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

[Yingwei Zheng via llvm-commits]

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@@ -564,6 +564,43 @@ 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 Value *CmpLHS = Cmp->getOperand(0);
+  const Value *CmpRHS = Cmp->getOperand(1);
+  const 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.
+  // Note: that the UMIN case is not possible as we canonicalize to addition.
+  if (CmpLHS == TVal) {
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dtcxzyw wrote:

Missing fold for commuted pattern: https://alive2.llvm.org/ce/z/bKhA-j


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