[Bug gas/10677] New: incorrect code generated with asm function pointers when compiled with -fPIC on x84_64

[scott dot sourceware dot 2009 at scottrix dot co dot uk]

This problem only occurs on x86_64, not i686.

Example code (a.c):

#include <stdio.h>

extern void my_asm_func(void);

asm(".text\n" \
    "my_asm_func:\n" \
    "  mov 1234,%rax\n" \
    "  ret\n" \
    ".previous\n");

int my_c_func() { return 1; }

int main()
{
   void *fred;

   fred=(void *)my_asm_func;
   printf("function = %p\n",fred);
   fred=(void *)my_c_func;
   printf("function = %p\n",fred);
   return 0;
}

if this is compiled with the line:

gcc -c -g -o a.o a.c

The assemble code for the two "fred=" function pointer assignments are correct:

   fred=(void *)my_asm_func;
  1c:   48 c7 45 f8 00 00 00    movq   $0x0,-0x8(%rbp)
  23:   00 

   fred=(void *)my_c_func;
  37:   48 c7 45 f8 00 00 00    movq   $0x0,-0x8(%rbp)
  3e:   00 

as expected.  However, when used with -fPIC:

gcc -fPIC -c -g -o a.o a.c

we get :

   fred=(void *)my_asm_func;
  1c:   48 8b 05 dd ff ff ff    mov    -0x23(%rip),%rax        # 0
<my_asm_func>
  23:   48 89 45 f8             mov    %rax,-0x8(%rbp)

   fred=(void *)my_c_func;
  3c:   48 8b 05 00 00 00 00    mov    0x0(%rip),%rax        # 43 <main+0x2f>
  43:   48 89 45 f8             mov    %rax,-0x8(%rbp)

For some reason the asm function pointer has already been fixed up with a value,
which is actually the location the of the function, but it will move the value
at that address into rax, not the address itself.  Looking at the intermediate
.s file:

movq  address@hidden(%rip), %rax

movq  address@hidden(%rip), %rax

Both functions have the same fixup type (assuming that is the correct
terminology).  However, they produce different code.  If I add a .global to the
assembler then it works, however, I don't think that I should need to, and on
i686 it works fine.

I have reproduced this problem on gcc 4.3.2 and 4.4.1, both with binutils
2.19.1.  I have only given objdump -S output for the relevant sections of code.
 If you require more information please let me know.

This problem was actually found while compiling valgrind for a 64 bit x86
target machine.

-- 
           Summary: incorrect code generated with asm function pointers when
                    compiled with -fPIC on x84_64
           Product: binutils
           Version: 2.19
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gas
        AssignedTo: unassigned at sources dot redhat dot com
        ReportedBy: scott dot sourceware dot 2009 at scottrix dot co dot uk
                CC: bug-binutils at gnu dot org


http://sourceware.org/bugzilla/show_bug.cgi?id=10677

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