[llvm] [InstCombine] Fold select pattern with sub and negation to abs intrinsic (PR #156246)

[Yingwei Zheng via llvm-commits]

================
@@ -1153,6 +1153,45 @@ 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)
+  // 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)))) {
+    return Builder.CreateBinaryIntrinsic(Intrinsic::abs, TI,
+                                         Builder.getFalse());
+  }
+
+  // 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)))) &&
+      match(FI, m_NSWSub(m_Specific(A), m_Specific(B)))) {
----------------
dtcxzyw wrote:

```suggestion
      match(FI, m_NSWSub(m_Specific(A), m_Specific(B))) &&
      match(TI, m_Neg(m_Specific(FI))) {
```


https://github.com/llvm/llvm-project/pull/156246