Re: libffi doesn't keep the stack aligned to 16 bytes

[Andrew Haley <aph at redhat dot com>]

On 07/01/2010 07:07 PM, Andrew Haley wrote:
> On 07/01/2010 06:47 PM, Neil Roberts wrote:
>> Hi
>>
>> GCC appears to assume by default that the stack is always aligned to 16
>> bytes when entering a function. Presumably this is to make it easier to
>> use the aligned access instructions to load data into SSE
>> registers. This means that if you have a variable with the aligned(16)
>> attribute then GCC knows that it doesn't have to add an extra
>> instruction to align the stack pointer because it will already be
>> aligned.
>>
>> However, libffi doesn't appear to respect this. When it calls a function
>> it allocates space on the stack by decrementing the stack pointer
>> according to the amount of space required. This means the depending on
>> the number of arguments the stack may or may not be aligned.
>>
>> This ends up causes crashes when calling libraries that use -mfpmath=sse
>> because then GCC will try to store temporary SSE variables on the stack
>> and it will assume it can use aligned access instructions.
>>
>> Attached is a test case which demonstates the problem and a patch to fix
>> it. We are using this patch in MeeGo because all of the libraries are
>> compiled with SSE math so we were getting crashes with gjs.
> 
> That looks right to me.

Should I just commit this patch as is?

Andrew.