[InstCombine] Fold select pattern with sub and negation to abs intrinsic

[AZero13]

%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:

%sub = sub nsw T %x, %y
%abs = call T @llvm.abs.T(T %sub, i1 false)

Alive2: https://alive2.llvm.org/ce/z/ApdJX8

https://alive2.llvm.org/ce/z/gRTmZk

[llvmbot]

@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 %neg

becomes:|

%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:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+9)
• (modified) llvm/test/Transforms/InstCombine/abs-intrinsic.ll (+71)

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
+}

[AZero13]

@dtcxzyw

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[dtcxzyw]

@zyw-bot mfuzz

[dtcxzyw]

Missing fold for another variant: https://alive2.llvm.org/ce/z/gRTmZk

[AZero13]

@dtcxzyw done!

[dtcxzyw]

LG w/ some nits. Please avoid force-pushing your branch, unless you need to rebase on top of other commits... The two-commit history is not enforced.

[AZero13]

Done @dtcxzyw

[AZero13]

@dtcxzyw Could you please merge?