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[LoopFusion] Move instructions from FC1.Preheader to FC0.Preheader when
proven safe.
Summary:
Currently LoopFusion give up when the second loop nest preheader is
not empty. For example:
for (int i = 0; i < 100; ++i) {}
x+=1;
for (int i = 0; i < 100; ++i) {}
The above example should be safe to fuse.
This PR moves instructions in FC1 preheader (e.g. x+=1; ) to
FC0 preheader, which then LoopFusion is able to fuse them.
Reviewer: kbarton, Meinersbur, jdoerfert, dmgreen, fhahn, hfinkel,
bmahjour, etiotto
Reviewed By: jdoerfert
Subscribers: hiraditya, llvm-commits
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D71821
This commit is contained in:
Whitney Tsang
parent
6a98aebfc4
commit
0b3ecba332
@ -41,6 +41,12 @@ bool isSafeToMoveBefore(Instruction &I, Instruction &InsertPoint,
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DominatorTree &DT, const PostDominatorTree &PDT,
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DependenceInfo &DI);
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/// Return true if all instructions (except the terminator) in \p BB can be
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/// safely moved before \p InsertPoint.
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bool isSafeToMoveBefore(BasicBlock &BB, Instruction &InsertPoint,
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DominatorTree &DT, const PostDominatorTree &PDT,
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DependenceInfo &DI);
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/// Move instructions, in an order-preserving manner, from \p FromBB to the
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/// beginning of \p ToBB when proven safe.
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void moveInstructionsToTheBeginning(BasicBlock &FromBB, BasicBlock &ToBB,
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@ -48,6 +54,12 @@ void moveInstructionsToTheBeginning(BasicBlock &FromBB, BasicBlock &ToBB,
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const PostDominatorTree &PDT,
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DependenceInfo &DI);
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/// Move instructions, in an order-preserving manner, from \p FromBB to the end
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/// of \p ToBB when proven safe.
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void moveInstructionsToTheEnd(BasicBlock &FromBB, BasicBlock &ToBB,
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DominatorTree &DT, const PostDominatorTree &PDT,
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DependenceInfo &DI);
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} // end namespace llvm
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#endif // LLVM_TRANSFORMS_UTILS_CODEMOVERUTILS_H
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@ -86,7 +86,9 @@ STATISTIC(UnknownTripCount, "Loop has unknown trip count");
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STATISTIC(UncomputableTripCount, "SCEV cannot compute trip count of loop");
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STATISTIC(NonEqualTripCount, "Loop trip counts are not the same");
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STATISTIC(NonAdjacent, "Loops are not adjacent");
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STATISTIC(NonEmptyPreheader, "Loop has a non-empty preheader");
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STATISTIC(
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NonEmptyPreheader,
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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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@ -738,19 +740,21 @@ private:
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continue;
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}
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// The following three 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 (!isEmptyPreheader(*FC1)) {
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LLVM_DEBUG(dbgs() << "Fusion candidate does not have empty "
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"preheader. Not fusing.\n");
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if (!isSafeToMoveBefore(*FC1->Preheader,
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*FC0->Preheader->getTerminator(), DT, PDT,
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DI)) {
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LLVM_DEBUG(dbgs() << "Fusion candidate contains unsafe "
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"instructions in preheader. Not fusing.\n");
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reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
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NonEmptyPreheader);
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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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@ -1166,6 +1170,10 @@ private:
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LLVM_DEBUG(dbgs() << "Fusion Candidate 0: \n"; FC0.dump();
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dbgs() << "Fusion Candidate 1: \n"; FC1.dump(););
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// Move instructions from the preheader of FC1 to the end of the preheader
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// of FC0.
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moveInstructionsToTheEnd(*FC1.Preheader, *FC0.Preheader, DT, PDT, DI);
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// Fusing guarded loops is handled slightly differently than non-guarded
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// loops and has been broken out into a separate method instead of trying to
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// intersperse the logic within a single method.
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@ -380,6 +380,17 @@ bool llvm::isSafeToMoveBefore(Instruction &I, Instruction &InsertPoint,
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return true;
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}
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bool llvm::isSafeToMoveBefore(BasicBlock &BB, Instruction &InsertPoint,
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DominatorTree &DT, const PostDominatorTree &PDT,
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DependenceInfo &DI) {
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return llvm::all_of(BB, [&](Instruction &I) {
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if (BB.getTerminator() == &I)
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return true;
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return isSafeToMoveBefore(I, InsertPoint, DT, PDT, DI);
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});
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}
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void llvm::moveInstructionsToTheBeginning(BasicBlock &FromBB, BasicBlock &ToBB,
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DominatorTree &DT,
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const PostDominatorTree &PDT,
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@ -394,3 +405,15 @@ void llvm::moveInstructionsToTheBeginning(BasicBlock &FromBB, BasicBlock &ToBB,
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I.moveBefore(MovePos);
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}
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}
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void llvm::moveInstructionsToTheEnd(BasicBlock &FromBB, BasicBlock &ToBB,
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DominatorTree &DT,
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const PostDominatorTree &PDT,
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DependenceInfo &DI) {
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Instruction *MovePos = ToBB.getTerminator();
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while (FromBB.size() > 1) {
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Instruction &I = FromBB.front();
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if (isSafeToMoveBefore(I, *MovePos, DT, PDT, DI))
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I.moveBefore(MovePos);
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}
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}
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@ -104,7 +104,7 @@ for.end15: ; preds = %for.cond.cleanup5
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ret void
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}
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; CHECK: remark: diagnostics_missed.c:38:3: [negative_dependence]: entry and for.end: Loop has a non-empty preheader
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; CHECK: remark: diagnostics_missed.c:38:3: [negative_dependence]: entry and for.end: Dependencies prevent fusion
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define void @negative_dependence(i32* noalias %A) !dbg !51 {
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entry:
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br label %for.body
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@ -185,6 +185,36 @@ for.end12: ; preds = %for.inc10
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; Function Attrs: nounwind readnone speculatable willreturn
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declare void @llvm.dbg.value(metadata, metadata, metadata) #0
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; CHECK: remark: diagnostics_missed.c:62:3: [unsafe_preheader]: for.first.preheader and for.second.preheader: Loop has a non-empty preheader with instructions that cannot be moved
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define void @unsafe_preheader(i32* noalias %A, i32* noalias %B) {
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for.first.preheader:
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br label %for.first, !dbg !80
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for.first:
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%i.02 = phi i64 [ 0, %for.first.preheader ], [ %inc, %for.first ]
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%arrayidx = getelementptr inbounds i32, i32* %A, i64 %i.02
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store i32 0, i32* %arrayidx, align 4
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%inc = add nsw i64 %i.02, 1
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%cmp = icmp slt i64 %inc, 100
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br i1 %cmp, label %for.first, label %for.second.preheader
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for.second.preheader:
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call void @bar()
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br label %for.second
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for.second:
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%j.01 = phi i64 [ 0, %for.second.preheader ], [ %inc6, %for.second ]
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%arrayidx4 = getelementptr inbounds i32, i32* %B, i64 %j.01
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store i32 0, i32* %arrayidx4, align 4
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%inc6 = add nsw i64 %j.01, 1
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%cmp2 = icmp slt i64 %inc6, 100
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br i1 %cmp2, label %for.second, 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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!llvm.dbg.cu = !{!2}
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@ -267,3 +297,7 @@ attributes #0 = { nounwind readnone speculatable willreturn }
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!74 = !DILocation(line: 51, column: 3, scope: !70)
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!75 = !DILocation(line: 52, column: 15, scope: !70)
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!76 = !DILocation(line: 57, column: 3, scope: !63)
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!77 = distinct !DISubprogram(name: "unsafe_preheader", scope: !3, file: !3, line: 60, type: !15, scopeLine: 60, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !78)
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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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@ -63,3 +63,59 @@ bb15: ; preds = %bb9
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bb12: ; preds = %bb15, %bb14
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ret void
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}
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; Test that `%add` is moved in for.first.preheader, and the two loops for.first
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; and for.second are fused.
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; CHECK: void @moveinsts_preheader
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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: %add = add nsw i32 %x, 1
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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_preheader(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 = phi i64 [ %inc.i, %for.first ], [ 0, %for.first.preheader ]
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%Ai = getelementptr inbounds i32, i32* %A, i64 %i
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store i32 0, i32* %Ai, align 4
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%inc.i = add nsw i64 %i, 1
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%cmp.i = icmp slt i64 %inc.i, %N
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br i1 %cmp.i, 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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br i1 %cmp.guard, label %for.second.preheader, label %for.end
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for.second.preheader:
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%add = add nsw i32 %x, 1
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br label %for.second
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for.second:
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%j = phi i64 [ %inc.j, %for.second ], [ 0, %for.second.preheader ]
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%Bj = getelementptr inbounds i32, i32* %B, i64 %j
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store i32 0, i32* %Bj, align 4
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%inc.j = add nsw i64 %j, 1
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%cmp.j = icmp slt i64 %inc.j, %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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@ -306,3 +306,88 @@ for.second.latch:
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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 basic block entry, and the two loops for.first
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; and for.second are fused.
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; CHECK: i32 @moveinsts_preheader
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; CHECK-LABEL: entry:
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; CHECK-NEXT: %add = add nsw i32 %x, 1
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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: ret i32 %add
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define i32 @moveinsts_preheader(i32* %A, i32 %x) {
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entry:
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br label %for.first
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for.first:
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%i = phi i64 [ 0, %entry ], [ %inc.i, %for.first ]
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%Ai = getelementptr inbounds i32, i32* %A, i64 %i
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store i32 0, i32* %Ai, align 4
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%inc.i = add nsw i64 %i, 1
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%cmp.i = icmp slt i64 %inc.i, 100
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br i1 %cmp.i, 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
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for.second:
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%j = phi i64 [ 0, %for.first.exit ], [ %inc.j, %for.second ]
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%Aj = getelementptr inbounds i32, i32* %A, i64 %j
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store i32 2, i32* %Aj, align 4
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%inc.j = add nsw i64 %j, 1
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%cmp.j = icmp slt i64 %inc.j, 100
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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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ret i32 %add
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}
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; Test that `%add` cannot be moved to basic block entry, as it uses %i, which
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; defined after basic block entry. And the two loops for.first and for.second
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; are not fused.
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; CHECK: i64 @unsafe_preheader
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; CHECK-LABEL: entry:
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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.i, label %for.first, label %for.first.exit
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; CHECK-LABEL: for.first.exit:
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; CHECK-NEXT: %add = add nsw i64 %x, %i
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; CHECK-NEXT: br label %for.second
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; CHECK-LABEL: for.second:
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; CHECK: br i1 %cmp.j, label %for.second, label %for.second.exit
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; CHECK-LABEL: for.second.exit:
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; CHECK-NEXT: ret i64 %add
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define i64 @unsafe_preheader(i32* %A, i64 %x) {
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entry:
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br label %for.first
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for.first:
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%i = phi i64 [ 0, %entry ], [ %inc.i, %for.first ]
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%Ai = getelementptr inbounds i32, i32* %A, i64 %i
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store i32 0, i32* %Ai, align 4
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%inc.i = add nsw i64 %i, 1
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%cmp.i = icmp slt i64 %inc.i, 100
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br i1 %cmp.i, label %for.first, label %for.first.exit
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for.first.exit:
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%add = add nsw i64 %x, %i
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br label %for.second
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for.second:
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%j = phi i64 [ 0, %for.first.exit ], [ %inc.j, %for.second ]
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%Aj = getelementptr inbounds i32, i32* %A, i64 %j
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store i32 2, i32* %Aj, align 4
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%inc.j = add nsw i64 %j, 1
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%cmp.j = icmp slt i64 %inc.j, 100
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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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ret i64 %add
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}
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