liblink: introduce TLS register on 386 and amd64

When I did the original 386 ports on Linux and OS X, I chose to
define GS-relative expressions like 4(GS) as relative to the actual
thread-local storage base, which was usually GS but might not be
(it might be FS, or it might be a different constant offset from GS or FS).

The original scope was limited but since then the rewrites have
gotten out of control. Sometimes GS is rewritten, sometimes FS.
Some ports do other rewrites to enable shared libraries and
other linking. At no point in the code is it clear whether you are
looking at the real GS/FS or some synthesized thing that will be
rewritten. The code manipulating all these is duplicated in many
places.

The first step to fixing issue 7719 is to make the code intelligible
again.

This CL adds an explicit TLS pseudo-register to the 386 and amd64.
As a register, TLS refers to the thread-local storage base, and it
can only be loaded into another register:

        MOVQ TLS, AX

An offset from the thread-local storage base is written off(reg)(TLS*1).
Semantically it is off(reg), but the (TLS*1) annotation marks this as
indexing from the loaded TLS base. This emits a relocation so that
if the linker needs to adjust the offset, it can. For example:

        MOVQ TLS, AX
        MOVQ 8(AX)(TLS*1), CX // load m into CX

On systems that support direct access to the TLS memory, this
pair of instructions can be reduced to a direct TLS memory reference:

        MOVQ 8(TLS), CX // load m into CX

The 2-instruction and 1-instruction forms correspond roughly to
ELF TLS initial exec mode and ELF TLS local exec mode, respectively.

Liblink applies this rewrite on systems that support the 1-instruction form.
The decision is made using only the operating system (and probably
the -shared flag, eventually), not the link mode. If some link modes
on a particular operating system require the 2-instruction form,
then all builds for that operating system will use the 2-instruction
form, so that the link mode decision can be delayed to link time.

Obviously it is late to be making changes like this, but I despair
of correcting issue 7719 and issue 7164 without it. To make sure
I am not changing existing behavior, I built a "hello world" program
for every GOOS/GOARCH combination we have and then worked
to make sure that the rewrite generates exactly the same binaries,
byte for byte. There are a handful of TODOs in the code marking
kludges to get the byte-for-byte property, but at least now I can
explain exactly how each binary is handled.

The targets I tested this way are:

        darwin-386
        darwin-amd64
        dragonfly-386
        dragonfly-amd64
        freebsd-386
        freebsd-amd64
        freebsd-arm
        linux-386
        linux-amd64
        linux-arm
        nacl-386
        nacl-amd64p32
        netbsd-386
        netbsd-amd64
        openbsd-386
        openbsd-amd64
        plan9-386
        plan9-amd64
        solaris-amd64
        windows-386
        windows-amd64

There were four exceptions to the byte-for-byte goal:

windows-386 and windows-amd64 have a time stamp
at bytes 137 and 138 of the header.

darwin-386 and plan9-386 have five or six modified
bytes in the middle of the Go symbol table, caused by
editing comments in runtime/sys_{darwin,plan9}_386.s.

Fixes #7164.

LGTM=iant
R=iant, aram, minux.ma, dave
CC=golang-codereviews
https://golang.org/cl/87920043

include/link.h

@@ -232,6 +232,8 @@ enum
 	R_CONST,
 	R_PCREL,
 	R_TLS,
+	R_TLS_LE, // TLS local exec offset from TLS segment register
+	R_TLS_IE, // TLS initial exec offset from TLS base pointer
 	R_GOTOFF,
 	R_PLT0,
 	R_PLT1,
@@ -340,7 +342,6 @@ struct	Link
 	char*	thestring; // full name of architecture ("arm", "amd64", ..)
 	int32	goarm; // for arm only, GOARM setting
 	int	headtype;
-	int	linkmode;
 
 	LinkArch*	arch;
 	int32	(*ignore)(char*);	// do not emit names satisfying this function

src/cmd/6a/lex.c

@@ -334,6 +334,7 @@ struct
 	"TR5",		LBREG,	D_TR+5,
 	"TR6",		LBREG,	D_TR+6,
 	"TR7",		LBREG,	D_TR+7,
+	"TLS",		LSREG,	D_TLS,
 
 	"AAA",		LTYPE0,	AAAA,
 	"AAD",		LTYPE0,	AAAD,

src/cmd/6c/txt.c

@@ -513,7 +513,7 @@ naddr(Node *n, Addr *a)
 		break;
 
 	case OEXREG:
-		a->type = D_INDIR + D_GS;
+		a->type = D_INDIR + D_TLS;
 		a->offset = n->reg - 1;
 		break;
 

src/cmd/6l/6.out.h

@@ -850,20 +850,18 @@ enum
 	D_DR		= 95,
 	D_TR		= 103,
 
-	D_NONE		= 111,
-
-	D_BRANCH	= 112,
-	D_EXTERN	= 113,
-	D_STATIC	= 114,
-	D_AUTO		= 115,
-	D_PARAM		= 116,
-	D_CONST		= 117,
-	D_FCONST	= 118,
-	D_SCONST	= 119,
-	D_ADDR		= 120,
-
-	D_FILE,
-	D_FILE1,
+	D_TLS		= 111,
+	D_NONE		= 112,
+
+	D_BRANCH	= 113,
+	D_EXTERN	= 114,
+	D_STATIC	= 115,
+	D_AUTO		= 116,
+	D_PARAM		= 117,
+	D_CONST		= 118,
+	D_FCONST	= 119,
+	D_SCONST	= 120,
+	D_ADDR		= 121,
 
 	D_INDIR,	/* additive */
 

src/cmd/6l/obj.c

@@ -79,7 +79,6 @@ archinit(void)
 	case Hsolaris:
 		break;
 	}
-	ctxt->linkmode = linkmode;
 
 	switch(HEADTYPE) {
 	default:

src/cmd/8a/lex.c

@@ -241,6 +241,7 @@ struct
 	"ES",		LSREG,	D_ES,
 	"FS",		LSREG,	D_FS,
 	"GS",		LSREG,	D_GS,
+	"TLS",		LSREG,	D_TLS,
 
 	"GDTR",		LBREG,	D_GDTR,
 	"IDTR",		LBREG,	D_IDTR,

src/cmd/8c/txt.c

@@ -464,7 +464,7 @@ naddr(Node *n, Addr *a)
 		break;
 
 	case OEXREG:
-		a->type = D_INDIR + D_GS;
+		a->type = D_INDIR + D_TLS;
 		a->offset = n->reg - 1;
 		break;
 

src/cmd/8l/8.out.h

@@ -636,21 +636,19 @@ enum
 	D_X5,
 	D_X6,
 	D_X7,
-
-	D_NONE		= 67,
-
-	D_BRANCH	= 68,
-	D_EXTERN	= 69,
-	D_STATIC	= 70,
-	D_AUTO		= 71,
-	D_PARAM		= 72,
-	D_CONST		= 73,
-	D_FCONST	= 74,
-	D_SCONST	= 75,
-	D_ADDR		= 76,
-
-	D_FILE,
-	D_FILE1,
+	
+	D_TLS		= 67,
+	D_NONE		= 68,
+
+	D_BRANCH	= 69,
+	D_EXTERN	= 70,
+	D_STATIC	= 71,
+	D_AUTO		= 72,
+	D_PARAM		= 73,
+	D_CONST		= 74,
+	D_FCONST	= 75,
+	D_SCONST	= 76,
+	D_ADDR		= 77,
 
 	D_INDIR,	/* additive */
 

src/cmd/8l/obj.c

@@ -69,7 +69,6 @@ archinit(void)
 	case Hopenbsd:
 		break;
 	}
-	ctxt->linkmode = linkmode;
 
 	switch(HEADTYPE) {
 	default:

src/cmd/dist/buildruntime.c

@@ -127,99 +127,22 @@ static struct {
 	char *goos;
 	char *hdr;
 } zasmhdr[] = {
-	{"386", "windows",
-		"#define	get_tls(r)	MOVL 0x14(FS), r\n"
-		"#define	g(r)	0(r)\n"
-		"#define	m(r)	4(r)\n"
-	},
-	{"386", "plan9",
-		"// Plan 9 does not have per-process segment descriptors with\n"
-		"// which to do thread-local storage. Instead, we will use a\n"
-		"// fixed offset from the per-process TOS struct address for\n"
-		"// the local storage. Since the process ID is contained in the\n"
-		"// TOS struct, we specify an offset for that here as well.\n"
-		"#define	get_tls(r)	MOVL _tos(SB), r \n"
-		"#define	g(r)	-8(r)\n"
-		"#define	m(r)	-4(r)\n"
-		"#define	procid(r)	48(r)\n"
-	},
-	{"386", "linux",
-		"// On Linux systems, what we call 0(GS) and 4(GS) for g and m\n"
-		"// turn into %gs:-8 and %gs:-4 (using gcc syntax to denote\n"
-		"// what the machine sees as opposed to 8l input).\n"
-		"// 8l rewrites 0(GS) and 4(GS) into these.\n"
-		"//\n"
-		"// On Linux Xen, it is not allowed to use %gs:-8 and %gs:-4\n"
-		"// directly.  Instead, we have to store %gs:0 into a temporary\n"
-		"// register and then use -8(%reg) and -4(%reg).  This kind\n"
-		"// of addressing is correct even when not running Xen.\n"
-		"//\n"
-		"// 8l can rewrite MOVL 0(GS), CX into the appropriate pair\n"
-		"// of mov instructions, using CX as the intermediate register\n"
-		"// (safe because CX is about to be written to anyway).\n"
-		"// But 8l cannot handle other instructions, like storing into 0(GS),\n"
-		"// which is where these macros come into play.\n"
-		"// get_tls sets up the temporary and then g and r use it.\n"
-		"//\n"
-		"// Another wrinkle is that get_tls needs to read from %gs:0,\n"
-		"// but in 8l input it's called 8(GS), because 8l is going to\n"
-		"// subtract 8 from all the offsets, as described above.\n"
-		"//\n"
-		"// The final wrinkle is that when generating an ELF .o file for\n"
-		"// external linking mode, we need to be able to relocate the\n"
-		"// -8(r) and -4(r) instructions. Tag them with an extra (GS*1)\n"
-		"// that is ignored by the linker except for that identification.\n"
-		"#define	get_tls(r)	MOVL 8(GS), r\n"
-		"#define	g(r)	-8(r)(GS*1)\n"
-		"#define	m(r)	-4(r)(GS*1)\n"
-	},
-	{"386", "nacl",
-		// Same as Linux above.
-		"#define	get_tls(r)	MOVL 8(GS), r\n"
-		"#define	g(r)	-8(r)(GS*1)\n"
-		"#define	m(r)	-4(r)(GS*1)\n"
-	},
 	{"386", "",
-		"#define	get_tls(r)\n"
-		"#define	g(r)	0(GS)\n"
-		"#define	m(r)	4(GS)\n"
-	},
-
-	{"amd64p32", "nacl",
-		"#define get_tls(r)\n"
-		"#define g(r) 0(GS)\n"
-		"#define m(r) 4(GS)\n"
-	},
-	{"amd64", "windows",
-		"#define	get_tls(r) MOVQ 0x28(GS), r\n"
-		"#define	g(r) 0(r)\n"
-		"#define	m(r) 8(r)\n"
-	},
-	{"amd64", "plan9",
-		"#define	get_tls(r)\n"
-		"#define	g(r) 0(GS)\n"
-		"#define	m(r) 8(GS)\n"
-		"#define	procid(r) 16(GS)\n"
+		"#define	get_tls(r)	MOVL TLS, r\n"
+		"#define	g(r)	0(r)(TLS*1)\n"
+		"#define	m(r)	4(r)(TLS*1)\n"
 	},
-	{"amd64", "solaris",
-		"#define	get_tls(r) MOVQ 0(FS), r\n"
-		"#define	g(r) -16(r)(FS*1)\n"
-		"#define	m(r) -8(r)(FS*1)\n"
-	},
-	// The TLS accessors here are defined here to use initial exec model.
-	// If the linker is not outputting a shared library, it will reduce
-	// the TLS accessors to the local exec model, effectively removing
-	// get_tls().
-	{"amd64", "linux",
-		"#define	get_tls(r) MOVQ runtime·tlsgm(SB), r\n"
-		"#define	g(r) 0(r)(GS*1)\n"
-		"#define	m(r) 8(r)(GS*1)\n"
+	{"amd64p32", "",
+		"#define	get_tls(r)	MOVL TLS, r\n"
+		"#define	g(r)	0(r)(TLS*1)\n"
+		"#define	m(r)	4(r)(TLS*1)\n"
 	},
 	{"amd64", "",
-		"#define get_tls(r)\n"
-		"#define g(r) 0(GS)\n"
-		"#define m(r) 8(GS)\n"
+		"#define	get_tls(r)	MOVQ TLS, r\n"
+		"#define	g(r)	0(r)(TLS*1)\n"
+		"#define	m(r)	8(r)(TLS*1)\n"
 	},	
+
 	{"arm", "",
 	"#define	LR	R14\n"
 	},

src/cmd/ld/data.c

@@ -183,6 +183,17 @@ relocsym(LSym *s)
 			if(thechar != '6')
 				o = r->add;
 			break;
+		case R_TLS_LE:
+			o = ctxt->tlsoffset + r->add;
+			break;
+		case R_TLS_IE:
+			if(iself || ctxt->headtype == Hplan9)
+				o = ctxt->tlsoffset + r->add;
+			else if(ctxt->headtype == Hwindows)
+				o = r->add;
+			else
+				sysfatal("unexpected R_TLS_IE relocation for %s", headstr(ctxt->headtype));
+			break;
 		case R_ADDR:
 			if(linkmode == LinkExternal && r->sym->type != SCONST) {
 				r->done = 0;
@@ -262,6 +273,10 @@ relocsym(LSym *s)
 		default:
 			ctxt->cursym = s;
 			diag("bad reloc size %#ux for %s", siz, r->sym->name);
+		case 1:
+			// TODO(rsc): Remove.
+			s->p[off] = (int8)o;
+			break;
 		case 4:
 			if(r->type == R_PCREL) {
 				if(o != (int32)o)
@@ -312,6 +327,8 @@ dynrelocsym(LSym *s)
 			return;
 		for(r=s->r; r<s->r+s->nr; r++) {
 			targ = r->sym;
+			if(targ == nil)
+				continue;
 			if(!targ->reachable)
 				diag("internal inconsistency: dynamic symbol %s is not reachable.", targ->name);
 			if(r->sym->plt == -2 && r->sym->got != -2) { // make dynimport JMP table for PE object files.

src/cmd/ld/pobj.c

@@ -143,7 +143,6 @@ main(int argc, char *argv[])
 		headstring = headstr(HEADTYPE);
 
 	archinit();
-	ctxt->linkmode = linkmode;
 	ctxt->debugfloat = debug['F'];
 
 	if(debug['v'])

src/liblink/asm6.c

@@ -114,6 +114,7 @@ enum
 	Ytr0,	Ytr1,	Ytr2,	Ytr3,	Ytr4,	Ytr5,	Ytr6,	Ytr7,	Yrl32,	Yrl64,
 	Ymr, Ymm,
 	Yxr, Yxm,
+	Ytls,
 	Ymax,
 
 	Zxxx		= 0,
@@ -1871,7 +1872,7 @@ instinit(void)
 }
 
 static int
-prefixof(Addr *a)
+prefixof(Link *ctxt, Addr *a)
 {
 	switch(a->type) {
 	case D_INDIR+D_CS:
@@ -1884,6 +1885,27 @@ prefixof(Addr *a)
 		return 0x64;
 	case D_INDIR+D_GS:
 		return 0x65;
+	case D_INDIR+D_TLS:
+		// NOTE: Systems listed here should be only systems that
+		// support direct TLS references like 8(TLS) implemented as
+		// direct references from FS or GS. Systems that require
+		// the initial-exec model, where you load the TLS base into
+		// a register and then index from that register, do not reach
+		// this code and should not be listed.
+		switch(ctxt->headtype) {
+		default:
+			sysfatal("unknown TLS base register for %s", headstr(ctxt->headtype));
+		case Hdragonfly:
+		case Hfreebsd:
+		case Hlinux:
+		case Hnetbsd:
+		case Hopenbsd:
+		case Hplan9:
+		case Hsolaris:
+			return 0x64; // FS
+		case Hdarwin:
+			return 0x65; // GS
+		}
 	}
 	switch(a->index) {
 	case D_CS:
@@ -2033,6 +2055,7 @@ oclass(Link *ctxt, Addr *a)
 	case D_ES:	return	Yes;
 	case D_FS:	return	Yfs;
 	case D_GS:	return	Ygs;
+	case D_TLS:	return	Ytls;
 
 	case D_GDTR:	return	Ygdtr;
 	case D_IDTR:	return	Yidtr;
@@ -2278,6 +2301,19 @@ vaddr(Link *ctxt, Addr *a, Reloc *r)
 				r->type = R_PCREL;
 		} else
 			r->type = R_ADDR;
+		break;
+	
+	case D_INDIR+D_TLS:
+		if(r == nil) {
+			ctxt->diag("need reloc for %D", a);
+			sysfatal("reloc");
+		}
+		r->type = R_TLS_LE;
+		r->siz = 4;
+		r->off = -1;	// caller must fill in
+		r->add = v;
+		v = 0;
+		break;
 	}
 	return v;
 }
@@ -2294,7 +2330,7 @@ asmandsz(Link *ctxt, Addr *a, int r, int rex, int m64)
 	v = a->offset;
 	t = a->type;
 	rel.siz = 0;
-	if(a->index != D_NONE && a->index != D_FS && a->index != D_GS) {
+	if(a->index != D_NONE && a->index != D_TLS) {
 		if(t < D_INDIR) { 
 			switch(t) {
 			default:
@@ -2360,9 +2396,11 @@ asmandsz(Link *ctxt, Addr *a, int r, int rex, int m64)
 		scale = 1;
 	} else
 		t -= D_INDIR;
+	if(t == D_TLS)
+		v = vaddr(ctxt, a, &rel);
 
 	ctxt->rexflag |= (regrex[t] & Rxb) | rex;
-	if(t == D_NONE || (D_CS <= t && t <= D_GS)) {
+	if(t == D_NONE || (D_CS <= t && t <= D_GS) || t == D_TLS) {
 		if((ctxt->flag_shared || ctxt->headtype == Hnacl) && t == D_NONE && (a->type == D_STATIC || a->type == D_EXTERN) || ctxt->asmode != 64) {
 			*ctxt->andptr++ = (0 << 6) | (5 << 0) | (r << 3);
 			goto putrelv;
@@ -2389,17 +2427,38 @@ asmandsz(Link *ctxt, Addr *a, int r, int rex, int m64)
 		goto putrelv;
 	}
 	if(t >= D_AX && t <= D_R15) {
-		if(v == 0 && t != D_BP && t != D_R13) {
+		// TODO: Remove Hwindows condition.
+		if(v == 0 && t != D_BP && t != D_R13 && (a->index != D_TLS || (ctxt->headtype == Hwindows && a->scale == 2))) {
 			*ctxt->andptr++ = (0 << 6) | (reg[t] << 0) | (r << 3);
 			return;
 		}
-		if(v >= -128 && v < 128) {
+		if(v >= -128 && v < 128 && (a->index != D_TLS || a->scale != 1)) {
 			ctxt->andptr[0] = (1 << 6) | (reg[t] << 0) | (r << 3);
+			if(a->index == D_TLS) {
+				Reloc *r;
+				memset(&rel, 0, sizeof rel);
+				rel.type = R_TLS_IE;
+				rel.siz = 1;
+				rel.sym = nil;
+				rel.add = v;
+				r = addrel(ctxt->cursym);
+				*r = rel;
+				r->off = ctxt->curp->pc + ctxt->andptr + 1 - ctxt->and;
+				v = 0;
+			}
 			ctxt->andptr[1] = v;
 			ctxt->andptr += 2;
 			return;
 		}
 		*ctxt->andptr++ = (2 << 6) | (reg[t] << 0) | (r << 3);
+		if(a->index == D_TLS) {
+			memset(&rel, 0, sizeof rel);
+			rel.type = R_TLS_IE;
+			rel.siz = 4;
+			rel.sym = nil;
+			rel.add = v;
+			v = 0;
+		}
 		goto putrelv;
 	}
 	goto bad;
@@ -2574,6 +2633,10 @@ static Movtab	ymovtab[] =
 	{ASHRQ,	Ycol,	Yml,	6,	Pw,0xac,0xad,0},
 	{ASHLW,	Ycol,	Yml,	6,	Pe,0xa4,0xa5,0},
 	{ASHRW,	Ycol,	Yml,	6,	Pe,0xac,0xad,0},
+
+/* load TLS base */
+	{AMOVQ,	Ytls,	Yrl,	7,	0,0,0,0},
+
 	0
 };
 
@@ -2664,10 +2727,10 @@ doasm(Link *ctxt, Prog *p)
 		return;
 	}
 	
-	pre = prefixof(&p->from);
+	pre = prefixof(ctxt, &p->from);
 	if(pre)
 		*ctxt->andptr++ = pre;
-	pre = prefixof(&p->to);
+	pre = prefixof(ctxt, &p->to);
 	if(pre)
 		*ctxt->andptr++ = pre;
 
@@ -3296,6 +3359,43 @@ mfound:
 			break;
 		}
 		break;
+	
+	case 7:	/* mov tls, r */
+		// NOTE: The systems listed here are the ones that use the "TLS initial exec" model,
+		// where you load the TLS base register into a register and then index off that
+		// register to access the actual TLS variables. Systems that allow direct TLS access
+		// are handled in prefixof above and should not be listed here.
+		switch(ctxt->headtype) {
+		default:
+			sysfatal("unknown TLS base location for %s", headstr(ctxt->headtype));
+
+		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;
+			pp.from.type = D_INDIR+D_NONE;
+			pp.from.offset = 0;
+			pp.from.index = D_NONE;
+			pp.from.scale = 0;
+			ctxt->rexflag |= Pw;
+			*ctxt->andptr++ = 0x64; // FS
+			*ctxt->andptr++ = 0x8B;
+			asmand(ctxt, &pp.from, &p->to);
+			break;
+		
+		case Hwindows:
+			// Windows TLS base is always 0x28(GS).
+			pp.from = p->from;
+			pp.from.type = D_INDIR+D_GS;
+			pp.from.offset = 0x28;
+			pp.from.index = D_NONE;
+			pp.from.scale = 0;
+			ctxt->rexflag |= Pw;
+			*ctxt->andptr++ = 0x65; // GS
+			*ctxt->andptr++ = 0x8B;
+			asmand(ctxt, &pp.from, &p->to);
+			break;
+		}
+		break;
 	}
 }
 

src/liblink/asm8.c

@@ -78,6 +78,7 @@ enum
 	Ym,
 	Ybr,
 	Ycol,
+	Ytls,
 
 	Ycs,	Yss,	Yds,	Yes,	Yfs,	Ygs,
 	Ygdtr,	Yidtr,	Yldtr,	Ymsw,	Ytask,
@@ -1441,7 +1442,7 @@ instinit(void)
 }
 
 static int
-prefixof(Addr *a)
+prefixof(Link *ctxt, Addr *a)
 {
 	switch(a->type) {
 	case D_INDIR+D_CS:
@@ -1454,6 +1455,23 @@ prefixof(Addr *a)
 		return 0x64;
 	case D_INDIR+D_GS:
 		return 0x65;
+	case D_INDIR+D_TLS:
+		// NOTE: Systems listed here should be only systems that
+		// support direct TLS references like 8(TLS) implemented as
+		// direct references from FS or GS. Systems that require
+		// the initial-exec model, where you load the TLS base into
+		// a register and then index from that register, do not reach
+		// this code and should not be listed.
+		switch(ctxt->headtype) {
+		default:
+			sysfatal("unknown TLS base register for %s", headstr(ctxt->headtype));
+		case Hdarwin:
+		case Hdragonfly:
+		case Hfreebsd:
+		case Hnetbsd:
+		case Hopenbsd:
+			return 0x65; // GS
+		}
 	}
 	return 0;
 }
@@ -1543,6 +1561,7 @@ oclass(Addr *a)
 	case D_ES:	return	Yes;
 	case D_FS:	return	Yfs;
 	case D_GS:	return	Ygs;
+	case D_TLS:	return	Ytls;
 
 	case D_GDTR:	return	Ygdtr;
 	case D_IDTR:	return	Yidtr;
@@ -1724,6 +1743,19 @@ vaddr(Link *ctxt, Addr *a, Reloc *r)
 			r->add = v;
 			v = 0;
 		}
+		break;
+	
+	case D_INDIR+D_TLS:
+		if(r == nil) {
+			ctxt->diag("need reloc for %D", a);
+			sysfatal("bad code");
+		}
+		r->type = R_TLS_LE;
+		r->siz = 4;
+		r->off = -1; // caller must fill in
+		r->add = v;
+		v = 0;
+		break;
 	}
 	return v;
 }
@@ -1738,7 +1770,7 @@ asmand(Link *ctxt, Addr *a, int r)
 	v = a->offset;
 	t = a->type;
 	rel.siz = 0;
-	if(a->index != D_NONE && a->index != D_FS && a->index != D_GS) {
+	if(a->index != D_NONE && a->index != D_TLS) {
 		if(t < D_INDIR || t >= 2*D_INDIR) {
 			switch(t) {
 			default:
@@ -1801,8 +1833,10 @@ asmand(Link *ctxt, Addr *a, int r)
 		scale = 1;
 	} else
 		t -= D_INDIR;
+	if(t == D_TLS)
+		v = vaddr(ctxt, a, &rel);
 
-	if(t == D_NONE || (D_CS <= t && t <= D_GS)) {
+	if(t == D_NONE || (D_CS <= t && t <= D_GS) || t == D_TLS) {
 		*ctxt->andptr++ = (0 << 6) | (5 << 0) | (r << 3);
 		goto putrelv;
 	}
@@ -1823,17 +1857,43 @@ asmand(Link *ctxt, Addr *a, int r)
 		goto putrelv;
 	}
 	if(t >= D_AX && t <= D_DI) {
-		if(v == 0 && rel.siz == 0 && t != D_BP) {
+		// TODO(rsc): Remove the Hwindows test.
+		// As written it produces the same byte-identical output as the code it replaced.
+		if(v == 0 && rel.siz == 0 && t != D_BP && (a->index != D_TLS || ctxt->headtype == Hwindows)) {
 			*ctxt->andptr++ = (0 << 6) | (reg[t] << 0) | (r << 3);
 			return;
 		}
-		if(v >= -128 && v < 128 && rel.siz == 0 && a->index != D_FS && a->index != D_GS) {
+		// TODO(rsc): Change a->index tests to check D_TLS.
+		// Then remove the if statement inside the body.
+		// As written the code is clearly incorrect for external linking,
+		// but as written it produces the same byte-identical output as the code it replaced.
+		if(v >= -128 && v < 128 && rel.siz == 0 && (a->index != D_TLS || ctxt->headtype == Hwindows || a->scale != 1))  {
 			ctxt->andptr[0] = (1 << 6) | (reg[t] << 0) | (r << 3);
+			if(a->index == D_TLS) {
+				Reloc *r;
+				memset(&rel, 0, sizeof rel);
+				rel.type = R_TLS_IE;
+				rel.siz = 1;
+				rel.sym = nil;
+				rel.add = v;
+				r = addrel(ctxt->cursym);
+				*r = rel;
+				r->off = ctxt->curp->pc + ctxt->andptr + 1 - ctxt->and;
+				v = 0;
+			}
 			ctxt->andptr[1] = v;
 			ctxt->andptr += 2;
 			return;
 		}
 		*ctxt->andptr++ = (2 << 6) | (reg[t] << 0) | (r << 3);
+		if(a->index == D_TLS) {
+			memset(&rel, 0, sizeof rel);
+			rel.type = R_TLS_IE;
+			rel.siz = 4;
+			rel.sym = nil;
+			rel.add = v;
+			v = 0;
+		}
 		goto putrelv;
 	}
 	goto bad;
@@ -1961,6 +2021,10 @@ static uchar	ymovtab[] =
 /* extra imul */
 	AIMULW,	Yml,	Yrl,	7,	Pq,0xaf,0,0,
 	AIMULL,	Yml,	Yrl,	7,	Pm,0xaf,0,0,
+
+/* load TLS base pointer */
+	AMOVL,	Ytls,	Yrl,	8,	0,0,0,0,
+
 	0
 };
 
@@ -2108,10 +2172,10 @@ doasm(Link *ctxt, Prog *p)
 	
 	ctxt->curp = p;	// TODO
 
-	pre = prefixof(&p->from);
+	pre = prefixof(ctxt, &p->from);
 	if(pre)
 		*ctxt->andptr++ = pre;
-	pre = prefixof(&p->to);
+	pre = prefixof(ctxt, &p->to);
 	if(pre)
 		*ctxt->andptr++ = pre;
 
@@ -2628,6 +2692,54 @@ mfound:
 		*ctxt->andptr++ = t[5];
 		asmand(ctxt, &p->from, reg[p->to.type]);
 		break;
+	
+	case 8: /* mov tls, r */
+		// NOTE: The systems listed here are the ones that use the "TLS initial exec" model,
+		// where you load the TLS base register into a register and then index off that
+		// register to access the actual TLS variables. Systems that allow direct TLS access
+		// are handled in prefixof above and should not be listed here.
+		switch(ctxt->headtype) {
+		default:
+			sysfatal("unknown TLS base location for %s", headstr(ctxt->headtype));
+
+		case Hlinux:
+		case Hnacl:
+			// ELF TLS base is 0(GS).
+			pp.from = p->from;
+			pp.from.type = D_INDIR+D_GS;
+			pp.from.offset = 0;
+			pp.from.index = D_NONE;
+			pp.from.scale = 0;
+			*ctxt->andptr++ = 0x65; // GS
+			*ctxt->andptr++ = 0x8B;
+			asmand(ctxt, &pp.from, reg[p->to.type]);
+			break;
+		
+		case Hplan9:
+			if(ctxt->plan9tos == nil)
+				ctxt->plan9tos = linklookup(ctxt, "_tos", 0);
+			memset(&pp.from, 0, sizeof pp.from);
+			pp.from.type = D_EXTERN;
+			pp.from.sym = ctxt->plan9tos;
+			pp.from.offset = 0;
+			pp.from.index = D_NONE;
+			*ctxt->andptr++ = 0x8B;
+			asmand(ctxt, &pp.from, reg[p->to.type]);
+			break;
+
+		case Hwindows:
+			// Windows TLS base is always 0x14(FS).
+			pp.from = p->from;
+			pp.from.type = D_INDIR+D_FS;
+			pp.from.offset = 0x14;
+			pp.from.index = D_NONE;
+			pp.from.scale = 0;
+			*ctxt->andptr++ = 0x64; // FS
+			*ctxt->andptr++ = 0x8B;
+			asmand(ctxt, &pp.from, reg[p->to.type]);
+			break;
+		}
+		break;
 	}
 }
 

src/liblink/list6.c

@@ -341,6 +341,7 @@ char*	regstr[] =
 	"TR6",
 	"TR7",
 
+	"TLS",	/* [D_TLS] */
 	"NONE",	/* [D_NONE] */
 };
 

src/liblink/list8.c

@@ -289,6 +289,7 @@ char*	regstr[] =
 	"X6",
 	"X7",
 
+	"TLS",	/* [D_TLS] */
 	"NONE",	/* [D_NONE] */
 };
 

src/liblink/obj6.c

@@ -99,6 +99,17 @@ settextflag(Prog *p, int f)
 
 static void nacladdr(Link*, Prog*, Addr*);
 
+static int
+canuselocaltls(Link *ctxt)
+{
+	switch(ctxt->headtype) {
+//	case Hlinux:
+	case Hwindows:
+		return 0;
+	}
+	return 1;
+}
+
 static void
 progedit(Link *ctxt, Prog *p)
 {
@@ -106,105 +117,98 @@ progedit(Link *ctxt, Prog *p)
 	LSym *s;
 	Prog *q;
 
-	if(ctxt->headtype == Hnacl) {
-		nacladdr(ctxt, p, &p->from);
-		nacladdr(ctxt, p, &p->to);
-	}
-
-	if(p->from.type == D_INDIR+D_GS || p->from.index == D_GS)
-		p->from.offset += ctxt->tlsoffset;
-	if(p->to.type == D_INDIR+D_GS || p->to.index == D_GS)
-		p->to.offset += ctxt->tlsoffset;
-
-	if(ctxt->gmsym == nil)
-		ctxt->gmsym = linklookup(ctxt, "runtime.tlsgm", 0);
-
-	if(ctxt->headtype == Hwindows) { 
-		// Windows
-		// Convert
-		//   op	  n(GS), reg
-		// to
-		//   MOVL 0x28(GS), reg
-		//   op	  n(reg), reg
-		// The purpose of this patch is to fix some accesses
-		// to extern register variables (TLS) on Windows, as
-		// a different method is used to access them.
-		if(p->from.type == D_INDIR+D_GS
-		&& p->to.type >= D_AX && p->to.type <= D_DI 
-		&& p->from.offset <= 8) {
-			q = appendp(ctxt, p);
-			q->from = p->from;
-			q->from.type = D_INDIR + p->to.type;
-			q->to = p->to;
-			q->as = p->as;
-			p->as = AMOVQ;
-			p->from.type = D_INDIR+D_GS;
-			p->from.offset = 0x28;
-		}
-	}
-	if(ctxt->headtype == Hlinux || ctxt->headtype == Hfreebsd
-	|| ctxt->headtype == Hopenbsd || ctxt->headtype == Hnetbsd
-	|| ctxt->headtype == Hplan9 || ctxt->headtype == Hdragonfly
-	|| ctxt->headtype == Hsolaris) {
-		// ELF uses FS instead of GS.
-		if(p->from.type == D_INDIR+D_GS)
-			p->from.type = D_INDIR+D_FS;
-		if(p->to.type == D_INDIR+D_GS)
-			p->to.type = D_INDIR+D_FS;
-		if(p->from.index == D_GS)
-			p->from.index = D_FS;
-		if(p->to.index == D_GS)
-			p->to.index = D_FS;
-	}
-	if(!ctxt->flag_shared) {
-		// Convert g() or m() accesses of the form
-		//   op n(reg)(GS*1), reg
-		// to
-		//   op n(GS*1), reg
-		if(p->from.index == D_FS || p->from.index == D_GS) {
-			p->from.type = D_INDIR + p->from.index;
+	// Thread-local storage references use the TLS pseudo-register.
+	// As a register, TLS refers to the thread-local storage base, and it
+	// can only be loaded into another register:
+	//
+	//         MOVQ TLS, AX
+	//
+	// An offset from the thread-local storage base is written off(reg)(TLS*1).
+	// Semantically it is off(reg), but the (TLS*1) annotation marks this as
+	// indexing from the loaded TLS base. This emits a relocation so that
+	// if the linker needs to adjust the offset, it can. For example:
+	//
+	//         MOVQ TLS, AX
+	//         MOVQ 8(AX)(TLS*1), CX // load m into CX
+	// 
+	// On systems that support direct access to the TLS memory, this
+	// pair of instructions can be reduced to a direct TLS memory reference:
+	// 
+	//         MOVQ 8(TLS), CX // load m into CX
+	//
+	// The 2-instruction and 1-instruction forms correspond roughly to
+	// ELF TLS initial exec mode and ELF TLS local exec mode, respectively.
+	// 
+	// We applies this rewrite on systems that support the 1-instruction form.
+	// The decision is made using only the operating system (and probably
+	// the -shared flag, eventually), not the link mode. If some link modes
+	// on a particular operating system require the 2-instruction form,
+	// then all builds for that operating system will use the 2-instruction
+	// form, so that the link mode decision can be delayed to link time.
+	//
+	// In this way, all supported systems use identical instructions to
+	// access TLS, and they are rewritten appropriately first here in
+	// liblink and then finally using relocations in the linker.
+
+	if(canuselocaltls(ctxt)) {
+		// Reduce TLS initial exec model to TLS local exec model.
+		// Sequences like
+		//	MOVQ TLS, BX
+		//	... off(BX)(TLS*1) ...
+		// become
+		//	NOP
+		//	... off(TLS) ...
+		//
+		// TODO(rsc): Remove the Hsolaris special case. It exists only to
+		// guarantee we are producing byte-identical binaries as before this code.
+		// But it should be unnecessary.
+		if((p->as == AMOVQ || p->as == AMOVL) && p->from.type == D_TLS && D_AX <= p->to.type && p->to.type <= D_R15 && ctxt->headtype != Hsolaris)
+			nopout(p);
+		if(p->from.index == D_TLS && D_INDIR+D_AX <= p->from.type && p->from.type <= D_INDIR+D_R15) {
+			p->from.type = D_INDIR+D_TLS;
+			p->from.scale = 0;
 			p->from.index = D_NONE;
 		}
-		// Convert g() or m() accesses of the form
-		//   op reg, n(reg)(GS*1)
-		// to
-		//   op reg, n(GS*1)
-		if(p->to.index == D_FS || p->to.index == D_GS) {
-			p->to.type = D_INDIR + p->to.index;
+		if(p->to.index == D_TLS && D_INDIR+D_AX <= p->to.type && p->to.type <= D_INDIR+D_R15) {
+			p->to.type = D_INDIR+D_TLS;
+			p->to.scale = 0;
 			p->to.index = D_NONE;
 		}
-		// Convert get_tls access of the form
-		//   op runtime.tlsgm(SB), reg
-		// to
-		//   NOP
-		if(ctxt->gmsym != nil && p->from.sym == ctxt->gmsym) {
-			p->as = ANOP;
-			p->from.type = D_NONE;
-			p->to.type = D_NONE;
-			p->from.sym = nil;
-			p->to.sym = nil;
-		}
 	} else {
-		// Convert TLS reads of the form
-		//   op n(GS), reg
-		// to
-		//   MOVQ $runtime.tlsgm(SB), reg
-		//   op n(reg)(GS*1), reg
-		if((p->from.type == D_INDIR+D_FS || p->from.type == D_INDIR + D_GS) && p->to.type >= D_AX && p->to.type <= D_DI) {
+		// As a courtesy to the C compilers, rewrite TLS local exec load as TLS initial exec load.
+		// The instruction
+		//	MOVQ off(TLS), BX
+		// becomes the sequence
+		//	MOVQ TLS, BX
+		//	MOVQ off(BX)(TLS*1), BX
+		// This allows the C compilers to emit references to m and g using the direct off(TLS) form.
+		if((p->as == AMOVQ || p->as == AMOVL) && p->from.type == D_INDIR+D_TLS && D_AX <= p->to.type && p->to.type <= D_R15) {
 			q = appendp(ctxt, p);
-			q->to = p->to;
 			q->as = p->as;
-			q->from.type = D_INDIR+p->to.type;
-			q->from.index = p->from.type - D_INDIR;
-			q->from.scale = 1;
-			q->from.offset = p->from.offset;
-			p->as = AMOVQ;
-			p->from.type = D_EXTERN;
-			p->from.sym = ctxt->gmsym;
+			q->from = p->from;
+			q->from.type = D_INDIR + p->to.type;
+			q->from.index = D_TLS;
+			q->from.scale = 2; // TODO: use 1
+			q->to = p->to;
+			p->from.type = D_TLS;
+			p->from.index = D_NONE;
 			p->from.offset = 0;
 		}
 	}
 
+	// TODO: Remove.
+	if(ctxt->headtype == Hwindows || ctxt->headtype == Hplan9) {
+		if(p->from.scale == 1 && p->from.index == D_TLS)
+			p->from.scale = 2;
+		if(p->to.scale == 1 && p->to.index == D_TLS)
+			p->to.scale = 2;
+	}
+
+	if(ctxt->headtype == Hnacl) {
+		nacladdr(ctxt, p, &p->from);
+		nacladdr(ctxt, p, &p->to);
+	}
+
 	// Maintain information about code generation mode.
 	if(ctxt->mode == 0)
 		ctxt->mode = 64;
@@ -315,9 +319,9 @@ nacladdr(Link *ctxt, Prog *p, Addr *a)
 		ctxt->diag("invalid address: %P", p);
 		return;
 	}
-	if(a->type == D_INDIR+D_GS)
+	if(a->type == D_INDIR+D_TLS)
 		a->type = D_INDIR+D_BP;
-	else if(a->type == D_GS)
+	else if(a->type == D_TLS)
 		a->type = D_BP;
 	if(D_INDIR <= a->type && a->type <= D_INDIR+D_INDIR) {
 		switch(a->type) {
@@ -632,48 +636,24 @@ indir_cx(Link *ctxt, Addr *a)
 // Returns last new instruction.
 static Prog*
 load_g_cx(Link *ctxt, Prog *p)
-{
-	if(ctxt->flag_shared) {
-		// Load TLS offset with MOVQ $runtime.tlsgm(SB), CX
-		p->as = AMOVQ;
-		p->from.type = D_EXTERN;
-		p->from.sym = ctxt->gmsym;
-		p->to.type = D_CX;
-		p = appendp(ctxt, p);
-	}
+{	
+	Prog *next;
+
 	p->as = AMOVQ;
-	if(ctxt->headtype == Hlinux || ctxt->headtype == Hfreebsd
-	|| ctxt->headtype == Hopenbsd || ctxt->headtype == Hnetbsd
-	|| ctxt->headtype == Hplan9 || ctxt->headtype == Hdragonfly
-	|| ctxt->headtype == Hsolaris)
-		// ELF uses FS
-		p->from.type = D_INDIR+D_FS;
-	else if(ctxt->headtype == Hnacl) {
+	if(ctxt->arch->ptrsize == 4)
 		p->as = AMOVL;
-		p->from.type = D_INDIR+D_BP;
-	} else
-		p->from.type = D_INDIR+D_GS;
-	if(ctxt->flag_shared) {
-		// Add TLS offset stored in CX
-		p->from.index = p->from.type - D_INDIR;
-		indir_cx(ctxt, &p->from);
-	}
-	p->from.offset = ctxt->tlsoffset+0;
+	p->from.type = D_INDIR+D_TLS;
+	p->from.offset = 0;
 	p->to.type = D_CX;
-	if(ctxt->headtype == Hwindows) {
-		// movq %gs:0x28, %rcx
-		// movq (%rcx), %rcx
-		p->as = AMOVQ;
-		p->from.type = D_INDIR+D_GS;
-		p->from.offset = 0x28;
-		p->to.type = D_CX;
+	
+	next = p->link;
+	progedit(ctxt, p);
+	while(p->link != next)
+		p = p->link;
+	
+	if(p->from.index == D_TLS)
+		p->from.scale = 2;
 
-		p = appendp(ctxt, p);
-		p->as = AMOVQ;
-		indir_cx(ctxt, &p->from);
-		p->from.offset = 0;
-		p->to.type = D_CX;
-	}
 	return p;
 }
 

src/liblink/obj8.c

@@ -91,80 +91,80 @@ settextflag(Prog *p, int f)
 	p->from.scale = f;
 }
 
+static int
+canuselocaltls(Link *ctxt)
+{
+	switch(ctxt->headtype) {
+	case Hlinux:
+	case Hnacl:
+	case Hplan9:
+	case Hwindows:
+		return 0;
+	}
+	return 1;
+}
+
 static void
 progedit(Link *ctxt, Prog *p)
 {
-	Prog *q;
 	char literal[64];
 	LSym *s;
-
-	if(p->from.type == D_INDIR+D_GS)
-		p->from.offset += ctxt->tlsoffset;
-	if(p->to.type == D_INDIR+D_GS)
-		p->to.offset += ctxt->tlsoffset;
-
-	if(ctxt->headtype == Hwindows) {
-		// Convert
-		//   op	  n(GS), reg
-		// to
-		//   MOVL 0x14(FS), reg
-		//   op	  n(reg), reg
-		// The purpose of this patch is to fix some accesses
-		// to extern register variables (TLS) on Windows, as
-		// a different method is used to access them.
-		if(p->from.type == D_INDIR+D_GS
-		&& p->to.type >= D_AX && p->to.type <= D_DI) {
-			q = appendp(ctxt, p);
-			q->from = p->from;
-			q->from.type = D_INDIR + p->to.type;
-			q->to = p->to;
-			q->as = p->as;
-			p->as = AMOVL;
-			p->from.type = D_INDIR+D_FS;
-			p->from.offset = 0x14;
+	Prog *q;
+	
+	// See obj6.c for discussion of TLS.
+	if(canuselocaltls(ctxt)) {
+		// Reduce TLS initial exec model to TLS local exec model.
+		// Sequences like
+		//	MOVL TLS, BX
+		//	... off(BX)(TLS*1) ...
+		// become
+		//	NOP
+		//	... off(TLS) ...
+		if(p->as == AMOVL && p->from.type == D_TLS && D_AX <= p->to.type && p->to.type <= D_DI) {
+			p->as = ANOP;
+			p->from.type = D_NONE;
+			p->to.type = D_NONE;
 		}
-	}
-	if(ctxt->headtype == Hlinux || ctxt->headtype == Hnacl) {
-		// Running binaries under Xen requires using
-		//	MOVL 0(GS), reg
-		// and then off(reg) instead of saying off(GS) directly
-		// when the offset is negative.
-		// In external mode we just produce a reloc.
-		if(p->from.type == D_INDIR+D_GS && p->from.offset < 0
-		&& p->to.type >= D_AX && p->to.type <= D_DI) {
-			if(ctxt->linkmode != LinkExternal) {
-				q = appendp(ctxt, p);
-				q->from = p->from;
-				q->from.type = D_INDIR + p->to.type;
-				q->to = p->to;
-				q->as = p->as;
-				p->as = AMOVL;
-				p->from.type = D_INDIR+D_GS;
-				p->from.offset = 0;
-			} else {
-				// Add signals to relocate.
-				p->from.index = D_GS;
-				p->from.scale = 1;
-			}
+		if(p->from.index == D_TLS && D_INDIR+D_AX <= p->from.type && p->from.type <= D_INDIR+D_DI) {
+			p->from.type = D_INDIR+D_TLS;
+			p->from.scale = 0;
+			p->from.index = D_NONE;
 		}
-	}
-	if(ctxt->headtype == Hplan9) {
-		if(p->from.type == D_INDIR+D_GS
-		&& p->to.type >= D_AX && p->to.type <= D_DI) {
-			if(ctxt->plan9tos == nil)
-				ctxt->plan9tos = linklookup(ctxt, "_tos", 0);
+		if(p->to.index == D_TLS && D_INDIR+D_AX <= p->to.type && p->to.type <= D_INDIR+D_DI) {
+			p->to.type = D_INDIR+D_TLS;
+			p->to.scale = 0;
+			p->to.index = D_NONE;
+		}
+	} else {
+		// As a courtesy to the C compilers, rewrite TLS local exec load as TLS initial exec load.
+		// The instruction
+		//	MOVL off(TLS), BX
+		// becomes the sequence
+		//	MOVL TLS, BX
+		//	MOVL off(BX)(TLS*1), BX
+		// This allows the C compilers to emit references to m and g using the direct off(TLS) form.
+		if(p->as == AMOVL && p->from.type == D_INDIR+D_TLS && D_AX <= p->to.type && p->to.type <= D_DI) {
 			q = appendp(ctxt, p);
+			q->as = p->as;
 			q->from = p->from;
 			q->from.type = D_INDIR + p->to.type;
+			q->from.index = D_TLS;
+			q->from.scale = 2; // TODO: use 1
 			q->to = p->to;
-			q->as = p->as;
-			p->as = AMOVL;
-			p->from.type = D_EXTERN;
-			p->from.sym = ctxt->plan9tos;
+			p->from.type = D_TLS;
+			p->from.index = D_NONE;
 			p->from.offset = 0;
 		}
 	}
 
+	// TODO: Remove.
+	if(ctxt->headtype == Hplan9) {
+		if(p->from.scale == 1 && p->from.index == D_TLS)
+			p->from.scale = 2;
+		if(p->to.scale == 1 && p->to.index == D_TLS)
+			p->to.scale = 2;
+	}
+
 	// Rewrite CALL/JMP/RET to symbol as D_BRANCH.
 	switch(p->as) {
 	case ACALL:
@@ -435,62 +435,21 @@ addstacksplit(Link *ctxt, LSym *cursym)
 static Prog*
 load_g_cx(Link *ctxt, Prog *p)
 {
-	switch(ctxt->headtype) {
-	case Hwindows:
-		p->as = AMOVL;
-		p->from.type = D_INDIR+D_FS;
-		p->from.offset = 0x14;
-		p->to.type = D_CX;
+	Prog *next;
 
-		p = appendp(ctxt, p);
-		p->as = AMOVL;
-		p->from.type = D_INDIR+D_CX;
-		p->from.offset = 0;
-		p->to.type = D_CX;
-		break;
+	p->as = AMOVL;
+	p->from.type = D_INDIR+D_TLS;
+	p->from.offset = 0;
+	p->to.type = D_CX;
+
+	next = p->link;
+	progedit(ctxt, p);
+	while(p->link != next)
+		p = p->link;
 	
-	case Hlinux:
-	case Hnacl:
-		if(ctxt->linkmode != LinkExternal) {
-			p->as = AMOVL;
-			p->from.type = D_INDIR+D_GS;
-			p->from.offset = 0;
-			p->to.type = D_CX;
+	if(p->from.index == D_TLS)
+		p->from.scale = 2;
 
-			p = appendp(ctxt, p);
-			p->as = AMOVL;
-			p->from.type = D_INDIR+D_CX;
-			p->from.offset = ctxt->tlsoffset + 0;
-			p->to.type = D_CX;
-		} else {
-			p->as = AMOVL;
-			p->from.type = D_INDIR+D_GS;
-			p->from.offset = ctxt->tlsoffset + 0;
-			p->to.type = D_CX;
-			p->from.index = D_GS;
-			p->from.scale = 1;
-		}
-		break;
-	
-	case Hplan9:
-		p->as = AMOVL;
-		p->from.type = D_EXTERN;
-		p->from.sym = ctxt->plan9tos;
-		p->to.type = D_CX;
-		
-		p = appendp(ctxt, p);
-		p->as = AMOVL;
-		p->from.type = D_INDIR+D_CX;
-		p->from.offset = ctxt->tlsoffset + 0;
-		p->to.type = D_CX;				
-		break;
-	
-	default:
-		p->as = AMOVL;
-		p->from.type = D_INDIR+D_GS;
-		p->from.offset = ctxt->tlsoffset + 0;
-		p->to.type = D_CX;
-	}
 	return p;
 }
 

src/liblink/objfile.c

@@ -274,6 +274,7 @@ writesym(Link *ctxt, Biobuf *b, LSym *s)
 	Pcln *pc;
 	Prog *p;
 	Auto *a;
+	char *name;
 
 	if(ctxt->debugasm) {
 		Bprint(ctxt->bso, "%s ", s->name);
@@ -308,7 +309,10 @@ writesym(Link *ctxt, Biobuf *b, LSym *s)
 		}
 		for(i=0; i<s->nr; i++) {
 			r = &s->r[i];
-			Bprint(ctxt->bso, "\trel %d+%d t=%d %s+%lld\n", (int)r->off, r->siz, r->type, r->sym->name, (vlong)r->add);
+			name = "";
+			if(r->sym != nil)
+				name = r->sym->name;
+			Bprint(ctxt->bso, "\trel %d+%d t=%d %s+%lld\n", (int)r->off, r->siz, r->type, name, (vlong)r->add);
 		}
 	}
 

src/liblink/sym.c

@@ -118,6 +118,7 @@ linknew(LinkArch *arch)
 		sysfatal("unknown goos %s", getgoos());
 	
 	// Record thread-local storage offset.
+	// TODO(rsc): Move tlsoffset back into the linker.
 	switch(ctxt->headtype) {
 	default:
 		sysfatal("unknown thread-local storage offset for %s", headstr(ctxt->headtype));

src/pkg/runtime/runtime.h

@@ -99,10 +99,10 @@ typedef	struct	DebugVars	DebugVars;
  *
  * "extern register" is a special storage class implemented by 6c, 8c, etc.
  * On the ARM, it is an actual register; elsewhere it is a slot in thread-
- * local storage indexed by a segment register. See zasmhdr in
+ * local storage indexed by a pseudo-register TLS. See zasmhdr in
  * src/cmd/dist/buildruntime.c for details, and be aware that the linker may
  * make further OS-specific changes to the compiler's output. For example,
- * 6l/linux rewrites 0(GS) as -16(FS).
+ * 6l/linux rewrites 0(TLS) as -16(FS).
  *
  * Every C file linked into a Go program must include runtime.h so that the
  * C compiler (6c, 8c, etc.) knows to avoid other uses of these dedicated

src/pkg/runtime/sys_darwin_386.s

@@ -457,8 +457,7 @@ TEXT runtime·setldt(SB),NOSPLIT,$32
 	 * we use its pthread_create and let it set up %gs
 	 * for us.  When we do that, the private storage
 	 * we get is not at 0(GS) but at 0x468(GS).
-	 * To insulate the rest of the tool chain from this ugliness,
-	 * 8l rewrites 0(GS) into 0x468(GS) for us.
+	 * 8l rewrites 0(TLS) into 0x468(GS) for us.
 	 * To accommodate that rewrite, we translate the
 	 * address and limit here so that 0x468(GS) maps to 0(address).
 	 *

src/pkg/runtime/sys_linux_386.s

@@ -383,7 +383,7 @@ TEXT runtime·setldt(SB),NOSPLIT,$32
 	 * for us.  When we do that, the private storage
 	 * we get is not at 0(GS), 4(GS), but -8(GS), -4(GS).
 	 * To insulate the rest of the tool chain from this
-	 * ugliness, 8l rewrites 0(GS) into -8(GS) for us.
+	 * ugliness, 8l rewrites 0(TLS) into -8(GS) for us.
 	 * To accommodate that rewrite, we translate
 	 * the address here and bump the limit to 0xffffffff (no limit)
 	 * so that -8(GS) maps to 0(address).

src/pkg/runtime/sys_nacl_amd64p32.s

@@ -13,7 +13,7 @@
 	MOVL $(0x10000 + ((code)<<5)), AX; JMP AX
 
 TEXT runtime·settls(SB),NOSPLIT,$0
-	MOVL	DI, GS // really BP
+	MOVL	DI, TLS // really BP
 	RET
 
 TEXT runtime·exit(SB),NOSPLIT,$0
@@ -173,7 +173,7 @@ TEXT runtime·nacl_thread_create(SB),NOSPLIT,$0
 TEXT runtime·mstart_nacl(SB),NOSPLIT,$0
 	NACL_SYSCALL(SYS_tls_get)
 	SUBL	$8, AX
-	MOVL	AX, GS
+	MOVL	AX, TLS
 	JMP runtime·mstart(SB)
 
 TEXT runtime·nacl_nanosleep(SB),NOSPLIT,$0
@@ -254,12 +254,12 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$80
 	// restore TLS register at time of execution,
 	// in case it's been smashed.
 	// the TLS register is really BP, but for consistency
-	// with non-NaCl systems it is referred to here as GS.
+	// with non-NaCl systems it is referred to here as TLS.
 	// NOTE: Cannot use SYS_tls_get here (like we do in mstart_nacl),
 	// because the main thread never calls tls_set.
 	LEAL ctxt+0(FP), AX
 	MOVL (16*4+5*8)(AX), AX
-	MOVL	AX, GS
+	MOVL	AX, TLS
 
 	// check that m exists
 	get_tls(CX)
@@ -305,7 +305,7 @@ sigtramp_ret:
 	MOVQ	16(SI), DX
 	MOVQ	24(SI), BX
 	MOVL	32(SI), SP	// MOVL for SP sandboxing
-	// 40(SI) is saved BP aka GS, already restored above
+	// 40(SI) is saved BP aka TLS, already restored above
 	// 48(SI) is saved SI, never to be seen again
 	MOVQ	56(SI), DI
 	MOVQ	64(SI), R8

src/pkg/runtime/sys_plan9_386.s

@@ -100,8 +100,9 @@ TEXT runtime·rfork(SB),NOSPLIT,$0
 	MOVL	DX, g(AX)
 	MOVL	BX, m(AX)
 
-	// Initialize AX from TOS struct.
-	MOVL	procid(AX), AX
+	// Initialize procid from TOS struct.
+	// TODO: Be explicit and insert a new MOVL _tos(SB), AX here.
+	MOVL	48(AX), AX // procid
 	MOVL	AX, m_procid(BX)	// save pid as m->procid
 	
 	CALL	runtime·stackcheck(SB)	// smashes AX, CX

src/pkg/runtime/sys_plan9_amd64.s

@@ -136,7 +136,7 @@ TEXT runtime·rfork(SB),NOSPLIT,$0
 	MOVQ	BX, m(AX)
 
 	// Initialize AX from pid in TLS.
-	MOVQ	procid(AX), AX
+	MOVQ	0(FS), AX
 	MOVQ	AX, m_procid(BX)	// save pid as m->procid
 	
 	CALL	runtime·stackcheck(SB)	// smashes AX, CX