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[LoopFusion] Move instructions from FC1.GuardBlock to FC0.GuardBlock and
from FC0.ExitBlock to FC1.ExitBlock when proven safe.
Summary:
Currently LoopFusion give up when the second loop nest guard
block or the first loop nest exit block is not empty. For example:
if (0 < N) {
for (int i = 0; i < N; ++i) {}
x+=1;
}
y+=1;
if (0 < N) {
for (int i = 0; i < N; ++i) {}
}
The above example should be safe to fuse.
This PR moves instructions in FC1 guard block (e.g. y+=1;) to
FC0 guard block, or instructions in FC0 exit block (e.g. x+=1;) to
FC1 exit block, which then LoopFusion is able to fuse them.
Reviewer: kbarton, jdoerfert, Meinersbur, dmgreen, fhahn, hfinkel,
bmahjour, etiotto
Reviewed By: jdoerfert
Subscribers: hiraditya, llvm-commits
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D73641
This commit is contained in:
Whitney Tsang
parent
402abf7a14
commit
7242e653d2
@ -91,8 +91,10 @@ STATISTIC(
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"Loop has a non-empty preheader with instructions that cannot be moved");
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STATISTIC(FusionNotBeneficial, "Fusion is not beneficial");
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STATISTIC(NonIdenticalGuards, "Candidates have different guards");
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STATISTIC(NonEmptyExitBlock, "Candidate has a non-empty exit block");
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STATISTIC(NonEmptyGuardBlock, "Candidate has a non-empty guard block");
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STATISTIC(NonEmptyExitBlock, "Candidate has a non-empty exit block with "
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"instructions that cannot be moved");
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STATISTIC(NonEmptyGuardBlock, "Candidate has a non-empty guard block with "
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"instructions that cannot be moved");
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STATISTIC(NotRotated, "Candidate is not rotated");
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enum FusionDependenceAnalysisChoice {
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@ -750,25 +752,30 @@ private:
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continue;
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}
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// The following two checks look for empty blocks in FC0 and FC1. If
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// any of these blocks are non-empty, we do not fuse. This is done
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// because we currently do not have the safety checks to determine if
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// it is safe to move the blocks past other blocks in the loop. Once
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// these checks are added, these conditions can be relaxed.
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if (FC0->GuardBranch && !isEmptyExitBlock(*FC0)) {
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LLVM_DEBUG(dbgs() << "Fusion candidate does not have empty exit "
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"block. Not fusing.\n");
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reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
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NonEmptyExitBlock);
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continue;
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}
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if (FC0->GuardBranch) {
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assert(FC1->GuardBranch && "Expecting valid FC1 guard branch");
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if (FC1->GuardBranch && !isEmptyGuardBlock(*FC1)) {
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LLVM_DEBUG(dbgs() << "Fusion candidate does not have empty guard "
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"block. Not fusing.\n");
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reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
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NonEmptyGuardBlock);
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continue;
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if (!isSafeToMoveBefore(*FC0->ExitBlock,
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*FC1->ExitBlock->getFirstNonPHIOrDbg(), DT,
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PDT, DI)) {
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LLVM_DEBUG(dbgs() << "Fusion candidate contains unsafe "
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"instructions in exit block. Not fusing.\n");
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reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
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NonEmptyExitBlock);
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continue;
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}
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if (!isSafeToMoveBefore(
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*FC1->GuardBranch->getParent(),
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*FC0->GuardBranch->getParent()->getTerminator(), DT, PDT,
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DI)) {
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LLVM_DEBUG(dbgs()
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<< "Fusion candidate contains unsafe "
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"instructions in guard block. Not fusing.\n");
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reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
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NonEmptyGuardBlock);
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continue;
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}
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}
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// Check the dependencies across the loops and do not fuse if it would
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@ -1079,38 +1086,6 @@ private:
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return (FC1.GuardBranch->getSuccessor(1) == FC1.Preheader);
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}
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/// Check that the guard for \p FC *only* contains the cmp/branch for the
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/// guard.
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/// Once we are able to handle intervening code, any code in the guard block
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/// for FC1 will need to be treated as intervening code and checked whether
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/// it can safely move around the loops.
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bool isEmptyGuardBlock(const FusionCandidate &FC) const {
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assert(FC.GuardBranch && "Expecting a fusion candidate with guard branch.");
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if (auto *CmpInst = dyn_cast<Instruction>(FC.GuardBranch->getCondition())) {
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auto *GuardBlock = FC.GuardBranch->getParent();
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// If the generation of the cmp value is in GuardBlock, then the size of
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// the guard block should be 2 (cmp + branch). If the generation of the
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// cmp value is in a different block, then the size of the guard block
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// should only be 1.
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if (CmpInst->getParent() == GuardBlock)
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return GuardBlock->size() == 2;
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else
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return GuardBlock->size() == 1;
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}
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return false;
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}
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bool isEmptyPreheader(const FusionCandidate &FC) const {
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assert(FC.Preheader && "Expecting a valid preheader");
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return FC.Preheader->size() == 1;
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}
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bool isEmptyExitBlock(const FusionCandidate &FC) const {
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assert(FC.ExitBlock && "Expecting a valid exit block");
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return FC.ExitBlock->size() == 1;
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}
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/// Simplify the condition of the latch branch of \p FC to true, when both of
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/// its successors are the same.
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void simplifyLatchBranch(const FusionCandidate &FC) const {
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@ -1390,6 +1365,14 @@ private:
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BasicBlock *FC0NonLoopBlock = FC0.getNonLoopBlock();
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BasicBlock *FC1NonLoopBlock = FC1.getNonLoopBlock();
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// Move instructions from the exit block of FC0 to the beginning of the exit
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// block of FC1.
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moveInstructionsToTheBeginning(*FC0.ExitBlock, *FC1.ExitBlock, DT, PDT, DI);
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// Move instructions from the guard block of FC1 to the end of the guard
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// block of FC0.
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moveInstructionsToTheEnd(*FC1GuardBlock, *FC0GuardBlock, DT, PDT, DI);
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assert(FC0NonLoopBlock == FC1GuardBlock && "Loops are not adjacent");
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SmallVector<DominatorTree::UpdateType, 8> TreeUpdates;
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@ -213,6 +213,93 @@ for.second:
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for.end:
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ret void
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}
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; CHECK: remark: diagnostics_missed.c:67:3: [unsafe_exitblock]: for.first.preheader and for.second.preheader: Candidate has a non-empty exit block with instructions that cannot be moved
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define void @unsafe_exitblock(i32* noalias %A, i32* noalias %B, i64 %N) {
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for.first.guard:
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%cmp3 = icmp slt i64 0, %N
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br i1 %cmp3, label %for.first.preheader, label %for.second.guard
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for.first.preheader:
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br label %for.first, !dbg !83
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for.first:
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%i.04 = phi i64 [ %inc, %for.first ], [ 0, %for.first.preheader ]
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%arrayidx = getelementptr inbounds i32, i32* %A, i64 %i.04
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store i32 0, i32* %arrayidx, align 4
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%inc = add nsw i64 %i.04, 1
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%cmp = icmp slt i64 %inc, %N
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br i1 %cmp, label %for.first, label %for.first.exit
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for.first.exit:
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call void @bar()
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br label %for.second.guard
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for.second.guard:
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%cmp21 = icmp slt i64 0, %N
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br i1 %cmp21, label %for.second.preheader, label %for.end
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for.second.preheader:
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br label %for.second
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for.second:
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%j.02 = phi i64 [ %inc6, %for.second ], [ 0, %for.second.preheader ]
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%arrayidx4 = getelementptr inbounds i32, i32* %B, i64 %j.02
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store i32 0, i32* %arrayidx4, align 4
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%inc6 = add nsw i64 %j.02, 1
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%cmp2 = icmp slt i64 %inc6, %N
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br i1 %cmp2, label %for.second, label %for.second.exit
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for.second.exit:
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br label %for.end
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for.end:
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ret void
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}
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; CHECK: remark: diagnostics_missed.c:72:3: [unsafe_guardblock]: for.first.preheader and for.second.preheader: Candidate has a non-empty guard block with instructions that cannot be moved
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define void @unsafe_guardblock(i32* noalias %A, i32* noalias %B, i64 %N) {
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for.first.guard:
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%cmp3 = icmp slt i64 0, %N
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br i1 %cmp3, label %for.first.preheader, label %for.second.guard
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for.first.preheader:
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br label %for.first, !dbg !86
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for.first:
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%i.04 = phi i64 [ %inc, %for.first ], [ 0, %for.first.preheader ]
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%arrayidx = getelementptr inbounds i32, i32* %A, i64 %i.04
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store i32 0, i32* %arrayidx, align 4
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%inc = add nsw i64 %i.04, 1
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%cmp = icmp slt i64 %inc, %N
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br i1 %cmp, label %for.first, label %for.first.exit
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for.first.exit:
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br label %for.second.guard
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for.second.guard:
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call void @bar()
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%cmp21 = icmp slt i64 0, %N
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br i1 %cmp21, label %for.second.preheader, label %for.end
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for.second.preheader:
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br label %for.second
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for.second:
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%j.02 = phi i64 [ %inc6, %for.second ], [ 0, %for.second.preheader ]
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%arrayidx4 = getelementptr inbounds i32, i32* %B, i64 %j.02
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store i32 0, i32* %arrayidx4, align 4
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%inc6 = add nsw i64 %j.02, 1
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%cmp2 = icmp slt i64 %inc6, %N
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br i1 %cmp2, label %for.second, label %for.second.exit
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for.second.exit:
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br label %for.end
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for.end:
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ret void
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}
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declare void @bar()
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attributes #0 = { nounwind readnone speculatable willreturn }
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@ -301,3 +388,9 @@ attributes #0 = { nounwind readnone speculatable willreturn }
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!78 = !{}
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!79 = distinct !DILexicalBlock(scope: !77, file: !3, line: 3, column: 5)
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!80 = !DILocation(line: 62, column: 3, scope: !79)
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!81 = distinct !DISubprogram(name: "unsafe_exitblock", scope: !3, file: !3, line: 65, type: !15, scopeLine: 60, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !78)
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!82 = distinct !DILexicalBlock(scope: !81, file: !3, line: 3, column: 5)
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!83 = !DILocation(line: 67, column: 3, scope: !82)
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!84 = distinct !DISubprogram(name: "unsafe_guardblock", scope: !3, file: !3, line: 70, type: !15, scopeLine: 60, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !78)
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!85 = distinct !DILexicalBlock(scope: !84, file: !3, line: 3, column: 5)
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!86 = !DILocation(line: 72, column: 3, scope: !85)
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@ -119,3 +119,116 @@ for.second.exit:
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for.end:
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ret void
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}
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; Test that `%add` is moved in for.second.exit, and the two loops for.first
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; and for.second are fused.
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; CHECK: void @moveinsts_exitblock
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; CHECK-LABEL: for.first.guard:
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; CHECK: br i1 %cmp.guard, label %for.first.preheader, label %for.end
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; CHECK-LABEL: for.first.preheader:
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; CHECK-NEXT: br label %for.first
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; CHECK-LABEL: for.first:
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; CHECK: br i1 %cmp.j, label %for.first, label %for.second.exit
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; CHECK-LABEL: for.second.exit:
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; CHECK-NEXT: %add = add nsw i32 %x, 1
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; CHECK-NEXT: br label %for.end
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; CHECK-LABEL: for.end:
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; CHECK-NEXT: ret void
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define void @moveinsts_exitblock(i32* noalias %A, i32* noalias %B, i64 %N, i32 %x) {
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for.first.guard:
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%cmp.guard = icmp slt i64 0, %N
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br i1 %cmp.guard, label %for.first.preheader, label %for.second.guard
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for.first.preheader:
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br label %for.first
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for.first:
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%i.04 = phi i64 [ %inc, %for.first ], [ 0, %for.first.preheader ]
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%arrayidx = getelementptr inbounds i32, i32* %A, i64 %i.04
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store i32 0, i32* %arrayidx, align 4
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%inc = add nsw i64 %i.04, 1
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%cmp = icmp slt i64 %inc, %N
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br i1 %cmp, label %for.first, label %for.first.exit
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for.first.exit:
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%add = add nsw i32 %x, 1
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br label %for.second.guard
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for.second.guard:
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br i1 %cmp.guard, label %for.second.preheader, label %for.end
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for.second.preheader:
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br label %for.second
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for.second:
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%j.02 = phi i64 [ %inc6, %for.second ], [ 0, %for.second.preheader ]
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%arrayidx4 = getelementptr inbounds i32, i32* %B, i64 %j.02
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store i32 0, i32* %arrayidx4, align 4
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%inc6 = add nsw i64 %j.02, 1
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%cmp.j = icmp slt i64 %inc6, %N
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br i1 %cmp.j, label %for.second, label %for.second.exit
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for.second.exit:
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br label %for.end
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for.end:
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ret void
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}
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; Test that `%add` is moved in for.first.guard, and the two loops for.first
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; and for.second are fused.
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; CHECK: void @moveinsts_guardblock
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; CHECK-LABEL: for.first.guard:
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; CHECK-NEXT: %cmp.guard = icmp slt i64 0, %N
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; CHECK-NEXT: %add = add nsw i32 %x, 1
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; CHECK: br i1 %cmp.guard, label %for.first.preheader, label %for.end
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; CHECK-LABEL: for.first.preheader:
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; CHECK-NEXT: br label %for.first
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; CHECK-LABEL: for.first:
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; CHECK: br i1 %cmp.j, label %for.first, label %for.second.exit
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; CHECK-LABEL: for.second.exit:
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; CHECK-NEXT: br label %for.end
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; CHECK-LABEL: for.end:
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; CHECK-NEXT: ret void
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define void @moveinsts_guardblock(i32* noalias %A, i32* noalias %B, i64 %N, i32 %x) {
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for.first.guard:
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%cmp.guard = icmp slt i64 0, %N
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br i1 %cmp.guard, label %for.first.preheader, label %for.second.guard
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for.first.preheader:
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br label %for.first
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for.first:
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%i.04 = phi i64 [ %inc, %for.first ], [ 0, %for.first.preheader ]
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%arrayidx = getelementptr inbounds i32, i32* %A, i64 %i.04
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store i32 0, i32* %arrayidx, align 4
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%inc = add nsw i64 %i.04, 1
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%cmp = icmp slt i64 %inc, %N
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br i1 %cmp, label %for.first, label %for.first.exit
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for.first.exit:
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br label %for.second.guard
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for.second.guard:
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%add = add nsw i32 %x, 1
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br i1 %cmp.guard, label %for.second.preheader, label %for.end
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for.second.preheader:
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br label %for.second
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for.second:
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%j.02 = phi i64 [ %inc6, %for.second ], [ 0, %for.second.preheader ]
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%arrayidx4 = getelementptr inbounds i32, i32* %B, i64 %j.02
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store i32 0, i32* %arrayidx4, align 4
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%inc6 = add nsw i64 %j.02, 1
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%cmp.j = icmp slt i64 %inc6, %N
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br i1 %cmp.j, label %for.second, label %for.second.exit
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for.second.exit:
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br label %for.end
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for.end:
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ret void
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}
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