liblink: introduce way to avoid pc-relative addressing

For Solaris. Sigh.

LGTM=dave
R=aram, iant, dave
CC=golang-codereviews
https://golang.org/cl/129540043

src/liblink/asm6.c

@@ -1535,6 +1535,19 @@ static Optab optab[] =
 static Optab*	opindex[ALAST+1];
 static vlong	vaddr(Link*, Addr*, Reloc*);
 
+// isextern reports whether s describes an external symbol that must avoid pc-relative addressing.
+// This happens on systems like Solaris that call .so functions instead of system calls.
+// It does not seem to be necessary for any other systems. This is probably working
+// around a Solaris-specific bug that should be fixed differently, but we don't know
+// what that bug is. And this does fix it.
+static int
+isextern(LSym *s)
+{
+	// All the Solaris dynamic imports from libc.so begin with "libc·", which
+	// the compiler rewrites to "libc." by the time liblink gets it.
+	return strncmp(s->name, "libc.", 5) == 0;
+}
+
 // single-instruction no-ops of various lengths.
 // constructed by hand and disassembled with gdb to verify.
 // see http://www.agner.org/optimize/optimizing_assembly.pdf for discussion.
@@ -1932,6 +1945,8 @@ oclass(Link *ctxt, Addr *a)
 				switch(a->index) {
 				case D_EXTERN:
 				case D_STATIC:
+					if(a->sym != nil && isextern(a->sym))
+						return Yi32;
 					return Yiauto; // use pc-relative addressing
 				case D_AUTO:
 				case D_PARAM:
@@ -2109,7 +2124,7 @@ oclass(Link *ctxt, Addr *a)
 				return Yi32;	/* unsigned */
 			return Yi64;
 		}
-		return Yi32;	/* TO DO: D_ADDR as Yi64 */
+		return Yi32;
 
 	case D_BRANCH:
 		return Ybr;
@@ -2282,12 +2297,17 @@ vaddr(Link *ctxt, Addr *a, Reloc *r)
 			ctxt->diag("need reloc for %D", a);
 			sysfatal("reloc");
 		}
-		r->siz = 4;	// TODO: 8 for external symbols
+		if(isextern(s)) {
+			r->siz = 4;
+			r->type = R_ADDR;
+		} else {
+			r->siz = 4;
+			r->type = R_PCREL;
+		}
 		r->off = -1;	// caller must fill in
 		r->sym = s;
 		r->add = v;
 		v = 0;
-		r->type = R_PCREL;
 		if(s->type == STLSBSS) {
 			r->xadd = r->add - r->siz;
 			r->type = R_TLS;
@@ -2327,6 +2347,13 @@ asmandsz(Link *ctxt, Addr *a, int r, int rex, int m64)
 			switch(t) {
 			default:
 				goto bad;
+			case D_EXTERN:
+			case D_STATIC:
+				if(!isextern(a->sym))
+					goto bad;
+				t = D_NONE;
+				v = vaddr(ctxt, a, &rel);
+				break;
 			case D_AUTO:
 			case D_PARAM:
 				t = D_SP;
@@ -2386,7 +2413,7 @@ asmandsz(Link *ctxt, Addr *a, int r, int rex, int m64)
 
 	ctxt->rexflag |= (regrex[t] & Rxb) | rex;
 	if(t == D_NONE || (D_CS <= t && t <= D_GS) || t == D_TLS) {
-		if(t == D_NONE && (a->type == D_STATIC || a->type == D_EXTERN) || ctxt->asmode != 64) {
+		if((a->sym == nil || !isextern(a->sym)) && t == D_NONE && (a->type == D_STATIC || a->type == D_EXTERN) || ctxt->asmode != 64) {
 			*ctxt->andptr++ = (0 << 6) | (5 << 0) | (r << 3);
 			goto putrelv;
 		}