From 45ca9ef5c1235a80ceffca459cbf1bd47032b35f Mon Sep 17 00:00:00 2001
From: David Chase <drchase@google.com>
Date: Wed, 31 Mar 2021 22:51:44 -0400
Subject: [PATCH] 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 |  5 ++-
 test/abi/fuzz_trailing_zero_field.go     | 39 ++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 1 deletion(-)
 create mode 100644 test/abi/fuzz_trailing_zero_field.go

diff --git a/src/cmd/compile/internal/abi/abiutils.go b/src/cmd/compile/internal/abi/abiutils.go
index 56e008830fe..50a818c025c 100644
--- a/src/cmd/compile/internal/abi/abiutils.go
+++ b/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:
diff --git a/test/abi/fuzz_trailing_zero_field.go b/test/abi/fuzz_trailing_zero_field.go
new file mode 100644
index 00000000000..ae7ad32aa22
--- /dev/null
+++ b/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)
+}