runtime, cmd/ld: make code more position-independent

Change the stack unwinding code to compensate for the dynamic
relocation of symbols.
Change the gc instruction GC_CALL to use a relative offset instead of
an absolute address.

R=golang-dev
CC=golang-dev
https://golang.org/cl/7248048

Author: Elias Naur

src/cmd/ld/data.c

@@ -748,7 +748,6 @@ setuint64(Sym *s, vlong r, uint64 v)
 	setuintxx(s, r, v, 8);
 }
 
-/*
 static vlong
 addaddrpcrelplus(Sym *s, Sym *t, int32 add)
 {
@@ -769,7 +768,6 @@ addaddrpcrelplus(Sym *s, Sym *t, int32 add)
 	r->add = add;
 	return i;
 }
-*/
 
 vlong
 addaddrplus(Sym *s, Sym *t, int32 add)
@@ -951,7 +949,7 @@ gcaddsym(Sym *gc, Sym *s, int32 off)
 		//print("gcaddsym:    %s    %d    %s\n", s->name, s->size, gotype->name);
 		adduintxx(gc, GC_CALL, PtrSize);
 		adduintxx(gc, off, PtrSize);
-		addaddrplus(gc, decodetype_gc(gotype), 1*PtrSize);
+		addaddrpcrelplus(gc, decodetype_gc(gotype), 4*PtrSize);
 	} else {
 		//print("gcaddsym:    %s    %d    <unknown type>\n", s->name, s->size);
 		for(a = -off&(PtrSize-1); a+PtrSize<=s->size; a+=PtrSize) {

src/pkg/runtime/mgc0.c

@@ -663,7 +663,7 @@ scanblock(Workbuf *wbuf, Obj *wp, uintptr nobj, bool keepworking)
 			// Stack push.
 			*stack_ptr-- = stack_top;
 			stack_top = (Frame){1, 0, stack_top.b + pc[1], pc+3 /*return address*/};
-			pc = (uintptr*)pc[2];  // target of the CALL instruction
+			pc = (uintptr*)((byte*)pc + (uintptr)pc[2]);  // target of the CALL instruction
 			continue;
 
 		case GC_MAP_PTR:

src/pkg/runtime/mgc0.h

@@ -12,6 +12,7 @@
 // Meaning of arguments:
 //   off      Offset (in bytes) from the start of the current object
 //   objgc    Pointer to GC info of an object
+//   objgcrel Offset to GC info of an object
 //   len      Length of an array
 //   elemsize Size (in bytes) of an element
 //   size     Size (in bytes)
@@ -21,7 +22,7 @@ enum {
 	GC_APTR,        // Pointer to an arbitrary object. Args: (off)
 	GC_ARRAY_START, // Start an array with a fixed length. Args: (off, len, elemsize)
 	GC_ARRAY_NEXT,  // The next element of an array. Args: none
-	GC_CALL,        // Call a subroutine. Args: (off, objgc)
+	GC_CALL,        // Call a subroutine. Args: (off, objgcrel)
 	GC_MAP_PTR,     // Go map. Args: (off, MapType*)
 	GC_STRING,      // Go string. Args: (off)
 	GC_EFACE,       // interface{}. Args: (off)

src/pkg/runtime/symtab.c

@@ -93,6 +93,7 @@ walksymtab(void (*fn)(Sym*))
 
 static Func *func;
 static int32 nfunc;
+extern byte reloffset[];
 
 static byte **fname;
 static int32 nfname;
@@ -118,7 +119,7 @@ dofunc(Sym *sym)
 		}
 		f = &func[nfunc++];
 		f->name = runtime·gostringnocopy(sym->name);
-		f->entry = sym->value;
+		f->entry = sym->value + (uint64)reloffset;
 		if(sym->symtype == 'L' || sym->symtype == 'l')
 			f->frame = -sizeof(uintptr);
 		break;