[Bug go/56431] -lpthread should be added to -lgo

["hjl.tools at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56431

--- Comment #4 from H.J. Lu <hjl.tools at gmail dot com> 2013-02-27 19:36:07 UTC ---
(In reply to comment #3)
> Object OBJ has a weak reference to SYM.
> 
> OBJ is linked against shared library S1.  S1 does not define SYM.
> 
> S1 happens to be linked against shared library S2.  S2 does define SYM.
> 
> That will work fine: at runtime OBJ will see that SYM is defined, because S1
> brings in S2.
> 

This isn't what happens.  Since gold doesn't check S2 at link-time, it
resolves SYM to 0:

[hjl@gnu-6 pr15149]$ cat foo.c 
#include <stdio.h>

extern int xxx () __attribute__((weak));

int
foo ()
{
  if (&xxx)
    return xxx ();
  return 1;
}

int
main ()
{
  printf ("xxx: %d\n", foo ());
  return 0;
}
[hjl@gnu-6 pr15149]$ cat xxx.c
int
xxx ()
{
  return 0;
}
[hjl@gnu-6 pr15149]$ cat yyy.c
/* Dummy */
[hjl@gnu-6 pr15149]$ make y
gcc -Wl,--no-copy-dt-needed-entries    -c -o foo.o foo.c
gcc -Wl,--no-copy-dt-needed-entries -shared -fPIC -o libxxx.so xxx.c
gcc -Wl,--no-copy-dt-needed-entries -shared -fPIC -o libyyy.so yyy.c libxxx.so
-Wl,-rpath,.
gcc -Wl,--no-copy-dt-needed-entries -fuse-ld=gold -B./ -Wl,--no-as-needed -o y
foo.o libyyy.so -Wl,-rpath,.
[hjl@gnu-6 pr15149]$ ./y
xxx: 1
[hjl@gnu-6 pr15149]$ 

38: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND xxx

If gold sees SYM defined in S2, it will create a R_X86_64_JUMP_SLO relocation,
set xxx value to the PLT entry and resolve xxx to the PLT entry.

000000401ad0  000700000007 R_X86_64_JUMP_SLO 0000000000400570 xxx + 0

7: 0000000000400570     0 FUNC    WEAK   DEFAULT  UND xxx

That is the difference between seeing SYM in S2 or not.