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[InstCombine] replace undef in vector constant for safe shift transform (PR45447)

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As noted in PR45447, we have a vector-constant-with-undef-element transform bug:
https://bugs.llvm.org/show_bug.cgi?id=45447

We replace undefs with a safe constant (0 or -1) based on the (non-)negative
predicate constraint.

So this is correct:
http://volta.cs.utah.edu:8080/z/WZE36H
...but this is not:
http://volta.cs.utah.edu:8080/z/boj8gJ

Previously, we were relying on getSafeVectorConstantForBinop() in the related fold (D76800).
But that's making an assumption about what qualifies as "safe", and that assumption may
not always hold.

Differential Revision: https://reviews.llvm.org/D77739
This commit is contained in:
Sanjay Patel 2020-04-09 07:55:55 -04:00
parent 941b9f5d86
commit de39bfc89c
2 changed files with 12 additions and 6 deletions
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16
lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
View File

@ -3071,16 +3071,22 @@ Instruction *InstCombiner::visitXor(BinaryOperator &I) {
Constant *C;
if (match(NotVal, m_AShr(m_Constant(C), m_Value(Y))) &&
match(C, m_Negative())) {
Constant *NewC = ConstantExpr::getNot(C);
if (C->getType()->isVectorTy())
NewC = getSafeVectorConstantForBinop(Instruction::LShr, NewC, false);
return BinaryOperator::CreateLShr(NewC, Y);
// We matched a negative constant, so propagating undef is unsafe.
// Clamp undef elements to -1.
Type *EltTy = C->getType()->getScalarType();
C = Constant::replaceUndefsWith(C, ConstantInt::getAllOnesValue(EltTy));
return BinaryOperator::CreateLShr(ConstantExpr::getNot(C), Y);
}
// ~(C >>u Y) --> ~C >>s Y (when inverting the replicated sign bits)
if (match(NotVal, m_LShr(m_Constant(C), m_Value(Y))) &&
match(C, m_NonNegative()))
match(C, m_NonNegative())) {
// We matched a non-negative constant, so propagating undef is unsafe.
// Clamp undef elements to 0.
Type *EltTy = C->getType()->getScalarType();
C = Constant::replaceUndefsWith(C, ConstantInt::getNullValue(EltTy));
return BinaryOperator::CreateAShr(ConstantExpr::getNot(C), Y);
}
// ~(X + C) --> -(C + 1) - X
if (match(Op0, m_Add(m_Value(X), m_Constant(C))))

2
test/Transforms/InstCombine/vector-xor.ll
View File

@ -172,7 +172,7 @@ define <4 x i32> @test_v4i32_not_lshr_nonnegative_const(<4 x i32> %a0) {
define <4 x i32> @test_v4i32_not_lshr_nonnegative_const_undef(<4 x i32> %a0) {
; CHECK-LABEL: @test_v4i32_not_lshr_nonnegative_const_undef(
; CHECK-NEXT: [[TMP1:%.*]] = ashr <4 x i32> <i32 -4, i32 -6, i32 undef, i32 -10>, [[A0:%.*]]
; CHECK-NEXT: [[TMP1:%.*]] = ashr <4 x i32> <i32 -4, i32 -6, i32 -1, i32 -10>, [[A0:%.*]]
; CHECK-NEXT: ret <4 x i32> [[TMP1]]
;
%1 = lshr <4 x i32> <i32 3, i32 5, i32 undef, i32 9>, %a0