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[InstCombine] Infer exact for lshr by cttz #136696

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Merged
merged 13 commits into from
Apr 29, 2025
Merged

[InstCombine] Infer exact for lshr by cttz #136696

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19aa8f2
Update InstCombineShifts.cpp
houngkoungting Apr 22, 2025
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houngkoungting Apr 23, 2025
213dc72
Update InstCombineShifts.cpp
houngkoungting Apr 27, 2025
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houngkoungting Apr 27, 2025
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Merge branch 'main' into main
houngkoungting Apr 27, 2025
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Update InstCombineShifts.cpp
houngkoungting Apr 27, 2025
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houngkoungting Apr 28, 2025
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Update cttz-shift-exact.ll
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Update InstCombineShifts.cpp
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Update cttz-shift-exact.ll
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6 changes: 6 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -994,6 +994,12 @@ static bool setShiftFlags(BinaryOperator &I, const SimplifyQuery &Q) {
I.setIsExact();
return true;
}
// Infer 'exact' flag if shift amount is cttz(x) on the same operand.
if (match(I.getOperand(1), m_Intrinsic<Intrinsic::cttz>(
m_Specific(I.getOperand(0)), m_Value()))) {
I.setIsExact();
return true;
}
}

// Compute what we know about shift count.
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41 changes: 41 additions & 0 deletions llvm/test/Transforms/InstCombine/cttz-shift-exact.ll
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@@ -0,0 +1,41 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
; RUN: opt -passes=instcombine -S < %s | FileCheck %s

declare i32 @llvm.cttz.i32(i32, i1)

define i32 @test_cttz_lshr(i32 %x) {
; CHECK-LABEL: define i32 @test_cttz_lshr(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[CTTZ:%.*]] = call range(i32 0, 33) i32 @llvm.cttz.i32(i32 [[X]], i1 true)
; CHECK-NEXT: [[SH:%.*]] = lshr exact i32 [[X]], [[CTTZ]]
; CHECK-NEXT: ret i32 [[SH]]
;
%cttz = call i32 @llvm.cttz.i32(i32 %x, i1 false)
%sh = lshr i32 %x, %cttz
ret i32 %sh
}

define i32 @test_cttz_ashr(i32 %x) {
; CHECK-LABEL: define i32 @test_cttz_ashr(
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[CTTZ:%.*]] = call range(i32 0, 33) i32 @llvm.cttz.i32(i32 [[X]], i1 true)
; CHECK-NEXT: [[SH:%.*]] = ashr exact i32 [[X]], [[CTTZ]]
; CHECK-NEXT: ret i32 [[SH]]
;
%cttz = call i32 @llvm.cttz.i32(i32 %x, i1 true)
%sh = ashr i32 %x, %cttz
ret i32 %sh
}

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Please also add one negative test where the cttz operand is an unrelated value.

define i32 @test_cttz_diff_operand(i32 %x, i32 %y) {
; CHECK-LABEL: define i32 @test_cttz_diff_operand(
; CHECK-SAME: i32 [[X:%.*]], i32 [[Y:%.*]]) {
; CHECK-NEXT: [[CTTZ:%.*]] = call range(i32 0, 33) i32 @llvm.cttz.i32(i32 [[Y]], i1 true)
; CHECK-NEXT: [[SH:%.*]] = lshr i32 [[X]], [[CTTZ]]
; CHECK-NEXT: ret i32 [[SH]]
;
%cttz = call i32 @llvm.cttz.i32(i32 %y, i1 true)
%sh = lshr i32 %x, %cttz
ret i32 %sh
}

2 changes: 1 addition & 1 deletion llvm/test/Transforms/InstCombine/select.ll
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Expand Up @@ -2736,7 +2736,7 @@ define i32 @pr47322_more_poisonous_replacement(i32 %arg) {
; CHECK-LABEL: @pr47322_more_poisonous_replacement(
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[ARG:%.*]], 0
; CHECK-NEXT: [[TRAILING:%.*]] = call range(i32 0, 33) i32 @llvm.cttz.i32(i32 [[ARG]], i1 true)
; CHECK-NEXT: [[SHIFTED:%.*]] = lshr i32 [[ARG]], [[TRAILING]]
; CHECK-NEXT: [[SHIFTED:%.*]] = lshr exact i32 [[ARG]], [[TRAILING]]
; CHECK-NEXT: [[R1_SROA_0_1:%.*]] = select i1 [[CMP]], i32 0, i32 [[SHIFTED]]
; CHECK-NEXT: ret i32 [[R1_SROA_0_1]]
;
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