[llvm] [X86] Lower mathlib call ldexp into scalef when avx512 is enabled (PR #166839)

[Kavin Gnanapandithan via llvm-commits]

================
@@ -19153,6 +19155,81 @@ SDValue X86TargetLowering::LowerINSERT_VECTOR_ELT(SDValue Op,
   return SDValue();
 }
 
+static SDValue LowerFLDEXP(SDValue Op, const X86Subtarget &Subtarget,
+                           SelectionDAG &DAG) {
+  SDLoc DL(Op);
+  SDValue X = Op.getOperand(0);
+  MVT XTy = X.getSimpleValueType();
+  SDValue Exp = Op.getOperand(1);
+  MVT ExtVT;
+
+  switch (XTy.SimpleTy) {
+  default:
+    return SDValue();
+  case MVT::f16:
+    if (!Subtarget.hasFP16())
+      X = DAG.getFPExtendOrRound(X, DL, MVT::f32);
+    [[fallthrough]];
+  case MVT::f32:
+  case MVT::f64: {
+    MVT VT = MVT::getVectorVT(X.getSimpleValueType(),
+                              128 / X.getSimpleValueType().getSizeInBits());
+    Exp = DAG.getNode(ISD::SINT_TO_FP, DL, X.getValueType(), Exp);
+    SDValue VX = DAG.getNode(ISD::SCALAR_TO_VECTOR, DL, VT, X);
+    SDValue VExp = DAG.getNode(ISD::SCALAR_TO_VECTOR, DL, VT, Exp);
+    SDValue Scalefs = DAG.getNode(X86ISD::SCALEFS, DL, VT, VX, VExp, VX);
+    SDValue Final = DAG.getExtractVectorElt(DL, X.getValueType(), Scalefs, 0);
+    if (X.getValueType() != XTy)
+      Final = DAG.getFPExtendOrRound(Final, DL, XTy);
+    return Final;
+  }
+  case MVT::v4f32:
+  case MVT::v2f64:
+  case MVT::v8f32:
+  case MVT::v4f64:
+  case MVT::v16f32:
+  case MVT::v8f64:
+    Exp = DAG.getNode(ISD::SINT_TO_FP, DL, XTy, Exp);
----------------
KavinTheG wrote:

After the switch, the only case where Exp is an integer is for vector fp16, but I do convert it to an fp after widening. I tried converting i16 vector exponents before widening, but that seemed very difficult. 

Would it be preferable to convert Exp  after the switch  statement like so? 

```
  case MVT::v8f64:
    // Exp = DAG.getNode(ISD::SINT_TO_FP, DL, XTy, Exp);
    if (XTy.getSizeInBits() == 512 || Subtarget.hasVLX()) {
      Exp = DAG.getNode(ISD::SINT_TO_FP, DL, XTy, Exp);
      return DAG.getNode(X86ISD::SCALEF, DL, XTy, X, Exp);
    }
    break;
    
 ...
 
  if (X.getValueType().getScalarType() != MVT::f16)
    Exp = DAG.getNode(ISD::SINT_TO_FP, DL, XTy, Exp);
  SDValue WideX = widenSubVector(X, true, Subtarget, DAG, DL, 512);
  SDValue WideExp = widenSubVector(Exp, true, Subtarget, DAG, DL, 512);

```


https://github.com/llvm/llvm-project/pull/166839