liblink, runtime: preliminary support for plan9/amd64

A TLS slot is reserved by _rt0_.*_plan9 as an automatic and
its address (which is static on Plan 9) is saved in the
global _privates symbol. The startup linkage now is exactly
like that from Plan 9 libc, and the way we access g is
exactly as if we'd have used privalloc(2).

Aside from making the code more standard, this change
drastically simplifies it, both for 386 and for amd64, and
makes the Plan 9 code in liblink common for both 386 and
amd64.

The amd64 runtime code was cleared of nxm assumptions, and
now runs on the standard Plan 9 kernel.

Note handling fixes will follow in a separate CL.

LGTM=rsc
R=golang-codereviews, rsc, bradfitz, dave
CC=0intro, ality, golang-codereviews, jas, minux.ma, mischief
https://golang.org/cl/101510049

5 files changed, 23 insertions, 9 deletions

diff --git a/src/liblink/asm6.c b/src/liblink/asm6.c
index 66afc7a12a..751c972154 100644
--- a/src/liblink/asm6.c
+++ b/src/liblink/asm6.c
@@ -1901,7 +1901,6 @@ prefixof(Link *ctxt, Addr *a)
 		case Hlinux:
 		case Hnetbsd:
 		case Hopenbsd:
-		case Hplan9:
 		case Hsolaris:
 			return 0x64; // FS
 		case Hdarwin:
@@ -3365,6 +3364,19 @@ mfound:
 		default:
 			sysfatal("unknown TLS base location for %s", headstr(ctxt->headtype));
 
+		case Hplan9:
+			if(ctxt->plan9privates == nil)
+				ctxt->plan9privates = linklookup(ctxt, "_privates", 0);
+			memset(&pp.from, 0, sizeof pp.from);
+			pp.from.type = D_EXTERN;
+			pp.from.sym = ctxt->plan9privates;
+			pp.from.offset = 0;
+			pp.from.index = D_NONE;
+			ctxt->rexflag |= Pw;
+			*ctxt->andptr++ = 0x8B;
+			asmand(ctxt, &pp.from, &p->to);
+			break;
+
 		case Hsolaris: // TODO(rsc): Delete Hsolaris from list. Should not use this code. See progedit in obj6.c.
 			// TLS base is 0(FS).
 			pp.from = p->from;
diff --git a/src/liblink/asm8.c b/src/liblink/asm8.c
index 3ab527ce89..62ef41e18a 100644
--- a/src/liblink/asm8.c
+++ b/src/liblink/asm8.c
@@ -2707,11 +2707,11 @@ mfound:
 			break;
 		
 		case Hplan9:
-			if(ctxt->plan9tos == nil)
-				ctxt->plan9tos = linklookup(ctxt, "_tos", 0);
+			if(ctxt->plan9privates == nil)
+				ctxt->plan9privates = linklookup(ctxt, "_privates", 0);
 			memset(&pp.from, 0, sizeof pp.from);
 			pp.from.type = D_EXTERN;
-			pp.from.sym = ctxt->plan9tos;
+			pp.from.sym = ctxt->plan9privates;
 			pp.from.offset = 0;
 			pp.from.index = D_NONE;
 			*ctxt->andptr++ = 0x8B;
diff --git a/src/liblink/obj6.c b/src/liblink/obj6.c
index d83f847851..a8538c9ecf 100644
--- a/src/liblink/obj6.c
+++ b/src/liblink/obj6.c
@@ -103,7 +103,7 @@ static int
 canuselocaltls(Link *ctxt)
 {
 	switch(ctxt->headtype) {
-//	case Hlinux:
+	case Hplan9:
 	case Hwindows:
 		return 0;
 	}
@@ -402,6 +402,10 @@ addstacksplit(Link *ctxt, LSym *cursym)
 		for(i=0; i<nelem(morename); i++)
 			ctxt->symmorestack[i] = linklookup(ctxt, morename[i], 0);
 	}
+
+	if(ctxt->headtype == Hplan9 && ctxt->plan9privates == nil)
+		ctxt->plan9privates = linklookup(ctxt, "_privates", 0);
+
 	ctxt->cursym = cursym;
 
 	if(cursym->text == nil || cursym->text->link == nil)
diff --git a/src/liblink/obj8.c b/src/liblink/obj8.c
index 72934c1499..6f96ebae95 100644
--- a/src/liblink/obj8.c
+++ b/src/liblink/obj8.c
@@ -270,8 +270,8 @@ addstacksplit(Link *ctxt, LSym *cursym)
 		ctxt->symmorestack[1] = linklookup(ctxt, "runtime.morestack_noctxt", 0);
 	}
 
-	if(ctxt->headtype == Hplan9 && ctxt->plan9tos == nil)
-		ctxt->plan9tos = linklookup(ctxt, "_tos", 0);
+	if(ctxt->headtype == Hplan9 && ctxt->plan9privates == nil)
+		ctxt->plan9privates = linklookup(ctxt, "_privates", 0);
 
 	ctxt->cursym = cursym;
 
diff --git a/src/liblink/sym.c b/src/liblink/sym.c
index 2b029ce4dc..c8da39ded1 100644
--- a/src/liblink/sym.c
+++ b/src/liblink/sym.c
@@ -128,8 +128,6 @@ linknew(LinkArch *arch)
 	default:
 		sysfatal("unknown thread-local storage offset for %s", headstr(ctxt->headtype));
 	case Hplan9:
-		ctxt->tlsoffset = -2*ctxt->arch->ptrsize;
-		break;
 	case Hwindows:
 		break;
 	case Hlinux: