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[InstCombine] add fold for (ShiftC >> X) >u C

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This is the 'ugt' sibling to:
0399473de8

Decrement the input compare constant (and implicitly
decrement the new compare constant):
https://alive2.llvm.org/ce/z/iELmct
This commit is contained in:
Sanjay Patel
2022-06-29 11:38:48 -04:00
parent 905b6b3edd
commit d4f39d8333
2 changed files with 21 additions and 9 deletions
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14
llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
View File

@@ -2226,13 +2226,19 @@ Instruction *InstCombinerImpl::foldICmpShrConstant(ICmpInst &Cmp,
// If the shifted constant is a power-of-2, test the shift amount directly:
// (ShiftValC >> X) >u C --> X <u (LZ(C) - LZ(ShiftValC))
// TODO: Handle ult.
if (!IsAShr && Pred == CmpInst::ICMP_UGT && ShiftValC->isPowerOf2()) {
// (ShiftValC >> X) <u C --> X >=u (LZ(C-1) - LZ(ShiftValC))
if (!IsAShr && ShiftValC->isPowerOf2() &&
(Pred == CmpInst::ICMP_UGT || Pred == CmpInst::ICMP_ULT)) {
bool IsUGT = Pred == CmpInst::ICMP_UGT;
assert(ShiftValC->ugt(C) && "Expected simplify of compare");
unsigned CmpLZ = C.countLeadingZeros();
assert(IsUGT || !C.isZero() && "Expected X u< 0 to simplify");
unsigned CmpLZ =
IsUGT ? C.countLeadingZeros() : (C - 1).countLeadingZeros();
unsigned ShiftLZ = ShiftValC->countLeadingZeros();
Constant *NewC = ConstantInt::get(Shr->getType(), CmpLZ - ShiftLZ);
return new ICmpInst(ICmpInst::ICMP_ULT, Shr->User::getOperand(1), NewC);
auto NewPred = IsUGT ? CmpInst::ICMP_ULT : CmpInst::ICMP_UGE;
return new ICmpInst(NewPred, Shr->getOperand(1), NewC);
}
}

16
llvm/test/Transforms/InstCombine/icmp-shr.ll
View File

@@ -1092,8 +1092,7 @@ define i1 @lshr_pow2_sgt(i8 %x) {
define i1 @lshr_pow2_ult(i8 %x) {
; CHECK-LABEL: @lshr_pow2_ult(
; CHECK-NEXT: [[S:%.*]] = lshr i8 4, [[X:%.*]]
; CHECK-NEXT: [[R:%.*]] = icmp ult i8 [[S]], 2
; CHECK-NEXT: [[R:%.*]] = icmp ugt i8 [[X:%.*]], 1
; CHECK-NEXT: ret i1 [[R]]
;
%s = lshr i8 4, %x
@@ -1105,7 +1104,7 @@ define i1 @lshr_pow2_ult_use(i8 %x) {
; CHECK-LABEL: @lshr_pow2_ult_use(
; CHECK-NEXT: [[S:%.*]] = lshr i8 -128, [[X:%.*]]
; CHECK-NEXT: call void @use(i8 [[S]])
; CHECK-NEXT: [[R:%.*]] = icmp ult i8 [[S]], 5
; CHECK-NEXT: [[R:%.*]] = icmp ugt i8 [[X]], 4
; CHECK-NEXT: ret i1 [[R]]
;
%s = lshr i8 128, %x
@@ -1116,8 +1115,7 @@ define i1 @lshr_pow2_ult_use(i8 %x) {
define <2 x i1> @lshr_pow2_ult_vec(<2 x i8> %x) {
; CHECK-LABEL: @lshr_pow2_ult_vec(
; CHECK-NEXT: [[S:%.*]] = lshr <2 x i8> <i8 8, i8 8>, [[X:%.*]]
; CHECK-NEXT: [[R:%.*]] = icmp ult <2 x i8> [[S]], <i8 6, i8 6>
; CHECK-NEXT: [[R:%.*]] = icmp ne <2 x i8> [[X:%.*]], zeroinitializer
; CHECK-NEXT: ret <2 x i1> [[R]]
;
%s = lshr <2 x i8> <i8 8, i8 8>, %x
@@ -1125,6 +1123,8 @@ define <2 x i1> @lshr_pow2_ult_vec(<2 x i8> %x) {
ret <2 x i1> %r
}
; negative test - need power-of-2
define i1 @lshr_not_pow2_ult(i8 %x) {
; CHECK-LABEL: @lshr_not_pow2_ult(
; CHECK-NEXT: [[S:%.*]] = lshr i8 3, [[X:%.*]]
@@ -1136,6 +1136,8 @@ define i1 @lshr_not_pow2_ult(i8 %x) {
ret i1 %r
}
; TODO: This should reduce to X != 0.
define i1 @lshr_pow2_ult_smin(i8 %x) {
; CHECK-LABEL: @lshr_pow2_ult_smin(
; CHECK-NEXT: [[S:%.*]] = lshr i8 -128, [[X:%.*]]
@@ -1147,6 +1149,8 @@ define i1 @lshr_pow2_ult_smin(i8 %x) {
ret i1 %r
}
; negative test - need logical shift
define i1 @ashr_pow2_ult(i8 %x) {
; CHECK-LABEL: @ashr_pow2_ult(
; CHECK-NEXT: [[S:%.*]] = ashr i8 -128, [[X:%.*]]
@@ -1158,6 +1162,8 @@ define i1 @ashr_pow2_ult(i8 %x) {
ret i1 %r
}
; negative test - need unsigned pred
define i1 @lshr_pow2_slt(i8 %x) {
; CHECK-LABEL: @lshr_pow2_slt(
; CHECK-NEXT: [[S:%.*]] = lshr i8 -128, [[X:%.*]]