[llvm] Missed optimization when `b - a` is known nonnegative inside `select` instruction (PR #187898)

[via llvm-commits]

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@@ -664,10 +664,49 @@ static Value *foldSelectICmpMinMax(const ICmpInst *Cmp, Value *TVal,
                                    Value *FVal,
                                    InstCombiner::BuilderTy &Builder,
                                    const SimplifyQuery &SQ) {
-  const Value *CmpLHS = Cmp->getOperand(0);
-  const Value *CmpRHS = Cmp->getOperand(1);
+  Value *CmpLHS = Cmp->getOperand(0);
+  Value *CmpRHS = Cmp->getOperand(1);
   ICmpInst::Predicate Pred = Cmp->getPredicate();
 
+  if (match(TVal, m_Zero())) {
+    // (X <= Y) ? 0 : (X - Y)
+    if ((Pred == CmpInst::ICMP_SLT || Pred == CmpInst::ICMP_SLE) &&
+        match(FVal, m_NSWSub(m_Specific(CmpLHS), m_Specific(CmpRHS))) &&
+        isGuaranteedNotToBeUndef(CmpLHS, nullptr, Cmp, nullptr)) {
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user1342234 wrote:

Thanks, I mis-read your previous suggestion.

https://github.com/llvm/llvm-project/pull/187898