Allow SIMD-returning calls as arguments

As of this change we handle all relevant ABI scenarios.

1) Windows x64:
   - SIMD8: returned and passed as "TYP_LONG", fine.
   - SIMD12 / SIMD16 / SIMD32: returned and passed via a return buffer, fine.
2) Unix x64:
   - SIMD8: returned and passed in one FP register, fine.
   - SIMD12 / SIMD16, Vector4: returned and passed in two FP registers, fine.
   - SIMD16, Vector128 / SIMD32: returned and passed via a return buffer, fine.
3) x86:
   - SIMD8: can be returned via two registers or a return buffer (and is always passed on stack), both are fine.
   - SIMD12/SIMD16/SIMD32: returned via a return buffer, passed on stack, fine.
4) ARM64:
   - SIMD8, Vector2: returned in two FP registers (and passed as such or "TYP_LONG" under Windows varargs), fine.
   - SIMD8, Vector64: returned in one FP register, can be passed as such or as "TYP_LONG" under Windows varargs.
     The latter case is now handled correctly in "Lowering::LowerArg".
   - SIMD12: returned in three FP registers, passed as such or in two integer registers under Windows varargs, fine.
   - SIMD16, Vector4: returned in four FP registers, passed as such, or in two integer registers under Windows varargs, fine.
   - SIMD16, Vector128: returned in one FP register, passed as such, or in two integer registers under Windows varargs, fine
     (morph will decompose the varargs case into a `FIELD_LIST` via a temp).

Author: SingleAccretion

src/coreclr/jit/lower.cpp

@@ -1419,7 +1419,7 @@ GenTree* Lowering::LowerFloatArg(GenTree** pArg, CallArg* callArg)
                     break;
                 }
                 GenTree* node = use.GetNode();
-                if (varTypeIsFloating(node))
+                if (varTypeUsesFloatReg(node))
                 {
                     GenTree* intNode = LowerFloatArgReg(node, currRegNumber);
                     assert(intNode != nullptr);
@@ -1441,7 +1441,7 @@ GenTree* Lowering::LowerFloatArg(GenTree** pArg, CallArg* callArg)
             // List fields were replaced in place.
             return arg;
         }
-        else if (varTypeIsFloating(arg))
+        else if (varTypeUsesFloatReg(arg))
         {
             GenTree* intNode = LowerFloatArgReg(arg, callArg->AbiInfo.GetRegNum());
             assert(intNode != nullptr);
@@ -1464,11 +1464,13 @@ GenTree* Lowering::LowerFloatArg(GenTree** pArg, CallArg* callArg)
 //
 GenTree* Lowering::LowerFloatArgReg(GenTree* arg, regNumber regNum)
 {
+    assert(varTypeUsesFloatReg(arg));
+
     var_types floatType = arg->TypeGet();
-    assert(varTypeIsFloating(floatType));
-    var_types intType = (floatType == TYP_DOUBLE) ? TYP_LONG : TYP_INT;
-    GenTree*  intArg  = comp->gtNewBitCastNode(intType, arg);
+    var_types intType   = (floatType == TYP_FLOAT) ? TYP_INT : TYP_LONG;
+    GenTree*  intArg    = comp->gtNewBitCastNode(intType, arg);
     intArg->SetRegNum(regNum);
+
 #ifdef TARGET_ARM
     if (floatType == TYP_DOUBLE)
     {
@@ -3828,6 +3830,11 @@ void Lowering::LowerCallStruct(GenTreeCall* call)
                 assert(user->TypeIs(origType) || (returnType == user->TypeGet()));
                 break;
 
+            case GT_CALL:
+                // Argument lowering will deal with register file mismatches if needed.
+                assert(varTypeIsSIMD(origType));
+                break;
+
             case GT_STOREIND:
 #ifdef FEATURE_SIMD
                 if (varTypeIsSIMD(user))