[llvm] [InstCombine] Fold max/min when incrementing/decrementing by 1 (PR #142466)

[Alex MacLean via llvm-commits]

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@@ -565,6 +565,59 @@ Instruction *InstCombinerImpl::foldSelectIntoOp(SelectInst &SI, Value *TrueVal,
   return nullptr;
 }
 
+/// Try to fold a select to a min/max intrinsic. Many cases are already handled
+/// by matchDecomposedSelectPattern but here we handle the cases where more
+/// exensive modification of the IR is required.
+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);
+  ICmpInst::Predicate Pred = Cmp->getPredicate();
+
+  // (X > Y) ? X : (Y - 1) ==> MIN(X, Y - 1)
+  // (X < Y) ? X : (Y + 1) ==> MAX(X, Y + 1)
+  // This transformation is valid when overflow corresponding to the sign of
+  // the comparison is poison and we must drop the non-matching overflow flag.
+  if (CmpRHS == TVal) {
+    std::swap(CmpLHS, CmpRHS);
+    Pred = CmpInst::getSwappedPredicate(Pred);
+  }
+
+  // TODO: consider handeling 'or disjoint' as well, though these would need to
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AlexMaclean wrote:

Fixed

https://github.com/llvm/llvm-project/pull/142466