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[InstCombine] visitMaskedMerge(): when unfolding, sanitize undef constants (PR45955)

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We can't leave undef vector element constants as-is,
it is a miscompile, so we need to sanitize them.

We have two vectors (C and ~C):
* We can't replace undef with 0 in both of them
* We can't replace undef with 0 in only one of them
* We could replace undef with -1 in both of them
* We could replace undef with -1 in only one(!) of them
* We could replace undef with -1 in one and 0 in another one of them.

Therefore, it seems best to go with the last option, since otherwise
we'd loose knowledge that C and ~C have no common bits set,
which seems more important than preserving partial undef knowledge.

Fixes https://bugs.llvm.org/show_bug.cgi?id=45955
This commit is contained in:
Roman Lebedev 2020-05-17 21:58:55 +03:00
parent dc9326e53b
commit b6c9370768
2 changed files with 8 additions and 5 deletions
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3
lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
View File

@ -3049,6 +3049,9 @@ static Instruction *visitMaskedMerge(BinaryOperator &I,
Constant *C;
if (D->hasOneUse() && match(M, m_Constant(C))) {
// Propagating undef is unsafe. Clamp undef elements to -1.
Type *EltTy = C->getType()->getScalarType();
C = Constant::replaceUndefsWith(C, ConstantInt::getAllOnesValue(EltTy));
// Unfold.
Value *LHS = Builder.CreateAnd(X, C);
Value *NotC = Builder.CreateNot(C);

10
test/Transforms/InstCombine/unfold-masked-merge-with-const-mask-vector.ll
View File

@ -21,8 +21,8 @@ define <2 x i4> @splat (<2 x i4> %x, <2 x i4> %y) {
define <3 x i4> @splat_undef (<3 x i4> %x, <3 x i4> %y) {
; CHECK-LABEL: @splat_undef(
; CHECK-NEXT: [[TMP1:%.*]] = and <3 x i4> [[X:%.*]], <i4 -2, i4 undef, i4 -2>
; CHECK-NEXT: [[TMP2:%.*]] = and <3 x i4> [[Y:%.*]], <i4 1, i4 undef, i4 1>
; CHECK-NEXT: [[TMP1:%.*]] = and <3 x i4> [[X:%.*]], <i4 -2, i4 -1, i4 -2>
; CHECK-NEXT: [[TMP2:%.*]] = and <3 x i4> [[Y:%.*]], <i4 1, i4 0, i4 1>
; CHECK-NEXT: [[R:%.*]] = or <3 x i4> [[TMP1]], [[TMP2]]
; CHECK-NEXT: ret <3 x i4> [[R]]
;
@ -85,8 +85,8 @@ define <2 x i4> @in_constant_varx_14_nonsplat(<2 x i4> %x, <2 x i4> %mask) {
define <3 x i4> @in_constant_varx_14_undef(<3 x i4> %x, <3 x i4> %mask) {
; CHECK-LABEL: @in_constant_varx_14_undef(
; CHECK-NEXT: [[TMP1:%.*]] = and <3 x i4> [[X:%.*]], <i4 1, i4 undef, i4 1>
; CHECK-NEXT: [[R:%.*]] = or <3 x i4> [[TMP1]], <i4 -2, i4 undef, i4 6>
; CHECK-NEXT: [[TMP1:%.*]] = and <3 x i4> [[X:%.*]], <i4 1, i4 -1, i4 1>
; CHECK-NEXT: [[R:%.*]] = or <3 x i4> [[TMP1]], <i4 -2, i4 0, i4 6>
; CHECK-NEXT: ret <3 x i4> [[R]]
;
%n0 = xor <3 x i4> %x, <i4 14, i4 undef, i4 7> ; %x
@ -133,7 +133,7 @@ define <2 x i4> @in_constant_14_vary_nonsplat(<2 x i4> %y, <2 x i4> %mask) {
define <3 x i4> @in_constant_14_vary_undef(<3 x i4> %y, <3 x i4> %mask) {
; CHECK-LABEL: @in_constant_14_vary_undef(
; CHECK-NEXT: [[TMP1:%.*]] = and <3 x i4> [[Y:%.*]], <i4 -2, i4 undef, i4 -2>
; CHECK-NEXT: [[TMP1:%.*]] = and <3 x i4> [[Y:%.*]], <i4 -2, i4 0, i4 -2>
; CHECK-NEXT: [[R:%.*]] = or <3 x i4> [[TMP1]], <i4 0, i4 undef, i4 1>
; CHECK-NEXT: ret <3 x i4> [[R]]
;