gotypes: fix argument size for signatures without return types (#212)

This fixes a bug in argument size calculation in the case where the function
has no return values. Previously it was padding the argument struct to max
alignment, but this only happens if there are return values following.

Updates #191

gotypes/signature.go

@@ -97,20 +97,22 @@ func (s *Signature) init() {
 	p := s.sig.Params()
 	r := s.sig.Results()
 
-	// Compute parameter offsets.
+	// Compute parameter offsets. Note that if the function has results,
+	// additional padding up to max align is inserted between parameters and
+	// results.
 	vs := tuplevars(p)
 	vs = append(vs, types.NewParam(token.NoPos, nil, "sentinel", types.Typ[types.Uint64]))
 	paramsoffsets := Sizes.Offsetsof(vs)
 	paramssize := paramsoffsets[p.Len()]
+	if r.Len() == 0 {
+		paramssize = structsize(vs[:p.Len()])
+	}
 	s.params = newTuple(p, paramsoffsets, paramssize, "arg")
 
 	// Result offsets.
 	vs = tuplevars(r)
 	resultsoffsets := Sizes.Offsetsof(vs)
-	var resultssize int64
-	if n := len(vs); n > 0 {
-		resultssize = resultsoffsets[n-1] + Sizes.Sizeof(vs[n-1].Type())
-	}
+	resultssize := structsize(vs)
 	for i := range resultsoffsets {
 		resultsoffsets[i] += paramssize
 	}
@@ -175,3 +177,17 @@ func tuplevars(t *types.Tuple) []*types.Var {
 	}
 	return vs
 }
+
+// structsize computes the size of a struct containing the given variables as
+// fields. It would be equivalent to calculating the size of types.NewStruct(vs,
+// nil), apart from the fact that NewStruct panics if multiple fields have the
+// same name, and this happens for example if the variables represent return
+// types from a function.
+func structsize(vs []*types.Var) int64 {
+	n := len(vs)
+	if n == 0 {
+		return 0
+	}
+	offsets := Sizes.Offsetsof(vs)
+	return offsets[n-1] + Sizes.Sizeof(vs[n-1].Type())
+}