runtime,cmd/dist,cmd/link: add cgo support on openbsd/arm

Add support for cgo on openbsd/arm.The gcc shipped with base OpenBSD armv7
is old/inadequate, so use clang by default.

Change-Id: I945a26d369378952d357727718e69249411e1127
Reviewed-on: https://go-review.googlesource.com/c/154381
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/dist/build.go

@@ -1448,7 +1448,7 @@ var cgoEnabled = map[string]bool{
 	"netbsd/arm":      true,
 	"openbsd/386":     true,
 	"openbsd/amd64":   true,
-	"openbsd/arm":     false,
+	"openbsd/arm":     true,
 	"plan9/386":       false,
 	"plan9/amd64":     false,
 	"plan9/arm":       false,

src/cmd/dist/main.go

@@ -65,6 +65,13 @@ func main() {
 	case "freebsd":
 		// Since FreeBSD 10 gcc is no longer part of the base system.
 		defaultclang = true
+	case "openbsd":
+		// The gcc available on OpenBSD armv7 is old/inadequate (for example, lacks
+		// __sync_fetch_and_*/__sync_*_and_fetch) and will likely be removed in the
+		// not-to-distant future - use clang instead.
+		if runtime.GOARCH == "arm" {
+			defaultclang = true
+		}
 	case "solaris":
 		// Even on 64-bit platform, solaris uname -m prints i86pc.
 		out := run("", CheckExit, "isainfo", "-n")

src/cmd/link/internal/ld/lib.go

@@ -535,6 +535,12 @@ func (ctxt *Link) loadlib() {
 			if *flagLibGCC == "" {
 				*flagLibGCC = ctxt.findLibPathCmd("--print-libgcc-file-name", "libgcc")
 			}
+			if runtime.GOOS == "openbsd" && *flagLibGCC == "libgcc.a" {
+				// On OpenBSD `clang --print-libgcc-file-name` returns "libgcc.a".
+				// In this case we fail to load libgcc.a and can encounter link
+				// errors - see if we can find libcompiler_rt.a instead.
+				*flagLibGCC = ctxt.findLibPathCmd("--print-file-name=libcompiler_rt.a", "libcompiler_rt")
+			}
 			if *flagLibGCC != "none" {
 				hostArchive(ctxt, *flagLibGCC)
 			}

src/runtime/cgo/gcc_openbsd_arm.c

@@ -0,0 +1,67 @@
+// Copyright 2018 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.
+
+#include <sys/types.h>
+#include <pthread.h>
+#include <signal.h>
+#include <string.h>
+#include "libcgo.h"
+#include "libcgo_unix.h"
+
+static void* threadentry(void*);
+static void (*setg_gcc)(void*);
+
+void
+x_cgo_init(G *g, void (*setg)(void*))
+{
+	pthread_attr_t attr;
+	size_t size;
+
+	setg_gcc = setg;
+	pthread_attr_init(&attr);
+	pthread_attr_getstacksize(&attr, &size);
+	g->stacklo = (uintptr)&attr - size + 4096;
+	pthread_attr_destroy(&attr);
+}
+
+void
+_cgo_sys_thread_start(ThreadStart *ts)
+{
+	pthread_attr_t attr;
+	sigset_t ign, oset;
+	pthread_t p;
+	size_t size;
+	int err;
+
+	sigfillset(&ign);
+	pthread_sigmask(SIG_SETMASK, &ign, &oset);
+
+	pthread_attr_init(&attr);
+	pthread_attr_getstacksize(&attr, &size);
+
+	// Leave stacklo=0 and set stackhi=size; mstart will do the rest.
+	ts->g->stackhi = size;
+	err = _cgo_try_pthread_create(&p, &attr, threadentry, ts);
+
+	pthread_sigmask(SIG_SETMASK, &oset, nil);
+
+	if (err != 0) {
+		fprintf(stderr, "runtime/cgo: pthread_create failed: %s\n", strerror(err));
+		abort();
+	}
+}
+
+extern void crosscall_arm1(void (*fn)(void), void (*setg_gcc)(void*), void *g);
+
+static void*
+threadentry(void *v)
+{
+	ThreadStart ts;
+
+	ts = *(ThreadStart*)v;
+	free(v);
+
+	crosscall_arm1(ts.fn, setg_gcc, (void*)ts.g);
+	return nil;
+}

src/runtime/sys_openbsd_arm.s

@@ -371,8 +371,9 @@ TEXT runtime·closeonexec(SB),NOSPLIT,$0
 TEXT ·publicationBarrier(SB),NOSPLIT|NOFRAME,$0-0
 	B	runtime·armPublicationBarrier(SB)
 
-// TODO(jsing): Implement.
 TEXT runtime·read_tls_fallback(SB),NOSPLIT|NOFRAME,$0
-	MOVW	$5, R0
-	MOVW	R0, (R0)
+	MOVM.WP	[R1, R2, R3, R12], (R13)
+	MOVW	$330, R12		// sys___get_tcb
+	SWI	$0
+	MOVM.IAW (R13), [R1, R2, R3, R12]
 	RET