6l/5l: PIC and shared library support for the linkers.

Added the -shared flag to 5l/6l to output a PIC executable with the required
dynamic relocations and RIP-relative addressing in machine code.
Added dummy support to 8l to avoid compilation errors

See also:
https://golang.org/cl/6822078
https://golang.org/cl/7064048

and

https://groups.google.com/d/topic/golang-nuts/P05BDjLcQ5k/discussion

R=rsc, iant
CC=golang-dev
https://golang.org/cl/6926049

src/cmd/5l/asm.c

@@ -93,6 +93,19 @@ braddoff(int32 a, int32 b)
 	return (((uint32)a) & 0xff000000U) | (0x00ffffffU & (uint32)(a + b));
 }
 
+Sym *
+lookuprel(void)
+{
+	return lookup(".rel", 0);
+}
+
+void
+adddynrela(Sym *rel, Sym *s, Reloc *r)
+{
+	addaddrplus(rel, s, r->off);
+	adduint32(rel, R_ARM_RELATIVE);
+}
+
 void
 adddynrel(Sym *s, Reloc *r)
 {
@@ -859,15 +872,22 @@ if(debug['G']) print("%ux: %s: arm %d\n", (uint32)(p->pc), p->from.sym->name, p-
 			rel = addrel(cursym);
 			rel->off = pc - cursym->value;
 			rel->siz = 4;
-			rel->type = D_ADDR;
 			rel->sym = p->to.sym;
 			rel->add = p->to.offset;
+			if(flag_shared) {
+				rel->type = D_PCREL;
+				rel->add += pc - p->pcrel->pc - 8;
+			} else
+				rel->type = D_ADDR;
 			o1 = 0;
 		}
 		break;
 
 	case 12:	/* movw $lcon, reg */
 		o1 = omvl(p, &p->from, p->to.reg);
+		if(o->flag & LPCREL) {
+			o2 = oprrr(AADD, p->scond) | p->to.reg | REGPC << 16 | p->to.reg << 12;
+		}
 		break;
 
 	case 13:	/* op $lcon, [R], R */
@@ -1172,13 +1192,23 @@ if(debug['G']) print("%ux: %s: arm %d\n", (uint32)(p->pc), p->from.sym->name, p-
 		break;
 
 	case 62:	/* case R -> movw	R<<2(PC),PC */
-		o1 = olrr(p->from.reg, REGPC, REGPC, p->scond);
-		o1 |= 2<<7;
+		if(o->flag & LPCREL) {
+			o1 = oprrr(AADD, p->scond) | immrot(1) | p->from.reg << 16 | REGTMP << 12;
+			o2 = olrr(REGTMP, REGPC, REGTMP, p->scond);
+			o2 |= 2<<7;
+			o3 = oprrr(AADD, p->scond) | REGTMP | REGPC << 16 | REGPC << 12;
+		} else {
+			o1 = olrr(p->from.reg, REGPC, REGPC, p->scond);
+			o1 |= 2<<7;
+		}
 		break;
 
 	case 63:	/* bcase */
-		if(p->cond != P)
+		if(p->cond != P) {
 			o1 = p->cond->pc;
+			if(flag_shared)
+				o1 = o1 - p->pcrel->pc - 16;
+		}
 		break;
 
 	/* reloc ops */
@@ -1187,6 +1217,10 @@ if(debug['G']) print("%ux: %s: arm %d\n", (uint32)(p->pc), p->from.sym->name, p-
 		if(!o1)
 			break;
 		o2 = osr(p->as, p->from.reg, 0, REGTMP, p->scond);
+		if(o->flag & LPCREL) {
+			o3 = o2;
+			o2 = oprrr(AADD, p->scond) | REGTMP | REGPC << 16 | REGTMP << 12;
+		}
 		break;
 
 	case 65:	/* mov/movbu addr,R */
@@ -1196,6 +1230,10 @@ if(debug['G']) print("%ux: %s: arm %d\n", (uint32)(p->pc), p->from.sym->name, p-
 		o2 = olr(0, REGTMP, p->to.reg, p->scond);
 		if(p->as == AMOVBU || p->as == AMOVB)
 			o2 |= 1<<22;
+		if(o->flag & LPCREL) {
+			o3 = o2;
+			o2 = oprrr(AADD, p->scond) | REGTMP | REGPC << 16 | REGTMP << 12;
+		}
 		break;
 
 	case 68:	/* floating point store -> ADDR */
@@ -1203,6 +1241,10 @@ if(debug['G']) print("%ux: %s: arm %d\n", (uint32)(p->pc), p->from.sym->name, p-
 		if(!o1)
 			break;
 		o2 = ofsr(p->as, p->from.reg, 0, REGTMP, p->scond, p);
+		if(o->flag & LPCREL) {
+			o3 = o2;
+			o2 = oprrr(AADD, p->scond) | REGTMP | REGPC << 16 | REGTMP << 12;
+		}
 		break;
 
 	case 69:	/* floating point load <- ADDR */
@@ -1210,6 +1252,10 @@ if(debug['G']) print("%ux: %s: arm %d\n", (uint32)(p->pc), p->from.sym->name, p-
 		if(!o1)
 			break;
 		o2 = ofsr(p->as, p->to.reg, 0, REGTMP, p->scond, p) | (1<<20);
+		if(o->flag & LPCREL) {
+			o3 = o2;
+			o2 = oprrr(AADD, p->scond) | REGTMP | REGPC << 16 | REGTMP << 12;
+		}
 		break;
 
 	/* ArmV4 ops: */
@@ -1406,12 +1452,20 @@ if(debug['G']) print("%ux: %s: arm %d\n", (uint32)(p->pc), p->from.sym->name, p-
 			o2 ^= (1<<5)|(1<<6);
 		else if(p->as == AMOVH)
 			o2 ^= (1<<6);
+		if(o->flag & LPCREL) {
+			o3 = o2;
+			o2 = oprrr(AADD, p->scond) | REGTMP | REGPC << 16 | REGTMP << 12;
+		}
 		break;
 	case 94:	/* movh/movhu R,addr -> strh */
 		o1 = omvl(p, &p->to, REGTMP);
 		if(!o1)
 			break;
 		o2 = oshr(p->from.reg, 0, REGTMP, p->scond);
+		if(o->flag & LPCREL) {
+			o3 = o2;
+			o2 = oprrr(AADD, p->scond) | REGTMP | REGPC << 16 | REGTMP << 12;
+		}
 		break;
 	case 95:	/* PLD off(reg) */
 		o1 = 0xf5d0f000;

src/cmd/5l/l.h

@@ -111,6 +111,7 @@ struct	Prog
 	} u0;
 	Prog*	cond;
 	Prog*	link;
+	Prog*	pcrel;
 	int32	pc;
 	int32	line;
 	int32	spadj;
@@ -175,6 +176,7 @@ struct	Sym
 	Reloc*	r;
 	int32	nr;
 	int32	maxr;
+	int 	rel_ro;
 };
 
 #define SIGNINTERN	(1729*325*1729)
@@ -197,6 +199,7 @@ struct	Optab
 	char	size;
 	char	param;
 	char	flag;
+	uchar	pcrelsiz;
 };
 struct	Oprang
 {
@@ -214,6 +217,7 @@ enum
 	LFROM		= 1<<0,
 	LTO		= 1<<1,
 	LPOOL		= 1<<2,
+	LPCREL		= 1<<3,
 
 	C_NONE		= 0,
 	C_REG,
@@ -228,6 +232,7 @@ enum
 	C_NCON,		/* ~RCON */
 	C_SCON,		/* 0xffff */
 	C_LCON,
+	C_LCONADDR,
 	C_ZFCON,
 	C_SFCON,
 	C_LFCON,
@@ -281,6 +286,7 @@ EXTERN	int32	INITDAT;		/* data location */
 EXTERN	int32	INITRND;		/* data round above text location */
 EXTERN	int32	INITTEXT;		/* text location */
 EXTERN	char*	INITENTRY;		/* entry point */
+EXTERN	char*	LIBINITENTRY;		/* shared library entry point */
 EXTERN	int32	autosize;
 EXTERN	Auto*	curauto;
 EXTERN	Auto*	curhist;

src/cmd/5l/list.c

@@ -435,6 +435,7 @@ cnames[] =
 	[C_LAUTO]	= "C_LAUTO",
 	[C_LBRA]	= "C_LBRA",
 	[C_LCON]	= "C_LCON",
+	[C_LCONADDR]	= "C_LCONADDR",
 	[C_LOREG]	= "C_LOREG",
 	[C_NCON]	= "C_NCON",
 	[C_NONE]	= "C_NONE",

src/cmd/5l/noop.c

@@ -45,6 +45,20 @@ static	Sym*	sym_divu;
 static	Sym*	sym_mod;
 static	Sym*	sym_modu;
 
+static void
+linkcase(Prog *casep)
+{
+	Prog *p;
+
+	for(p = casep; p != P; p = p->link){
+		if(p->as == ABCASE) {
+			for(; p != P && p->as == ABCASE; p = p->link)
+				p->pcrel = casep;
+			break;
+		}
+	}
+}
+
 void
 noops(void)
 {
@@ -76,6 +90,11 @@ noops(void)
 	for(cursym = textp; cursym != nil; cursym = cursym->next) {
 		for(p = cursym->text; p != P; p = p->link) {
 			switch(p->as) {
+			case ACASE:
+				if(flag_shared)
+					linkcase(p);
+				break;
+
 			case ATEXT:
 				p->mark |= LEAF;
 				break;

src/cmd/5l/obj.c

@@ -80,6 +80,7 @@ main(int argc, char *argv[])
 	INITDAT = -1;
 	INITRND = -1;
 	INITENTRY = 0;
+	LIBINITENTRY = 0;
 	nuxiinit();
 	
 	p = getgoarm();
@@ -123,6 +124,7 @@ main(int argc, char *argv[])
 	flagcount("u", "reject unsafe packages", &debug['u']);
 	flagcount("v", "print link trace", &debug['v']);
 	flagcount("w", "disable DWARF generation", &debug['w']);
+	flagcount("shared", "generate shared object", &flag_shared);
 	
 	flagparse(&argc, &argv, usage);
 

src/cmd/5l/optab.c

@@ -77,10 +77,12 @@ Optab	optab[] =
 	{ ASWI,		C_NONE,	C_NONE,	C_LCON,		10, 4, 0 },
 
 	{ AWORD,	C_NONE,	C_NONE,	C_LCON,		11, 4, 0 },
+	{ AWORD,	C_NONE,	C_NONE,	C_LCONADDR,	11, 4, 0 },
 	{ AWORD,	C_NONE,	C_NONE,	C_ADDR,		11, 4, 0 },
 
 	{ AMOVW,	C_NCON,	C_NONE,	C_REG,		12, 4, 0 },
 	{ AMOVW,	C_LCON,	C_NONE,	C_REG,		12, 4, 0,	LFROM },
+	{ AMOVW,	C_LCONADDR,	C_NONE,	C_REG,	12, 4, 0,	LFROM | LPCREL, 4},
 
 	{ AADD,		C_NCON,	C_REG,	C_REG,		13, 8, 0 },
 	{ AADD,		C_NCON,	C_NONE,	C_REG,		13, 8, 0 },
@@ -119,20 +121,20 @@ Optab	optab[] =
 
 	{ AMOVW,	C_REG,	C_NONE,	C_LAUTO,	30, 8, REGSP,	LTO },
 	{ AMOVW,	C_REG,	C_NONE,	C_LOREG,	30, 8, 0,	LTO },
-	{ AMOVW,	C_REG,	C_NONE,	C_ADDR,		64, 8, 0,	LTO },
+	{ AMOVW,	C_REG,	C_NONE,	C_ADDR,		64, 8, 0,	LTO | LPCREL, 4 },
 	{ AMOVB,	C_REG,	C_NONE,	C_LAUTO,	30, 8, REGSP,	LTO },
 	{ AMOVB,	C_REG,	C_NONE,	C_LOREG,	30, 8, 0,	LTO },
-	{ AMOVB,	C_REG,	C_NONE,	C_ADDR,		64, 8, 0,	LTO },
+	{ AMOVB,	C_REG,	C_NONE,	C_ADDR,		64, 8, 0,	LTO | LPCREL, 4 },
 	{ AMOVBU,	C_REG,	C_NONE,	C_LAUTO,	30, 8, REGSP,	LTO },
 	{ AMOVBU,	C_REG,	C_NONE,	C_LOREG,	30, 8, 0,	LTO },
-	{ AMOVBU,	C_REG,	C_NONE,	C_ADDR,		64, 8, 0,	LTO },
+	{ AMOVBU,	C_REG,	C_NONE,	C_ADDR,		64, 8, 0,	LTO | LPCREL, 4 },
 
 	{ AMOVW,	C_LAUTO,C_NONE,	C_REG,		31, 8, REGSP,	LFROM },
 	{ AMOVW,	C_LOREG,C_NONE,	C_REG,		31, 8, 0,	LFROM },
-	{ AMOVW,	C_ADDR,	C_NONE,	C_REG,		65, 8, 0,	LFROM },
+	{ AMOVW,	C_ADDR,	C_NONE,	C_REG,		65, 8, 0,	LFROM | LPCREL, 4 },
 	{ AMOVBU,	C_LAUTO,C_NONE,	C_REG,		31, 8, REGSP,	LFROM },
 	{ AMOVBU,	C_LOREG,C_NONE,	C_REG,		31, 8, 0,	LFROM },
-	{ AMOVBU,	C_ADDR,	C_NONE,	C_REG,		65, 8, 0,	LFROM },
+	{ AMOVBU,	C_ADDR,	C_NONE,	C_REG,		65, 8, 0,	LFROM | LPCREL, 4 },
 
 	{ AMOVW,	C_LACON,C_NONE,	C_REG,		34, 8, REGSP,	LFROM },
 
@@ -159,8 +161,8 @@ Optab	optab[] =
 	{ AMOVF,	C_LAUTO,C_NONE,	C_FREG,		53, 12, REGSP,	LFROM },
 	{ AMOVF,	C_LOREG,C_NONE,	C_FREG,		53, 12, 0,	LFROM },
 
-	{ AMOVF,	C_FREG,	C_NONE,	C_ADDR,		68, 8, 0,	LTO },
-	{ AMOVF,	C_ADDR,	C_NONE,	C_FREG,		69, 8, 0,	LFROM },
+	{ AMOVF,	C_FREG,	C_NONE,	C_ADDR,		68, 8, 0,	LTO | LPCREL, 4 },
+	{ AMOVF,	C_ADDR,	C_NONE,	C_FREG,		69, 8, 0,	LFROM | LPCREL, 4},
 
 	{ AADDF,	C_FREG,	C_NONE,	C_FREG,		54, 4, 0 },
 	{ AADDF,	C_FREG,	C_REG,	C_FREG,		54, 4, 0 },
@@ -178,7 +180,7 @@ Optab	optab[] =
 	{ AMOVB,	C_REG,	C_NONE,	C_SHIFT,	61, 4, 0 },
 	{ AMOVBU,	C_REG,	C_NONE,	C_SHIFT,	61, 4, 0 },
 
-	{ ACASE,	C_REG,	C_NONE,	C_NONE,		62, 4, 0 },
+	{ ACASE,	C_REG,	C_NONE,	C_NONE,		62, 4, 0, LPCREL, 8 },
 	{ ABCASE,	C_NONE, C_NONE, C_SBRA,		63, 4, 0 },
 
 	{ AMOVH,	C_REG,	C_NONE, C_HAUTO,	70, 4, REGSP,	0 },
@@ -195,20 +197,20 @@ Optab	optab[] =
 
 	{ AMOVH,	C_REG,	C_NONE, C_LAUTO,	72, 8, REGSP,	LTO },
 	{ AMOVH,	C_REG,	C_NONE,	C_LOREG,	72, 8, 0,	LTO },
-	{ AMOVH,	C_REG,	C_NONE,	C_ADDR,	94, 8, 0,	LTO },
+	{ AMOVH,	C_REG,	C_NONE,	C_ADDR,	94, 8, 0,	LTO | LPCREL, 4 },
 	{ AMOVHU,	C_REG,	C_NONE, C_LAUTO,	72, 8, REGSP,	LTO },
 	{ AMOVHU,	C_REG,	C_NONE,	C_LOREG,	72, 8, 0,	LTO },
-	{ AMOVHU,	C_REG,	C_NONE,	C_ADDR,	94, 8, 0,	LTO },
+	{ AMOVHU,	C_REG,	C_NONE,	C_ADDR,	94, 8, 0,	LTO | LPCREL, 4 },
 
 	{ AMOVB,	C_LAUTO,C_NONE,	C_REG,		73, 8, REGSP,	LFROM },
 	{ AMOVB,	C_LOREG,C_NONE,	C_REG,		73, 8, 0,	LFROM },
-	{ AMOVB,	C_ADDR,	C_NONE,	C_REG,		93, 8, 0,	LFROM },
+	{ AMOVB,	C_ADDR,	C_NONE,	C_REG,		93, 8, 0,	LFROM | LPCREL, 4 },
 	{ AMOVH,	C_LAUTO,C_NONE, C_REG,		73, 8, REGSP,	LFROM },
 	{ AMOVH,	C_LOREG,C_NONE,	C_REG,		73, 8, 0,	LFROM },
-	{ AMOVH,	C_ADDR,	C_NONE,	C_REG,		93, 8, 0,	LFROM },
+	{ AMOVH,	C_ADDR,	C_NONE,	C_REG,		93, 8, 0,	LFROM | LPCREL, 4 },
 	{ AMOVHU,	C_LAUTO,C_NONE, C_REG,		73, 8, REGSP,	LFROM },
 	{ AMOVHU,	C_LOREG,C_NONE,	C_REG,		73, 8, 0,	LFROM },
-	{ AMOVHU,	C_ADDR,	C_NONE,	C_REG,		93, 8, 0,	LFROM },
+	{ AMOVHU,	C_ADDR,	C_NONE,	C_REG,		93, 8, 0,	LFROM | LPCREL, 4 },
 
 	{ ALDREX,	C_SOREG,C_NONE,	C_REG,		77, 4, 0 },
 	{ ASTREX,	C_SOREG,C_REG,	C_REG,		78, 4, 0 },

src/cmd/5l/span.c

@@ -330,9 +330,11 @@ addpool(Prog *p, Adr *a)
 	switch(c) {
 	default:
 		t.to = *a;
+		if(flag_shared && t.to.sym != S)
+			t.pcrel = p;
 		break;
 
-	case	C_SROREG:
+	case C_SROREG:
 	case C_LOREG:
 	case C_ROREG:
 	case C_FOREG:
@@ -347,11 +349,13 @@ addpool(Prog *p, Adr *a)
 		break;
 	}
 
-	for(q = blitrl; q != P; q = q->link)	/* could hash on t.t0.offset */
-		if(memcmp(&q->to, &t.to, sizeof(t.to)) == 0) {
-			p->cond = q;
-			return;
-		}
+	if(t.pcrel == P) {
+		for(q = blitrl; q != P; q = q->link)	/* could hash on t.t0.offset */
+			if(q->pcrel == P && memcmp(&q->to, &t.to, sizeof(t.to)) == 0) {
+				p->cond = q;
+				return;
+			}
+	}
 
 	q = prg();
 	*q = t;
@@ -570,7 +574,10 @@ aclass(Adr *a)
 			if(s == S)
 				break;
 			instoffset = 0;	// s.b. unused but just in case
-			return C_LCON;
+			if(flag_shared)
+				return C_LCONADDR;
+			else
+				return C_LCON;
 
 		case D_AUTO:
 			instoffset = autosize + a->offset;
@@ -736,8 +743,14 @@ buildop(void)
 	for(i=0; i<C_GOK; i++)
 		for(n=0; n<C_GOK; n++)
 			xcmp[i][n] = cmp(n, i);
-	for(n=0; optab[n].as != AXXX; n++)
-		;
+	for(n=0; optab[n].as != AXXX; n++) {
+		if((optab[n].flag & LPCREL) != 0) {
+			if(flag_shared)
+				optab[n].size += optab[n].pcrelsiz;
+			else
+				optab[n].flag &= ~LPCREL;
+		}
+	}
 	qsort(optab, n, sizeof(optab[0]), ocmp);
 	for(i=0; i<n; i++) {
 		r = optab[i].as;

src/cmd/6l/asm.c

@@ -99,6 +99,20 @@ int nelfsym = 1;
 static void addpltsym(Sym*);
 static void addgotsym(Sym*);
 
+Sym *
+lookuprel(void)
+{
+	return lookup(".rela", 0);
+}
+
+void
+adddynrela(Sym *rela, Sym *s, Reloc *r)
+{
+	addaddrplus(rela, s, r->off);
+	adduint64(rela, R_X86_64_RELATIVE);
+	addaddrplus(rela, r->sym, r->add); // Addend
+}
+
 void
 adddynrel(Sym *s, Reloc *r)
 {
@@ -463,7 +477,7 @@ adddynsym(Sym *s)
 			addaddr(d, s);
 	
 		/* size of object */
-		adduint64(d, 0);
+		adduint64(d, s->size);
 	
 		if(!s->dynexport && s->dynimplib && needlib(s->dynimplib)) {
 			elfwritedynent(lookup(".dynamic", 0), DT_NEEDED,

src/cmd/6l/l.h

@@ -181,6 +181,7 @@ struct	Sym
 	Reloc*	r;
 	int32	nr;
 	int32	maxr;
+	int 	rel_ro;
 };
 struct	Optab
 {
@@ -320,6 +321,7 @@ EXTERN	int32	INITRND;
 EXTERN	int64	INITTEXT;
 EXTERN	int64	INITDAT;
 EXTERN	char*	INITENTRY;		/* entry point */
+EXTERN	char*	LIBINITENTRY;		/* shared library entry point */
 EXTERN	char*	pcstr;
 EXTERN	Auto*	curauto;
 EXTERN	Auto*	curhist;

src/cmd/6l/obj.c

@@ -82,6 +82,7 @@ main(int argc, char *argv[])
 	INITDAT = -1;
 	INITRND = -1;
 	INITENTRY = 0;
+	LIBINITENTRY = 0;
 	nuxiinit();
 
 	flagcount("1", "use alternate profiling code", &debug['1']);
@@ -117,6 +118,7 @@ main(int argc, char *argv[])
 	flagcount("u", "reject unsafe packages", &debug['u']);
 	flagcount("v", "print link trace", &debug['v']);
 	flagcount("w", "disable DWARF generation", &debug['w']);
+	flagcount("shared", "generate shared object", &flag_shared);
 	
 	flagparse(&argc, &argv, usage);
 

src/cmd/6l/span.c

@@ -372,7 +372,10 @@ oclass(Adr *a)
 				switch(a->index) {
 				case D_EXTERN:
 				case D_STATIC:
-					return Yi32;	/* TO DO: Yi64 */
+					if(flag_shared)
+						return Yiauto;
+					else
+						return Yi32;	/* TO DO: Yi64 */
 				case D_AUTO:
 				case D_PARAM:
 					return Yiauto;
@@ -731,7 +734,10 @@ vaddr(Adr *a, Reloc *r)
 			diag("need reloc for %D", a);
 			errorexit();
 		}
-		r->type = D_ADDR;
+		if(flag_shared)
+			r->type = D_PCREL;
+		else
+			r->type = D_ADDR;
 		r->siz = 4;	// TODO: 8 for external symbols
 		r->off = -1;	// caller must fill in
 		r->sym = s;
@@ -760,6 +766,8 @@ asmandsz(Adr *a, int r, int rex, int m64)
 				goto bad;
 			case D_STATIC:
 			case D_EXTERN:
+				if(flag_shared)
+					goto bad;
 				t = D_NONE;
 				v = vaddr(a, &rel);
 				break;
@@ -820,7 +828,7 @@ asmandsz(Adr *a, int r, int rex, int m64)
 
 	rexflag |= (regrex[t] & Rxb) | rex;
 	if(t == D_NONE || (D_CS <= t && t <= D_GS)) {
-		if(asmode != 64){
+		if(flag_shared && t == D_NONE && (a->type == D_STATIC || a->type == D_EXTERN) || asmode != 64) {
 			*andptr++ = (0 << 6) | (5 << 0) | (r << 3);
 			goto putrelv;
 		}
@@ -1776,13 +1784,17 @@ asmins(Prog *p)
 			if(c != 0xf2 && c != 0xf3 && (c < 0x64 || c > 0x67) && c != 0x2e && c != 0x3e && c != 0x26)
 				break;
 		}
-		for(r=cursym->r+cursym->nr; r-- > cursym->r; ) {
-			if(r->off < p->pc)
-				break;
-			r->off++;
-		}
 		memmove(and+np+1, and+np, n-np);
 		and[np] = 0x40 | rexflag;
 		andptr++;
 	}
+	n = andptr - and;
+	for(r=cursym->r+cursym->nr; r-- > cursym->r; ) {
+		if(r->off < p->pc)
+			break;
+		if(rexflag)
+			r->off++;
+		if(r->type == D_PCREL)
+			r->add -= p->pc + n - (r->off + r->siz);
+	}
 }

src/cmd/8l/asm.c

@@ -95,6 +95,18 @@ int	nelfsym = 1;
 static void	addpltsym(Sym*);
 static void	addgotsym(Sym*);
 
+Sym *
+lookuprel(void)
+{
+	return lookup(".rel", 0);
+}
+
+void
+adddynrela(Sym *rela, Sym *s, Reloc *r)
+{
+	sysfatal("adddynrela not implemented");
+}
+
 void
 adddynrel(Sym *s, Reloc *r)
 {

src/cmd/8l/l.h

@@ -163,6 +163,7 @@ struct	Sym
 	Reloc*	r;
 	int32	nr;
 	int32	maxr;
+	int 	rel_ro;
 };
 struct	Optab
 {
@@ -275,6 +276,7 @@ EXTERN	int32	INITRND;
 EXTERN	int32	INITTEXT;
 EXTERN	int32	INITDAT;
 EXTERN	char*	INITENTRY;		/* entry point */
+EXTERN	char*	LIBINITENTRY;		/* shared library entry point */
 EXTERN	int32	casepc;
 EXTERN	char*	pcstr;
 EXTERN	Auto*	curauto;

src/cmd/8l/obj.c

@@ -89,6 +89,7 @@ main(int argc, char *argv[])
 	INITDAT = -1;
 	INITRND = -1;
 	INITENTRY = 0;
+	LIBINITENTRY = 0;
 	nuxiinit();
 
 	flagcount("1", "use alternate profiling code", &debug['1']);

src/cmd/ld/data.c

@@ -232,7 +232,9 @@ void
 dynrelocsym(Sym *s)
 {
 	Reloc *r;
-
+	Sym *rel;
+	Sym *got;
+	
 	if(HEADTYPE == Hwindows) {
 		Sym *rel, *targ;
 
@@ -268,9 +270,23 @@ dynrelocsym(Sym *s)
 		return;
 	}
 
-	for(r=s->r; r<s->r+s->nr; r++)
+	got = rel = nil;
+	if(flag_shared) {
+		rel = lookuprel();
+		got = lookup(".got", 0);
+	}
+	s->rel_ro = 0;
+	for(r=s->r; r<s->r+s->nr; r++) {
 		if(r->sym != S && r->sym->type == SDYNIMPORT || r->type >= 256)
 			adddynrel(s, r);
+		if(flag_shared && r->sym != S && (r->sym->dynimpname == nil || r->sym->dynexport) && r->type == D_ADDR
+				&& (s == got || s->type == SDATA || s->type == SGOSTRING || s->type == STYPE || s->type == SRODATA)) {
+			// Create address based RELATIVE relocation
+			adddynrela(rel, s, r);
+			if(s->type < SNOPTRDATA)
+				s->rel_ro = 1;
+		}
+	}
 }
 
 void
@@ -714,6 +730,29 @@ setuint64(Sym *s, vlong r, uint64 v)
 	setuintxx(s, r, v, 8);
 }
 
+/*
+static vlong
+addaddrpcrelplus(Sym *s, Sym *t, int32 add)
+{
+	vlong i;
+	Reloc *r;
+
+	if(s->type == 0)
+		s->type = SDATA;
+	s->reachable = 1;
+	i = s->size;
+	s->size += PtrSize;
+	symgrow(s, s->size);
+	r = addrel(s);
+	r->sym = t;
+	r->off = i;
+	r->siz = PtrSize;
+	r->type = D_PCREL;
+	r->add = add;
+	return i;
+}
+*/
+
 vlong
 addaddrplus(Sym *s, Sym *t, int32 add)
 {
@@ -968,6 +1007,12 @@ dodata(void)
 	}
 	*l = nil;
 
+	if(flag_shared) {
+		for(s=datap; s != nil; s = s->next) {
+			if(s->rel_ro)
+				s->type = SDATARELRO;
+		}
+	}
 	datap = datsort(datap);
 
 	/*
@@ -1004,7 +1049,7 @@ dodata(void)
 	sect->vaddr = datsize;
 	lookup("noptrdata", 0)->sect = sect;
 	lookup("enoptrdata", 0)->sect = sect;
-	for(; s != nil && s->type < SDATA; s = s->next) {
+	for(; s != nil && s->type < SDATARELRO; s = s->next) {
 		s->sect = sect;
 		s->type = SDATA;
 		t = alignsymsize(s->size);
@@ -1015,12 +1060,34 @@ dodata(void)
 	sect->len = datsize - sect->vaddr;
 	datsize = rnd(datsize, PtrSize);
 
+	/* dynamic relocated rodata */
+	if(flag_shared) {
+		sect = addsection(&segdata, ".data.rel.ro", 06);
+		sect->vaddr = datsize;
+		lookup("datarelro", 0)->sect = sect;
+		lookup("edatarelro", 0)->sect = sect;
+		for(; s != nil && s->type == SDATARELRO; s = s->next) {
+			if(s->align != 0)
+				datsize = rnd(datsize, s->align);
+			s->sect = sect;
+			s->type = SDATA;
+			s->value = datsize;
+			datsize += rnd(s->size, PtrSize);
+		}
+		sect->len = datsize - sect->vaddr;
+		datsize = rnd(datsize, PtrSize);
+	}
+
 	/* data */
 	sect = addsection(&segdata, ".data", 06);
 	sect->vaddr = datsize;
 	lookup("data", 0)->sect = sect;
 	lookup("edata", 0)->sect = sect;
 	for(; s != nil && s->type < SBSS; s = s->next) {
+		if(s->type == SDATARELRO) {
+			cursym = s;
+			diag("unexpected symbol type %d", s->type);
+		}
 		s->sect = sect;
 		s->type = SDATA;
 		t = alignsymsize(s->size);
@@ -1080,6 +1147,7 @@ dodata(void)
 	sect->vaddr = 0;
 	lookup("rodata", 0)->sect = sect;
 	lookup("erodata", 0)->sect = sect;
+	lookup("reloffset", 0)->sect = sect;
 	datsize = 0;
 	s = datap;
 	for(; s != nil && s->type < STYPELINK; s = s->next) {
@@ -1227,7 +1295,7 @@ textaddress(void)
 void
 address(void)
 {
-	Section *s, *text, *data, *rodata, *symtab, *pclntab, *noptr, *bss, *noptrbss;
+	Section *s, *text, *data, *rodata, *symtab, *pclntab, *noptr, *bss, *noptrbss, *datarelro;
 	Section *gcdata, *gcbss, *typelink;
 	Sym *sym, *sub;
 	uvlong va;
@@ -1257,6 +1325,7 @@ address(void)
 	noptr = nil;
 	bss = nil;
 	noptrbss = nil;
+	datarelro = nil;
 	for(s=segdata.sect; s != nil; s=s->next) {
 		s->vaddr = va;
 		va += s->len;
@@ -1270,6 +1339,8 @@ address(void)
 			bss = s;
 		if(strcmp(s->name, ".noptrbss") == 0)
 			noptrbss = s;
+		if(strcmp(s->name, ".data.rel.ro") == 0)
+			datarelro = s;
 	}
 	segdata.filelen -= bss->len + noptrbss->len; // deduct .bss
 
@@ -1297,6 +1368,10 @@ address(void)
 	xdefine("erodata", SRODATA, rodata->vaddr + rodata->len);
 	xdefine("typelink", SRODATA, typelink->vaddr);
 	xdefine("etypelink", SRODATA, typelink->vaddr + typelink->len);
+	if(datarelro != nil) {
+		xdefine("datarelro", SRODATA, datarelro->vaddr);
+		xdefine("edatarelro", SRODATA, datarelro->vaddr + datarelro->len);
+	}
 	xdefine("gcdata", SGCDATA, gcdata->vaddr);
 	xdefine("egcdata", SGCDATA, gcdata->vaddr + gcdata->len);
 	xdefine("gcbss", SGCBSS, gcbss->vaddr);

src/cmd/ld/elf.c

@@ -794,6 +794,8 @@ doelf(void)
 	addstring(shstrtab, ".elfdata");
 	addstring(shstrtab, ".rodata");
 	addstring(shstrtab, ".typelink");
+	if(flag_shared)
+		addstring(shstrtab, ".data.rel.ro");
 	addstring(shstrtab, ".gcdata");
 	addstring(shstrtab, ".gcbss");
 	addstring(shstrtab, ".gosymtab");
@@ -927,6 +929,13 @@ doelf(void)
 		
 		elfwritedynent(s, DT_DEBUG, 0);
 
+		if(flag_shared) {
+			Sym *init_sym = lookup(LIBINITENTRY, 0);
+			if(init_sym->type != STEXT)
+				diag("entry not text: %s", init_sym->name);
+			elfwritedynentsym(s, DT_INIT, init_sym);
+		}
+
 		// Do not write DT_NULL.  elfdynhash will finish it.
 	}
 }
@@ -1277,7 +1286,11 @@ asmbelf(vlong symo)
 	eh->ident[EI_DATA] = ELFDATA2LSB;
 	eh->ident[EI_VERSION] = EV_CURRENT;
 
-	eh->type = ET_EXEC;
+	if(flag_shared)
+		eh->type = ET_DYN;
+	else
+		eh->type = ET_EXEC;
+
 	eh->version = EV_CURRENT;
 	eh->entry = entryvalue();
 

src/cmd/ld/go.c

@@ -732,6 +732,8 @@ deadcode(void)
 		Bprint(&bso, "%5.2f deadcode\n", cputime());
 
 	mark(lookup(INITENTRY, 0));
+	if(flag_shared)
+		mark(lookup(LIBINITENTRY, 0));
 	for(i=0; i<nelem(morename); i++)
 		mark(lookup(morename[i], 0));
 

src/cmd/ld/lib.c

@@ -104,6 +104,13 @@ libinit(void)
 		sprint(INITENTRY, "_rt0_%s_%s", goarch, goos);
 	}
 	lookup(INITENTRY, 0)->type = SXREF;
+	if(flag_shared) {
+		if(LIBINITENTRY == nil) {
+			LIBINITENTRY = mal(strlen(goarch)+strlen(goos)+20);
+			sprint(LIBINITENTRY, "_rt0_%s_%s_lib", goarch, goos);
+		}
+		lookup(LIBINITENTRY, 0)->type = SXREF;
+	}
 }
 
 void
@@ -305,7 +312,7 @@ loadlib(void)
 	//
 	// Exception: on OS X, programs such as Shark only work with dynamic
 	// binaries, so leave it enabled on OS X (Mach-O) binaries.
-	if(!havedynamic && HEADTYPE != Hdarwin)
+	if(!flag_shared && !havedynamic && HEADTYPE != Hdarwin)
 		debug['d'] = 1;
 	
 	importcycles();

src/cmd/ld/lib.h

@@ -47,6 +47,7 @@ enum
 	SELFROSECT,
 	SELFSECT,
 	SNOPTRDATA,
+	SDATARELRO,
 	SDATA,
 	SMACHO,	/* Mach-O __nl_symbol_ptr */
 	SMACHOGOT,
@@ -138,6 +139,7 @@ EXTERN	int	havedynamic;
 EXTERN	int	iscgo;
 EXTERN	int	elfglobalsymndx;
 EXTERN	int	flag_race;
+EXTERN	int flag_shared;
 EXTERN	char*	tracksym;
 EXTERN	char*	interpreter;
 
@@ -176,6 +178,8 @@ void	symtab(void);
 void	Lflag(char *arg);
 void	usage(void);
 void	adddynrel(Sym*, Reloc*);
+void	adddynrela(Sym*, Sym*, Reloc*);
+Sym*	lookuprel();
 void	ldobj1(Biobuf *f, char*, int64 len, char *pn);
 void	ldobj(Biobuf*, char*, int64, char*, int);
 void	ldelf(Biobuf*, char*, int64, char*);

src/cmd/ld/symtab.c

@@ -387,6 +387,11 @@ symtab(void)
 	xdefine("etypelink", SRODATA, 0);
 	xdefine("rodata", SRODATA, 0);
 	xdefine("erodata", SRODATA, 0);
+	xdefine("reloffset", SRODATA, 0);
+	if(flag_shared) {
+		xdefine("datarelro", SDATARELRO, 0);
+		xdefine("edatarelro", SDATARELRO, 0);
+	}
 	xdefine("gcdata", SGCDATA, 0);
 	xdefine("egcdata", SGCDATA, 0);
 	xdefine("gcbss", SGCBSS, 0);