cmd/cgo: remove -O options when generating compiler errors

The cgo tool generates compiler errors to find out what kind of name it
is using.  Turning on optimization can confuse that process by producing
new unexpected messages.

Fixes #14669.

Change-Id: Idc8e35fd259711ecc9638566b691c11d17140325
Reviewed-on: https://go-review.googlesource.com/23231
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

misc/cgo/errors/issue14669.go

+// Copyright 2016 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.
+
+// Issue 14669: test that fails when build with CGO_CFLAGS selecting
+// optimization.
+
+package p
+
+/*
+const int E = 1;
+
+typedef struct s {
+	int       c;
+} s;
+*/
+import "C"
+
+func F() {
+	_ = C.s{
+		c: C.E,
+	}
+}

misc/cgo/errors/test.bash

@@ -45,6 +45,13 @@ expect issue13129.go C.ushort
 check issue13423.go
 expect issue13635.go C.uchar C.schar C.ushort C.uint C.ulong C.longlong C.ulonglong C.complexfloat C.complexdouble
 
+if ! go build issue14669.go; then
+	exit 1
+fi
+if ! CGO_CFLAGS="-O" go build issue14669.go; then
+	exit 1
+fi
+
 if ! go run ptr.go; then
 	exit 1
 fi

src/cmd/cgo/gcc.go

@@ -1243,12 +1243,20 @@ func (p *Package) gccErrors(stdin []byte) string {
 	// TODO(rsc): require failure
 	args := p.gccCmd()
 
+	// Optimization options can confuse the error messages; remove them.
+	nargs := make([]string, 0, len(args))
+	for _, arg := range args {
+		if !strings.HasPrefix(arg, "-O") {
+			nargs = append(nargs, arg)
+		}
+	}
+
 	if *debugGcc {
-		fmt.Fprintf(os.Stderr, "$ %s <<EOF\n", strings.Join(args, " "))
+		fmt.Fprintf(os.Stderr, "$ %s <<EOF\n", strings.Join(nargs, " "))
 		os.Stderr.Write(stdin)
 		fmt.Fprint(os.Stderr, "EOF\n")
 	}
-	stdout, stderr, _ := run(stdin, args)
+	stdout, stderr, _ := run(stdin, nargs)
 	if *debugGcc {
 		os.Stderr.Write(stdout)
 		os.Stderr.Write(stderr)