cmd/ld: emit TLS relocations during external linking

This CL was written by rsc.  I just tweaked 8l.

This CL adds TLS relocation to the ELF .o file we write during external linking,
so that the host linker (gcc) can decide the final location of m and g.

Similar relocations are not necessary on OS X because we use an alternate
program start-time mechanism to acquire thread-local storage.

Similar relocations are not necessary on ARM or Plan 9 or Windows
because external linking mode is not yet supported on those systems.

On almost all ELF systems, the references we use are like %fs:-0x4 or %gs:-0x4,
which we write in 6a/8a as -0x4(FS) or -0x4(GS). On Linux/ELF, however,
Xen's lack of support for this mode forced us long ago to use a two-instruction
sequence: first we load %gs:0x0 into a register r, and then we use -0x4(r).
(The ELF program loader arranges that %gs:0x0 contains a regular pointer to
that same memory location.) In order to relocate those -0x4(r) references,
the linker must know where they are. This CL adds the equivalent notation
-0x4(r)(GS*1) for this purpose: it assembles to the same encoding as -0x4(r)
but the (GS*1) indicates to the linker that this is one of those thread-local
references that needs relocation.

Thanks to Elias Naur for reminding me about this missing piece and
also for writing the test.

R=r
CC=golang-dev
https://codereview.appspot.com/7891047

3 files changed, 60 insertions, 0 deletions

diff --git a/misc/cgo/testtls/tls.go b/misc/cgo/testtls/tls.go
new file mode 100644
index 000000000..a9546a61c
--- /dev/null
+++ b/misc/cgo/testtls/tls.go
@@ -0,0 +1,28 @@
+// Copyright 2013 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.
+
+package cgotlstest
+
+// #include <pthread.h>
+// extern void setTLS(int);
+// extern int getTLS();
+import "C"
+
+import (
+	"runtime"
+	"testing"
+)
+
+func testTLS(t *testing.T) {
+	var keyVal C.int = 1234
+
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+	C.setTLS(C.int(keyVal))
+	storedVal := C.getTLS()
+
+	if storedVal != keyVal {
+		t.Fatalf("stored %d want %d", storedVal, keyVal)
+	}
+}
diff --git a/misc/cgo/testtls/tls_test.go b/misc/cgo/testtls/tls_test.go
new file mode 100644
index 000000000..3076c2d59
--- /dev/null
+++ b/misc/cgo/testtls/tls_test.go
@@ -0,0 +1,13 @@
+// Copyright 2013 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 !windows
+
+package cgotlstest
+
+import "testing"
+
+func TestTLS(t *testing.T) {
+	testTLS(t)
+}
diff --git a/misc/cgo/testtls/tls_unix.c b/misc/cgo/testtls/tls_unix.c
new file mode 100644
index 000000000..545951631
--- /dev/null
+++ b/misc/cgo/testtls/tls_unix.c
@@ -0,0 +1,19 @@
+// Copyright 2013 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>
+
+static __thread int tls;
+
+void
+setTLS(int v)
+{
+	tls = v;
+}
+
+int
+getTLS()
+{
+	return tls;
+}