liblink, cmd/5l: restore flag_shared

CL 56120043 fixed and cleaned up TLS on ARM after introducing liblink, but
left flag_shared broken. This CL restores the (unsupported) flag_shared
behaviour by simply rewriting access to $runtime.tlsgm(SB) with
runtime.tlsgm(SB), to compensate for the extra indirection when going from
the R_ARM_TLS_LE32 relocation to the R_ARM_TLS_IE32 relocation.

Also, remove unnecessary symbol lookup left after 56120043.

LGTM=iant
R=iant, rsc
CC=golang-codereviews
https://golang.org/cl/57000043

src/cmd/5l/5.out.h

@@ -279,7 +279,7 @@ enum
 	D_PLT1 = (D_NONE+44), // R_ARM_PLT32, 2nd inst: add ip, ip, #0xNN000
 	D_PLT2 = (D_NONE+45), // R_ARM_PLT32, 3rd inst: ldr pc, [ip, #0xNNN]!
 	D_CALL = (D_NONE+46), // R_ARM_PLT32/R_ARM_CALL/R_ARM_JUMP24, bl xxxxx or b yyyyy
-	D_TLS = (D_NONE+47), // R_ARM_TLS_LE32
+	D_TLS = (D_NONE+47), // R_ARM_TLS_LE32/R_ARM_TLS_IE32
 };
 
 /*

src/liblink/asm5.c

@@ -1371,11 +1371,10 @@ if(0 /*debug['G']*/) print("%ux: %s: arm %d\n", (uint32)(p->pc), p->from.sym->na
 			// 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.
-			// The TLS flag_shared case is not tested and probably now wrong.
 			if(rel->sym == ctxt->gmsym) {
 				rel->type = D_TLS;
 				if(ctxt->flag_shared)
-					rel->add += ctxt->pc - p->pcrel->pc - 8 - rel->siz; // TODO: probably wrong
+					rel->add += ctxt->pc - p->pcrel->pc - 8 - rel->siz;
 				rel->xadd = rel->add;
 				rel->xsym = rel->sym;
 			} else if(ctxt->flag_shared) {

src/liblink/obj5.c

@@ -167,6 +167,21 @@ progedit(Link *ctxt, Prog *p)
 		}
 		break;
 	}
+
+	if(ctxt->flag_shared) {
+		// Shared libraries use R_ARM_TLS_IE32 instead of 
+		// R_ARM_TLS_LE32, replacing the link time constant TLS offset in
+		// runtime.tlsgm with an address to a GOT entry containing the 
+		// offset. Rewrite $runtime.tlsgm(SB) to runtime.tlsgm(SB) to
+		// compensate.
+		if(ctxt->gmsym == nil)
+			ctxt->gmsym = linklookup(ctxt, "runtime.tlsgm", 0);
+
+		if(p->from.type == D_CONST && p->from.name == D_EXTERN && p->from.sym == ctxt->gmsym)
+			p->from.type = D_OREG;
+		if(p->to.type == D_CONST && p->to.name == D_EXTERN && p->to.sym == ctxt->gmsym)
+			p->to.type = D_OREG;
+	}
 }
 
 static Prog*
@@ -225,8 +240,6 @@ addstacksplit(Link *ctxt, LSym *cursym)
 	if(ctxt->symmorestack[0] == nil)
 		ctxt->symmorestack[0] = linklookup(ctxt, "runtime.morestack", 0);
 	
-	if(ctxt->gmsym == nil)
-		ctxt->gmsym = linklookup(ctxt, "runtime.tlsgm", 0);
 	q = nil;
 	
 	ctxt->cursym = cursym;
@@ -302,7 +315,6 @@ addstacksplit(Link *ctxt, LSym *cursym)
 	 * strip NOPs
 	 * expand RET
 	 * expand BECOME pseudo
-	 * fixup TLS
 	 */
 
 	for(p = cursym->text; p != nil; p = p->link) {