[clang] [llvm] [InstCombine] Fold `X Pred C2 ? X BOp C1 : C2 BOp C1` to `min/max(X, C2) BOp C1` (PR #116888)

[Nikita Popov via cfe-commits]

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@@ -1898,6 +1882,60 @@ static Instruction *foldSelectICmpEq(SelectInst &SI, ICmpInst *ICI,
   return nullptr;
 }
 
+/// Fold `X Pred C1 ? X BOp C2 : C1 BOp C2` to `min/max(X, C1) BOp C2`.
+/// This allows for better canonicalization.
+static Value *foldSelectWithConstOpToBinOp(ICmpInst *Cmp, Value *TrueVal,
+                                           Value *FalseVal,
+                                           IRBuilderBase &Builder) {
+  BinaryOperator *BOp;
+  Constant *C1, *C2, *C3;
+  Value *X;
+  ICmpInst::Predicate Predicate;
+
+  if (!match(Cmp, m_ICmp(Predicate, m_Value(X), m_Constant(C1))))
+    return nullptr;
+
+  if (!ICmpInst::isRelational(Predicate))
+    return nullptr;
+
+  if (match(TrueVal, m_Constant())) {
+    std::swap(FalseVal, TrueVal);
+    Predicate = ICmpInst::getInversePredicate(Predicate);
+  }
+
+  if (!match(TrueVal, m_BinOp(BOp)) || !match(FalseVal, m_Constant(C3)))
+    return nullptr;
+
+  unsigned Opcode = BOp->getOpcode();
+
+  if (Instruction::isIntDivRem(Opcode))
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nikic wrote:

This check deserves a comment -- am I understanding correctly that this is profitability heuristic, not a correctness check?

https://github.com/llvm/llvm-project/pull/116888