Re: [Qemu-devel] gcc auto-omit-frame-pointer vs msvc longjmp

[Bob Breuer <breuerr@mc.net>]

Richard Henderson wrote:
> On 10/17/2011 07:09 AM, Bob Breuer wrote:
>> I don't think this is a free/g_free issue.  If I use the following
>> patch, then I at least get the openbios messages:
>>
>> diff --git a/cpu-exec.c b/cpu-exec.c
>> index a9fa608..dfbd6ea 100644
>> --- a/cpu-exec.c
>> +++ b/cpu-exec.c
>> @@ -180,6 +180,7 @@ static void cpu_handle_debug_exception(CPUState
>>  /* main execution loop */
>>
>>  volatile sig_atomic_t exit_request;
>> +register void *ebp asm("ebp");
>>
>>  int cpu_exec(CPUState *env)
>>  {
>> @@ -233,6 +234,8 @@ int cpu_exec(CPUState *env)
>>
>>      /* prepare setjmp context for exception handling */
>>      for(;;) {
>> +        int dummy = 0;
>> +        ebp = &dummy;
> 
> See if
> 
>   asm("" : : : "ebp");
> 
> also solves the problem.

No, that doesn't fix it.

> 
>> Google finds a mention of longjmp failing with -fomit-frame-pointer:
>> http://lua-users.org/lists/lua-l/2005-02/msg00158.html
>>
>> Looks like gcc 4.6 turns on -fomit-frame-pointer by default.
> 
> Hmm.  This is the first I've heard of a longjmp implementation 
> failing without a frame pointer.  Presumably this is with the
> mingw i.e. msvc libc?

Yeah, mingw from www.mingw.org which I believe uses msvcrt.dll, package
gcc-core-4.6.1-2-mingw32-bin.

> This is something that could be worked around in gcc, I suppose.
> We recognize longjmp for some things, we could force the use of
> a frame pointer for msvc targets too.
> 
> For now it might be best to simply force -fno-omit-frame-pointer
> for mingw host in the configure script.

Here's a testcase that crashes on the longjmp:

#include <stdio.h>
#include <setjmp.h>

jmp_buf env;

int test(void)
{
  int i;

  asm("xor %%ebp,%%ebp" ::: "ebp");

  i = setjmp(env);
  printf("i = %d\n", i);

  if (i == 0)
    longjmp(env, 2);

  return i;
}

int main(void)
{
  return test();
}

Remove the asm statement to make it not crash.  Obviously with
omit-frame-pointer, gcc can shove anything into ebp.

Bob