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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 2020-01-29 15:02:21 +00:00
parent 0bec0e7151
commit da58e68fdf
6 changed files with 228 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
llvm/include/llvm/Transforms/Utils/CodeMoverUtils.h
View File

@ -41,6 +41,12 @@ bool isSafeToMoveBefore(Instruction &I, Instruction &InsertPoint,
DominatorTree &DT, const PostDominatorTree &PDT,
DependenceInfo &DI);
/// Return true if all instructions (except the terminator) in \p BB can be
/// safely moved before \p InsertPoint.
bool isSafeToMoveBefore(BasicBlock &BB, Instruction &InsertPoint,
DominatorTree &DT, const PostDominatorTree &PDT,
DependenceInfo &DI);
/// Move instructions, in an order-preserving manner, from \p FromBB to the
/// beginning of \p ToBB when proven safe.
void moveInstructionsToTheBeginning(BasicBlock &FromBB, BasicBlock &ToBB,
@ -48,6 +54,12 @@ void moveInstructionsToTheBeginning(BasicBlock &FromBB, BasicBlock &ToBB,
const PostDominatorTree &PDT,
DependenceInfo &DI);
/// Move instructions, in an order-preserving manner, from \p FromBB to the end
/// of \p ToBB when proven safe.
void moveInstructionsToTheEnd(BasicBlock &FromBB, BasicBlock &ToBB,
DominatorTree &DT, const PostDominatorTree &PDT,
DependenceInfo &DI);
} // end namespace llvm
#endif // LLVM_TRANSFORMS_UTILS_CODEMOVERUTILS_H

26
llvm/lib/Transforms/Scalar/LoopFuse.cpp
View File

@ -86,7 +86,9 @@ STATISTIC(UnknownTripCount, "Loop has unknown trip count");
STATISTIC(UncomputableTripCount, "SCEV cannot compute trip count of loop");
STATISTIC(NonEqualTripCount, "Loop trip counts are not the same");
STATISTIC(NonAdjacent, "Loops are not adjacent");
STATISTIC(NonEmptyPreheader, "Loop has a non-empty preheader");
STATISTIC(
NonEmptyPreheader,
"Loop has a non-empty preheader with instructions that cannot be moved");
STATISTIC(FusionNotBeneficial, "Fusion is not beneficial");
STATISTIC(NonIdenticalGuards, "Candidates have different guards");
STATISTIC(NonEmptyExitBlock, "Candidate has a non-empty exit block");
@ -738,19 +740,21 @@ private:
continue;
}
// The following three checks look for empty blocks in FC0 and FC1. If
// any of these blocks are non-empty, we do not fuse. This is done
// because we currently do not have the safety checks to determine if
// it is safe to move the blocks past other blocks in the loop. Once
// these checks are added, these conditions can be relaxed.
if (!isEmptyPreheader(*FC1)) {
LLVM_DEBUG(dbgs() << "Fusion candidate does not have empty "
"preheader. Not fusing.\n");
if (!isSafeToMoveBefore(*FC1->Preheader,
*FC0->Preheader->getTerminator(), DT, PDT,
DI)) {
LLVM_DEBUG(dbgs() << "Fusion candidate contains unsafe "
"instructions in preheader. Not fusing.\n");
reportLoopFusion<OptimizationRemarkMissed>(*FC0, *FC1,
NonEmptyPreheader);
continue;
}
// The following two checks look for empty blocks in FC0 and FC1. If
// any of these blocks are non-empty, we do not fuse. This is done
// because we currently do not have the safety checks to determine if
// it is safe to move the blocks past other blocks in the loop. Once
// these checks are added, these conditions can be relaxed.
if (FC0->GuardBranch && !isEmptyExitBlock(*FC0)) {
LLVM_DEBUG(dbgs() << "Fusion candidate does not have empty exit "
"block. Not fusing.\n");
@ -1166,6 +1170,10 @@ private:
LLVM_DEBUG(dbgs() << "Fusion Candidate 0: \n"; FC0.dump();
dbgs() << "Fusion Candidate 1: \n"; FC1.dump(););
// Move instructions from the preheader of FC1 to the end of the preheader
// of FC0.
moveInstructionsToTheEnd(*FC1.Preheader, *FC0.Preheader, DT, PDT, DI);
// Fusing guarded loops is handled slightly differently than non-guarded
// loops and has been broken out into a separate method instead of trying to
// intersperse the logic within a single method.

23
llvm/lib/Transforms/Utils/CodeMoverUtils.cpp
View File

@ -380,6 +380,17 @@ bool llvm::isSafeToMoveBefore(Instruction &I, Instruction &InsertPoint,
return true;
}
bool llvm::isSafeToMoveBefore(BasicBlock &BB, Instruction &InsertPoint,
DominatorTree &DT, const PostDominatorTree &PDT,
DependenceInfo &DI) {
return llvm::all_of(BB, [&](Instruction &I) {
if (BB.getTerminator() == &I)
return true;
return isSafeToMoveBefore(I, InsertPoint, DT, PDT, DI);
});
}
void llvm::moveInstructionsToTheBeginning(BasicBlock &FromBB, BasicBlock &ToBB,
DominatorTree &DT,
const PostDominatorTree &PDT,
@ -394,3 +405,15 @@ void llvm::moveInstructionsToTheBeginning(BasicBlock &FromBB, BasicBlock &ToBB,
I.moveBefore(MovePos);
}
}
void llvm::moveInstructionsToTheEnd(BasicBlock &FromBB, BasicBlock &ToBB,
DominatorTree &DT,
const PostDominatorTree &PDT,
DependenceInfo &DI) {
Instruction *MovePos = ToBB.getTerminator();
while (FromBB.size() > 1) {
Instruction &I = FromBB.front();
if (isSafeToMoveBefore(I, *MovePos, DT, PDT, DI))
I.moveBefore(MovePos);
}
}

36
llvm/test/Transforms/LoopFusion/diagnostics_missed.ll
View File

@ -104,7 +104,7 @@ for.end15: ; preds = %for.cond.cleanup5
ret void
}
; CHECK: remark: diagnostics_missed.c:38:3: [negative_dependence]: entry and for.end: Loop has a non-empty preheader
; CHECK: remark: diagnostics_missed.c:38:3: [negative_dependence]: entry and for.end: Dependencies prevent fusion
define void @negative_dependence(i32* noalias %A) !dbg !51 {
entry:
br label %for.body
@ -185,6 +185,36 @@ for.end12: ; preds = %for.inc10
; Function Attrs: nounwind readnone speculatable willreturn
declare void @llvm.dbg.value(metadata, metadata, metadata) #0
; 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
define void @unsafe_preheader(i32* noalias %A, i32* noalias %B) {
for.first.preheader:
br label %for.first, !dbg !80
for.first:
%i.02 = phi i64 [ 0, %for.first.preheader ], [ %inc, %for.first ]
%arrayidx = getelementptr inbounds i32, i32* %A, i64 %i.02
store i32 0, i32* %arrayidx, align 4
%inc = add nsw i64 %i.02, 1
%cmp = icmp slt i64 %inc, 100
br i1 %cmp, label %for.first, label %for.second.preheader
for.second.preheader:
call void @bar()
br label %for.second
for.second:
%j.01 = phi i64 [ 0, %for.second.preheader ], [ %inc6, %for.second ]
%arrayidx4 = getelementptr inbounds i32, i32* %B, i64 %j.01
store i32 0, i32* %arrayidx4, align 4
%inc6 = add nsw i64 %j.01, 1
%cmp2 = icmp slt i64 %inc6, 100
br i1 %cmp2, label %for.second, label %for.end
for.end:
ret void
}
declare void @bar()
attributes #0 = { nounwind readnone speculatable willreturn }
!llvm.dbg.cu = !{!2}
@ -267,3 +297,7 @@ attributes #0 = { nounwind readnone speculatable willreturn }
!74 = !DILocation(line: 51, column: 3, scope: !70)
!75 = !DILocation(line: 52, column: 15, scope: !70)
!76 = !DILocation(line: 57, column: 3, scope: !63)
!77 = distinct !DISubprogram(name: "unsafe_preheader", scope: !3, file: !3, line: 60, type: !15, scopeLine: 60, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !2, retainedNodes: !78)
!78 = !{}
!79 = distinct !DILexicalBlock(scope: !77, file: !3, line: 3, column: 5)
!80 = !DILocation(line: 62, column: 3, scope: !79)

56
llvm/test/Transforms/LoopFusion/guarded.ll
View File

@ -63,3 +63,59 @@ bb15: ; preds = %bb9
bb12: ; preds = %bb15, %bb14
ret void
}
; Test that `%add` is moved in for.first.preheader, and the two loops for.first
; and for.second are fused.
; CHECK: void @moveinsts_preheader
; CHECK-LABEL: for.first.guard:
; CHECK: br i1 %cmp.guard, label %for.first.preheader, label %for.end
; CHECK-LABEL: for.first.preheader:
; CHECK-NEXT: %add = add nsw i32 %x, 1
; CHECK-NEXT: br label %for.first
; CHECK-LABEL: for.first:
; CHECK: br i1 %cmp.j, label %for.first, label %for.second.exit
; CHECK-LABEL: for.second.exit:
; CHECK-NEXT: br label %for.end
; CHECK-LABEL: for.end:
; CHECK-NEXT: ret void
define void @moveinsts_preheader(i32* noalias %A, i32* noalias %B, i64 %N, i32 %x) {
for.first.guard:
%cmp.guard = icmp slt i64 0, %N
br i1 %cmp.guard, label %for.first.preheader, label %for.second.guard
for.first.preheader:
br label %for.first
for.first:
%i = phi i64 [ %inc.i, %for.first ], [ 0, %for.first.preheader ]
%Ai = getelementptr inbounds i32, i32* %A, i64 %i
store i32 0, i32* %Ai, align 4
%inc.i = add nsw i64 %i, 1
%cmp.i = icmp slt i64 %inc.i, %N
br i1 %cmp.i, label %for.first, label %for.first.exit
for.first.exit:
br label %for.second.guard
for.second.guard:
br i1 %cmp.guard, label %for.second.preheader, label %for.end
for.second.preheader:
%add = add nsw i32 %x, 1
br label %for.second
for.second:
%j = phi i64 [ %inc.j, %for.second ], [ 0, %for.second.preheader ]
%Bj = getelementptr inbounds i32, i32* %B, i64 %j
store i32 0, i32* %Bj, align 4
%inc.j = add nsw i64 %j, 1
%cmp.j = icmp slt i64 %inc.j, %N
br i1 %cmp.j, label %for.second, label %for.second.exit
for.second.exit:
br label %for.end
for.end:
ret void
}

85
llvm/test/Transforms/LoopFusion/simple.ll
View File

@ -306,3 +306,88 @@ for.second.latch:
for.end:
ret void
}
; Test that `%add` is moved in basic block entry, and the two loops for.first
; and for.second are fused.
; CHECK: i32 @moveinsts_preheader
; CHECK-LABEL: entry:
; CHECK-NEXT: %add = add nsw i32 %x, 1
; CHECK-NEXT: br label %for.first
; CHECK-LABEL: for.first:
; CHECK: br i1 %cmp.j, label %for.first, label %for.second.exit
; CHECK-LABEL: for.second.exit:
; CHECK-NEXT: ret i32 %add
define i32 @moveinsts_preheader(i32* %A, i32 %x) {
entry:
br label %for.first
for.first:
%i = phi i64 [ 0, %entry ], [ %inc.i, %for.first ]
%Ai = getelementptr inbounds i32, i32* %A, i64 %i
store i32 0, i32* %Ai, align 4
%inc.i = add nsw i64 %i, 1
%cmp.i = icmp slt i64 %inc.i, 100
br i1 %cmp.i, label %for.first, label %for.first.exit
for.first.exit:
%add = add nsw i32 %x, 1
br label %for.second
for.second:
%j = phi i64 [ 0, %for.first.exit ], [ %inc.j, %for.second ]
%Aj = getelementptr inbounds i32, i32* %A, i64 %j
store i32 2, i32* %Aj, align 4
%inc.j = add nsw i64 %j, 1
%cmp.j = icmp slt i64 %inc.j, 100
br i1 %cmp.j, label %for.second, label %for.second.exit
for.second.exit:
ret i32 %add
}
; Test that `%add` cannot be moved to basic block entry, as it uses %i, which
; defined after basic block entry. And the two loops for.first and for.second
; are not fused.
; CHECK: i64 @unsafe_preheader
; CHECK-LABEL: entry:
; CHECK-NEXT: br label %for.first
; CHECK-LABEL: for.first:
; CHECK: br i1 %cmp.i, label %for.first, label %for.first.exit
; CHECK-LABEL: for.first.exit:
; CHECK-NEXT: %add = add nsw i64 %x, %i
; CHECK-NEXT: br label %for.second
; CHECK-LABEL: for.second:
; CHECK: br i1 %cmp.j, label %for.second, label %for.second.exit
; CHECK-LABEL: for.second.exit:
; CHECK-NEXT: ret i64 %add
define i64 @unsafe_preheader(i32* %A, i64 %x) {
entry:
br label %for.first
for.first:
%i = phi i64 [ 0, %entry ], [ %inc.i, %for.first ]
%Ai = getelementptr inbounds i32, i32* %A, i64 %i
store i32 0, i32* %Ai, align 4
%inc.i = add nsw i64 %i, 1
%cmp.i = icmp slt i64 %inc.i, 100
br i1 %cmp.i, label %for.first, label %for.first.exit
for.first.exit:
%add = add nsw i64 %x, %i
br label %for.second
for.second:
%j = phi i64 [ 0, %for.first.exit ], [ %inc.j, %for.second ]
%Aj = getelementptr inbounds i32, i32* %A, i64 %j
store i32 2, i32* %Aj, align 4
%inc.j = add nsw i64 %j, 1
%cmp.j = icmp slt i64 %inc.j, 100
br i1 %cmp.j, label %for.second, label %for.second.exit
for.second.exit:
ret i64 %add
}