[llvm] [InstCombine] InstCombine should fold frexp of select to select of frexp (PR #121227)

[Matt Arsenault via llvm-commits]

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@@ -4043,6 +4043,52 @@ InstCombinerImpl::foldExtractOfOverflowIntrinsic(ExtractValueInst &EV) {
   return nullptr;
 }
 
+static Value *foldFrexpOfSelect(ExtractValueInst &EV, CallInst *FrexpCall,
+                                SelectInst *SelectInst,
+                                InstCombiner::BuilderTy &Builder) {
+  // Helper to fold frexp of select to select of frexp.
+  Value *Cond = SelectInst->getCondition();
+  Value *TrueVal = SelectInst->getTrueValue();
+  Value *FalseVal = SelectInst->getFalseValue();
+  ConstantFP *ConstOp = nullptr;
+  Value *VarOp = nullptr;
+  bool ConstIsTrue = false;
+
+  if (auto *TrueConst = dyn_cast<ConstantFP>(TrueVal)) {
+    ConstOp = TrueConst;
+    VarOp = FalseVal;
+    ConstIsTrue = true;
+  } else if (auto *FalseConst = dyn_cast<ConstantFP>(FalseVal)) {
+    ConstOp = FalseConst;
+    VarOp = TrueVal;
+    ConstIsTrue = false;
+  }
+
+  if (!ConstOp || !VarOp)
+    return nullptr;
+
+  CallInst *NewFrexp =
+      Builder.CreateCall(FrexpCall->getCalledFunction(), {VarOp}, "frexp");
+
+  Value *NewEV = Builder.CreateExtractValue(NewFrexp, 0, "mantissa");
+
+  APFloat ConstVal = ConstOp->getValueAPF();
+  int Exp = 0;
+  APFloat Mantissa = ConstVal;
+
+  if (ConstVal.isFiniteNonZero()) {
+    Mantissa = frexp(ConstVal, Exp, APFloat::rmNearestTiesToEven);
+  }
+
+  Constant *ConstantMantissa = ConstantFP::get(ConstOp->getType(), Mantissa);
+
+  Value *NewSel = Builder.CreateSelect(
+      Cond, ConstIsTrue ? ConstantMantissa : NewEV,
+      ConstIsTrue ? NewEV : ConstantMantissa, "select.frexp");
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arsenm wrote:

Can this preserve the fast math flags? 

https://github.com/llvm/llvm-project/pull/121227