[Bug inline-asm/41422] New: incorrect code generated with asm function pointers when compiled with -fPIC on x84_64

[Bug inline-asm/41422] New: incorrect code generated with asm function pointers when compiled with -fPIC on x84_64

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

I have only seen this problem ono x86_64, cannot reproduce on 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 

Values will be fixed up at link time:

gcc -g -o a a.o

Giving:

   fred=(void *)my_asm_func;
  400528:       48 c7 45 f8 0c 05 40    movq   $0x40050c,-0x8(%rbp)
  40052f:       00 

   fred=(void *)my_c_func;
  400543:       48 c7 45 f8 15 05 40    movq   $0x400515,-0x8(%rbp)
  40054a:       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 point 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.  Linking with:

 gcc -fPIC -g -o a a.o

gives:

   fred=(void *)my_asm_func;
  400568:       48 8b 05 dd ff ff ff    mov    -0x23(%rip),%rax        # 40054c
<my_asm_func>
  40056f:       48 89 45 f8             mov    %rax,-0x8(%rbp)

   fred=(void *)my_c_func;
  400588:       48 8b 05 51 0a 20 00    mov    0x200a51(%rip),%rax        #
600fe0 <_DYNAMIC+0x1a8>
  40058f:       48 89 45 f8             mov    %rax,-0x8(%rbp)

showing that the c function has correctly been fixed up, the asm one is still
incorrect.

I have reproduced this problem on gcc 4.3.2 and 4.4.1.  I have only given
objdump -S output for the relevant sections of code.  If you require more
information please 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: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: inline-asm
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: scott dot gccbugs dot 2009 at scottrix dot co dot uk


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41422

[Bug inline-asm/41422] incorrect code generated with asm function pointers when compiled with -fPIC on x84_64

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2009-09-20 22:01 -------
GCC outputs:
        movq    my_asm_func@GOTPCREL(%rip), %rsi

Which looks correct but since my_asm_func is local to the object file only, the
assembler/linker decides something different.

If you do:
static void my_asm_func(void); instead
it works the way you want it to work.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41422

[Bug inline-asm/41422] incorrect code generated with asm function pointers when compiled with -fPIC on x84_64

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

------- Comment #2 from scott dot gccbugs dot 2009 at scottrix dot co dot uk  2009-09-21 06:25 -------
I have changed 

extern void my_asm_func(void);

to

static void my_asm_func(void);

This gives me a warning:

a.c:3: warning: 'my_asm_func' used but never defined

and still produces incorrect code, as before.

I am more than happy to accept that this is an assembler problem (it's not the
linker since the problem is in the .o file.)  How do I generate the code that
you gave ? (movq    my_asm_func@GOTPCREL(%rip), %rsi)

thanks...


-- 

scott dot gccbugs dot 2009 at scottrix dot co dot uk changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41422

[Bug inline-asm/41422] incorrect code generated with asm function pointers when compiled with -fPIC on x84_64

[jakub at gcc dot gnu dot org]

------- Comment #3 from jakub at gcc dot gnu dot org  2009-09-21 06:58 -------
I think it is an assembler bug, unless GOTPCREL is only allowed for non-local
symbols (then it would be testcase author's fault).
GOTPCREL which is address of a pointer to the symbol should never be resolved
to
the actual address of the symbol.
You can always add .global my_asm_func to the asm, or that plus .hidden
my_asm_func to avoid exporting it from the current file.


-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41422

[Bug inline-asm/41422] incorrect code generated with asm function pointers when compiled with -fPIC on x84_64

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

------- Comment #4 from scott dot gccbugs dot 2009 at scottrix dot co dot uk  2009-09-21 08:46 -------
Thanks for the help, I have got the intermediate files out and can see what you
mean.  I'll raise the issue with binutils.  Again, thanks for the help and very
quick response.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41422