[Ffmpeg-devel] [BUG] Compilation failure when using --disable-opts

[Panagiotis Issaris]

M?ns Rullg?rd schreef:
> Panagiotis Issaris <takis at issaris.org> writes:
> 
>> Hi M?ns,
>>
>> M?ns Rullg?rd schreef:
>>> Panagiotis Issaris <takis at issaris.org> writes:
>>>> Diego Biurrun schreef:
>>>>> On Wed, Mar 14, 2007 at 09:32:51PM +0100, Panagiotis Issaris wrote:
>>>>>> M?ns Rullg?rd schreef:
>>>>>>> Panagiotis Issaris <takis.issaris at uhasselt.be> writes:
>>>>>>>
>>>>>>>> +int main(){
>>>>>>>> +    volatile int i=0;
>>>>>>>> +    asm volatile (
>>>>>>>> +        "xorl %%ebp, %%ebp"
>>>>>>>> +    ::: "%ebp");
>>>>>>>> +    return i;
>>>>>>>> +}
>>>>>>>> +EOF
>>>>>>> And what's the "int i" good for?
>>>>>> The 'i' is set at the beginning of the program, and so base pointer will
>>>>>> be used to initialize it (it is on the stack). At the end of the program
>>>>>> the the contents of 'i' is the returnvalue of the program and so the
>>>>>> base pointer will again be used to get at the value of 'i'. As it is set
>>>>>> to zero by the inline assembly code, a 0+offset pointer will be
>>>>>> dereferenced caused a segfault. Without the 'i' or with
>>>>>> -fomit-frame-pointer this won't happen.
>>>>> Could you add a comment?  This is non-intuitive..
>>>> Sure.
>>>>
>>>> Would you prefer the shorted but less clear:
>>>>
>>>> +# check whether EBP is available on x86
>>>> +# As 'i' is stored on the stack, this program will crash
>>>> +# if a the base pointer is used to access it because the
>>>> +# base pointer is cleared in the inline assembly code.
>>> That will do nicely.
>> Updated patch attached. I've removed the x86_64 test for now and moved the
>> comment inside the conditional as the other patch adds the other test in
>> the same conditional. (And I got rid of the forgotten "a" in the "if a
>> the base pointer".)
> 
> Why not merge the patches?  They're quite small and not entirely
> unrelated.
Fine by me. I only posted them separately because I figured I?d get the
comment to split them up :) In fact, I generated them using interdiff
from a combined patch =)


With friendly regards,
Takis

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