cmd/cgo: don't translate bitfields into Go fields
The cgo tool would sometimes emit a bitfield at an offset that did not
correspond to the C offset, such as for the example in the new test.
Change-Id: I61b2ca10ee44a42f81c13ed12865f2060168fed5
Reviewed-on: https://go-review.googlesource.com/c/go/+/252378
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/doc/go1.16.html b/doc/go1.16.html
index f177226..0167030 100644
--- a/doc/go1.16.html
+++ b/doc/go1.16.html
@@ -86,6 +86,16 @@
by <code>go</code> <code>mod</code> <code>vendor</code> since Go 1.11.
</p>
+<h3 id="cgo">Cgo</h3>
+
+<p> <!-- CL 252378 -->
+ The <a href="/cmd/cgo">cgo</a> tool will no longer try to translate
+ C struct bitfields into Go struct fields, even if their size can be
+ represented in Go. The order in which C bitfields appear in memory
+ is implementation dependent, so in some cases the cgo tool produced
+ results that were silently incorrect.
+</p>
+
<p>
TODO
</p>
diff --git a/misc/cgo/testgodefs/testdata/bitfields.go b/misc/cgo/testgodefs/testdata/bitfields.go
new file mode 100644
index 0000000..6a9724d
--- /dev/null
+++ b/misc/cgo/testgodefs/testdata/bitfields.go
@@ -0,0 +1,31 @@
+// Copyright 2020 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.
+//
+// +build ignore
+
+package main
+
+// This file tests that we don't generate an incorrect field location
+// for a bitfield that appears aligned.
+
+/*
+struct bitfields {
+ unsigned int B1 : 5;
+ unsigned int B2 : 1;
+ unsigned int B3 : 1;
+ unsigned int B4 : 1;
+ unsigned int Short1 : 16; // misaligned on 8 bit boundary
+ unsigned int B5 : 1;
+ unsigned int B6 : 1;
+ unsigned int B7 : 1;
+ unsigned int B8 : 1;
+ unsigned int B9 : 1;
+ unsigned int B10 : 3;
+ unsigned int Short2 : 16; // alignment is OK
+ unsigned int Short3 : 16; // alignment is OK
+};
+*/
+import "C"
+
+type bitfields C.struct_bitfields
diff --git a/misc/cgo/testgodefs/testdata/main.go b/misc/cgo/testgodefs/testdata/main.go
index 2e1ad33..4a3f6a7 100644
--- a/misc/cgo/testgodefs/testdata/main.go
+++ b/misc/cgo/testgodefs/testdata/main.go
@@ -4,6 +4,12 @@
package main
+import (
+ "fmt"
+ "os"
+ "reflect"
+)
+
// Test that the struct field in anonunion.go was promoted.
var v1 T
var v2 = v1.L
@@ -23,4 +29,26 @@
var _ = issue38649{X: 0}
func main() {
+ pass := true
+
+ // The Go translation of bitfields should not have any of the
+ // bitfield types. The order in which bitfields are laid out
+ // in memory is implementation defined, so we can't easily
+ // know how a bitfield should correspond to a Go type, even if
+ // it appears to be aligned correctly.
+ bitfieldType := reflect.TypeOf(bitfields{})
+ check := func(name string) {
+ _, ok := bitfieldType.FieldByName(name)
+ if ok {
+ fmt.Fprintf(os.Stderr, "found unexpected bitfields field %s\n", name)
+ pass = false
+ }
+ }
+ check("Short1")
+ check("Short2")
+ check("Short3")
+
+ if !pass {
+ os.Exit(1)
+ }
}
diff --git a/misc/cgo/testgodefs/testgodefs_test.go b/misc/cgo/testgodefs/testgodefs_test.go
index e4085f9..4c2312c 100644
--- a/misc/cgo/testgodefs/testgodefs_test.go
+++ b/misc/cgo/testgodefs/testgodefs_test.go
@@ -19,6 +19,7 @@
// import "C" block. Add more tests here.
var filePrefixes = []string{
"anonunion",
+ "bitfields",
"issue8478",
"fieldtypedef",
"issue37479",
diff --git a/src/cmd/cgo/gcc.go b/src/cmd/cgo/gcc.go
index 9179b54..eb6c1a5 100644
--- a/src/cmd/cgo/gcc.go
+++ b/src/cmd/cgo/gcc.go
@@ -2831,21 +2831,11 @@
tgo := t.Go
size := t.Size
talign := t.Align
- if f.BitSize > 0 {
- switch f.BitSize {
- case 8, 16, 32, 64:
- default:
- continue
- }
- size = f.BitSize / 8
- name := tgo.(*ast.Ident).String()
- if strings.HasPrefix(name, "int") {
- name = "int"
- } else {
- name = "uint"
- }
- tgo = ast.NewIdent(name + fmt.Sprint(f.BitSize))
- talign = size
+ if f.BitOffset > 0 || f.BitSize > 0 {
+ // The layout of bitfields is implementation defined,
+ // so we don't know how they correspond to Go fields
+ // even if they are aligned at byte boundaries.
+ continue
}
if talign > 0 && f.ByteOffset%talign != 0 {
