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[DSE] Bail out on MemoryPhis when deleting stores at end of function.

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When deleting stores at the end of a function, we have to do PHI
translation, otherwise we might miss reads in different iterations of a
loop. See multiblock-loop-carried-dependence.ll for details.

This fixes a mis-compile and surprisingly also increases the number of
eliminated stores from 26047 to 26572 for MultiSource/SPEC2000/SPEC2006
on X86 with -O3 -flto. This is most likely because we save budget by not
exploring through MemoryPhis, which are less likely to result in valid
candidates for elimination.

The issue was reported post-commit for fb109c42d91c.
This commit is contained in:
Florian Hahn 2020-09-12 18:57:26 +01:00
parent 2fae14a0b6
commit ad8e41fb76
4 changed files with 28 additions and 4 deletions
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10
lib/Transforms/Scalar/DeadStoreElimination.cpp
View File

@ -1776,10 +1776,12 @@ struct DSEState {
}
MemoryAccess *UseAccess = WorkList[I];
if (isa<MemoryPhi>(UseAccess)) {
PushMemUses(UseAccess);
continue;
}
// Simply adding the users of MemoryPhi to the worklist is not enough,
// because we might miss read clobbers in different iterations of a loop,
// for example.
// TODO: Add support for phi translation to handle the loop case.
if (isa<MemoryPhi>(UseAccess))
return false;
// TODO: Checking for aliasing is expensive. Consider reducing the amount
// of times this is called and/or caching it.

3
test/Transforms/DeadStoreElimination/MSSA/multiblock-loop-carried-dependence.ll
View File

@ -29,6 +29,9 @@ define void @test.1() {
; CHECK-NEXT: [[PTR_IV_2:%.*]] = getelementptr inbounds [100 x i32], [100 x i32]* [[A]], i64 0, i64 [[IV_2]]
; CHECK-NEXT: [[L_0:%.*]] = load i32, i32* [[PTR_IV_2]], align 4
; CHECK-NEXT: call void @use(i32 [[L_0]])
; CHECK-NEXT: [[ADD:%.*]] = add nsw i64 [[IV_2]], 1
; CHECK-NEXT: [[PTR_IV_2_ADD_1:%.*]] = getelementptr inbounds [100 x i32], [100 x i32]* [[A]], i64 0, i64 [[ADD]]
; CHECK-NEXT: store i32 10, i32* [[PTR_IV_2_ADD_1]], align 4
; CHECK-NEXT: [[L_1:%.*]] = load i32, i32* [[PTR_IV_2]], align 4
; CHECK-NEXT: call void @use(i32 [[L_1]])
; CHECK-NEXT: [[IV_2_NEXT]] = add nsw i64 [[IV_2]], 1

1
test/Transforms/DeadStoreElimination/MSSA/multiblock-malloc-free.ll
View File

@ -180,6 +180,7 @@ define void @test27() {
; CHECK-NEXT: br i1 true, label [[BB2:%.*]], label [[BB3:%.*]]
; CHECK: bb2:
; CHECK-NEXT: [[M:%.*]] = call noalias i8* @malloc(i64 10)
; CHECK-NEXT: store i8 1, i8* [[M]], align 1
; CHECK-NEXT: br label [[BB3]]
; CHECK: bb3:
; CHECK-NEXT: [[R:%.*]] = phi i8* [ null, [[BB1:%.*]] ], [ [[M]], [[BB2]] ]

18
test/Transforms/DeadStoreElimination/MSSA/multiblock-memintrinsics.ll
View File

@ -123,10 +123,18 @@ bb3:
define void @alloca_1(i1 %c) {
; CHECK-LABEL: @alloca_1(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[P_ALLOCA:%.*]] = alloca [32 x i32], align 4
; CHECK-NEXT: [[P:%.*]] = bitcast [32 x i32]* [[P_ALLOCA]] to i32*
; CHECK-NEXT: [[ARRAYIDX0:%.*]] = getelementptr inbounds i32, i32* [[P]], i64 1
; CHECK-NEXT: [[P3:%.*]] = bitcast i32* [[ARRAYIDX0]] to i8*
; CHECK-NEXT: [[TMP0:%.*]] = getelementptr inbounds i8, i8* [[P3]], i64 4
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* align 4 [[TMP0]], i8 0, i64 24, i1 false)
; CHECK-NEXT: br i1 [[C:%.*]], label [[BB1:%.*]], label [[BB2:%.*]]
; CHECK: bb1:
; CHECK-NEXT: br label [[BB3:%.*]]
; CHECK: bb2:
; CHECK-NEXT: [[ARRAYIDX1:%.*]] = getelementptr inbounds i32, i32* [[P]], i64 1
; CHECK-NEXT: store i32 1, i32* [[ARRAYIDX1]], align 4
; CHECK-NEXT: br label [[BB3]]
; CHECK: bb3:
; CHECK-NEXT: ret void
@ -152,10 +160,20 @@ bb3:
define void @alloca_2(i1 %c) {
; CHECK-LABEL: @alloca_2(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[P_ALLOCA:%.*]] = alloca [32 x i32], align 4
; CHECK-NEXT: [[P:%.*]] = bitcast [32 x i32]* [[P_ALLOCA]] to i32*
; CHECK-NEXT: [[ARRAYIDX0:%.*]] = getelementptr inbounds i32, i32* [[P]], i64 1
; CHECK-NEXT: [[P3:%.*]] = bitcast i32* [[ARRAYIDX0]] to i8*
; CHECK-NEXT: [[TMP0:%.*]] = getelementptr inbounds i8, i8* [[P3]], i64 4
; CHECK-NEXT: call void @llvm.memset.p0i8.i64(i8* align 4 [[TMP0]], i8 0, i64 24, i1 false)
; CHECK-NEXT: br i1 [[C:%.*]], label [[BB1:%.*]], label [[BB2:%.*]]
; CHECK: bb1:
; CHECK-NEXT: [[ARRAYIDX1:%.*]] = getelementptr inbounds i32, i32* [[P]], i64 1
; CHECK-NEXT: store i32 1, i32* [[ARRAYIDX1]], align 4
; CHECK-NEXT: br label [[BB3:%.*]]
; CHECK: bb2:
; CHECK-NEXT: [[ARRAYIDX2:%.*]] = getelementptr inbounds i32, i32* [[P]], i64 1
; CHECK-NEXT: store i32 1, i32* [[ARRAYIDX2]], align 4
; CHECK-NEXT: br label [[BB3]]
; CHECK: bb3:
; CHECK-NEXT: ret void