ld: weak symbols

A reference to the address of weak.foo resolves at link time
to the address of the symbol foo if foo would end up in the
binary anyway, or to zero if foo would not be in the binary.

For example:

        int xxx = 1;
        int yyy = 2;
        int weak·xxx;
        int weak·yyy;

        void main·main(void) {
                runtime·printf("%p %p %p\n", &xxx, &weak·xxx, &weak·yyy);
        }

prints the same non-nil address twice, then 0 (because yyy is not
referenced so it was dropped from the binary).

This will be used by the reflection tables.

R=iant
CC=golang-dev
https://golang.org/cl/4223044

src/cmd/5l/obj.c

@@ -280,6 +280,7 @@ main(int argc, char *argv[])
 	symtab();
 	dodata();
 	address();
+	doweak();
 	reloc();
 	asmb();
 	undef();

src/cmd/5l/pass.c

@@ -35,18 +35,6 @@
 
 static void xfol(Prog*, Prog**);
 
-void
-undef(void)
-{
-	int i;
-	Sym *s;
-
-	for(i=0; i<NHASH; i++)
-	for(s = hash[i]; s != S; s = s->hash)
-		if(s->type == SXREF)
-			diag("%s: not defined", s->name);
-}
-
 Prog*
 brchain(Prog *p)
 {

src/cmd/6l/obj.c

@@ -267,6 +267,7 @@ main(int argc, char *argv[])
 	symtab();
 	dodata();
 	address();
+	doweak();
 	reloc();
 	asmb();
 	undef();

src/cmd/6l/pass.c

@@ -718,15 +718,3 @@ atolwhex(char *s)
 		n = -n;
 	return n;
 }
-
-void
-undef(void)
-{
-	int i;
-	Sym *s;
-
-	for(i=0; i<NHASH; i++)
-	for(s = hash[i]; s != S; s = s->hash)
-		if(s->type == SXREF)
-			diag("%s: not defined", s->name);
-}

src/cmd/8l/obj.c

@@ -316,6 +316,7 @@ main(int argc, char *argv[])
 	symtab();
 	dodata();
 	address();
+	doweak();
 	reloc();
 	asmb();
 	undef();

src/cmd/8l/pass.c

@@ -666,15 +666,3 @@ atolwhex(char *s)
 		n = -n;
 	return n;
 }
-
-void
-undef(void)
-{
-	int i;
-	Sym *s;
-
-	for(i=0; i<NHASH; i++)
-	for(s = hash[i]; s != S; s = s->hash)
-		if(s->type == SXREF)
-			diag("%s(%d): not defined", s->name, s->version);
-}

src/cmd/ld/go.c

@@ -550,6 +550,8 @@ mark(Sym *s)
 
 	if(s == S || s->reachable)
 		return;
+	if(strncmp(s->name, "weak.", 5) == 0)
+		return;
 	s->reachable = 1;
 	if(s->text)
 		marktext(s);
@@ -654,6 +656,35 @@ deadcode(void)
 		textp = nil;
 	else
 		last->next = nil;
+	
+	for(i=0; i<NHASH; i++)
+	for(s = hash[i]; s != S; s = s->hash)
+		if(strncmp(s->name, "weak.", 5) == 0) {
+			s->special = 1;  // do not lay out in data segment
+			s->reachable = 1;
+		}
+}
+
+void
+doweak(void)
+{
+	int i;
+	Sym *s, *t;
+
+	// resolve weak references only if
+	// target symbol will be in binary anyway.
+	for(i=0; i<NHASH; i++)
+	for(s = hash[i]; s != S; s = s->hash) {
+		if(strncmp(s->name, "weak.", 5) == 0) {
+			t = lookup(s->name+5, s->version);
+			if(t->type != 0 && t->reachable) {
+				s->value = t->value;
+				s->type = t->type;
+			} else
+				s->value = 0;
+			continue;
+		}
+	}
 }
 
 void

src/cmd/ld/lib.c

@@ -1279,3 +1279,15 @@ headtype(char *name)
 	errorexit();
 	return -1;  // not reached
 }
+
+void
+undef(void)
+{
+	int i;
+	Sym *s;
+
+	for(i=0; i<NHASH; i++)
+	for(s = hash[i]; s != S; s = s->hash)
+		if(s->type == SXREF)
+			diag("%s(%d): not defined", s->name, s->version);
+}

src/cmd/ld/lib.h

@@ -167,6 +167,8 @@ int	archreloc(Reloc*, Sym*, vlong*);
 void	adddynsym(Sym*);
 void	addexport(void);
 void	dostkcheck(void);
+void	undef(void);
+void	doweak(void);
 
 int	pathchar(void);
 void*	mal(uint32);