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[InstCombine] Add CTLZ -> CTTZ simplification #164733

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merged 3 commits into from
Oct 24, 2025
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[InstCombine] Add CTLZ -> CTTZ simplification #164733

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f422303
[InstCombine] Add failing test for CTLZ -> CTTZ simplification
MihailMihov Oct 23, 2025
171ee2b
[InstCombine] Add CTLZ -> CTTZ simplification
MihailMihov Oct 23, 2025
fee6a0e
Merge branch 'main' into instcombine-ctlz-cttz-simplification
MihailMihov Oct 23, 2025
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12 changes: 12 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -582,6 +582,18 @@ static Instruction *foldCttzCtlz(IntrinsicInst &II, InstCombinerImpl &IC) {
IC.Builder.CreateBinaryIntrinsic(Intrinsic::ctlz, C, Op1);
return BinaryOperator::CreateSub(ConstCtlz, X);
}

// ctlz(~x & (x - 1)) -> bitwidth - cttz(x, false)
if (Op0->hasOneUse() &&
match(Op0,
m_c_And(m_Not(m_Value(X)), m_Add(m_Deferred(X), m_AllOnes())))) {
Type *Ty = II.getType();
unsigned BitWidth = Ty->getScalarSizeInBits();
auto *Cttz = IC.Builder.CreateIntrinsic(Intrinsic::cttz, Ty,
{X, IC.Builder.getFalse()});
auto *Bw = ConstantInt::get(Ty, APInt(BitWidth, BitWidth));
return IC.replaceInstUsesWith(II, IC.Builder.CreateSub(Bw, Cttz));
}
}

// cttz(Pow2) -> Log2(Pow2)
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145 changes: 145 additions & 0 deletions llvm/test/Transforms/InstCombine/ctlz-cttz.ll
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@@ -0,0 +1,145 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
; RUN: opt < %s -S -passes=instcombine | FileCheck %s

; ctpop(~i & (i - 1)) -> bitwidth - cttz(i, false)
define i8 @ctlz_to_sub_bw_cttz(i8 %a0) {
; CHECK-LABEL: define i8 @ctlz_to_sub_bw_cttz(
; CHECK-SAME: i8 [[A0:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[A0]], i1 false)
; CHECK-NEXT: [[CLZ:%.*]] = sub nuw nsw i8 8, [[TMP1]]
; CHECK-NEXT: ret i8 [[CLZ]]
;
%dec = add i8 %a0, -1
%not = xor i8 %a0, -1
%and = and i8 %dec, %not
%clz = tail call i8 @llvm.ctlz.i8(i8 %and, i1 false)
ret i8 %clz
}

define i8 @ctlz_to_sub_bw_cttz_poison(i8 %a0) {
; CHECK-LABEL: define i8 @ctlz_to_sub_bw_cttz_poison(
; CHECK-SAME: i8 [[A0:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[A0]], i1 false)
; CHECK-NEXT: [[CLZ:%.*]] = sub nuw nsw i8 8, [[TMP1]]
; CHECK-NEXT: ret i8 [[CLZ]]
;
%dec = add i8 %a0, -1
%not = xor i8 %a0, -1
%and = and i8 %dec, %not
%clz = tail call i8 @llvm.ctlz.i8(i8 %and, i1 true)
ret i8 %clz
}

define i8 @ctlz_to_sub_bw_cttz_different_add(i8 %a0) {
; CHECK-LABEL: define i8 @ctlz_to_sub_bw_cttz_different_add(
; CHECK-SAME: i8 [[A0:%.*]]) {
; CHECK-NEXT: [[DEC:%.*]] = add i8 [[A0]], 1
; CHECK-NEXT: [[NOT:%.*]] = xor i8 [[A0]], -1
; CHECK-NEXT: [[AND:%.*]] = and i8 [[DEC]], [[NOT]]
; CHECK-NEXT: [[CLZ:%.*]] = tail call range(i8 0, 9) i8 @llvm.ctlz.i8(i8 [[AND]], i1 false)
; CHECK-NEXT: ret i8 [[CLZ]]
;
%dec = add i8 %a0, 1
%not = xor i8 %a0, -1
%and = and i8 %dec, %not
%clz = tail call i8 @llvm.ctlz.i8(i8 %and, i1 false)
ret i8 %clz
}

define i8 @ctlz_to_sub_bw_cttz_different_xor(i8 %a0) {
; CHECK-LABEL: define i8 @ctlz_to_sub_bw_cttz_different_xor(
; CHECK-SAME: i8 [[A0:%.*]]) {
; CHECK-NEXT: [[DEC:%.*]] = add i8 [[A0]], -1
; CHECK-NEXT: [[NOT:%.*]] = xor i8 [[A0]], 1
; CHECK-NEXT: [[AND:%.*]] = and i8 [[DEC]], [[NOT]]
; CHECK-NEXT: [[CLZ:%.*]] = tail call range(i8 0, 9) i8 @llvm.ctlz.i8(i8 [[AND]], i1 false)
; CHECK-NEXT: ret i8 [[CLZ]]
;
%dec = add i8 %a0, -1
%not = xor i8 %a0, 1
%and = and i8 %dec, %not
%clz = tail call i8 @llvm.ctlz.i8(i8 %and, i1 false)
ret i8 %clz
}

declare void @use(i8)

define i8 @ctlz_to_sub_bw_cttz_multi_use_dec(i8 %a0) {
; CHECK-LABEL: define i8 @ctlz_to_sub_bw_cttz_multi_use_dec(
; CHECK-SAME: i8 [[A0:%.*]]) {
; CHECK-NEXT: [[DEC:%.*]] = add i8 [[A0]], -1
; CHECK-NEXT: call void @use(i8 [[DEC]])
; CHECK-NEXT: [[TMP1:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[A0]], i1 false)
; CHECK-NEXT: [[CLZ:%.*]] = sub nuw nsw i8 8, [[TMP1]]
; CHECK-NEXT: ret i8 [[CLZ]]
;
%dec = add i8 %a0, -1
call void @use(i8 %dec)
%not = xor i8 %a0, -1
%and = and i8 %dec, %not
%clz = tail call i8 @llvm.ctlz.i8(i8 %and, i1 false)
ret i8 %clz
}

define i8 @ctlz_to_sub_bw_cttz_multi_use_not(i8 %a0) {
; CHECK-LABEL: define i8 @ctlz_to_sub_bw_cttz_multi_use_not(
; CHECK-SAME: i8 [[A0:%.*]]) {
; CHECK-NEXT: [[NOT:%.*]] = xor i8 [[A0]], -1
; CHECK-NEXT: call void @use(i8 [[NOT]])
; CHECK-NEXT: [[TMP1:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[A0]], i1 false)
; CHECK-NEXT: [[CLZ:%.*]] = sub nuw nsw i8 8, [[TMP1]]
; CHECK-NEXT: ret i8 [[CLZ]]
;
%dec = add i8 %a0, -1
%not = xor i8 %a0, -1
call void @use(i8 %not)
%and = and i8 %dec, %not
%clz = tail call i8 @llvm.ctlz.i8(i8 %and, i1 false)
ret i8 %clz
}

define i8 @ctlz_to_sub_bw_cttz_multi_use_and(i8 %a0) {
; CHECK-LABEL: define i8 @ctlz_to_sub_bw_cttz_multi_use_and(
; CHECK-SAME: i8 [[A0:%.*]]) {
; CHECK-NEXT: [[DEC:%.*]] = add i8 [[A0]], -1
; CHECK-NEXT: [[NOT:%.*]] = xor i8 [[A0]], -1
; CHECK-NEXT: [[AND:%.*]] = and i8 [[DEC]], [[NOT]]
; CHECK-NEXT: call void @use(i8 [[AND]])
; CHECK-NEXT: [[CLZ:%.*]] = tail call range(i8 0, 9) i8 @llvm.ctlz.i8(i8 [[AND]], i1 false)
; CHECK-NEXT: ret i8 [[CLZ]]
;
%dec = add i8 %a0, -1
%not = xor i8 %a0, -1
%and = and i8 %dec, %not
call void @use(i8 %and)
%clz = tail call i8 @llvm.ctlz.i8(i8 %and, i1 false)
ret i8 %clz
}

define i8 @ctlz_to_sub_bw_cttz_commute_and(i8 %a0) {
; CHECK-LABEL: define i8 @ctlz_to_sub_bw_cttz_commute_and(
; CHECK-SAME: i8 [[A0:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = call range(i8 0, 9) i8 @llvm.cttz.i8(i8 [[A0]], i1 false)
; CHECK-NEXT: [[CLZ:%.*]] = sub nuw nsw i8 8, [[TMP1]]
; CHECK-NEXT: ret i8 [[CLZ]]
;
%dec = add i8 %a0, -1
%not = xor i8 %a0, -1
%and = and i8 %not, %dec
%clz = tail call i8 @llvm.ctlz.i8(i8 %and, i1 false)
ret i8 %clz
}

define <2 x i8> @ctlz_to_sub_bw_cttz_vec_splat(<2 x i8> %a0) {
; CHECK-LABEL: define <2 x i8> @ctlz_to_sub_bw_cttz_vec_splat(
; CHECK-SAME: <2 x i8> [[A0:%.*]]) {
; CHECK-NEXT: [[TMP1:%.*]] = call range(i8 0, 9) <2 x i8> @llvm.cttz.v2i8(<2 x i8> [[A0]], i1 false)
; CHECK-NEXT: [[CLZ:%.*]] = sub nuw nsw <2 x i8> splat (i8 8), [[TMP1]]
; CHECK-NEXT: ret <2 x i8> [[CLZ]]
;
%dec = add <2 x i8> %a0, <i8 -1, i8 -1>
%not = xor <2 x i8> %a0, <i8 -1, i8 -1>
%and = and <2 x i8> %dec, %not
%clz = tail call <2 x i8>@llvm.ctlz.v2i8(<2 x i8> %and, i1 false)
ret <2 x i8> %clz
}