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[InstCombine] Let combineLoadToNewType preserve ABI alignment of the load (PR44543) 

Summary:
If aligment on `LoadInst` isn't specified, load is assumed to be ABI-aligned.
And said aligment may be different for different types.
So if we change load type, but don't pay extra attention to the aligment
(i.e. keep it unspecified), we may either overpromise (if the default aligment
of the new type is higher), or underpromise (if the default aligment
of the new type is smaller).

Thus, if no alignment is specified, we need to manually preserve the implied ABI alignment.

This addresses https://bugs.llvm.org/show_bug.cgi?id=44543 by making combineLoadToNewType preserve ABI alignment of the load.

Reviewers: spatel, lebedev.ri

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72710
This commit is contained in:
Juneyoung Lee 2020-01-15 01:11:20 +09:00
parent 40c5bd4212
commit 3e32b7e127
2 changed files with 11 additions and 4 deletions
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9
llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
View File

@ -462,8 +462,15 @@ static LoadInst *combineLoadToNewType(InstCombiner &IC, LoadInst &LI, Type *NewT
NewPtr->getType()->getPointerAddressSpace() == AS))
NewPtr = IC.Builder.CreateBitCast(Ptr, NewTy->getPointerTo(AS));
unsigned Align = LI.getAlignment();
if (!Align)
// If old load did not have an explicit alignment specified,
// manually preserve the implied (ABI) alignment of the load.
// Else we may inadvertently incorrectly over-promise alignment.
Align = IC.getDataLayout().getABITypeAlignment(LI.getType());
LoadInst *NewLoad = IC.Builder.CreateAlignedLoad(
NewTy, NewPtr, LI.getAlignment(), LI.isVolatile(), LI.getName() + Suffix);
NewTy, NewPtr, Align, LI.isVolatile(), LI.getName() + Suffix);
NewLoad->setAtomic(LI.getOrdering(), LI.getSyncScopeID());
copyMetadataForLoad(*NewLoad, LI);
return NewLoad;

6
llvm/test/Transforms/InstCombine/load-bitcast64.ll
View File

@ -8,7 +8,7 @@ define i64* @test1(i8* %x) {
; CHECK-LABEL: @test1(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[TMP0:%.*]] = bitcast i8* [[X:%.*]] to i64**
; CHECK-NEXT: [[B1:%.*]] = load i64*, i64** [[TMP0]], align 8
; CHECK-NEXT: [[B1:%.*]] = load i64*, i64** [[TMP0]], align 4
; CHECK-NEXT: ret i64* [[B1]]
;
entry:
@ -57,7 +57,7 @@ define i64 @test4(i8* %x) {
; CHECK-LABEL: @test4(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[TMP0:%.*]] = bitcast i8* [[X:%.*]] to i64*
; CHECK-NEXT: [[B1:%.*]] = load i64, i64* [[TMP0]], align 4
; CHECK-NEXT: [[B1:%.*]] = load i64, i64* [[TMP0]], align 8
; CHECK-NEXT: ret i64 [[B1]]
;
entry:
@ -88,7 +88,7 @@ define i64 @test6(i8* %x) {
; CHECK-LABEL: @test6(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[TMP0:%.*]] = bitcast i8* [[X:%.*]] to i64*
; CHECK-NEXT: [[B1:%.*]] = load i64, i64* [[TMP0]], align 4
; CHECK-NEXT: [[B1:%.*]] = load i64, i64* [[TMP0]], align 8
; CHECK-NEXT: ret i64 [[B1]]
;
entry: