AMDGPU: Make v2f32 -> v2f16 legal when target supports v_cvt_pk_f16_f32 (#139956)

If targets support v_cvt_pk_f16_f32 instruction, v2f32 -> v2f16 should
be legal. However, SelectionDAG does not allow us to specify the source
type in the legalization rules. To workaround this, we make FP_ROUND
Custom for v2f16 then set up v2f32 -> v2f16 to be legal during custom
lowering.

Fixes: SWDEV-532608 -- expected v_cvt_pk_f16_f32 was not generated.

Author: changpeng

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -919,6 +919,9 @@ SITargetLowering::SITargetLowering(const TargetMachine &TM,
     setOperationAction(ISD::BUILD_VECTOR, MVT::v2bf16, Legal);
   }
 
+  if (Subtarget->hasCvtPkF16F32Inst())
+    setOperationAction(ISD::FP_ROUND, MVT::v2f16, Custom);
+
   setTargetDAGCombine({ISD::ADD,
                        ISD::UADDO_CARRY,
                        ISD::SUB,
@@ -6899,10 +6902,16 @@ SDValue SITargetLowering::getFPExtOrFPRound(SelectionDAG &DAG, SDValue Op,
 SDValue SITargetLowering::lowerFP_ROUND(SDValue Op, SelectionDAG &DAG) const {
   SDValue Src = Op.getOperand(0);
   EVT SrcVT = Src.getValueType();
+  EVT DstVT = Op.getValueType();
+
+  if (DstVT == MVT::v2f16) {
+    assert(Subtarget->hasCvtPkF16F32Inst() && "support v_cvt_pk_f16_f32");
+    return SrcVT == MVT::v2f32 ? Op : SDValue();
+  }
+
   if (SrcVT.getScalarType() != MVT::f64)
     return Op;
 
-  EVT DstVT = Op.getValueType();
   SDLoc DL(Op);
   if (DstVT == MVT::f16) {
     // TODO: Handle strictfp

llvm/test/CodeGen/AMDGPU/fptrunc.v2f16.no.fast.path.ll renamed to llvm/test/CodeGen/AMDGPU/fptrunc.v2f16.no.fast.math.ll

@@ -12,6 +12,20 @@ define <2 x half> @v_test_cvt_v2f32_v2f16(<2 x float> %src) {
   ret <2 x half> %res
 }
 
+define half @fptrunc_v2f32_v2f16_then_extract(<2 x float> %src) {
+; GFX950-LABEL: fptrunc_v2f32_v2f16_then_extract:
+; GFX950:       ; %bb.0:
+; GFX950-NEXT:    s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
+; GFX950-NEXT:    v_cvt_pk_f16_f32 v0, v0, v1
+; GFX950-NEXT:    v_add_f16_sdwa v0, v0, v0 dst_sel:DWORD dst_unused:UNUSED_PAD src0_sel:WORD_1 src1_sel:DWORD
+; GFX950-NEXT:    s_setpc_b64 s[30:31]
+  %vec_half = fptrunc <2 x float> %src to <2 x half>
+  %first = extractelement <2 x half> %vec_half, i64 1
+  %second = extractelement <2 x half> %vec_half, i64 0
+  %res = fadd half %first, %second
+  ret half %res
+}
+
 define <2 x half> @v_test_cvt_v2f64_v2f16(<2 x double> %src) {
 ; GFX950-SDAG-LABEL: v_test_cvt_v2f64_v2f16:
 ; GFX950-SDAG:       ; %bb.0: