runtime/cgo, go/build: cgo support for FreeBSD/ARM

This is the last CL for FreeBSD/ARM support.
Also update cmd/ld/doc.go for 5l support of -Hfreebsd.

R=rsc
CC=golang-dev
https://golang.org/cl/6650051

src/cmd/ld/doc.go

@@ -33,7 +33,7 @@ Options new in this version:
 		Write Apple Mach-O binaries (default when $GOOS is darwin)
 	-Hlinux
 		Write Linux ELF binaries (default when $GOOS is linux)
-	-Hfreebsd    (only in 6l/8l)
+	-Hfreebsd
 		Write FreeBSD ELF binaries (default when $GOOS is freebsd)
 	-Hnetbsd     (only in 6l/8l)
 		Write NetBSD ELF binaries (default when $GOOS is netbsd)

src/pkg/go/build/build.go

@@ -216,6 +216,7 @@ var cgoEnabled = map[string]bool{
 	"darwin/amd64":  true,
 	"freebsd/386":   true,
 	"freebsd/amd64": true,
+	"freebsd/arm":   true,
 	"linux/386":     true,
 	"linux/amd64":   true,
 	"linux/arm":     true,

src/pkg/runtime/cgo/gcc_freebsd_arm.c

+// Copyright 2012 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 <pthread.h>
+#include <string.h>
+#include "libcgo.h"
+
+static void *threadentry(void*);
+
+// We have to resort to TLS variable to save g(R10) and
+// m(R9). One reason is that external code might trigger
+// SIGSEGV, and our runtime.sigtramp don't even know we
+// are in external code, and will continue to use R10/R9,
+// this might as well result in another SIGSEGV.
+// Note: all three functions will clobber R0, and the last
+// two can be called from 5c ABI code.
+void __aeabi_read_tp(void) __attribute__((naked));
+void cgo_tls_set_gm(void) __attribute__((naked));
+void cgo_tls_get_gm(void) __attribute__((naked));
+void __aeabi_read_tp(void) {
+	// read @ 0xffff1000
+	__asm__ __volatile__ (
+		"ldr r0, =0xffff1000\n\t"
+		"ldr r0, [r0]\n\t"
+		"mov pc, lr\n\t"
+	);
+}
+// g (R10) at 8(TP), m (R9) at 12(TP)
+void cgo_tls_get_gm(void) {
+	__asm__ __volatile__ (
+		"push {lr}\n\t"
+		"bl __aeabi_read_tp\n\t"
+		"ldr r10, [r0, #8]\n\t"
+		"ldr r9, [r0, #12]\n\t"
+		"pop {pc}\n\t"
+	);
+}
+void cgo_tls_set_gm(void) {
+	__asm__ __volatile__ (
+		"push {lr}\n\t"
+		"bl __aeabi_read_tp\n\t"
+		"str r10, [r0, #8]\n\t"
+		"str r9, [r0, #12]\n\t"
+		"pop {pc}\n\t"
+	);
+}
+// both cgo_tls_{get,set}_gm can be called from runtime
+void (*cgo_load_gm)(void) = cgo_tls_get_gm;
+void (*cgo_save_gm)(void) = cgo_tls_set_gm;
+
+static void
+xinitcgo(G *g)
+{
+	pthread_attr_t attr;
+	size_t size;
+	cgo_tls_set_gm(); // save g and m for the initial thread
+
+	pthread_attr_init(&attr);
+	pthread_attr_getstacksize(&attr, &size);
+	g->stackguard = (uintptr)&attr - size + 4096;
+	pthread_attr_destroy(&attr);
+}
+
+void (*initcgo)(G*) = xinitcgo;
+
+void
+libcgo_sys_thread_start(ThreadStart *ts)
+{
+	pthread_attr_t attr;
+	pthread_t p;
+	size_t size;
+	int err;
+
+	// Not sure why the memset is necessary here,
+	// but without it, we get a bogus stack size
+	// out of pthread_attr_getstacksize.  C'est la Linux.
+	memset(&attr, 0, sizeof attr);
+	pthread_attr_init(&attr);
+	size = 0;
+	pthread_attr_getstacksize(&attr, &size);
+	ts->g->stackguard = size;
+	err = pthread_create(&p, &attr, threadentry, ts);
+	if (err != 0) {
+		fprintf(stderr, "runtime/cgo: pthread_create failed: %s\n", strerror(err));
+		abort();
+	}
+}
+
+extern void crosscall_arm2(void (*fn)(void), void *g, void *m);
+static void*
+threadentry(void *v)
+{
+	ThreadStart ts;
+
+	ts = *(ThreadStart*)v;
+	free(v);
+
+	ts.g->stackbase = (uintptr)&ts;
+
+	/*
+	 * libcgo_sys_thread_start set stackguard to stack size;
+	 * change to actual guard pointer.
+	 */
+	ts.g->stackguard = (uintptr)&ts - ts.g->stackguard + 4096 * 2;
+
+	crosscall_arm2(ts.fn, (void *)ts.g, (void *)ts.m);
+	return nil;
+}