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-rw-r--r--llvm/lib/Transforms/Scalar/LICM.cpp327
1 files changed, 312 insertions, 15 deletions
diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index e30c25da6b4..6a442774912 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -31,6 +31,7 @@
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/LICM.h"
+#include "llvm/ADT/SetOperations.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.h"
@@ -41,6 +42,7 @@
#include "llvm/Analysis/GuardUtils.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemorySSA.h"
@@ -75,6 +77,8 @@ using namespace llvm;
#define DEBUG_TYPE "licm"
+STATISTIC(NumCreatedBlocks, "Number of blocks created");
+STATISTIC(NumClonedBranches, "Number of branches cloned");
STATISTIC(NumSunk, "Number of instructions sunk out of loop");
STATISTIC(NumHoisted, "Number of instructions hoisted out of loop");
STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
@@ -86,6 +90,10 @@ static cl::opt<bool>
DisablePromotion("disable-licm-promotion", cl::Hidden, cl::init(false),
cl::desc("Disable memory promotion in LICM pass"));
+static cl::opt<bool> ControlFlowHoisting(
+ "licm-control-flow-hoisting", cl::Hidden, cl::init(false),
+ cl::desc("Enable control flow (and PHI) hoisting in LICM"));
+
static cl::opt<uint32_t> MaxNumUsesTraversed(
"licm-max-num-uses-traversed", cl::Hidden, cl::init(8),
cl::desc("Max num uses visited for identifying load "
@@ -103,7 +111,7 @@ static bool isNotUsedOrFreeInLoop(const Instruction &I, const Loop *CurLoop,
const LoopSafetyInfo *SafetyInfo,
TargetTransformInfo *TTI, bool &FreeInLoop);
static void hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
- ICFLoopSafetyInfo *SafetyInfo,
+ BasicBlock *Dest, ICFLoopSafetyInfo *SafetyInfo,
OptimizationRemarkEmitter *ORE);
static bool sink(Instruction &I, LoopInfo *LI, DominatorTree *DT,
const Loop *CurLoop, ICFLoopSafetyInfo *SafetyInfo,
@@ -437,6 +445,226 @@ bool llvm::sinkRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
return Changed;
}
+// This is a helper class for hoistRegion to make it able to hoist control flow
+// in order to be able to hoist phis. The way this works is that we initially
+// start hoisting to the loop preheader, and when we see a loop invariant branch
+// we make note of this. When we then come to hoist an instruction that's
+// conditional on such a branch we duplicate the branch and the relevant control
+// flow, then hoist the instruction into the block corresponding to its original
+// block in the duplicated control flow.
+class ControlFlowHoister {
+private:
+ // Information about the loop we are hoisting from
+ LoopInfo *LI;
+ DominatorTree *DT;
+ Loop *CurLoop;
+
+ // A map of blocks in the loop to the block their instructions will be hoisted
+ // to.
+ DenseMap<BasicBlock *, BasicBlock *> HoistDestinationMap;
+
+ // The branches that we can hoist, mapped to the block that marks a
+ // convergence point of their control flow.
+ DenseMap<BranchInst *, BasicBlock *> HoistableBranches;
+
+public:
+ ControlFlowHoister(LoopInfo *LI, DominatorTree *DT, Loop *CurLoop)
+ : LI(LI), DT(DT), CurLoop(CurLoop) {}
+
+ void registerPossiblyHoistableBranch(BranchInst *BI) {
+ // We can only hoist conditional branches with loop invariant operands.
+ if (!ControlFlowHoisting || !BI->isConditional() ||
+ !CurLoop->hasLoopInvariantOperands(BI))
+ return;
+
+ // The branch destinations need to be in the loop, and we don't gain
+ // anything by duplicating conditional branches with duplicate successors,
+ // as it's essentially the same as an unconditional branch.
+ BasicBlock *TrueDest = BI->getSuccessor(0);
+ BasicBlock *FalseDest = BI->getSuccessor(1);
+ if (!CurLoop->contains(TrueDest) || !CurLoop->contains(FalseDest) ||
+ TrueDest == FalseDest)
+ return;
+
+ // We can hoist BI if one branch destination is the successor of the other,
+ // or both have common successor which we check by seeing if the
+ // intersection of their successors is non-empty.
+ // TODO: This could be expanded to allowing branches where both ends
+ // eventually converge to a single block.
+ SmallPtrSet<BasicBlock *, 4> TrueDestSucc, FalseDestSucc;
+ TrueDestSucc.insert(succ_begin(TrueDest), succ_end(TrueDest));
+ FalseDestSucc.insert(succ_begin(FalseDest), succ_end(FalseDest));
+ BasicBlock *CommonSucc = nullptr;
+ if (TrueDestSucc.count(FalseDest)) {
+ CommonSucc = FalseDest;
+ } else if (FalseDestSucc.count(TrueDest)) {
+ CommonSucc = TrueDest;
+ } else {
+ set_intersect(TrueDestSucc, FalseDestSucc);
+ // If there's one common successor use that.
+ if (TrueDestSucc.size() == 1)
+ CommonSucc = *TrueDestSucc.begin();
+ // If there's more than one pick whichever appears first in the block list
+ // (we can't use the value returned by TrueDestSucc.begin() as it's
+ // unpredicatable which element gets returned).
+ else if (!TrueDestSucc.empty()) {
+ Function *F = TrueDest->getParent();
+ auto IsSucc = [&](BasicBlock &BB) { return TrueDestSucc.count(&BB); };
+ auto It = std::find_if(F->begin(), F->end(), IsSucc);
+ assert(It != F->end() && "Could not find successor in function");
+ CommonSucc = &*It;
+ }
+ }
+ // The common successor has to be dominated by the branch, as otherwise
+ // there will be some other path to the successor that will not be
+ // controlled by this branch so any phi we hoist would be controlled by the
+ // wrong condition. This also takes care of avoiding hoisting of loop back
+ // edges.
+ // TODO: In some cases this could be relaxed if the successor is dominated
+ // by another block that's been hoisted and we can guarantee that the
+ // control flow has been replicated exactly.
+ if (CommonSucc && DT->dominates(BI, CommonSucc))
+ HoistableBranches[BI] = CommonSucc;
+ }
+
+ bool canHoistPHI(PHINode *PN) {
+ // The phi must have loop invariant operands.
+ if (!ControlFlowHoisting || !CurLoop->hasLoopInvariantOperands(PN))
+ return false;
+ // We can hoist phis if the block they are in is the target of hoistable
+ // branches which cover all of the predecessors of the block.
+ SmallPtrSet<BasicBlock *, 8> PredecessorBlocks;
+ BasicBlock *BB = PN->getParent();
+ for (BasicBlock *PredBB : predecessors(BB))
+ PredecessorBlocks.insert(PredBB);
+ // If we have less predecessor blocks than predecessors then the phi will
+ // have more than one incoming value for the same block which we can't
+ // handle.
+ // TODO: This could be handled be erasing some of the duplicate incoming
+ // values.
+ if (PredecessorBlocks.size() != pred_size(BB))
+ return false;
+ for (auto &Pair : HoistableBranches) {
+ if (Pair.second == BB) {
+ // Which blocks are predecessors via this branch depends on if the
+ // branch is triangle-like or diamond-like.
+ if (Pair.first->getSuccessor(0) == BB) {
+ PredecessorBlocks.erase(Pair.first->getParent());
+ PredecessorBlocks.erase(Pair.first->getSuccessor(1));
+ } else if (Pair.first->getSuccessor(1) == BB) {
+ PredecessorBlocks.erase(Pair.first->getParent());
+ PredecessorBlocks.erase(Pair.first->getSuccessor(0));
+ } else {
+ PredecessorBlocks.erase(Pair.first->getSuccessor(0));
+ PredecessorBlocks.erase(Pair.first->getSuccessor(1));
+ }
+ }
+ }
+ // PredecessorBlocks will now be empty if for every predecessor of BB we
+ // found a hoistable branch source.
+ return PredecessorBlocks.empty();
+ }
+
+ BasicBlock *getOrCreateHoistedBlock(BasicBlock *BB) {
+ // If BB has already been hoisted, return that
+ if (HoistDestinationMap.count(BB))
+ return HoistDestinationMap[BB];
+
+ // Check if this block is conditional based on a pending branch
+ auto HasBBAsSuccessor =
+ [&](DenseMap<BranchInst *, BasicBlock *>::value_type &Pair) {
+ return BB != Pair.second && (Pair.first->getSuccessor(0) == BB ||
+ Pair.first->getSuccessor(1) == BB);
+ };
+ auto It = std::find_if(HoistableBranches.begin(), HoistableBranches.end(),
+ HasBBAsSuccessor);
+
+ // If not involved in a pending branch, hoist to preheader
+ BasicBlock *InitialPreheader = CurLoop->getLoopPreheader();
+ if (It == HoistableBranches.end()) {
+ LLVM_DEBUG(dbgs() << "LICM using " << InitialPreheader->getName()
+ << " as hoist destination for " << BB->getName()
+ << "\n");
+ HoistDestinationMap[BB] = InitialPreheader;
+ return InitialPreheader;
+ }
+ BranchInst *BI = It->first;
+ assert(std::find_if(++It, HoistableBranches.end(), HasBBAsSuccessor) ==
+ HoistableBranches.end() &&
+ "BB is expected to be the target of at most one branch");
+
+ LLVMContext &C = BB->getContext();
+ BasicBlock *TrueDest = BI->getSuccessor(0);
+ BasicBlock *FalseDest = BI->getSuccessor(1);
+ BasicBlock *CommonSucc = HoistableBranches[BI];
+ BasicBlock *HoistTarget = getOrCreateHoistedBlock(BI->getParent());
+
+ // Create hoisted versions of blocks that currently don't have them
+ auto CreateHoistedBlock = [&](BasicBlock *Orig) {
+ if (HoistDestinationMap.count(Orig))
+ return HoistDestinationMap[Orig];
+ BasicBlock *New =
+ BasicBlock::Create(C, Orig->getName() + ".licm", Orig->getParent());
+ HoistDestinationMap[Orig] = New;
+ DT->addNewBlock(New, HoistTarget);
+ if (CurLoop->getParentLoop())
+ CurLoop->getParentLoop()->addBasicBlockToLoop(New, *LI);
+ ++NumCreatedBlocks;
+ LLVM_DEBUG(dbgs() << "LICM created " << New->getName()
+ << " as hoist destination for " << Orig->getName()
+ << "\n");
+ return New;
+ };
+ BasicBlock *HoistTrueDest = CreateHoistedBlock(TrueDest);
+ BasicBlock *HoistFalseDest = CreateHoistedBlock(FalseDest);
+ BasicBlock *HoistCommonSucc = CreateHoistedBlock(CommonSucc);
+
+ // Link up these blocks with branches.
+ if (!HoistCommonSucc->getTerminator()) {
+ // The new common successor we've generated will branch to whatever that
+ // hoist target branched to.
+ BasicBlock *TargetSucc = HoistTarget->getSingleSuccessor();
+ assert(TargetSucc && "Expected hoist target to have a single successor");
+ HoistCommonSucc->moveBefore(TargetSucc);
+ BranchInst::Create(TargetSucc, HoistCommonSucc);
+ }
+ if (!HoistTrueDest->getTerminator()) {
+ HoistTrueDest->moveBefore(HoistCommonSucc);
+ BranchInst::Create(HoistCommonSucc, HoistTrueDest);
+ }
+ if (!HoistFalseDest->getTerminator()) {
+ HoistFalseDest->moveBefore(HoistCommonSucc);
+ BranchInst::Create(HoistCommonSucc, HoistFalseDest);
+ }
+
+ // If BI is being cloned to what was originally the preheader then
+ // HoistCommonSucc will now be the new preheader.
+ if (HoistTarget == InitialPreheader) {
+ // Phis in the loop header now need to use the new preheader.
+ InitialPreheader->replaceSuccessorsPhiUsesWith(HoistCommonSucc);
+ // The new preheader dominates the loop header.
+ DomTreeNode *PreheaderNode = DT->getNode(HoistCommonSucc);
+ DomTreeNode *HeaderNode = DT->getNode(CurLoop->getHeader());
+ DT->changeImmediateDominator(HeaderNode, PreheaderNode);
+ // The preheader hoist destination is now the new preheader, with the
+ // exception of the hoist destination of this branch.
+ for (auto &Pair : HoistDestinationMap)
+ if (Pair.second == InitialPreheader && Pair.first != BI->getParent())
+ Pair.second = HoistCommonSucc;
+ }
+
+ // Now finally clone BI.
+ ReplaceInstWithInst(
+ HoistTarget->getTerminator(),
+ BranchInst::Create(HoistTrueDest, HoistFalseDest, BI->getCondition()));
+ ++NumClonedBranches;
+
+ assert(CurLoop->getLoopPreheader() &&
+ "Hoisting blocks should not have destroyed preheader");
+ return HoistDestinationMap[BB];
+ }
+};
+
/// Walk the specified region of the CFG (defined by all blocks dominated by
/// the specified block, and that are in the current loop) in depth first
/// order w.r.t the DominatorTree. This allows us to visit definitions before
@@ -451,13 +679,19 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
CurLoop != nullptr && CurAST != nullptr && SafetyInfo != nullptr &&
"Unexpected input to hoistRegion");
- // We want to visit parents before children. We will enque all the parents
- // before their children in the worklist and process the worklist in order.
- SmallVector<DomTreeNode *, 16> Worklist = collectChildrenInLoop(N, CurLoop);
+ ControlFlowHoister CFH(LI, DT, CurLoop);
+ // Keep track of instructions that have been hoisted, as they may need to be
+ // re-hoisted if they end up not dominating all of their uses.
+ SmallVector<Instruction *, 16> HoistedInstructions;
+
+ // For PHI hoisting to work we need to hoist blocks before their successors.
+ // We can do this by iterating through the blocks in the loop in reverse
+ // post-order.
+ LoopBlocksRPO Worklist(CurLoop);
+ Worklist.perform(LI);
bool Changed = false;
- for (DomTreeNode *DTN : Worklist) {
- BasicBlock *BB = DTN->getBlock();
+ for (BasicBlock *BB : Worklist) {
// Only need to process the contents of this block if it is not part of a
// subloop (which would already have been processed).
if (inSubLoop(BB, CurLoop, LI))
@@ -483,13 +717,16 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
// Try hoisting the instruction out to the preheader. We can only do
// this if all of the operands of the instruction are loop invariant and
// if it is safe to hoist the instruction.
- //
+ // TODO: It may be safe to hoist if we are hoisting to a conditional block
+ // and we have accurately duplicated the control flow from the loop header
+ // to that block.
if (CurLoop->hasLoopInvariantOperands(&I) &&
canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, true, ORE) &&
isSafeToExecuteUnconditionally(
I, DT, CurLoop, SafetyInfo, ORE,
CurLoop->getLoopPreheader()->getTerminator())) {
- hoist(I, DT, CurLoop, SafetyInfo, ORE);
+ hoist(I, DT, CurLoop, CFH.getOrCreateHoistedBlock(BB), SafetyInfo, ORE);
+ HoistedInstructions.push_back(&I);
Changed = true;
continue;
}
@@ -514,7 +751,9 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
I.replaceAllUsesWith(Product);
eraseInstruction(I, *SafetyInfo, CurAST);
- hoist(*ReciprocalDivisor, DT, CurLoop, SafetyInfo, ORE);
+ hoist(*ReciprocalDivisor, DT, CurLoop, CFH.getOrCreateHoistedBlock(BB),
+ SafetyInfo, ORE);
+ HoistedInstructions.push_back(ReciprocalDivisor);
Changed = true;
continue;
}
@@ -526,13 +765,67 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
CurLoop->hasLoopInvariantOperands(&I) &&
SafetyInfo->isGuaranteedToExecute(I, DT, CurLoop) &&
SafetyInfo->doesNotWriteMemoryBefore(I, CurLoop)) {
- hoist(I, DT, CurLoop, SafetyInfo, ORE);
+ hoist(I, DT, CurLoop, CFH.getOrCreateHoistedBlock(BB), SafetyInfo, ORE);
+ HoistedInstructions.push_back(&I);
Changed = true;
continue;
}
+
+ if (PHINode *PN = dyn_cast<PHINode>(&I)) {
+ if (CFH.canHoistPHI(PN)) {
+ // Redirect incoming blocks first to ensure that we create hoisted
+ // versions of those blocks before we hoist the phi.
+ for (unsigned int i = 0; i < PN->getNumIncomingValues(); ++i)
+ PN->setIncomingBlock(
+ i, CFH.getOrCreateHoistedBlock(PN->getIncomingBlock(i)));
+ hoist(*PN, DT, CurLoop, CFH.getOrCreateHoistedBlock(BB), SafetyInfo,
+ ORE);
+ assert(DT->dominates(PN, BB) && "Conditional PHIs not expected");
+ Changed = true;
+ continue;
+ }
+ }
+
+ // Remember possibly hoistable branches so we can actually hoist them
+ // later if needed.
+ if (BranchInst *BI = dyn_cast<BranchInst>(&I))
+ CFH.registerPossiblyHoistableBranch(BI);
+ }
+ }
+
+ // If we hoisted instructions to a conditional block they may not dominate
+ // their uses that weren't hoisted (such as phis where some operands are not
+ // loop invariant). If so make them unconditional by moving them to their
+ // immediate dominator. We iterate through the instructions in reverse order
+ // which ensures that when we rehoist an instruction we rehoist its operands,
+ // and also keep track of where in the block we are rehoisting to to make sure
+ // that we rehoist instructions before the instructions that use them.
+ Instruction *HoistPoint = nullptr;
+ for (Instruction *I : reverse(HoistedInstructions)) {
+ if (!llvm::all_of(I->uses(), [&](Use &U) { return DT->dominates(I, U); })) {
+ BasicBlock *Dominator =
+ DT->getNode(I->getParent())->getIDom()->getBlock();
+ LLVM_DEBUG(dbgs() << "LICM rehoisting to " << Dominator->getName() << ": "
+ << *I << "\n");
+ if (!HoistPoint || HoistPoint->getParent() != Dominator) {
+ if (HoistPoint)
+ assert(DT->dominates(Dominator, HoistPoint->getParent()) &&
+ "New hoist point expected to dominate old hoist point");
+ HoistPoint = Dominator->getTerminator();
+ }
+ moveInstructionBefore(*I, *HoistPoint, *SafetyInfo);
+ HoistPoint = I;
+ Changed = true;
}
}
+ // Now that we've finished hoisting make sure that LI and DT are still valid.
+#ifndef NDEBUG
+ assert(DT->verify(DominatorTree::VerificationLevel::Fast) &&
+ "Dominator tree verification failed");
+ LI->verify(*DT);
+#endif
+
return Changed;
}
@@ -1100,9 +1393,9 @@ static bool sink(Instruction &I, LoopInfo *LI, DominatorTree *DT,
/// is safe to hoist, this instruction is called to do the dirty work.
///
static void hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
- ICFLoopSafetyInfo *SafetyInfo, OptimizationRemarkEmitter *ORE) {
- auto *Preheader = CurLoop->getLoopPreheader();
- LLVM_DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": " << I
+ BasicBlock *Dest, ICFLoopSafetyInfo *SafetyInfo,
+ OptimizationRemarkEmitter *ORE) {
+ LLVM_DEBUG(dbgs() << "LICM hoisting to " << Dest->getName() << ": " << I
<< "\n");
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "Hoisted", &I) << "hoisting "
@@ -1120,8 +1413,12 @@ static void hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
!SafetyInfo->isGuaranteedToExecute(I, DT, CurLoop))
I.dropUnknownNonDebugMetadata();
- // Move the new node to the Preheader, before its terminator.
- moveInstructionBefore(I, *Preheader->getTerminator(), *SafetyInfo);
+ if (isa<PHINode>(I))
+ // Move the new node to the end of the phi list in the destination block.
+ moveInstructionBefore(I, *Dest->getFirstNonPHI(), *SafetyInfo);
+ else
+ // Move the new node to the destination block, before its terminator.
+ moveInstructionBefore(I, *Dest->getTerminator(), *SafetyInfo);
// Do not retain debug locations when we are moving instructions to different
// basic blocks, because we want to avoid jumpy line tables. Calls, however,
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