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[InstCombine] Remove buggy zext of icmp eq with pow2 fold (PR57899) 

For the case where the constant is a power of two rather than zero,
the fold is incorrect, because it fails to check that the bit set
in the LHS matches the bit in the RHS.

Rather than fixing this, remove the power of two handling entirely,
as a different fold will already canonicalize such comparisons to
use a zero constant.

Fixes https://github.com/llvm/llvm-project/issues/57899.
This commit is contained in:
Nikita Popov 2022-09-22 16:35:07 +02:00
parent 76fd4bf675
commit 8df376db72
2 changed files with 3 additions and 10 deletions
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10
llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
View File

@ -1014,15 +1014,9 @@ Instruction *InstCombinerImpl::transformZExtICmp(ICmpInst *Cmp, ZExtInst &Zext)
// zext (X == 0) to i32 --> X^1 iff X has only the low bit set.
// zext (X == 0) to i32 --> (X>>1)^1 iff X has only the 2nd bit set.
// zext (X == 1) to i32 --> X iff X has only the low bit set.
// zext (X == 2) to i32 --> X>>1 iff X has only the 2nd bit set.
// zext (X != 0) to i32 --> X iff X has only the low bit set.
// zext (X != 0) to i32 --> X>>1 iff X has only the 2nd bit set.
// zext (X != 1) to i32 --> X^1 iff X has only the low bit set.
// zext (X != 2) to i32 --> (X>>1)^1 iff X has only the 2nd bit set.
if ((Op1CV->isZero() || Op1CV->isPowerOf2()) &&
// This only works for EQ and NE
Cmp->isEquality()) {
if (Op1CV->isZero() && Cmp->isEquality()) {
// If Op1C some other power of two, convert:
KnownBits Known = computeKnownBits(Cmp->getOperand(0), 0, &Zext);
@ -1038,7 +1032,7 @@ Instruction *InstCombinerImpl::transformZExtICmp(ICmpInst *Cmp, ZExtInst &Zext)
In->getName() + ".lobit");
}
if (!Op1CV->isZero() == isNE) { // Toggle the low bit.
if (!isNE) { // Toggle the low bit.
Constant *One = ConstantInt::get(In->getType(), 1);
In = Builder.CreateXor(In, One);
}

3
llvm/test/Transforms/InstCombine/zext.ll
View File

@ -511,7 +511,6 @@ define i8 @disguised_signbit_clear_test(i64 %x) {
ret i8 %t6
}
; FIXME: Currently miscompiled.
define i16 @pr57899(i1 %c, i32 %x) {
; CHECK-LABEL: @pr57899(
; CHECK-NEXT: entry:
@ -519,7 +518,7 @@ define i16 @pr57899(i1 %c, i32 %x) {
; CHECK: if:
; CHECK-NEXT: br label [[JOIN]]
; CHECK: join:
; CHECK-NEXT: ret i16 0
; CHECK-NEXT: ret i16 1
;
entry:
br i1 %c, label %if, label %join