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[InstCombine] Fold zext-of-icmp with no shift #68503

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Oct 9, 2023
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[InstCombine] Fold zext-of-icmp with no shift #68503

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17 changes: 8 additions & 9 deletions llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -904,19 +904,18 @@ Instruction *InstCombinerImpl::transformZExtICmp(ICmpInst *Cmp,
// zext (X == 0) to i32 --> (X>>1)^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.
if (Op1CV->isZero() && Cmp->isEquality() &&
(Cmp->getOperand(0)->getType() == Zext.getType() ||
Cmp->getPredicate() == ICmpInst::ICMP_NE)) {
// If Op1C some other power of two, convert:
KnownBits Known = computeKnownBits(Cmp->getOperand(0), 0, &Zext);

if (Op1CV->isZero() && Cmp->isEquality()) {
// Exactly 1 possible 1? But not the high-bit because that is
// canonicalized to this form.
KnownBits Known = computeKnownBits(Cmp->getOperand(0), 0, &Zext);
APInt KnownZeroMask(~Known.Zero);
if (KnownZeroMask.isPowerOf2() &&
(Zext.getType()->getScalarSizeInBits() !=
KnownZeroMask.logBase2() + 1)) {
uint32_t ShAmt = KnownZeroMask.logBase2();
uint32_t ShAmt = KnownZeroMask.logBase2();
bool IsExpectShAmt = KnownZeroMask.isPowerOf2() &&
(Zext.getType()->getScalarSizeInBits() != ShAmt + 1);
if (IsExpectShAmt &&
(Cmp->getOperand(0)->getType() == Zext.getType() ||
Cmp->getPredicate() == ICmpInst::ICMP_NE || ShAmt == 0)) {
Value *In = Cmp->getOperand(0);
if (ShAmt) {
// Perform a logical shr by shiftamt.
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3 changes: 2 additions & 1 deletion llvm/test/Transforms/InstCombine/zext.ll
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Expand Up @@ -749,10 +749,11 @@ define i64 @zext_icmp_ne_bool_1(ptr %ptr) {
ret i64 %len
}

; https://alive2.llvm.org/ce/z/k7qosS
define i32 @zext_icmp_eq0_no_shift(ptr %ptr ) {
; CHECK-LABEL: @zext_icmp_eq0_no_shift(
; CHECK-NEXT: [[X:%.*]] = load i8, ptr [[PTR:%.*]], align 1, !range [[RNG1:![0-9]+]]
; CHECK-NEXT: [[TMP1:%.*]] = icmp eq i8 [[X]], 0
; CHECK-NEXT: [[TMP1:%.*]] = xor i8 [[X]], 1
; CHECK-NEXT: [[RES:%.*]] = zext i8 [[TMP1]] to i32
; CHECK-NEXT: ret i32 [[RES]]
;
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