[llvm] [X86] Lower bf16->f32/f64 fpext without a GPR round-trip (PR #218359)

[via llvm-commits]

================
@@ -22973,6 +22975,39 @@ static SDValue LowerFP_TO_FP16(SDValue Op, SelectionDAG &DAG) {
   return Res;
 }
 
+SDValue X86TargetLowering::LowerBF16_TO_FP(SDValue Op,
+                                           SelectionDAG &DAG) const {
+  SDLoc DL(Op);
+  SDValue Src = Op.getOperand(0);
+  // Operand is usually already softened to i16 by type legalization.
+  if (Src.getValueType() == MVT::bf16)
+    Src = DAG.getBitcast(MVT::i16, Src);
+  else
+    Src = DAG.getAnyExtOrTrunc(Src, DL, MVT::i16);
+
+  // Peek through to bf16's underlying f16 register
+  MVT VecVT = MVT::v8i16;
+  SDValue VecSrc = Src;
+  if (Src.getOpcode() == ISD::BITCAST &&
+      Src.getOperand(0).getValueType() == MVT::f16) {
+    VecSrc = Src.getOperand(0);
+    VecVT = MVT::v8f16;
+  }
----------------
tfzee wrote:

Without the bitcast check there's some edgecases with when having AVX512FP16 which will fail and generate a GPR roundtrip. Maybe this isn't the cleanest way to handle it however. 
For example fptosi_bf16_to_i32 will go from
```
; CHECK-LABEL: fptosi_bf16_to_i32:
-; CHECK:       # %bb.0:
-; NEXT:    vpslld $16, %xmm0, %xmm0
-; NEXT:    vcvttss2si %xmm0, %eax
-; NEXT:    retq
```
to
```
+; AVX10_2-LABEL: fptosi_bf16_to_i32:
+; NEXT:    vmovw %xmm0, %eax
+; NEXT:    vmovw %eax, %xmm0
+; NEXT:    vpslld $16, %xmm0, %xmm0
+; NEXT:    vcvttss2si %xmm0, %eax
+; NEXT:    retq
```
Or add2 example will go from

```
-; NEXT:    vpslld $16, %xmm1, %xmm1
-; NEXT:    vpslld $16, %xmm0, %xmm0
-; NEXT:    vaddss %xmm1, %xmm0, %xmm0
-; NEXT:    vcvtneps2bf16 %xmm0, %xmm0
-; NEXT:    retq
```
to
```
+; NEXT:    vmovw %xmm0, %eax
+; NEXT:    vmovw %xmm1, %ecx
+; NEXT:    vmovw %ecx, %xmm0
+; NEXT:    vpslld $16, %xmm0, %xmm0
+; NEXT:    vmovw %eax, %xmm1
+; NEXT:    vpslld $16, %xmm1, %xmm1
+; NEXT:    vaddss %xmm0, %xmm1, %xmm0
+; NEXT:    vcvtneps2bf16 %xmm0, %xmm0
+; NEXT:    retq
```


https://github.com/llvm/llvm-project/pull/218359