[gcc r13-2902] runtime: synchronize empty struct field handling

[Ian Lance Taylor <ian@gcc.gnu.org>]

https://gcc.gnu.org/g:f38162977e2b7efaa75233a0cba2a30a2b7f5132

commit r13-2902-gf38162977e2b7efaa75233a0cba2a30a2b7f5132
Author: melonedo <funanzeng@gmail.com>
Date:   Mon Sep 19 16:01:04 2022 +0800

    runtime: synchronize empty struct field handling
    
    In GCCGO and gollvm, the logic for allocating one byte for the last field is:
    1. the last field has zero size
    2. the struct itself does not have zero size
    3. the last field is not blank
    this commit adds the last two conditions to runtime.structToFFI.
    
    For golang/go#55146
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/431735

Diff:
---
 gcc/go/gofrontend/MERGE |  2 +-
 libgo/go/runtime/ffi.go | 14 ++++++++++----
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/gcc/go/gofrontend/MERGE b/gcc/go/gofrontend/MERGE
index f7a7985287d..73aa712dbdf 100644
--- a/gcc/go/gofrontend/MERGE
+++ b/gcc/go/gofrontend/MERGE
@@ -1,4 +1,4 @@
-42efec8c126cf3787bc7c89d9c7f224eff7c5a21
+0140cca9bc0fad1108c7ed369376ac71cc4bfecf
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
diff --git a/libgo/go/runtime/ffi.go b/libgo/go/runtime/ffi.go
index cd8479ef551..86ce5b85d04 100644
--- a/libgo/go/runtime/ffi.go
+++ b/libgo/go/runtime/ffi.go
@@ -4,6 +4,7 @@
 
 // Only build this file if libffi is supported.
 
+//go:build libffi
 // +build libffi
 
 package runtime
@@ -221,9 +222,6 @@ func stringToFFI() *__ffi_type {
 // structToFFI returns an ffi_type for a Go struct type.
 func structToFFI(typ *structtype) *__ffi_type {
 	c := len(typ.fields)
-	if c == 0 {
-		return emptyStructToFFI()
-	}
 	if typ.typ.kind&kindDirectIface != 0 {
 		return ffi_type_pointer()
 	}
@@ -231,6 +229,7 @@ func structToFFI(typ *structtype) *__ffi_type {
 	fields := make([]*__ffi_type, 0, c+1)
 	checkPad := false
 	lastzero := false
+	sawnonzero := false
 	for i, v := range typ.fields {
 		// Skip zero-sized fields; they confuse libffi,
 		// and there is no value to pass in any case.
@@ -239,10 +238,13 @@ func structToFFI(typ *structtype) *__ffi_type {
 		// next field.
 		if v.typ.size == 0 {
 			checkPad = true
-			lastzero = true
+			if v.name == nil || *v.name != "_" {
+				lastzero = true
+			}
 			continue
 		}
 		lastzero = false
+		sawnonzero = true
 
 		if checkPad {
 			off := uintptr(0)
@@ -263,6 +265,10 @@ func structToFFI(typ *structtype) *__ffi_type {
 		fields = append(fields, typeToFFI(v.typ))
 	}
 
+	if !sawnonzero {
+		return emptyStructToFFI()
+	}
+
 	if lastzero {
 		// The compiler adds one byte padding to non-empty struct ending
 		// with a zero-sized field (types.cc:get_backend_struct_fields).