liblink, cmd/ld, runtime: set the type of runtime.tlsg in runtime

In the old code, liblink, cmd/ld and runtime all have code determine
whether runtime.tlsg is an actual variable or a placeholder for TLS
relocation. This change consolidate them into one: the runtime/tls_arm.s
will ultimately determine the type of that variable.

Change-Id: I3b3f80791a1db4c2b7318f81a115972cd2237e43
Reviewed-on: https://go-review.googlesource.com/2118Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/cmd/ld/lib.c

@@ -257,7 +257,13 @@ loadlib(void)
 	}
 	
 	tlsg = linklookup(ctxt, "runtime.tlsg", 0);
-	tlsg->type = STLSBSS;
+	// For most ports, runtime.tlsg is a placeholder symbol for TLS
+	// relocation. However, the Android and Darwin ports need it to
+	// be a real variable. Instead of hard-coding which platforms
+	// need it to be a real variable, we set the type to STLSBSS only
+	// when the runtime has not declared its type already.
+	if(tlsg->type == 0)
+		tlsg->type = STLSBSS;
 	tlsg->size = PtrSize;
 	tlsg->hide = 1;
 	tlsg->reachable = 1;

src/liblink/asm5.c

@@ -1638,8 +1638,9 @@ if(0 /*debug['G']*/) print("%ux: %s: arm %d\n", (uint32)(p->pc), p->from.sym->na
 			// runtime.tlsg is special.
 			// Its "address" is the offset from the TLS thread pointer
 			// to the thread-local g and m pointers.
-			// Emit a TLS relocation instead of a standard one.
-			if(rel->sym == ctxt->tlsg) {
+			// Emit a TLS relocation instead of a standard one if it's
+			// typed STLSBSS.
+			if(rel->sym == ctxt->tlsg && ctxt->tlsg->type == STLSBSS) {
 				rel->type = R_TLS;
 				if(ctxt->flag_shared)
 					rel->add += ctxt->pc - p->pcrel->pc - 8 - rel->siz;

src/runtime/tls_arm.s

@@ -17,6 +17,9 @@
 
 // On android, runtime.tlsg is a normal variable.
 // TLS offset is computed in x_cgo_inittls.
+#ifdef GOOS_android
+#define TLSG_IS_VARIABLE
+#endif
 
 // save_g saves the g register into pthread-provided
 // thread-local memory, so that we can call externally compiled
@@ -37,7 +40,7 @@ TEXT runtime·save_g(SB),NOSPLIT,$-4
 	// $runtime.tlsg(SB) is a special linker symbol.
 	// It is the offset from the TLS base pointer to our
 	// thread-local storage for g.
-#ifdef GOOS_android
+#ifdef TLSG_IS_VARIABLE
 	MOVW	runtime·tlsg(SB), R11
 #else
 	MOVW	$runtime·tlsg(SB), R11
@@ -60,7 +63,7 @@ TEXT runtime·load_g(SB),NOSPLIT,$0
 	// $runtime.tlsg(SB) is a special linker symbol.
 	// It is the offset from the TLS base pointer to our
 	// thread-local storage for g.
-#ifdef GOOS_android
+#ifdef TLSG_IS_VARIABLE
 	MOVW	runtime·tlsg(SB), R11
 #else
 	MOVW	$runtime·tlsg(SB), R11
@@ -68,3 +71,7 @@ TEXT runtime·load_g(SB),NOSPLIT,$0
 	ADD	R11, R0
 	MOVW	0(R0), g
 	RET
+
+#ifdef TLSG_IS_VARIABLE
+GLOBL runtime·tlsg+0(SB), NOPTR, $4
+#endif