cmd/compile: fix register/offset calculation for trailing empty field case.

Includes test.
Long term, need to make the offending code be more in terms
of official types package offsets, instead of duplicating that
logic.

For #40724.

Change-Id: Id33a153f10aed3289cc48d1f99a8e0f6ece9474d
Reviewed-on: https://go-review.googlesource.com/c/go/+/306469
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/compile/internal/abi/abiutils.go

@@ -211,8 +211,11 @@ func appendParamOffsets(offsets []int64, at int64, t *types.Type) ([]int64, int6
 				offsets, at = appendParamOffsets(offsets, at, t.Elem())
 			}
 		case types.TSTRUCT:
-			for _, f := range t.FieldSlice() {
+			for i, f := range t.FieldSlice() {
 				offsets, at = appendParamOffsets(offsets, at, f.Type)
+				if f.Type.Width == 0 && i == t.NumFields()-1 {
+					at++ // last field has zero width
+				}
 			}
 			at = align(at, t) // type size is rounded up to its alignment
 		case types.TSLICE:

test/abi/fuzz_trailing_zero_field.go

@@ -0,0 +1,39 @@
+// run
+
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+var p0exp = S1{
+	F1: complex(float64(2.3640607624715027), float64(-0.2717825524109192)),
+	F2: S2{F1: 9},
+	F3: 103050709,
+}
+
+type S1 struct {
+	F1 complex128
+	F2 S2
+	F3 uint64
+}
+
+type S2 struct {
+	F1 uint64
+	F2 empty
+}
+
+type empty struct {
+}
+
+//go:noinline
+//go:registerparams
+func callee(p0 S1) {
+	if p0 != p0exp {
+		panic("bad p0")
+	}
+}
+
+func main() {
+	callee(p0exp)
+}