[InstCombine] Fold select pattern with sub and negation to abs intrinsic #156246
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@llvm/pr-subscribers-llvm-transforms Author: AZero13 (AZero13) Changes%sub = sub nsw T %x, %y
%cmp = icmp sgt T %x, %y ; or sge
%neg = sub T 0, %sub
%abs = select i1 %cmp, T %sub, T %negbecomes:| %sub = sub nsw T %x, %y
%abs = call T @<!-- -->llvm.abs.T(T %sub, i1 false)0940842e61b4 Full diff: https://github.com/llvm/llvm-project/pull/156246.diff 2 Files Affected:
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index eb4332fbc0959..d75e34f90d469 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1153,6 +1153,15 @@ static Value *foldAbsDiff(ICmpInst *Cmp, Value *TVal, Value *FVal,
return Builder.CreateBinaryIntrinsic(Intrinsic::abs, TI, Builder.getTrue());
}
+ // Match: (A > B) ? (A - B) : (0 - (A - B)) --> abs(A - B)
+ // Recognize the pattern where we have sub, icmp, neg(sub), select
+ if (Pred == CmpInst::ICMP_SGT &&
+ match(TI, m_Sub(m_Specific(A), m_Specific(B))) &&
+ match(FI, m_Neg(m_Specific(TI))) &&
+ TI->hasNoSignedWrap()) {
+ return Builder.CreateBinaryIntrinsic(Intrinsic::abs, TI, Builder.getFalse());
+ }
+
return nullptr;
}
diff --git a/llvm/test/Transforms/InstCombine/abs-intrinsic.ll b/llvm/test/Transforms/InstCombine/abs-intrinsic.ll
index d32f0e4690502..b58038bbec7ad 100644
--- a/llvm/test/Transforms/InstCombine/abs-intrinsic.ll
+++ b/llvm/test/Transforms/InstCombine/abs-intrinsic.ll
@@ -847,3 +847,74 @@ cond.end:
%r = phi i32 [ %0, %cond.true ], [ 0, %entry ]
ret i32 %r
}
+
+define i32 @abs_diff(i32 %x, i32 %y) {
+; CHECK-LABEL: @abs_diff(
+; CHECK-NEXT: [[SUB:%.*]] = sub nsw i32 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT: [[COND:%.*]] = call i32 @llvm.abs.i32(i32 [[SUB]], i1 false)
+; CHECK-NEXT: ret i32 [[COND]]
+;
+ %sub = sub nsw i32 %x, %y
+ %cmp = icmp sgt i32 %x, %y
+ %sub1 = sub i32 0, %sub
+ %cond = select i1 %cmp, i32 %sub, i32 %sub1
+ ret i32 %cond
+}
+
+define i32 @abs_diff_neg_no_nsw_neg(i32 %x, i32 %y) {
+; CHECK-LABEL: @abs_diff_neg_no_nsw_neg(
+; CHECK-NEXT: [[SUB:%.*]] = sub i32 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp sgt i32 [[X]], [[Y]]
+; CHECK-NEXT: [[SUB1:%.*]] = sub i32 0, [[SUB]]
+; CHECK-NEXT: [[COND:%.*]] = select i1 [[CMP]], i32 [[SUB]], i32 [[SUB1]]
+; CHECK-NEXT: ret i32 [[COND]]
+;
+ %sub = sub i32 %x, %y
+ %cmp = icmp sgt i32 %x, %y
+ %sub1 = sub i32 0, %sub
+ %cond = select i1 %cmp, i32 %sub, i32 %sub1
+ ret i32 %cond
+}
+
+define i32 @abs_diff_neg(i32 %x, i32 %y) {
+; CHECK-LABEL: @abs_diff_neg(
+; CHECK-NEXT: [[SUB:%.*]] = sub nsw i32 [[Y:%.*]], [[X:%.*]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp sgt i32 [[X]], [[Y]]
+; CHECK-NEXT: [[SUB1:%.*]] = sub i32 0, [[SUB]]
+; CHECK-NEXT: [[COND:%.*]] = select i1 [[CMP]], i32 [[SUB]], i32 [[SUB1]]
+; CHECK-NEXT: ret i32 [[COND]]
+;
+ %sub = sub nsw i32 %y, %x
+ %cmp = icmp sgt i32 %x, %y
+ %sub1 = sub i32 0, %sub
+ %cond = select i1 %cmp, i32 %sub, i32 %sub1
+ ret i32 %cond
+}
+
+define i32 @abs_diff_neg_no_nsw(i32 %x, i32 %y) {
+; CHECK-LABEL: @abs_diff_neg_no_nsw(
+; CHECK-NEXT: [[SUB:%.*]] = sub i32 [[Y:%.*]], [[X:%.*]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp sgt i32 [[X]], [[Y]]
+; CHECK-NEXT: [[SUB1:%.*]] = sub i32 0, [[SUB]]
+; CHECK-NEXT: [[COND:%.*]] = select i1 [[CMP]], i32 [[SUB]], i32 [[SUB1]]
+; CHECK-NEXT: ret i32 [[COND]]
+;
+ %sub = sub i32 %y, %x
+ %cmp = icmp sgt i32 %x, %y
+ %sub1 = sub i32 0, %sub
+ %cond = select i1 %cmp, i32 %sub, i32 %sub1
+ ret i32 %cond
+}
+
+define i32 @abs_diff_ge(i32 %x, i32 %y) {
+; CHECK-LABEL: @abs_diff_ge(
+; CHECK-NEXT: [[SUB:%.*]] = sub nsw i32 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT: [[COND:%.*]] = call i32 @llvm.abs.i32(i32 [[SUB]], i1 false)
+; CHECK-NEXT: ret i32 [[COND]]
+;
+ %sub = sub nsw i32 %x, %y
+ %cmp = icmp sge i32 %x, %y
+ %sub1 = sub i32 0, %sub
+ %cond = select i1 %cmp, i32 %sub, i32 %sub1
+ ret i32 %cond
+}
|
You can test this locally with the following command:git-clang-format --diff origin/main HEAD --extensions cpp -- llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
View the diff from clang-format here.diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 73a9f53bb..6d661c087 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1154,7 +1154,8 @@ static Value *foldAbsDiff(ICmpInst *Cmp, Value *TVal, Value *FVal,
}
// Match: (A > B) ? (A - B) : (0 - (A - B)) --> abs(A - B)
- // Also handles commuted cases like (B < A), (A >= B), (B <= A) after normalization
+ // Also handles commuted cases like (B < A), (A >= B), (B <= A) after
+ // normalization
if (Pred == CmpInst::ICMP_SGT &&
match(TI, m_NSWSub(m_Specific(A), m_Specific(B))) &&
match(FI, m_Neg(m_Specific(TI)))) {
@@ -1162,7 +1163,7 @@ static Value *foldAbsDiff(ICmpInst *Cmp, Value *TVal, Value *FVal,
Builder.getFalse());
}
- // Match: (A < B) ? (0 - (A - B)) : (A - B) --> abs(A - B)
+ // Match: (A < B) ? (0 - (A - B)) : (A - B) --> abs(A - B)
// This handles cases like (A <= B) after normalization
if (Pred == CmpInst::ICMP_SLT &&
match(TI, m_Neg(m_NSWSub(m_Specific(A), m_Specific(B)))) &&
|
|
@zyw-bot mfuzz |
dtcxzyw
reviewed
Sep 7, 2025
This patch adds a new fold in InstCombine to recognize and optimize a common absolute value pattern. ```llvm %sub = sub nsw T %x, %y %cmp = icmp sgt T %x, %y ; or sge %neg = sub T 0, %sub %abs = select i1 %cmp, T %sub, T %neg ``` becomes:| ```llvm %sub = sub nsw T %x, %y %abs = call T @llvm.abs.T(T %sub, i1 false) ```
|
Missing fold for another variant: https://alive2.llvm.org/ce/z/gRTmZk |
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becomes:
Alive2: https://alive2.llvm.org/ce/z/ApdJX8