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Date: Sun, 20 Sep 2009 21:43:00 -0000
Subject: [Bug inline-asm/41422]  New: incorrect code generated with asm function pointers when compiled with -fPIC on x84_64
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From: "scott dot gccbugs dot 2009 at scottrix dot co dot uk" <gcc-bugzilla@gcc.gnu.org>
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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