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[InstCombine] Restrict a GEP transform to avoid changing provenance

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This is an alternative to D98120. Herein, instead of deleting the transformation entirely, we check
that the underlying objects are both the same and therefore this transformation wouldn't incur a
provenance change, if applied.

https://alive2.llvm.org/ce/z/SYF_yv

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D98588
This commit is contained in:
Simonas Kazlauskas 2021-03-13 21:55:26 +02:00
parent 5bf1dc2e73
commit 1101d7f026
2 changed files with 14 additions and 9 deletions
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17
lib/Transforms/InstCombine/InstructionCombining.cpp
View File

@ -2173,14 +2173,15 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
Matched = true;
}
if (Matched) {
// Canonicalize (gep i8* X, (ptrtoint Y)-(ptrtoint X))
// to (bitcast Y)
Value *Y;
if (match(V, m_Sub(m_PtrToInt(m_Value(Y)),
m_PtrToInt(m_Specific(GEP.getOperand(0))))))
return CastInst::CreatePointerBitCastOrAddrSpaceCast(Y, GEPType);
}
// Canonicalize (gep i8* X, (ptrtoint Y)-(ptrtoint X)) to (bitcast Y), but
// only if both point to the same underlying object (otherwise provenance
// is not necessarily retained).
Value *Y;
Value *X = GEP.getOperand(0);
if (Matched &&
match(V, m_Sub(m_PtrToInt(m_Value(Y)), m_PtrToInt(m_Specific(X)))) &&
getUnderlyingObject(X) == getUnderlyingObject(Y))
return CastInst::CreatePointerBitCastOrAddrSpaceCast(Y, GEPType);
}
}

6
test/Transforms/InstCombine/getelementptr.ll
View File

@ -1087,7 +1087,11 @@ define %struct.C* @test44i(%struct.C* %c1, %struct.C* %c2) {
define %struct.C* @test45(%struct.C* %c1, %struct.C** %c2) {
; CHECK-LABEL: @test45(
; CHECK-NEXT: [[GEP:%.*]] = bitcast %struct.C** [[C2:%.*]] to %struct.C*
; CHECK-NEXT: [[PTRTOINT1:%.*]] = ptrtoint %struct.C* [[C1:%.*]] to i64
; CHECK-NEXT: [[PTRTOINT2:%.*]] = ptrtoint %struct.C** [[C2:%.*]] to i64
; CHECK-NEXT: [[SUB:%.*]] = sub i64 [[PTRTOINT2]], [[PTRTOINT1]]
; CHECK-NEXT: [[SHR:%.*]] = sdiv i64 [[SUB]], 7
; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds [[STRUCT_C:%.*]], %struct.C* [[C1]], i64 [[SHR]]
; CHECK-NEXT: ret %struct.C* [[GEP]]
;
%ptrtoint1 = ptrtoint %struct.C* %c1 to i64