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

[Blue Swirl <blauwirbel@gmail.com>]

On Mon, Oct 17, 2011 at 5:22 PM, Richard Henderson <rth@twiddle.net> 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.
>
>> 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?
>
> 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.

IIRC buggy versions of alloca() could also fail without a frame pointer.