[WebAssembly] Support BUILD_VECTOR with F16x8. (#108117)

Convert BUILD_VECTORS with FP16x8 to I16x8 since there's no FP16 scalar
value to intialize v128.const.

Author: brendandahl

llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp

@@ -212,6 +212,9 @@ WebAssemblyTargetLowering::WebAssemblyTargetLowering(
                    MVT::v2f64})
       setOperationAction(ISD::BUILD_VECTOR, T, Custom);
 
+    if (Subtarget->hasFP16())
+      setOperationAction(ISD::BUILD_VECTOR, MVT::f16, Custom);
+
     // We have custom shuffle lowering to expose the shuffle mask
     for (auto T : {MVT::v16i8, MVT::v8i16, MVT::v4i32, MVT::v4f32, MVT::v2i64,
                    MVT::v2f64})
@@ -2059,6 +2062,18 @@ static SDValue LowerConvertLow(SDValue Op, SelectionDAG &DAG) {
 
 SDValue WebAssemblyTargetLowering::LowerBUILD_VECTOR(SDValue Op,
                                                      SelectionDAG &DAG) const {
+  MVT VT = Op.getSimpleValueType();
+  if (VT == MVT::v8f16) {
+    // BUILD_VECTOR can't handle FP16 operands since Wasm doesn't have a scaler
+    // FP16 type, so cast them to I16s.
+    MVT IVT = VT.changeVectorElementType(MVT::i16);
+    SmallVector<SDValue, 8> NewOps;
+    for (unsigned I = 0, E = Op.getNumOperands(); I < E; ++I)
+      NewOps.push_back(DAG.getBitcast(MVT::i16, Op.getOperand(I)));
+    SDValue Res = DAG.getNode(ISD::BUILD_VECTOR, SDLoc(), IVT, NewOps);
+    return DAG.getBitcast(VT, Res);
+  }
+
   if (auto ConvertLow = LowerConvertLow(Op, DAG))
     return ConvertLow;
 

llvm/test/CodeGen/WebAssembly/half-precision.ll

@@ -27,6 +27,13 @@ define <8 x half> @splat_v8f16(float %x) {
   ret <8 x half> %v
 }
 
+; CHECK-LABEL: const_splat_v8f16:
+; CHECK:       v128.const      $push0=, 20800, 0, 0, 0, 0, 0, 0, 20800
+; CHECK-NEXT:  return $pop0
+define <8 x half> @const_splat_v8f16() {
+  ret <8 x half> <half 42., half 0., half 0., half 0., half 0., half 0., half 0., half 42.>
+}
+
 ; CHECK-LABEL: extract_lane_v8f16:
 ; CHECK:       f16x8.extract_lane $push0=, $0, 1
 ; CHECK-NEXT:  return $pop0