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[InstCombine] Fold select(X >s 0, 0, -X) | smax(X, 0) to abs(X) #165200

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Merged
merged 13 commits into from
Nov 2, 2025
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Merged

[InstCombine] Fold select(X >s 0, 0, -X) | smax(X, 0) to abs(X) #165200

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[InstCombine] Fold select(X >s 0, 0, -X) | smax(X, 0) to abs(X)
wenju-he Oct 27, 2025
af62083
named value, add negative test with multiple uses of @llvm.smax
wenju-he Oct 27, 2025
21ca09a
rename %e -> %add
wenju-he Oct 27, 2025
1e9a8ce
add TODO: fold to smax(neg, fold two smax
wenju-he Oct 28, 2025
e5d284b
Update llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
wenju-he Oct 28, 2025
e19950b
Update llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
wenju-he Oct 28, 2025
28f1987
clang-format
wenju-he Oct 28, 2025
bd8b791
select(X <s 0, -X, 0) | smax(X, 0) --> abs(X)
wenju-he Oct 28, 2025
154c2c1
update clang/test/CodeGen/SystemZ/builtins-systemz-zvector.c
wenju-he Oct 28, 2025
3aaa55a
Merge branch 'main' into instcombine-or-select-smax-to-abs
wenju-he Oct 29, 2025
c01eb5a
rename or_lgt_select_smax_to_abs -> or_slt_select_smax_to_abs
wenju-he Oct 30, 2025
4c92c7c
Update llvm/test/Transforms/InstCombine/or.ll
wenju-he Oct 31, 2025
d6eecb3
use m_SMax
wenju-he Oct 31, 2025
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8 changes: 4 additions & 4 deletions clang/test/CodeGen/SystemZ/builtins-systemz-zvector.c
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Original file line number Diff line number Diff line change
Expand Up @@ -3584,13 +3584,13 @@ void test_integer(void) {
// CHECK-ASM: vsrlb

vsc = vec_abs(vsc);
// CHECK-ASM: vlcb
// CHECK-ASM: vlpb
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@uweigand uweigand Oct 28, 2025

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The SystemZ changes LGTM. In fact, this fixes a regression I hadn't even been aware of, which was introduced here: 1a60ae0

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@wenju-he wenju-he Oct 30, 2025

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The SystemZ changes LGTM. In fact, this fixes a regression I hadn't even been aware of, which was introduced here: 1a60ae0

thanks @uweigand for review

vss = vec_abs(vss);
// CHECK-ASM: vlch
// CHECK-ASM: vlph
vsi = vec_abs(vsi);
// CHECK-ASM: vlcf
// CHECK-ASM: vlpf
vsl = vec_abs(vsl);
// CHECK-ASM: vlcg
// CHECK-ASM: vlpg

vsc = vec_max(vsc, vsc);
// CHECK-ASM: vmxb
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2 changes: 1 addition & 1 deletion clang/test/CodeGen/SystemZ/builtins-systemz-zvector5.c
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Expand Up @@ -246,7 +246,7 @@ void test_integer(void) {
// CHECK-ASM: vctzq

vslll = vec_abs(vslll);
// CHECK-ASM: vlcq
// CHECK-ASM: vlpq

vslll = vec_avg(vslll, vslll);
// CHECK: call i128 @llvm.s390.vavgq(i128 %{{.*}}, i128 %{{.*}})
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24 changes: 24 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
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Expand Up @@ -3997,6 +3997,27 @@ static Value *foldOrUnsignedUMulOverflowICmp(BinaryOperator &I,
return nullptr;
}

/// Fold select(X >s 0, 0, -X) | smax(X, 0) --> abs(X)
/// select(X <s 0, -X, 0) | smax(X, 0) --> abs(X)
static Value *FoldOrOfSelectSmaxToAbs(BinaryOperator &I,
InstCombiner::BuilderTy &Builder) {
Value *X;
Value *Sel;
if (match(&I, m_c_Or(m_Value(Sel), m_OneUse(m_Intrinsic<Intrinsic::smax>(
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@arsenm arsenm Oct 31, 2025

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Use m_SMax

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@wenju-he wenju-he Oct 31, 2025

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Use m_SMax

done

m_Value(X), m_ZeroInt()))))) {
auto NegX = m_Neg(m_Specific(X));
if (match(Sel, m_Select(m_SpecificICmp(ICmpInst::ICMP_SGT, m_Specific(X),
m_ZeroInt()),
m_ZeroInt(), NegX)) ||
match(Sel, m_Select(m_SpecificICmp(ICmpInst::ICMP_SLT, m_Specific(X),
m_ZeroInt()),
NegX, m_ZeroInt())))
return Builder.CreateBinaryIntrinsic(Intrinsic::abs, X,
Builder.getFalse());
}
return nullptr;
}

// FIXME: We use commutative matchers (m_c_*) for some, but not all, matches
// here. We should standardize that construct where it is needed or choose some
// other way to ensure that commutated variants of patterns are not missed.
Expand Down Expand Up @@ -4545,6 +4566,9 @@ Instruction *InstCombinerImpl::visitOr(BinaryOperator &I) {
if (Value *V = SimplifyAddWithRemainder(I))
return replaceInstUsesWith(I, V);

if (Value *Res = FoldOrOfSelectSmaxToAbs(I, Builder))
return replaceInstUsesWith(I, Res);

return nullptr;
}

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95 changes: 95 additions & 0 deletions llvm/test/Transforms/InstCombine/or.ll
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Expand Up @@ -2113,3 +2113,98 @@ define <4 x i32> @or_zext_nneg_minus_constant_splat(<4 x i8> %a) {
%or = or <4 x i32> %zext, splat (i32 -9)
ret <4 x i32> %or
}

define i8 @or_positive_minus_non_positive_to_abs(i8 %a){
; CHECK-LABEL: @or_positive_minus_non_positive_to_abs(
; CHECK-NEXT: [[TMP2:%.*]] = call i8 @llvm.abs.i8(i8 [[A:%.*]], i1 false)
; CHECK-NEXT: ret i8 [[TMP2]]
;
%b = icmp sgt i8 %a, 0
%mask = sext i1 %b to i8
%neg = sub i8 0, %a
%mask_inv = xor i8 %mask, -1
%c = and i8 %neg, %mask_inv
%d = and i8 %a, %mask
%or = or i8 %c, %d
ret i8 %or
}

; TODO Fold to smax https://alive2.llvm.org/ce/z/wDiDh2
define i8 @or_select_smax_neg_to_abs(i8 %a){
; CHECK-LABEL: @or_select_smax_neg_to_abs(
; CHECK-NEXT: [[SGT0:%.*]] = icmp sgt i8 [[A:%.*]], 0
; CHECK-NEXT: [[NEG:%.*]] = sub nsw i8 0, [[A]]
; CHECK-NEXT: [[OR:%.*]] = select i1 [[SGT0]], i8 0, i8 [[NEG]]
; CHECK-NEXT: ret i8 [[OR]]
;
%sgt0 = icmp sgt i8 %a, 0
%neg = sub nsw i8 0, %a
%sel = select i1 %sgt0, i8 0, i8 %neg
ret i8 %sel
}

; TODO Fold to abs https://alive2.llvm.org/ce/z/DybfHG
define i8 @or_select_smax_smax_to_abs(i8 %a){
; CHECK-LABEL: @or_select_smax_smax_to_abs(
; CHECK-NEXT: [[NEG:%.*]] = sub nsw i8 0, [[A:%.*]]
; CHECK-NEXT: [[SEL:%.*]] = call i8 @llvm.smax.i8(i8 [[NEG]], i8 0)
; CHECK-NEXT: [[MAX:%.*]] = call i8 @llvm.smax.i8(i8 [[A]], i8 0)
; CHECK-NEXT: [[OR:%.*]] = or i8 [[SEL]], [[MAX]]
; CHECK-NEXT: ret i8 [[OR]]
;
%neg = sub nsw i8 0, %a
%sel = call i8 @llvm.smax.i8(i8 %neg, i8 0)
%max = call i8 @llvm.smax.i8(i8 %a, i8 0)
%or = or i8 %sel, %max
ret i8 %or
}

declare i8 @llvm.abs.i8(i8, i1)
declare <2 x i8> @llvm.abs.v2i8(<2 x i8>, i1)

define <2 x i8> @or_sgt_select_smax_to_abs(<2 x i8> %a){
; CHECK-LABEL: @or_sgt_select_smax_to_abs(
; CHECK-NEXT: [[OR:%.*]] = call <2 x i8> @llvm.abs.v2i8(<2 x i8> [[A:%.*]], i1 false)
; CHECK-NEXT: ret <2 x i8> [[OR]]
;
%sgt0 = icmp sgt <2 x i8> %a, zeroinitializer
%neg = sub <2 x i8> zeroinitializer, %a
%sel = select <2 x i1> %sgt0, <2 x i8> zeroinitializer, <2 x i8> %neg
%max = call <2 x i8> @llvm.smax.v2i8(<2 x i8> %a, <2 x i8> zeroinitializer)
%or = or <2 x i8> %sel, %max
ret <2 x i8> %or
}

define <2 x i8> @or_lgt_select_smax_to_abs(<2 x i8> %a){
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@dtcxzyw dtcxzyw Oct 30, 2025

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Suggested change
define <2 x i8> @or_lgt_select_smax_to_abs(<2 x i8> %a){
define <2 x i8> @or_slt_select_smax_to_abs(<2 x i8> %a){

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@wenju-he wenju-he Oct 30, 2025

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Suggested change
define <2 x i8> @or_lgt_select_smax_to_abs(<2 x i8> %a){
define <2 x i8> @or_slt_select_smax_to_abs(<2 x i8> %a){

done, thanks @dtcxzyw

; CHECK-LABEL: @or_lgt_select_smax_to_abs(
; CHECK-NEXT: [[OR:%.*]] = call <2 x i8> @llvm.abs.v2i8(<2 x i8> [[A:%.*]], i1 false)
; CHECK-NEXT: ret <2 x i8> [[OR]]
;
%slt0 = icmp slt <2 x i8> %a, zeroinitializer
%neg = sub <2 x i8> zeroinitializer, %a
%sel = select <2 x i1> %slt0, <2 x i8> %neg, <2 x i8> zeroinitializer
%max = call <2 x i8> @llvm.smax.v2i8(<2 x i8> %a, <2 x i8> zeroinitializer)
%or = or <2 x i8> %sel, %max
ret <2 x i8> %or
}

; negative test - %d has multiple uses. %or is not folded to abs.

define <2 x i8> @or_select_smax_multi_uses(<2 x i8> %a){
; CHECK-LABEL: @or_select_smax_multi_uses(
; CHECK-NEXT: [[B:%.*]] = icmp sgt <2 x i8> [[A:%.*]], zeroinitializer
; CHECK-NEXT: [[NEG:%.*]] = sub <2 x i8> zeroinitializer, [[A]]
; CHECK-NEXT: [[C:%.*]] = select <2 x i1> [[B]], <2 x i8> zeroinitializer, <2 x i8> [[NEG]]
; CHECK-NEXT: [[D:%.*]] = call <2 x i8> @llvm.smax.v2i8(<2 x i8> [[A]], <2 x i8> zeroinitializer)
; CHECK-NEXT: [[OR1:%.*]] = or <2 x i8> [[C]], [[D]]
; CHECK-NEXT: [[OR:%.*]] = add <2 x i8> [[OR1]], [[D]]
; CHECK-NEXT: ret <2 x i8> [[OR]]
;
%sgt0 = icmp sgt <2 x i8> %a, zeroinitializer
%neg = sub <2 x i8> zeroinitializer, %a
%sel = select <2 x i1> %sgt0, <2 x i8> zeroinitializer, <2 x i8> %neg
%max = call <2 x i8> @llvm.smax.v2i8(<2 x i8> %a, <2 x i8> zeroinitializer)
%or = or <2 x i8> %sel, %max
%add = add <2 x i8> %or, %max
ret <2 x i8> %add
}
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@arsenm arsenm Oct 27, 2025

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Missing negative test for the multiple use case

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@wenju-he wenju-he Oct 27, 2025

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Missing negative test for the multiple use case

done

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