[llvm] [InstCombine] Transform (fcmp + fadd + sel) into (fcmp + sel + fadd) (PR #106492)

[Matt Arsenault via llvm-commits]

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@@ -3633,6 +3633,65 @@ static bool hasAffectedValue(Value *V, SmallPtrSetImpl<Value *> &Affected,
   return false;
 }
 
+// This transformation enables the possibility of transforming fcmp + sel into
+// a fmaxnum/fminnum intrinsic.
+static Value *foldSelectIntoAddConstant(SelectInst &SI,
+                                        InstCombiner::BuilderTy &Builder) {
+  // Do this transformation only when select instruction gives NaN and NSZ
+  // guarantee.
+  auto *SIFOp = dyn_cast<FPMathOperator>(&SI);
+  if (!SIFOp || !SIFOp->hasNoSignedZeros() || !SIFOp->hasNoNaNs())
+    return nullptr;
+
+  // select((fcmp Pred, X, 0), (fadd X, C), C)
+  //      => fadd((select (fcmp Pred, X, 0), X, 0), C)
+  //
+  // Pred := OGT, OGE, OLT, OLE, UGT, UGE, ULT, and ULE
+  Instruction *FAdd;
+  Constant *C;
+  Value *X, *Z;
+  CmpInst::Predicate Pred;
+
+  // Note: OneUse check for `Cmp` is necessary because it makes sure that other
+  // InstCombine folds don't undo this transformation and cause an infinite
+  // loop.
+  if (match(&SI, m_Select(m_OneUse(m_FCmp(Pred, m_Value(X), m_Value(Z))),
+                          m_OneUse(m_Instruction(FAdd)), m_Constant(C))) ||
+      match(&SI, m_Select(m_OneUse(m_FCmp(Pred, m_Value(X), m_Value(Z))),
+                          m_Constant(C), m_OneUse(m_Instruction(FAdd))))) {
+    if (!match(Z, m_AnyZeroFP()))
+      return nullptr;
+
+    // Only these Predicates can be transformed into fmaxnum/fminnum intrinsic.
+    switch (Pred) {
+    default:
+      return nullptr;
+    case FCmpInst::FCMP_OGT:
+    case FCmpInst::FCMP_OGE:
+    case FCmpInst::FCMP_OLT:
+    case FCmpInst::FCMP_OLE:
+    case FCmpInst::FCMP_UGT:
+    case FCmpInst::FCMP_UGE:
+    case FCmpInst::FCMP_ULT:
+    case FCmpInst::FCMP_ULE:
+      break;
+    }
+
+    if (!match(FAdd, m_FAdd(m_Specific(X), m_Specific(C))))
+      return nullptr;
+
+    Value *NewSelect = Builder.CreateSelect(SI.getCondition(), X, Z,
+                                            SI.getName() + ".new", &SI);
+    cast<Instruction>(NewSelect)->setFastMathFlags(SI.getFastMathFlags());
+
+    Value *NewFAdd =
+        Builder.CreateFAddFMF(NewSelect, C, FAdd, FAdd->getName() + ".new");
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arsenm wrote:

Constants canonically should go to the RHS, swap these operands to save a step later 

https://github.com/llvm/llvm-project/pull/106492