranchInst &BI, DominatorTree &DT,
LoopInfo &LI) {
assert(BI.isConditional() && "Can only unswitch a conditional branch!");
DEBUG(dbgs() << " Trying to unswitch branch: " << BI << "\n");
Value *LoopCond = BI.getCondition();
// Need a trivial loop condition to unswitch.
if (!L.isLoopInvariant(LoopCond))
return false;
// FIXME: We should compute this once at the start and update it!
SmallVector<BasicBlock *, 16> ExitBlocks;
SmallPtrSet<BasicBlock *, 16> ExitBlockSet(ExitBlocks.begin(),
// Check to see if a successor of the branch is guaranteed to
// exit through a unique exit block without having any
// side-effects. If so, determine the value of Cond that causes
// it to do this.
ConstantInt *CondVal = ConstantInt::getTrue(BI.getContext());
ConstantInt *Replacement = ConstantInt::getFalse(BI.getContext());
int LoopExitSuccIdx = 0;
auto *LoopExitBB = BI.getSuccessor(0);
if (!ExitBlockSet.count(LoopExitBB)) {
std::swap(CondVal, Replacement);
LoopExitSuccIdx = 1;
LoopExitBB = BI.getSuccessor(1);
if (!ExitBlockSet.count(LoopExitBB))
return false;
auto *ContinueBB = BI.getSuccessor(1 - LoopExitSuccIdx);
assert(L.contains(ContinueBB) &&
"Cannot have both successors exit and still be in the loop!");
auto *ParentBB = BI.getParent();
if (!areLoopExitPHIsLoopInvariant(L, *ParentBB, *LoopExitBB))
return false;
DEBUG(dbgs() << " unswitching trivial branch when: " << CondVal
<< " == " << LoopCond << "\n");
// Split the preheader, so that we know that there is a safe place to insert
// the conditional branch. We will change the preheader to have a conditional
// branch on LoopCond.
BasicBlock *OldPH = L.getLoopPreheader();
BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI);
// Now that we have a place to insert the conditional branch, create a place
// to branch to: this is the exit block out of the loop that we are
// unswitching. We need to split this if there are other loop predecessors.
// Because the loop is in simplified form, *any* other predecessor is enough.
BasicBlock *UnswitchedBB;
if (BasicBlock *PredBB = LoopExitBB->getUniquePredecessor()) {
assert(PredBB == BI.getParent() &&
"A branch's parent isn't a predecessor!");
UnswitchedBB = LoopExitBB;
} else {
UnswitchedBB = SplitBlock(LoopExitBB, &LoopExitBB->front(), &DT, &LI);
// Now splice the branch to gate reaching the new preheader and re-point its
// successors.
BI.getParent()->getInstList(), BI);
BI.setSuccessor(LoopExitSuccIdx, UnswitchedBB);
BI.setSuccessor(1 - LoopExitSuccIdx, NewPH);
// Create a new unconditional branch that will continue the loop as a new
// terminator.
BranchInst::Create(ContinueBB, ParentBB);
// Rewrite the relevant PHI nodes.
if (UnswitchedBB == LoopExitBB)
rewritePHINodesForUnswitchedExitBlock(*UnswitchedBB, *ParentBB, *OldPH);
rewritePHINodesForExitAndUnswitchedBlocks(*LoopExitBB, *UnswitchedBB,
*ParentBB, *OldPH);
// Now we need to update the dominator tree.
updateDTAfterUnswitch(UnswitchedBB, OldPH, DT);
// But if we split something off of the loop exit block then we also removed
// one of the predecessors for the loop exit block and may need to update its
// idom.
if (UnswitchedBB != LoopExitBB)
updateIDomWithKnownCommonDominator(LoopExitBB, L.getHeader(), DT);
// Since this is an i1 condition we can also trivially replace uses of it
// within the loop with a constant.
replaceLoopUsesWithConstant(L, *LoopCond, *Replacement);
return true;
/// Unswitch a trivial switch if the condition is loop invariant.
/// This routine should only be called when loop code leading to the switch has
/// been validated as trivial (no side effects). This routine checks if the
/// condition is invariant and that at least one of the successors is a loop
/// exit. This allows us to unswitch without duplicating the loop, making it
/// trivial.
/// If this routine fails to unswitch the switch it returns false.
/// If the switch can be unswitched, this routine splits the preheader and
/// copies the switch above that split. If the default case is one of the
/// exiting cases, it copies the non-exiting cases and points them at the new
/// preheader. If the default case is not exiting, it copies the exiting cases
/// and points the default at the preheader. It preserves loop simplified form
/// (splitting the exit blocks as necessary). It simplifies the switch within
/// the loop by removing now-dead cases. If the default case is one of those
/// unswitched, it replaces its destination with a new basic block containing
/// only unreachable. Such basic blocks, while technically loop exits, are not
/// considered for unswitching so this is a stable transform and the same
/// switch will not be revisited. If after unswitching there is only a single
/// in-loop successor, the switch is further simplified to an unconditional
/// branch. Still more cleanup can be done with some simplify-cfg like pass.
static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT,
LoopInfo &LI) {
DEBUG(dbgs() << " Trying to unswitch switch: " << SI << "\n");
Value *LoopCond = SI.getCondition();
// If this isn't switching on an invariant condition, we can't unswitch it.
if (!L.isLoopInvariant(LoopCond))
return false;
auto *ParentBB = SI.getParent();
// FIXME: We should compute this once at the start and update it!
SmallVector<BasicBlock *, 16> ExitBlocks;
SmallPtrSet<BasicBlock *, 16> ExitBlockSet(ExitBlocks.begin(),
SmallVector<int, 4> ExitCaseIndices;
for (auto Case : SI.cases()) {
auto *SuccBB = Case.getCaseSuccessor();
if (ExitBlockSet.count(SuccBB) &&
areLoopExitPHIsLoopInvariant(L, *ParentBB, *SuccBB))
BasicBlock *DefaultExitBB = nullptr;
if (ExitBlockSet.count(SI.getDefaultDest()) &&
areLoopExitPHIsLoopInvariant(L, *ParentBB, *SI.getDefaultDest()) &&
DefaultExitBB = SI.getDefaultDest();
else if (ExitCaseIndices.empty())
return false;
DEBUG(dbgs() << " unswitching trivial cases...\n");
SmallVector<std::pair<ConstantInt *, BasicBlock *>, 4> ExitCases;
// We walk the case indices backwards so that we remove the last case first
// and don't disrupt the earlier indices.
for (unsigned Index : reverse(ExitCaseIndices)) {
auto CaseI = SI.case_begin() + Index;
// Save the value of this case.
ExitCases.push_back({CaseI->getCaseValue(), CaseI->getCaseSuccessor()});
// Delete the unswitched cases.
// Check if after this all of the remaining cases point at the same
// successor.
BasicBlock *CommonSuccBB = nullptr;
if (SI.getNumCases() > 0 &&
std::all_of(std::next(SI.case_begin()), SI.case_end(),
[&SI](const SwitchInst::CaseHandle &Case) {
return Case.getCaseSuccessor() ==
CommonSuccBB = SI.case_begin()->getCaseSuccessor();
if (DefaultExitBB) {
// We can't remove the default edge so replace it with an edge to either
// the single common remaining successor (if we have one) or an unreachable
// block.
if (CommonSuccBB) {
} else {
BasicBlock *UnreachableBB = BasicBlock::Create(
Twine(ParentBB->getName()) + ".unreachable_default",
new UnreachableInst(ParentBB->getContext(), UnreachableBB);
DT.addNewBlock(UnreachableBB, ParentBB);
} else {
// If we're not unswitching the default, we need it to match any cases to
// have a common successor or if we have no cases it is the common
// successor.
if (SI.getNumCases() == 0)
CommonSuccBB = SI.getDefaultDest();
else if (SI.getDefaultDest() != CommonSuccBB)
CommonSuccBB = nullptr;
// Split the preheader, so that we know that there is a safe place to insert
// the switch.
BasicBlock *OldPH = L.getLoopPreheader();
BasicBlock *NewPH = SplitEdge(OldPH, L.getHeader(), &DT, &LI);
// Now add the unswitched switch.
auto *NewSI = SwitchInst::Create(LoopCond, NewPH, ExitCases.size(), OldPH);
// Rewrite the IR for the unswitched basic blocks. This requires two steps.
// First, we split any exit blocks with remaining in-loop predecessors. Then
// we update the PHIs in one of two ways depending on if there was a split.
// We walk in reverse so that we split in the same order as the cases
// appeared. This is purely for convenience of reading the resulting IR, but
// it doesn't cost anything really.
SmallPtrSet<BasicBlock *, 2> UnswitchedExitBBs;
SmallDenseMap<BasicBlock *, BasicBlock *, 2> SplitExitBBMap;
// Handle the default exit if necessary.
// FIXME: It'd be great if we could merge this with the loop below but LLVM's
// ranges aren't quite powerful enough yet.
if (DefaultExitBB) {
if (pred_empty(DefaultExitBB)) {
rewritePHINodesForUnswitchedExitBlock(*DefaultExitBB, *ParentBB, *OldPH);
} else {
auto *SplitBB =
SplitBlock(DefaultExitBB, &DefaultExitBB->front(), &DT, &LI);
rewritePHINodesForExitAndUnswitchedBlocks(*DefaultExitBB, *SplitBB,
*ParentBB, *OldPH);
updateIDomWithKnownCommonDominator(DefaultExitBB, L.getHeader(), DT);
DefaultExitBB = SplitExitBBMap[DefaultExitBB] = SplitBB;
// Note that we must use a reference in the for loop so that we update the
// container.
for (auto &CasePair : reverse(ExitCases)) {
// Grab a reference to the exit block in the pair so that we can update it.
BasicBlock *ExitBB = CasePair.second;
// If this case is the last edge into the exit block, we can simply reuse it
// as it will no longer be a loop exit. No mapping necessary.
if (pred_empty(ExitBB)) {
// Only rewrite once.
if (UnswitchedExitBBs.insert(ExitBB).second)
rewritePHINodesForUnswitchedExitBlock(*ExitBB, *ParentBB, *OldPH);
// Otherwise we need to split the exit block so that we retain an exit
// block from the loop and a target for the unswitched condition.
BasicBlock *&SplitExitBB = SplitExitBBMap[ExitBB];
if (!SplitExitBB) {
// If this is the first time we see this, do the split and remember it.
SplitExitBB = SplitBlock(ExitBB, &ExitBB->front(), &DT, &LI);
rewritePHINodesForExitAndUnswitchedBlocks(*ExitBB, *SplitExitBB,
*ParentBB, *OldPH);
updateIDomWithKnownCommonDominator(ExitBB, L.getHeader(), DT);
// Update the case pair to point to the split block.
CasePair.second = SplitExitBB;
// Now add the unswitched cases. We do this in reverse order as we built them
// in reverse order.
for (auto CasePair : reverse(ExitCases)) {
ConstantInt *CaseVal = CasePair.first;
BasicBlock *UnswitchedBB = CasePair.second;
NewSI->addCase(CaseVal, UnswitchedBB);
updateDTAfterUnswitch(UnswitchedBB, OldPH, DT);
// If the default was unswitched, re-point it and add explicit cases for
// entering the loop.
if (DefaultExitBB) {
updateDTAfterUnswitch(DefaultExitBB, OldPH, DT);
// We removed all the exit cases, so we just copy the cases to the
// unswitched switch.
for (auto Case : SI.cases())
NewSI->addCase(Case.getCaseValue(), NewPH);
// If we ended up with a common successor for every path through the switch
// after unswitching, rewrite it to an unconditional branch to make it easy
// to recognize. Otherwise we potentially have to recognize the default case
// pointing at unreachable and other complexity.
if (CommonSuccBB) {
BasicBlock *BB = SI.getParent();
BranchInst::Create(CommonSuccBB, BB);
return true;
/// This routine scans the loop to find a branch or switch which occurs before
/// any side effects occur. These can potentially be unswitched without
/// duplicating the loop. If a branch or switch is successfully unswitched the
/// scanning continues to see if subsequent branches or switches have become
/// trivial. Once all trivial candidates have been unswitched, this routine
/// returns.
/// The return value indicates whether anything was unswitched (and therefore
/// changed).
static bool unswitchAllTrivialConditions(Loop &L, DominatorTree &DT,
LoopInfo &LI) {
bool Changed = false;
// If loop header has only one reachable successor we should keep looking for
// trivial condition candidates in the successor as well. An alternative is
// to constant fold conditions and merge successors into loop header (then we
// only need to check header's terminator). The reason for not doing this in
// LoopUnswitch pass is that it could potentially break LoopPassManager's
// invariants. Folding dead branches could either eliminate the current loop
// or make other loops unreachable. LCSSA form might also not be preserved
// after deleting branches. The following code keeps traversing loop header's
// successors until it finds the trivial condition candidate (condition that
// is not a constant). Since unswitching generates branches with constant
// conditions, this scenario could be very common in practice.
BasicBlock *CurrentBB = L.getHeader();
SmallPtrSet<BasicBlock *, 8> Visited;
do {
// Check if there are any side-effecting instructions (e.g. stores, calls,
// volatile loads) in the part of the loop that the code *would* execute
// without unswitching.
if (llvm::any_of(*CurrentBB,
[](Instruction &I) { return I.mayHaveSideEffects(); }))
return Changed;
TerminatorInst *CurrentTerm = CurrentBB->getTerminator();
if (auto *SI = dyn_cast<SwitchInst>(CurrentTerm)) {
// Don't bother trying to unswitch past a switch with a constant
// condition. This should be removed prior to running this pass by
// simplify-cfg.
if (isa<Constant>(SI->getCondition()))
return Changed;
if (!unswitchTrivialSwitch(L, *SI, DT, LI))
// Coludn't unswitch this one so we're done.
return Changed;
// Mark that we managed to unswitch something.
Changed = true;
// If unswitching turned the terminator into an unconditional branch then
// we can continue. The unswitching logic specifically works to fold any
// cases it can into an unconditional branch to make it easier to
// recognize here.
auto *BI = dyn_cast<BranchInst>(CurrentBB->getTerminator());
if (!BI || BI->isConditional())
return Changed;
CurrentBB = BI->getSuccessor(0);
auto *BI = dyn_cast<BranchInst>(CurrentTerm);
if (!BI)
// We do not understand other terminator instructions.
return Changed;
// Don't bother trying to unswitch past an unconditional branch or a branch
// with a constant value. These should be removed by simplify-cfg prior to
// running this pass.
if (!BI->isConditional() || isa<Constant>(BI->getCondition()))
return Changed;
// Found a trivial condition candidate: non-foldable conditional branch. If
// we fail to unswitch this, we can't do anything else that is trivial.
if (!unswitchTrivialBranch(L, *BI, DT, LI))
return Changed;
// Mark that we managed to unswitch something.
Changed = true;
// We unswitched the branch. This should always leave us with an
// unconditional branch that we can follow now.
BI = cast<BranchInst>(CurrentBB->getTerminator());
assert(!BI->isConditional() &&
"Cannot form a conditional branch by unswitching1");
CurrentBB = BI->getSuccessor(0);
// When continuing, if we exit the loop or reach a previous visited block,
// then we can not reach any trivial condition candidates (unfoldable
// branch instructions or switch instructions) and no unswitch can happen.
} while (L.contains(CurrentBB) && Visited.insert(CurrentBB).second);
return Changed;
/// Build the cloned blocks for an unswitched copy of the given loop.
/// The cloned blocks are inserted before the loop preheader (`LoopPH`) and
/// after the split block (`SplitBB`) that will be used to select between the
/// cloned and original loop.
/// This routine handles cloning all of the necessary loop blocks and exit
/// blocks including rewriting their instructions and the relevant PHI nodes.
/// It skips loop and exit blocks that are not necessary based on the provided
/// set. It also correctly creates the unconditional branch in the cloned
/// unswitched parent block to only point at the unswitched successor.
/// This does not handle most of the necessary updates to `LoopInfo`. Only exit
/// block splitting is correctly reflected in `LoopInfo`, essentially all of
/// the cloned blocks (and their loops) are left without full `LoopInfo`
/// updates. This also doesn't fully update `DominatorTree`. It adds the cloned
/// blocks to them but doesn't create the cloned `DominatorTree` structure and
/// instead the caller must recompute an accurate DT. It *does* correctly
/// update the `AssumptionCache` provided in `AC`.
static BasicBlock *buildClonedLoopBlocks(
Loop &L, BasicBlock *LoopPH, BasicBlock *SplitBB,
ArrayRef<BasicBlock *> ExitBlocks, BasicBlock *ParentBB,
BasicBlock *UnswitchedSuccBB, BasicBlock *ContinueSuccBB,
const SmallPtrSetImpl<BasicBlock *> &SkippedLoopAndExitBlocks,
ValueToValueMapTy &VMap, AssumptionCache &AC, DominatorTree &DT,
LoopInfo &LI) {
SmallVector<BasicBlock *, 4> NewBlocks;
NewBlocks.reserve(L.getNumBlocks() + ExitBlocks.size());
// We will need to clone a bunch of blocks, wrap up the clone operation in
// a helper.
auto CloneBlock = [&](BasicBlock *OldBB) {
// Clone the basic block and insert it before the new preheader.
BasicBlock *NewBB = CloneBasicBlock(OldBB, VMap, ".us", OldBB->getParent());
// Record this block and the mapping.
VMap[OldBB] = NewBB;
// Add the block to the domtree. We'll move it to the correct position
// below.
DT.addNewBlock(NewBB, SplitBB);
return NewBB;
// First, clone the preheader.
auto *ClonedPH = CloneBlock(LoopPH);
// Then clone all the loop blocks, skipping the ones that aren't necessary.
for (auto *LoopBB : L.blocks())
if (!SkippedLoopAndExitBlocks.count(LoopBB))
// Split all the loop exit edges so that when we clone the exit blocks, if
// any of the exit blocks are *also* a preheader for some other loop, we
// don't create multiple predecessors entering the loop header.
for (auto *ExitBB : ExitBlocks) {
if (SkippedLoopAndExitBlocks.count(ExitBB))
// When we are going to clone an exit, we don't need to clone all the
// instructions in the exit block and we want to ensure we have an easy
// place to merge the CFG, so split the exit first. This is always safe to
// do because there cannot be any non-loop predecessors of a loop exit in
// loop simplified form.
auto *MergeBB = SplitBlock(ExitBB, &ExitBB->front(), &DT, &LI);
// Rearrange the names to make it easier to write test cases by having the
// exit block carry the suffix rather than the merge block carrying the
// suffix.
ExitBB->setName(Twine(MergeBB->getName()) + ".split");
// Now clone the original exit block.
auto *ClonedExitBB = CloneBlock(ExitBB);
assert(ClonedExitBB->getTerminator()->getNumSuccessors() == 1 &&
"Exit block should have been split to have one successor!");
assert(ClonedExitBB->getTerminator()->getSuccessor(0) == MergeBB &&
"Cloned exit block has the wrong successor!");
// Move the merge block's idom to be the split point as one exit is
// dominated by one header, and the other by another, so we know the split
// point dominates both. While the dominator tree isn't fully accurate, we
// want sub-trees within the original loop to be correctly reflect
// dominance within that original loop (at least) and that requires moving
// the merge block out of that subtree.
// FIXME: This is very brittle as we essentially have a partial contract on
// the dominator tree. We really need to instead update it and keep it
// valid or stop relying on it.
DT.changeImmediateDominator(MergeBB, SplitBB);
// Remap any cloned instructions and create a merge phi node for them.
for (auto ZippedInsts : llvm::zip_first(
llvm::make_range(ExitBB->begin(), std::prev(ExitBB->end())),
std::prev(ClonedExitBB->end())))) {
Instruction &I = std::get<0>(ZippedInsts);
Instruction &ClonedI = std::get<1>(ZippedInsts);
// The only instructions in the exit block should be PHI nodes and
// potentially a landing pad.
(isa<PHINode>(I) || isa<LandingPadInst>(I) || isa<CatchPadInst>(I)) &&
"Bad instruction in exit block!");
// We should have a value map between the instruction and its clone.
assert(VMap.lookup(&I) == &ClonedI && "Mismatch in the value map!");
auto *MergePN =
PHINode::Create(I.getType(), /*NumReservedValues*/ 2, ".us-phi",
MergePN->addIncoming(&I, ExitBB);
MergePN->addIncoming(&ClonedI, ClonedExitBB);
// Rewrite the instructions in the cloned blocks to refer to the instructions
// in the cloned blocks. We have to do this as a second pass so that we have
// everything available. Also, we have inserted new instructions which may
// include assume intrinsics, so we update the assumption cache while
// processing this.
for (auto *ClonedBB : NewBlocks)
for (Instruction &I : *ClonedBB) {
RemapInstruction(&I, VMap,
RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
if (auto *II = dyn_cast<IntrinsicInst>(&I))
if (II->getIntrinsicID() == Intrinsic::assume)
// Remove the cloned parent as a predecessor of the cloned continue successor
// if we did in fact clone it.
auto *ClonedParentBB = cast<BasicBlock>(VMap.lookup(ParentBB));
if (auto *ClonedContinueSuccBB =
/*DontDeleteUselessPHIs*/ true);
// Replace the cloned branch with an unconditional branch to the cloneed
// unswitched successor.
auto *ClonedSuccBB = cast<BasicBlock>(VMap.lookup(UnswitchedSuccBB));
BranchInst::Create(ClonedSuccBB, ClonedParentBB);
// Update any PHI nodes in the cloned successors of the skipped blocks to not
// have spurious incoming values.
for (auto *LoopBB : L.blocks())
if (SkippedLoopAndExitBlocks.count(LoopBB))
for (auto *SuccBB : successors(LoopBB))
if (auto *ClonedSuccBB = cast_or_null<BasicBlock>(VMap.lookup(SuccBB)))
for (PHINode &PN : ClonedSuccBB->phis())
PN.removeIncomingValue(LoopBB, /*DeletePHIIfEmpty*/ false);
return ClonedPH;
/// Recursively clone the specified loop and all of its children.
/// The target parent loop for the clone should be provided, or can be null if
/// the clone is a top-level loop. While cloning, all the blocks are mapped
/// with the provided value map. The entire original loop must be present in
/// the value map. The cloned loop is returned.
static Loop *cloneLoopNest(Loop &OrigRootL, Loop *RootParentL,
const ValueToValueMapTy &VMap, LoopInfo &LI) {
auto AddClonedBlocksToLoop = [&](Loop &OrigL, Loop &ClonedL) {
assert(ClonedL.getBlocks().empty() && "Must start with an empty loop!");
for (auto *BB : OrigL.blocks()) {
auto *ClonedBB = cast<BasicBlock>(VMap.lookup(BB));
if (LI.getLoopFor(BB) == &OrigL) {
assert(!LI.getLoopFor(ClonedBB) &&
"Should not have an existing loop for this block!");
LI.changeLoopFor(ClonedBB, &ClonedL);
// We specially handle the first loop because it may get cloned into
// a different parent and because we most commonly are cloning leaf loops.
Loop *ClonedRootL = LI.AllocateLoop();
if (RootParentL)
AddClonedBlocksToLoop(OrigRootL, *ClonedRootL);
if (OrigRootL.empty())
return ClonedRootL;
// If we have a nest, we can quickly clone the entire loop nest using an
// iterative approach because it is a tree. We keep the cloned parent in the
// data structure to avoid repeatedly querying through a map to find it.
SmallVector<std::pair<Loop *, Loop *>, 16> LoopsToClone;
// Build up the loops to clone in reverse order as we'll clone them from the
// back.
for (Loop *ChildL : llvm::reverse(OrigRootL))
LoopsToClone.push_back({ClonedRootL, ChildL});
do {
Loop *ClonedParentL, *L;
std::tie(ClonedParentL, L) = LoopsToClone.pop_back_val();
Loop *ClonedL = LI.AllocateLoop();
AddClonedBlocksToLoop(*L, *ClonedL);
for (Loop *ChildL : llvm::reverse(*L))
LoopsToClone.push_back({ClonedL, ChildL});
} while (!LoopsToClone.empty());
return ClonedRootL;
/// Build the cloned loops of an original loop from unswitching.
/// Because unswitching simplifies the CFG of the loop, this isn't a trivial
/// operation. We need to re-verify that there even is a loop (as the backedge
/// may not have been cloned), and even if there are remaining backedges the
/// backedge set may be different. However, we know that each child loop is
/// undisturbed, we only need to find where to place each child loop within
/// either any parent loop or within a cloned version of the original loop.
/// Because child loops may end up cloned outside of any cloned version of the
/// original loop, multiple cloned sibling loops may be created. All of them
/// are returned so that the newly introduced loop nest roots can be
/// identified.
static Loop *buildClonedLoops(Loop &OrigL, ArrayRef<BasicBlock *> ExitBlocks,
const ValueToValueMapTy &VMap, LoopInfo &LI,
SmallVectorImpl<Loop *> &NonChildClonedLoops) {
Loop *ClonedL = nullptr;
auto *OrigPH = OrigL.getLoopPreheader();
auto *OrigHeader = OrigL.getHeader();
auto *ClonedPH = cast<BasicBlock>(VMap.lookup(OrigPH));
auto *ClonedHeader = cast<BasicBlock>(VMap.lookup(OrigHeader));
// We need to know the loops of the cloned exit blocks to even compute the
// accurate parent loop. If we only clone exits to some parent of the
// original parent, we want to clone into that outer loop. We also keep track
// of the loops that our cloned exit blocks participate in.
Loop *ParentL = nullptr;
SmallVector<BasicBlock *, 4> ClonedExitsInLoops;
SmallDenseMap<BasicBlock *, Loop *, 16> ExitLoopMap;
for (auto *ExitBB : ExitBlocks)
if (auto *ClonedExitBB = cast_or_null<BasicBlock>(VMap.lookup(ExitBB)))
if (Loop *ExitL = LI.getLoopFor(ExitBB)) {
ExitLoopMap[ClonedExitBB] = ExitL;
if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL)))
ParentL = ExitL;
assert((!ParentL || ParentL == OrigL.getParentLoop() ||
ParentL->contains(OrigL.getParentLoop())) &&
"The computed parent loop should always contain (or be) the parent of "
"the original loop.");
// We build the set of blocks dominated by the cloned header from the set of
// cloned blocks out of the original loop. While not all of these will
// necessarily be in the cloned loop, it is enough to establish that they
// aren't in unreachable cycles, etc.
SmallSetVector<BasicBlock *, 16> ClonedLoopBlocks;
for (auto *BB : OrigL.blocks())
if (auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB)))
// Rebuild the set of blocks that will end up in the cloned loop. We may have
// skipped cloning some region of this loop which can in turn skip some of
// the backedges so we have to rebuild the blocks in the loop based on the
// backedges that remain after cloning.
SmallVector<BasicBlock *, 16> Worklist;
SmallPtrSet<BasicBlock *, 16> BlocksInClonedLoop;
for (auto *Pred : predecessors(ClonedHeader)) {
// The only possible non-loop header predecessor is the preheader because
// we know we cloned the loop in simplified form.
if (Pred == ClonedPH)
// Because the loop was in simplified form, the only non-loop predecessor
// should be the preheader.
assert(ClonedLoopBlocks.count(Pred) && "Found a predecessor of the loop "
"header other than the preheader "
"that is not part of the loop!");
// Insert this block into the loop set and on the first visit (and if it
// isn't the header we're currently walking) put it into the worklist to
// recurse through.
if (BlocksInClonedLoop.insert(Pred).second && Pred != ClonedHeader)
// If we had any backedges then there *is* a cloned loop. Put the header into
// the loop set and then walk the worklist backwards to find all the blocks
// that remain within the loop after cloning.
if (!BlocksInClonedLoop.empty()) {
while (!Worklist.empty()) {
BasicBlock *BB = Worklist.pop_back_val();
assert(BlocksInClonedLoop.count(BB) &&
"Didn't put block into the loop set!");
// Insert any predecessors that are in the possible set into the cloned
// set, and if the insert is successful, add them to the worklist. Note
// that we filter on the blocks that are definitely reachable via the
// backedge to the loop header so we may prune out dead code within the
// cloned loop.
for (auto *Pred : predecessors(BB))
if (ClonedLoopBlocks.count(Pred) &&
ClonedL = LI.AllocateLoop();
if (ParentL) {
ParentL->addBasicBlockToLoop(ClonedPH, LI);
} else {
// We don't want to just add the cloned loop blocks based on how we
// discovered them. The original order of blocks was carefully built in
// a way that doesn't rely on predecessor ordering. Rather than re-invent
// that logic, we just re-walk the original blocks (and those of the child
// loops) and filter them as we add them into the cloned loop.
for (auto *BB : OrigL.blocks()) {
auto *ClonedBB = cast_or_null<BasicBlock>(VMap.lookup(BB));
if (!ClonedBB || !BlocksInClonedLoop.count(ClonedBB))
// Directly add the blocks that are only in this loop.
if (LI.getLoopFor(BB) == &OrigL) {
ClonedL->addBasicBlockToLoop(ClonedBB, LI);
// We want to manually add it to this loop and parents.
// Registering it with LoopInfo will happen when we clone the top
// loop for this block.
for (Loop *PL = ClonedL; PL; PL = PL->getParentLoop())
// Now add each child loop whose header remains within the cloned loop. All
// of the blocks within the loop must satisfy the same constraints as the
// header so once we pass the header checks we can just clone the entire
// child loop nest.
for (Loop *ChildL : OrigL) {
auto *ClonedChildHeader =
if (!ClonedChildHeader || !BlocksInClonedLoop.count(ClonedChildHeader))
#ifndef NDEBUG
// We should never have a cloned child loop header but fail to have
// all of the blocks for that child loop.
for (auto *ChildLoopBB : ChildL->blocks())
cast<BasicBlock>(VMap.lookup(ChildLoopBB))) &&
"Child cloned loop has a header within the cloned outer "
"loop but not all of its blocks!");
cloneLoopNest(*ChildL, ClonedL, VMap, LI);
// Now that we've handled all the components of the original loop that were
// cloned into a new loop, we still need to handle anything from the original
// loop that wasn't in a cloned loop.
// Figure out what blocks are left to place within any loop nest containing
// the unswitched loop. If we never formed a loop, the cloned PH is one of
// them.
SmallPtrSet<BasicBlock *, 16> UnloopedBlockSet;
if (BlocksInClonedLoop.empty())
for (auto *ClonedBB : ClonedLoopBlocks)
if (!BlocksInClonedLoop.count(ClonedBB))
// Copy the cloned exits and sort them in ascending loop depth, we'll work
// backwards across these to process them inside out. The order shouldn't
// matter as we're just trying to build up the map from inside-out; we use
// the map in a more stably ordered way below.
auto OrderedClonedExitsInLoops = ClonedExitsInLoops;
std::sort(OrderedClonedExitsInLoops.begin(), OrderedClonedExitsInLoops.end(),
[&](BasicBlock *LHS, BasicBlock *RHS) {
return ExitLoopMap.lookup(LHS)->getLoopDepth() <
// Populate the existing ExitLoopMap with everything reachable from each
// exit, starting from the inner most exit.
while (!UnloopedBlockSet.empty() && !OrderedClonedExitsInLoops.empty()) {
assert(Worklist.empty() && "Didn't clear worklist!");
BasicBlock *ExitBB = OrderedClonedExitsInLoops.pop_back_val();
Loop *ExitL = ExitLoopMap.lookup(ExitBB);
// Walk the CFG back until we hit the cloned PH adding everything reachable
// and in the unlooped set to this exit block's loop.
do {
BasicBlock *BB = Worklist.pop_back_val();
// We can stop recursing at the cloned preheader (if we get there).
if (BB == ClonedPH)
for (BasicBlock *PredBB : predecessors(BB)) {
// If this pred has already been moved to our set or is part of some
// (inner) loop, no update needed.
if (!UnloopedBlockSet.erase(PredBB)) {
(BlocksInClonedLoop.count(PredBB) || ExitLoopMap.count(PredBB)) &&
"Predecessor not mapped to a loop!");
// We just insert into the loop set here. We'll add these blocks to the
// exit loop after we build up the set in an order that doesn't rely on
// predecessor order (which in turn relies on use list order).
bool Inserted = ExitLoopMap.insert({PredBB, ExitL}).second;
assert(Inserted && "Should only visit an unlooped block once!");
// And recurse through to its predecessors.
} while (!Worklist.empty());
// Now that the ExitLoopMap gives as mapping for all the non-looping cloned
// blocks to their outer loops, walk the cloned blocks and the cloned exits
// in their original order adding them to the correct loop.
// We need a stable insertion order. We use the order of the original loop
// order and map into the correct parent loop.
for (auto *BB : llvm::concat<BasicBlock *const>(
makeArrayRef(ClonedPH), ClonedLoopBlocks, ClonedExitsInLoops))
if (Loop *OuterL = ExitLoopMap.lookup(BB))
OuterL->addBasicBlockToLoop(BB, LI);
#ifndef NDEBUG
for (auto &BBAndL : ExitLoopMap) {
auto *BB = BBAndL.first;
auto *OuterL = BBAndL.second;
assert(LI.getLoopFor(BB) == OuterL &&
"Failed to put all blocks into outer loops!");
// Now that all the blocks are placed into the correct containing loop in the
// absence of child loops, find all the potentially cloned child loops and
// clone them into whatever outer loop we placed their header into.
for (Loop *ChildL : OrigL) {
auto *ClonedChildHeader =
if (!ClonedChildHeader || BlocksInClonedLoop.count(ClonedChildHeader))
#ifndef NDEBUG
for (auto *ChildLoopBB : ChildL->blocks())
assert(VMap.count(ChildLoopBB) &&
"Cloned a child loop header but not all of that loops blocks!");
*ChildL, ExitLoopMap.lookup(ClonedChildHeader), VMap, LI));
// Return the main cloned loop if any.
return ClonedL;
static void deleteDeadBlocksFromLoop(Loop &L, BasicBlock *DeadSubtreeRoot,
SmallVectorImpl<BasicBlock *> &ExitBlocks,
DominatorTree &DT, LoopInfo &LI) {
// Walk the dominator tree to build up the set of blocks we will delete here.
// The order is designed to allow us to always delete bottom-up and avoid any
// dangling uses.
SmallSetVector<BasicBlock *, 16> DeadBlocks;
for (int i = 0; i < (int)DeadBlocks.size(); ++i)
for (DomTreeNode *ChildN : *DT[DeadBlocks[i]]) {
// FIXME: This assert should pass and that means we don't change nearly
// as much below! Consider rewriting all of this to avoid deleting
// blocks. They are always cloned before being deleted, and so instead
// could just be moved.
// FIXME: This in turn means that we might actually be more able to
// update the domtree.
assert((L.contains(ChildN->getBlock()) ||
llvm::find(ExitBlocks, ChildN->getBlock()) != ExitBlocks.end()) &&
"Should never reach beyond the loop and exits when deleting!");
// Filter out the dead blocks from the exit blocks list so that it can be
// used in the caller.
[&](BasicBlock *BB) { return DeadBlocks.count(BB); });
// Remove these blocks from their successors.
for (auto *BB : DeadBlocks)
for (BasicBlock *SuccBB : successors(BB))
SuccBB->removePredecessor(BB, /*DontDeleteUselessPHIs*/ true);
// Walk from this loop up through its parents removing all of the dead blocks.
for (Loop *ParentL = &L; ParentL; ParentL = ParentL->getParentLoop()) {
for (auto *BB : DeadBlocks)
[&](BasicBlock *BB) { return DeadBlocks.count(BB); });
// Now delete the dead child loops. This raw delete will clear them
// recursively.
llvm::erase_if(L.getSubLoopsVector(), [&](Loop *ChildL) {
if (!DeadBlocks.count(ChildL->getHeader()))
return false;
[&](BasicBlock *ChildBB) {
return DeadBlocks.count(ChildBB);
}) &&
"If the child loop header is dead all blocks in the child loop must "
"be dead as well!");
return true;
// Remove the mappings for the dead blocks.
for (auto *BB : DeadBlocks)
LI.changeLoopFor(BB, nullptr);
// Drop all the references from these blocks to others to handle cyclic
// references as we start deleting the blocks themselves.
for (auto *BB : DeadBlocks)
for (auto *BB : llvm::reverse(DeadBlocks)) {
/// Recompute the set of blocks in a loop after unswitching.
/// This walks from the original headers predecessors to rebuild the loop. We
/// take advantage of the fact that new blocks can't have been added, and so we
/// filter by the original loop's blocks. This also handles potentially
/// unreachable code that we don't want to explore but might be found examining
/// the predecessors of the header.
/// If the original loop is no longer a loop, this will return an empty set. If
/// it remains a loop, all the blocks within it will be added to the set
/// (including those blocks in inner loops).
static SmallPtrSet<const BasicBlock *, 16> recomputeLoopBlockSet(Loop &L,
LoopInfo &LI) {
SmallPtrSet<const BasicBlock *, 16> LoopBlockSet;
auto *PH = L.getLoopPreheader();
auto *Header = L.getHeader();
// A worklist to use while walking backwards from the header.
SmallVector<BasicBlock *, 16> Worklist;
// First walk the predecessors of the header to find the backedges. This will
// form the basis of our walk.
for (auto *Pred : predecessors(Header)) {
// Skip the preheader.
if (Pred == PH)
// Because the loop was in simplified form, the only non-loop predecessor
// is the preheader.
assert(L.contains(Pred) && "Found a predecessor of the loop header other "
"than the preheader that is not part of the "
// Insert this block into the loop set and on the first visit and, if it
// isn't the header we're currently walking, put it into the worklist to
// recurse through.
if (LoopBlockSet.insert(Pred).second && Pred != Header)
// If no backedges were found, we're done.
if (LoopBlockSet.empty())
return LoopBlockSet;
// Add the loop header to the set.
// We found backedges, recurse through them to identify the loop blocks.
while (!Worklist.empty()) {
BasicBlock *BB = Worklist.pop_back_val();
assert(LoopBlockSet.count(BB) && "Didn't put block into the loop set!");
// Because we know the inner loop structure remains valid we can use the
// loop structure to jump immediately across the entire nested loop.
// Further, because it is in loop simplified form, we can directly jump
// to its preheader afterward.
if (Loop *InnerL = LI.getLoopFor(BB))
if (InnerL != &L) {
assert(L.contains(InnerL) &&
"Should not reach a loop *outside* this loop!");
// The preheader is the only possible predecessor of the loop so
// insert it into the set and check whether it was already handled.
auto *InnerPH = InnerL->getLoopPreheader();
assert(L.contains(InnerPH) && "Cannot contain an inner loop block "
"but not contain the inner loop "
if (!LoopBlockSet.insert(InnerPH).second)
// The only way to reach the preheader is through the loop body
// itself so if it has been visited the loop is already handled.
// Insert all of the blocks (other than those already present) into
// the loop set. The only block we expect to already be in the set is
// the one we used to find this loop as we immediately handle the
// others the first time we encounter the loop.
for (auto *InnerBB : InnerL->blocks()) {
if (InnerBB == BB) {
assert(LoopBlockSet.count(InnerBB) &&
"Block should already be in the set!");
bool Inserted = LoopBlockSet.insert(InnerBB).second;
assert(Inserted && "Should only insert an inner loop once!");
// Add the preheader to the worklist so we will continue past the
// loop body.
// Insert any predecessors that were in the original loop into the new
// set, and if the insert is successful, add them to the worklist.
for (auto *Pred : predecessors(BB))
if (L.contains(Pred) && LoopBlockSet.insert(Pred).second)
// We've found all the blocks participating in the loop, return our completed
// set.
return LoopBlockSet;
/// Rebuild a loop after unswitching removes some subset of blocks and edges.
/// The removal may have removed some child loops entirely but cannot have
/// disturbed any remaining child loops. However, they may need to be hoisted
/// to the parent loop (or to be top-level loops). The original loop may be
/// completely removed.
/// The sibling loops resulting from this update are returned. If the original
/// loop remains a valid loop, it will be the first entry in this list with all
/// of the newly sibling loops following it.
/// Returns true if the loop remains a loop after unswitching, and false if it
/// is no longer a loop after unswitching (and should not continue to be
/// referenced).
static bool rebuildLoopAfterUnswitch(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
LoopInfo &LI,
SmallVectorImpl<Loop *> &HoistedLoops) {
auto *PH = L.getLoopPreheader();
// Compute the actual parent loop from the exit blocks. Because we may have
// pruned some exits the loop may be different from the original parent.
Loop *ParentL = nullptr;
SmallVector<Loop *, 4> ExitLoops;
SmallVector<BasicBlock *, 4> ExitsInLoops;
for (auto *ExitBB : ExitBlocks)
if (Loop *ExitL = LI.getLoopFor(ExitBB)) {
if (!ParentL || (ParentL != ExitL && ParentL->contains(ExitL)))
ParentL = ExitL;
// Recompute the blocks participating in this loop. This may be empty if it
// is no longer a loop.
auto LoopBlockSet = recomputeLoopBlockSet(L, LI);
// If we still have a loop, we need to re-set the loop's parent as the exit
// block set changing may have moved it within the loop nest. Note that this
// can only happen when this loop has a parent as it can only hoist the loop
// *up* the nest.
if (!LoopBlockSet.empty() && L.getParentLoop() != ParentL) {
// Remove this loop's (original) blocks from all of the intervening loops.
for (Loop *IL = L.getParentLoop(); IL != ParentL;
IL = IL->getParentLoop()) {
for (auto *BB : L.blocks())
llvm::erase_if(IL->getBlocksVector(), [&](BasicBlock *BB) {
return BB == PH || L.contains(BB);
LI.changeLoopFor(PH, ParentL);
if (ParentL)
// Now we update all the blocks which are no longer within the loop.
auto &Blocks = L.getBlocksVector();
auto BlocksSplitI =
? Blocks.begin()
: std::stable_partition(
Blocks.begin(), Blocks.end(),
[&](BasicBlock *BB) { return LoopBlockSet.count(BB); });
// Before we erase the list of unlooped blocks, build a set of them.
SmallPtrSet<BasicBlock *, 16> UnloopedBlocks(BlocksSplitI, Blocks.end());
if (LoopBlockSet.empty())
// Now erase these blocks from the loop.
for (auto *BB : make_range(BlocksSplitI, Blocks.end()))
Blocks.erase(BlocksSplitI, Blocks.end());
// Sort the exits in ascending loop depth, we'll work backwards across these
// to process them inside out.
std::stable_sort(ExitsInLoops.begin(), ExitsInLoops.end(),
[&](BasicBlock *LHS, BasicBlock *RHS) {
return LI.getLoopDepth(LHS) < LI.getLoopDepth(RHS);
// We'll build up a set for each exit loop.
SmallPtrSet<BasicBlock *, 16> NewExitLoopBlocks;
Loop *PrevExitL = L.getParentLoop(); // The deepest possible exit loop.
auto RemoveUnloopedBlocksFromLoop =
[](Loop &L, SmallPtrSetImpl<BasicBlock *> &UnloopedBlocks) {
for (auto *BB : UnloopedBlocks)
llvm::erase_if(L.getBlocksVector(), [&](BasicBlock *BB) {
return UnloopedBlocks.count(BB);
SmallVector<BasicBlock *, 16> Worklist;
while (!UnloopedBlocks.empty() && !ExitsInLoops.empty()) {
assert(Worklist.empty() && "Didn't clear worklist!");
assert(NewExitLoopBlocks.empty() && "Didn't clear loop set!");
// Grab the next exit block, in decreasing loop depth order.
BasicBlock *ExitBB = ExitsInLoops.pop_back_val();
Loop &ExitL = *LI.getLoopFor(ExitBB);
assert(ExitL.contains(&L) && "Exit loop must contain the inner loop!");
// Erase all of the unlooped blocks from the loops between the previous
// exit loop and this exit loop. This works because the ExitInLoops list is
// sorted in increasing order of loop depth and thus we visit loops in
// decreasing order of loop depth.
for (; PrevExitL != &ExitL; PrevExitL = PrevExitL->getParentLoop())
RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
// Walk the CFG back until we hit the cloned PH adding everything reachable
// and in the unlooped set to this exit block's loop.
do {
BasicBlock *BB = Worklist.pop_back_val();
// We can stop recursing at the cloned preheader (if we get there).
if (BB == PH)
for (BasicBlock *PredBB : predecessors(BB)) {
// If this pred has already been moved to our set or is part of some
// (inner) loop, no update needed.
if (!UnloopedBlocks.erase(PredBB)) {
assert((NewExitLoopBlocks.count(PredBB) ||
ExitL.contains(LI.getLoopFor(PredBB))) &&
"Predecessor not in a nested loop (or already visited)!");
// We just insert into the loop set here. We'll add these blocks to the
// exit loop after we build up the set in a deterministic order rather
// than the predecessor-influenced visit order.
bool Inserted = NewExitLoopBlocks.insert(PredBB).second;
assert(Inserted && "Should only visit an unlooped block once!");
// And recurse through to its predecessors.
} while (!Worklist.empty());
// If blocks in this exit loop were directly part of the original loop (as
// opposed to a child loop) update the map to point to this exit loop. This
// just updates a map and so the fact that the order is unstable is fine.
for (auto *BB : NewExitLoopBlocks)
if (Loop *BBL = LI.getLoopFor(BB))
if (BBL == &L || !L.contains(BBL))
LI.changeLoopFor(BB, &ExitL);
// We will remove the remaining unlooped blocks from this loop in the next
// iteration or below.
// Any remaining unlooped blocks are no longer part of any loop unless they
// are part of some child loop.
for (; PrevExitL; PrevExitL = PrevExitL->getParentLoop())
RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
for (auto *BB : UnloopedBlocks)
if (Loop *BBL = LI.getLoopFor(BB))
if (BBL == &L || !L.contains(BBL))
LI.changeLoopFor(BB, nullptr);
// Sink all the child loops whose headers are no longer in the loop set to
// the parent (or to be top level loops). We reach into the loop and directly
// update its subloop vector to make this batch update efficient.
auto &SubLoops = L.getSubLoopsVector();
auto SubLoopsSplitI =
? SubLoops.begin()
: std::stable_partition(
SubLoops.begin(), SubLoops.end(), [&](Loop *SubL) {
return LoopBlockSet.count(SubL->getHeader());
for (auto *HoistedL : make_range(SubLoopsSplitI, SubLoops.end())) {
// To compute the new parent of this hoisted loop we look at where we
// placed the preheader above. We can't lookup the header itself because we
// retained the mapping from the header to the hoisted loop. But the
// preheader and header should have the exact same new parent computed
// based on the set of exit blocks from the original loop as the preheader
// is a predecessor of the header and so reached in the reverse walk. And
// because the loops were all in simplified form the preheader of the
// hoisted loop can't be part of some *other* loop.
if (auto *NewParentL = LI.getLoopFor(HoistedL->getLoopPreheader()))
SubLoops.erase(SubLoopsSplitI, SubLoops.end());
// Actually delete the loop if nothing remained within it.
if (Blocks.empty()) {
assert(SubLoops.empty() &&
"Failed to remove all subloops from the original loop!");
if (Loop *ParentL = L.getParentLoop())
ParentL->removeChildLoop(llvm::find(*ParentL, &L));
LI.removeLoop(llvm::find(LI, &L));
return false;
return true;
/// Helper to visit a dominator subtree, invoking a callable on each node.
/// Returning false at any point will stop walking past that node of the tree.
template <typename CallableT>
void visitDomSubTree(DominatorTree &DT, BasicBlock *BB, CallableT Callable) {
SmallVector<DomTreeNode *, 4> DomWorklist;
#ifndef NDEBUG
SmallPtrSet<DomTreeNode *, 4> Visited;
do {
DomTreeNode *N = DomWorklist.pop_back_val();
// Visit this node.
if (!Callable(N->getBlock()))
// Accumulate the child nodes.
for (DomTreeNode *ChildN : *N) {
assert(Visited.insert(ChildN).second &&
"Cannot visit a node twice when walking a tree!");
} while (!DomWorklist.empty());
/// Take an invariant branch that has been determined to be safe and worthwhile
/// to unswitch despite being non-trivial to do so and perform the unswitch.
/// This directly updates the CFG to hoist the predicate out of the loop, and
/// clone the necessary parts of the loop to maintain behavior.
/// It also updates both dominator tree and loopinfo based on the unswitching.
/// Once unswitching has been performed it runs the provided callback to report
/// the new loops and no-longer valid loops to the caller.
static bool unswitchInvariantBranch(
Loop &L, BranchInst &BI, DominatorTree &DT, LoopInfo &LI,
AssumptionCache &AC,
function_ref<void(bool, ArrayRef<Loop *>)> NonTrivialUnswitchCB) {
assert(BI.isConditional() && "Can only unswitch a conditional branch!");
assert(L.isLoopInvariant(BI.getCondition()) &&
"Can only unswitch an invariant branch condition!");
// Constant and BBs tracking the cloned and continuing successor.
const int ClonedSucc = 0;
auto *ParentBB = BI.getParent();
auto *UnswitchedSuccBB = BI.getSuccessor(ClonedSucc);
auto *ContinueSuccBB = BI.getSuccessor(1 - ClonedSucc);
assert(UnswitchedSuccBB != ContinueSuccBB &&
"Should not unswitch a branch that always goes to the same place!");
// The branch should be in this exact loop. Any inner loop's invariant branch
// should be handled by unswitching that inner loop. The caller of this
// routine should filter out any candidates that remain (but were skipped for
// whatever reason).
assert(LI.getLoopFor(ParentBB) == &L && "Branch in an inner loop!");
SmallVector<BasicBlock *, 4> ExitBlocks;
// We cannot unswitch if exit blocks contain a cleanuppad instruction as we
// don't know how to split those exit blocks.
// FIXME: We should teach SplitBlock to handle this and remove this
// restriction.
for (auto *ExitBB : ExitBlocks)
if (isa<CleanupPadInst>(ExitBB->getFirstNonPHI()))
return false;
SmallPtrSet<BasicBlock *, 4> ExitBlockSet(ExitBlocks.begin(),
// Compute the parent loop now before we start hacking on things.
Loop *ParentL = L.getParentLoop();
// Compute the outer-most loop containing one of our exit blocks. This is the
// furthest up our loopnest which can be mutated, which we will use below to
// update things.
Loop *OuterExitL = &L;
for (auto *ExitBB : ExitBlocks) {
Loop *NewOuterExitL = LI.getLoopFor(ExitBB);
if (!NewOuterExitL) {
// We exited the entire nest with this block, so we're done.
OuterExitL = nullptr;
if (NewOuterExitL != OuterExitL && NewOuterExitL->contains(OuterExitL))
OuterExitL = NewOuterExitL;
// If the edge we *aren't* cloning in the unswitch (the continuing edge)
// dominates its target, we can skip cloning the dominated region of the loop
// and its exits. We compute this as a set of nodes to be skipped.
SmallPtrSet<BasicBlock *, 4> SkippedLoopAndExitBlocks;
if (ContinueSuccBB->getUniquePredecessor() ||
llvm::all_of(predecessors(ContinueSuccBB), [&](BasicBlock *PredBB) {
return PredBB == ParentBB || DT.dominates(ContinueSuccBB, PredBB);
})) {
visitDomSubTree(DT, ContinueSuccBB, [&](BasicBlock *BB) {
return true;
// Similarly, if the edge we *are* cloning in the unswitch (the unswitched
// edge) dominates its target, we will end up with dead nodes in the original
// loop and its exits that will need to be deleted. Here, we just retain that
// the property holds and will compute the deleted set later.
bool DeleteUnswitchedSucc =
UnswitchedSuccBB->getUniquePredecessor() ||
llvm::all_of(predecessors(UnswitchedSuccBB), [&](BasicBlock *PredBB) {
return PredBB == ParentBB || DT.dominates(UnswitchedSuccBB, PredBB);
// Split the preheader, so that we know that there is a safe place to insert
// the conditional branch. We will change the preheader to have a conditional
// branch on LoopCond. The original preheader will become the split point
// between the unswitched versions, and we will have a new preheader for the
// original loop.
BasicBlock *SplitBB = L.getLoopPreheader();
BasicBlock *LoopPH = SplitEdge(SplitBB, L.getHeader(), &DT, &LI);
// Keep a mapping for the cloned values.
ValueToValueMapTy VMap;
// Build the cloned blocks from the loop.
auto *ClonedPH = buildClonedLoopBlocks(
L, LoopPH, SplitBB, ExitBlocks, ParentBB, UnswitchedSuccBB,
ContinueSuccBB, SkippedLoopAndExitBlocks, VMap, AC, DT, LI);
// Build the cloned loop structure itself. This may be substantially
// different from the original structure due to the simplified CFG. This also
// handles inserting all the cloned blocks into the correct loops.
SmallVector<Loop *, 4> NonChildClonedLoops;
Loop *ClonedL =
buildClonedLoops(L, ExitBlocks, VMap, LI, NonChildClonedLoops);
// Remove the parent as a predecessor of the unswitched successor.
UnswitchedSuccBB->removePredecessor(ParentBB, /*DontDeleteUselessPHIs*/ true);
// Now splice the branch from the original loop and use it to select between
// the two loops.
SplitBB->getInstList().splice(SplitBB->end(), ParentBB->getInstList(), BI);
BI.setSuccessor(ClonedSucc, ClonedPH);
BI.setSuccessor(1 - ClonedSucc, LoopPH);
// Create a new unconditional branch to the continuing block (as opposed to
// the one cloned).
BranchInst::Create(ContinueSuccBB, ParentBB);
// Delete anything that was made dead in the original loop due to
// unswitching.
if (DeleteUnswitchedSucc)
deleteDeadBlocksFromLoop(L, UnswitchedSuccBB, ExitBlocks, DT, LI);
SmallVector<Loop *, 4> HoistedLoops;
bool IsStillLoop = rebuildLoopAfterUnswitch(L, ExitBlocks, LI, HoistedLoops);
// This will have completely invalidated the dominator tree. We can't easily
// bound how much is invalid because in some cases we will refine the
// predecessor set of exit blocks of the loop which can move large unrelated
// regions of code into a new subtree.
// FIXME: Eventually, we should use an incremental update utility that
// leverages the existing information in the dominator tree (and potentially
// the nature of the change) to more efficiently update things.
// We can change which blocks are exit blocks of all the cloned sibling
// loops, the current loop, and any parent loops which shared exit blocks
// with the current loop. As a consequence, we need to re-form LCSSA for
// them. But we shouldn't need to re-form LCSSA for any child loops.
// FIXME: This could be made more efficient by tracking which exit blocks are
// new, and focusing on them, but that isn't likely to be necessary.
// In order to reasonably rebuild LCSSA we need to walk inside-out across the
// loop nest and update every loop that could have had its exits changed. We
// also need to cover any intervening loops. We add all of these loops to
// a list and sort them by loop depth to achieve this without updating
// unnecessary loops.
auto UpdateLCSSA = [&](Loop &UpdateL) {
#ifndef NDEBUG
for (Loop *ChildL : UpdateL)
assert(ChildL->isRecursivelyLCSSAForm(DT, LI) &&
"Perturbed a child loop's LCSSA form!");
formLCSSA(UpdateL, DT, &LI, nullptr);
// For non-child cloned loops and hoisted loops, we just need to update LCSSA
// and we can do it in any order as they don't nest relative to each other.
for (Loop *UpdatedL : llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops))
// If the original loop had exit blocks, walk up through the outer most loop
// of those exit blocks to update LCSSA and form updated dedicated exits.
if (OuterExitL != &L) {
SmallVector<Loop *, 4> OuterLoops;
// We start with the cloned loop and the current loop if they are loops and
// move toward OuterExitL. Also, if either the cloned loop or the current
// loop have become top level loops we need to walk all the way out.
if (ClonedL) {
if (!ClonedL->getParentLoop())
OuterExitL = nullptr;
if (IsStillLoop) {
if (!L.getParentLoop())
OuterExitL = nullptr;
// Grab all of the enclosing loops now.
for (Loop *OuterL = ParentL; OuterL != OuterExitL;
OuterL = OuterL->getParentLoop())
// Finally, update our list of outer loops. This is nicely ordered to work
// inside-out.
for (Loop *OuterL : OuterLoops) {
// First build LCSSA for this loop so that we can preserve it when
// forming dedicated exits. We don't want to perturb some other loop's
// LCSSA while doing that CFG edit.
// For loops reached by this loop's original exit blocks we may
// introduced new, non-dedicated exits. At least try to re-form dedicated
// exits for these loops. This may fail if they couldn't have dedicated
// exits to start with.
formDedicatedExitBlocks(OuterL, &DT, &LI, /*PreserveLCSSA*/ true);
#ifndef NDEBUG
// Verify the entire loop structure to catch any incorrect updates before we
// progress in the pass pipeline.
// Now that we've unswitched something, make callbacks to report the changes.
// For that we need to merge together the updated loops and the cloned loops
// and check whether the original loop survived.
SmallVector<Loop *, 4> SibLoops;
for (Loop *UpdatedL : llvm::concat<Loop *>(NonChildClonedLoops, HoistedLoops))
if (UpdatedL->getParentLoop() == ParentL)
NonTrivialUnswitchCB(IsStillLoop, SibLoops);
return true;
/// Recursively compute the cost of a dominator subtree based on the per-block
/// cost map provided.
/// The recursive computation is memozied into the provided DT-indexed cost map
/// to allow querying it for most nodes in the domtree without it becoming
/// quadratic.
static int
computeDomSubtreeCost(DomTreeNode &N,
const SmallDenseMap<BasicBlock *, int, 4> &BBCostMap,
SmallDenseMap<DomTreeNode *, int, 4> &DTCostMap) {
// Don't accumulate cost (or recurse through) blocks not in our block cost
// map and thus not part of the duplication cost being considered.
auto BBCostIt = BBCostMap.find(N.getBlock());
if (BBCostIt == BBCostMap.end())
return 0;
// Lookup this node to see if we already computed its cost.
auto DTCostIt = DTCostMap.find(&N);
if (DTCostIt != DTCostMap.end())
return DTCostIt->second;
// If not, we have to compute it. We can't use insert above and update
// because computing the cost may insert more things into the map.
int Cost = std::accumulate(
N.begin(), N.end(), BBCostIt->second, [&](int Sum, DomTreeNode *ChildN) {
return Sum + computeDomSubtreeCost(*ChildN, BBCostMap, DTCostMap);
bool Inserted = DTCostMap.insert({&N, Cost}).second;
assert(Inserted && "Should not insert a node while visiting children!");
return Cost;
/// Unswitch control flow predicated on loop invariant conditions.
/// This first hoists all branches or switches which are trivial (IE, do not
/// require duplicating any part of the loop) out of the loop body. It then
/// looks at other loop invariant control flows and tries to unswitch those as
/// well by cloning the loop if the result is small enough.
static bool
unswitchLoop(Loop &L, DominatorTree &DT, LoopInfo &LI, AssumptionCache &AC,
TargetTransformInfo &TTI, bool NonTrivial,
function_ref<void(bool, ArrayRef<Loop *>)> NonTrivialUnswitchCB) {
assert(L.isRecursivelyLCSSAForm(DT, LI) &&
"Loops must be in LCSSA form before unswitching.");
bool Changed = false;
// Must be in loop simplified form: we need a preheader and dedicated exits.
if (!L.isLoopSimplifyForm())
return false;
// Try trivial unswitch first before loop over other basic blocks in the loop.
Changed |= unswitchAllTrivialConditions(L, DT, LI);
// If we're not doing non-trivial unswitching, we're done. We both accept
// a parameter but also check a local flag that can be used for testing
// a debugging.
if (!NonTrivial && !EnableNonTrivialUnswitch)
return Changed;
// Collect all remaining invariant branch conditions within this loop (as
// opposed to an inner loop which would be handled when visiting that inner
// loop).
SmallVector<TerminatorInst *, 4> UnswitchCandidates;
for (auto *BB : L.blocks())
if (LI.getLoopFor(BB) == &L)
if (auto *BI = dyn_cast<BranchInst>(BB->getTerminator()))
if (BI->isConditional() && L.isLoopInvariant(BI->getCondition()) &&
BI->getSuccessor(0) != BI->getSuccessor(1))
// If we didn't find any candidates, we're done.
if (UnswitchCandidates.empty())
return Changed;
DEBUG(dbgs() << "Considering " << UnswitchCandidates.size()
<< " non-trivial loop invariant conditions for unswitching.\n");
// Given that unswitching these terminators will require duplicating parts of
// the loop, so we need to be able to model that cost. Compute the ephemeral
// values and set up a data structure to hold per-BB costs. We cache each
// block's cost so that we don't recompute this when considering different
// subsets of the loop for duplication during unswitching.
SmallPtrSet<const Value *, 4> EphValues;
CodeMetrics::collectEphemeralValues(&L, &AC, EphValues);
SmallDenseMap<BasicBlock *, int, 4> BBCostMap;
// Compute the cost of each block, as well as the total loop cost. Also, bail
// out if we see instructions which are incompatible with loop unswitching
// (convergent, noduplicate, or cross-basic-block tokens).
// FIXME: We might be able to safely handle some of these in non-duplicated
// regions.
int LoopCost = 0;
for (auto *BB : L.blocks()) {
int Cost = 0;
for (auto &I : *BB) {
if (EphValues.count(&I))
if (I.getType()->isTokenTy() && I.isUsedOutsideOfBlock(BB))
return Changed;
if (auto CS = CallSite(&I))
if (CS.isConvergent() || CS.cannotDuplicate())
return Changed;
Cost += TTI.getUserCost(&I);
assert(Cost >= 0 && "Must not have negative costs!");
LoopCost += Cost;
assert(LoopCost >= 0 && "Must not have negative loop costs!");
BBCostMap[BB] = Cost;
DEBUG(dbgs() << " Total loop cost: " << LoopCost << "\n");
// Now we find the best candidate by searching for the one with the following
// properties in order:
// 1) An unswitching cost below the threshold
// 2) The smallest number of duplicated unswitch candidates (to avoid
// creating redundant subsequent unswitching)
// 3) The smallest cost after unswitching.
// We prioritize reducing fanout of unswitch candidates provided the cost
// remains below the threshold because this has a multiplicative effect.
// This requires memoizing each dominator subtree to avoid redundant work.
// FIXME: Need to actually do the number of candidates part above.
SmallDenseMap<DomTreeNode *, int, 4> DTCostMap;
// Given a terminator which might be unswitched, computes the non-duplicated
// cost for that terminator.
auto ComputeUnswitchedCost = [&](TerminatorInst *TI) {
BasicBlock &BB = *TI->getParent();
SmallPtrSet<BasicBlock *, 4> Visited;
int Cost = LoopCost;
for (BasicBlock *SuccBB : successors(&BB)) {
// Don't count successors more than once.
if (!Visited.insert(SuccBB).second)
// This successor's domtree will not need to be duplicated after
// unswitching if the edge to the successor dominates it (and thus the
// entire tree). This essentially means there is no other path into this
// subtree and so it will end up live in only one clone of the loop.
if (SuccBB->getUniquePredecessor() ||
llvm::all_of(predecessors(SuccBB), [&](BasicBlock *PredBB) {
return PredBB == &BB || DT.dominates(SuccBB, PredBB);
})) {
Cost -= computeDomSubtreeCost(*DT[SuccBB], BBCostMap, DTCostMap);
assert(Cost >= 0 &&
"Non-duplicated cost should never exceed total loop cost!");
// Now scale the cost by the number of unique successors minus one. We
// subtract one because there is already at least one copy of the entire
// loop. This is computing the new cost of unswitching a condition.
assert(Visited.size() > 1 &&
"Cannot unswitch a condition without multiple distinct successors!");
return Cost * (Visited.size() - 1);
TerminatorInst *BestUnswitchTI = nullptr;
int BestUnswitchCost;
for (TerminatorInst *CandidateTI : UnswitchCandidates) {
int CandidateCost = ComputeUnswitchedCost(CandidateTI);
DEBUG(dbgs() << " Computed cost of " << CandidateCost
<< " for unswitch candidate: " << *CandidateTI << "\n");
if (!BestUnswitchTI || CandidateCost < BestUnswitchCost) {
BestUnswitchTI = CandidateTI;
BestUnswitchCost = CandidateCost;
if (BestUnswitchCost < UnswitchThreshold) {
DEBUG(dbgs() << " Trying to unswitch non-trivial (cost = "
<< BestUnswitchCost << ") branch: " << *BestUnswitchTI
<< "\n");
Changed |= unswitchInvariantBranch(L, cast<BranchInst>(*BestUnswitchTI), DT,
LI, AC, NonTrivialUnswitchCB);
} else {
DEBUG(dbgs() << "Cannot unswitch, lowest cost found: " << BestUnswitchCost
<< "\n");
return Changed;
PreservedAnalyses SimpleLoopUnswitchPass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR,
LPMUpdater &U) {
Function &F = *L.getHeader()->getParent();
DEBUG(dbgs() << "Unswitching loop in " << F.getName() << ": " << L << "\n");
// Save the current loop name in a variable so that we can report it even
// after it has been deleted.
std::string LoopName = L.getName();
auto NonTrivialUnswitchCB = [&L, &U, &LoopName](bool CurrentLoopValid,
ArrayRef<Loop *> NewLoops) {
// If we did a non-trivial unswitch, we have added new (cloned) loops.
// If the current loop remains valid, we should revisit it to catch any
// other unswitch opportunities. Otherwise, we need to mark it as deleted.
if (CurrentLoopValid)
U.markLoopAsDeleted(L, LoopName);
if (!unswitchLoop(L, AR.DT, AR.LI, AR.AC, AR.TTI, NonTrivial,
return PreservedAnalyses::all();
#ifndef NDEBUG
// Historically this pass has had issues with the dominator tree so verify it
// in asserts builds.
return getLoopPassPreservedAnalyses();
namespace {
class SimpleLoopUnswitchLegacyPass : public LoopPass {
bool NonTrivial;
static char ID; // Pass ID, replacement for typeid
explicit SimpleLoopUnswitchLegacyPass(bool NonTrivial = false)
: LoopPass(ID), NonTrivial(NonTrivial) {
bool runOnLoop(Loop *L, LPPassManager &LPM) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
} // end anonymous namespace
bool SimpleLoopUnswitchLegacyPass::runOnLoop(Loop *L, LPPassManager &LPM) {
if (skipLoop(L))
return false;
Function &F = *L->getHeader()->getParent();
DEBUG(dbgs() << "Unswitching loop in " << F.getName() << ": " << *L << "\n");
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
auto NonTrivialUnswitchCB = [&L, &LPM](bool CurrentLoopValid,
ArrayRef<Loop *> NewLoops) {
// If we did a non-trivial unswitch, we have added new (cloned) loops.
for (auto *NewL : NewLoops)
// If the current loop remains valid, re-add it to the queue. This is
// a little wasteful as we'll finish processing the current loop as well,
// but it is the best we can do in the old PM.
if (CurrentLoopValid)
bool Changed =
unswitchLoop(*L, DT, LI, AC, TTI, NonTrivial, NonTrivialUnswitchCB);
// If anything was unswitched, also clear any cached information about this
// loop.
#ifndef NDEBUG
// Historically this pass has had issues with the dominator tree so verify it
// in asserts builds.
return Changed;
char SimpleLoopUnswitchLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(SimpleLoopUnswitchLegacyPass, "simple-loop-unswitch",
"Simple unswitch loops", false, false)
INITIALIZE_PASS_END(SimpleLoopUnswitchLegacyPass, "simple-loop-unswitch",
"Simple unswitch loops", false, false)
Pass *llvm::createSimpleLoopUnswitchLegacyPass(bool NonTrivial) {
return new SimpleLoopUnswitchLegacyPass(NonTrivial);