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[InstCombine] Prefer to keep power-of-2 constants when combining ashr exact and slt/ult of a constant #86111

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Merged
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May 10, 2024
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[InstCombine] Prefer to keep power-of-2 constants when combining ashr exact and slt/ult of a constant #86111

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10 changes: 10 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2479,6 +2479,16 @@ Instruction *InstCombinerImpl::foldICmpShrConstant(ICmpInst &Cmp,
// those conditions rather than checking them. This is difficult because of
// undef/poison (PR34838).
if (IsAShr && Shr->hasOneUse()) {
if (IsExact && (Pred == CmpInst::ICMP_SLT || Pred == CmpInst::ICMP_ULT) &&
(C - 1).isPowerOf2() && C.countLeadingZeros() > ShAmtVal) {
// When C - 1 is a power of two and the transform can be legally
// performed, prefer this form so the produced constant is close to a
// power of two.
// icmp slt/ult (ashr exact X, ShAmtC), C
// --> icmp slt/ult X, (C - 1) << ShAmtC) + 1
APInt ShiftedC = (C - 1).shl(ShAmtVal) + 1;
return new ICmpInst(Pred, X, ConstantInt::get(ShrTy, ShiftedC));
}
if (IsExact || Pred == CmpInst::ICMP_SLT || Pred == CmpInst::ICMP_ULT) {
// When ShAmtC can be shifted losslessly:
// icmp PRED (ashr exact X, ShAmtC), C --> icmp PRED X, (C << ShAmtC)
Expand Down
10 changes: 4 additions & 6 deletions llvm/test/Transforms/InstCombine/icmp-shr-lt-gt.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -3379,7 +3379,7 @@ define i1 @ashrslt_01_01_exact(i4 %x) {

define i1 @ashrslt_01_02_exact(i4 %x) {
; CHECK-LABEL: @ashrslt_01_02_exact(
; CHECK-NEXT: [[C:%.*]] = icmp slt i4 [[X:%.*]], 4
; CHECK-NEXT: [[C:%.*]] = icmp slt i4 [[X:%.*]], 3
; CHECK-NEXT: ret i1 [[C]]
;
%s = ashr exact i4 %x, 1
Expand All @@ -3389,7 +3389,7 @@ define i1 @ashrslt_01_02_exact(i4 %x) {

define i1 @ashrslt_01_03_exact(i4 %x) {
; CHECK-LABEL: @ashrslt_01_03_exact(
; CHECK-NEXT: [[C:%.*]] = icmp slt i4 [[X:%.*]], 6
; CHECK-NEXT: [[C:%.*]] = icmp slt i4 [[X:%.*]], 5
; CHECK-NEXT: ret i1 [[C]]
;
%s = ashr exact i4 %x, 1
Expand Down Expand Up @@ -3800,11 +3800,9 @@ define i1 @ashrslt_03_15_exact(i4 %x) {
ret i1 %c
}

; TODO: The resulting compared constant can be safely replaced with one that
; is closer to a power of two.
define i1 @ashr_slt_exact_near_pow2_cmpval(i8 %x) {
; CHECK-LABEL: @ashr_slt_exact_near_pow2_cmpval(
; CHECK-NEXT: [[C:%.*]] = icmp slt i8 [[X:%.*]], 10
; CHECK-NEXT: [[C:%.*]] = icmp slt i8 [[X:%.*]], 9
; CHECK-NEXT: ret i1 [[C]]
;
%s = ashr exact i8 %x, 1
Expand All @@ -3814,7 +3812,7 @@ define i1 @ashr_slt_exact_near_pow2_cmpval(i8 %x) {

define i1 @ashr_ult_exact_near_pow2_cmpval(i8 %x) {
; CHECK-LABEL: @ashr_ult_exact_near_pow2_cmpval(
; CHECK-NEXT: [[C:%.*]] = icmp ult i8 [[X:%.*]], 10
; CHECK-NEXT: [[C:%.*]] = icmp ult i8 [[X:%.*]], 9
; CHECK-NEXT: ret i1 [[C]]
;
%s = ashr exact i8 %x, 1
Expand Down
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