runtime,runtime/cgo: set up TLS storage for Android Q without cgo

Android Q frees a static TLS slot for us to use. Use the offset of
that slot as the default for our TLS offset.

As a result, runtime/cgo is no more a requirement for Android Q and
newer.

Updates #31343
Updates #29674

Change-Id: I759049b2e2865bd3d4fdc05a8cfc6db8b0da1f5d
Reviewed-on: https://go-review.googlesource.com/c/go/+/170955
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/asm_386.s

@@ -1564,5 +1564,8 @@ TEXT runtime·panicExtendSlice3CU(SB),NOSPLIT,$0-12
 	JMP	runtime·goPanicExtendSlice3CU(SB)
 
 #ifdef GOOS_android
+// Use the free TLS_SLOT_APP slot #2 on Android Q.
+// Earlier androids are set up in gcc_android.c.
+DATA runtime·tls_g+0(SB)/4, $8
 GLOBL runtime·tls_g+0(SB), NOPTR, $4
 #endif

src/runtime/asm_amd64.s

@@ -1713,5 +1713,8 @@ TEXT runtime·panicSlice3CU(SB),NOSPLIT,$0-16
 	JMP	runtime·goPanicSlice3CU(SB)
 
 #ifdef GOOS_android
+// Use the free TLS_SLOT_APP slot #2 on Android Q.
+// Earlier androids are set up in gcc_android.c.
+DATA runtime·tls_g+0(SB)/8, $16
 GLOBL runtime·tls_g+0(SB), NOPTR, $8
 #endif

src/runtime/cgo/gcc_android.c

@@ -47,7 +47,7 @@ inittls(void **tlsg, void **tlsbase)
 {
 	pthread_key_t k;
 	int i, err;
-	void *handle, *get_ver;
+	void *handle, *get_ver, *off;
 
 	// Check for Android Q where we can use the free TLS_SLOT_APP slot.
 	handle = dlopen("libc.so", RTLD_LAZY);
@@ -60,7 +60,11 @@ inittls(void **tlsg, void **tlsbase)
 	get_ver = dlsym(handle, "android_get_device_api_level");
 	dlclose(handle);
 	if (get_ver != NULL) {
-		*tlsg = (void *)(TLS_SLOT_APP*sizeof(void *));
+		off = (void *)(TLS_SLOT_APP*sizeof(void *));
+		// tlsg is initialized to Q's free TLS slot. Verify it while we're here.
+		if (*tlsg != off) {
+			fatalf("tlsg offset wrong, got %ld want %ld\n", *tlsg, off);
+		}
 		return;
 	}
 

src/runtime/tls_arm.s

@@ -103,6 +103,11 @@ TEXT setg_gcc<>(SB),NOSPLIT,$0
 	B		runtime·save_g(SB)
 
 #ifdef TLSG_IS_VARIABLE
+#ifdef GOOS_android
+// Use the free TLS_SLOT_APP slot #2 on Android Q.
+// Earlier androids are set up in gcc_android.c.
+DATA runtime·tls_g+0(SB)/4, $8
+#endif
 GLOBL runtime·tls_g+0(SB), NOPTR, $4
 #else
 GLOBL runtime·tls_g+0(SB), TLSBSS, $4

src/runtime/tls_arm64.s

@@ -43,6 +43,11 @@ nocgo:
 	RET
 
 #ifdef TLSG_IS_VARIABLE
+#ifdef GOOS_android
+// Use the free TLS_SLOT_APP slot #2 on Android Q.
+// Earlier androids are set up in gcc_android.c.
+DATA runtime·tls_g+0(SB)/8, $16
+#endif
 GLOBL runtime·tls_g+0(SB), NOPTR, $8
 #else
 GLOBL runtime·tls_g+0(SB), TLSBSS, $8