Misalignment of local SSE variables in thread function

[Tim Prince]

Jonathan Kinsey wrote:
> Tim Prince wrote:
>   
>> Jonathan Kinsey wrote:
>>     
>>> Tim Prince wrote:
>>>  
>>>       
>>>> I don't remember what was said in the past, but any gcc function which
>>>> uses SSE parallel instructions must be called from a gcc function which
>>>> passes a 16-byte aligned stack.  So you may be able to overcome it by
>>>> interposing a wrapper function at the thread entry point (which does not
>>>> use parallel SSE), with all gcc functions compiled with normal options
>>>> (not -Os, which sets a smaller value in -mpreferred-stack-alignment).
>>>> To speculate further, possibly your pthreads library was compiled with
>>>> options which don't pass 16-byte aligned stack.
>>>>     
>>>>         
>>> Thanks for the reply - no help though:
>>>
>>> By a wrapper function, if you mean for f() to call say g() - this makes
>>> no difference (command line: gcc -msse test.c -otest.exe).
>>>
>>> gcc doesn't seem to recognize -mpreferred-stack-alignment=8 as a valid
>>> option.
>>>
>>> I'm not using pthreads, so it's not that either.
>>>
>>>
>>>   
>>>       
>> Sorry, you should have checked me.  'info gcc'
>> -mpreferred-stack-boundary=4     (2^4 == 16) is needed, and would be set
>> by options other than -O2.  So, not specifying any relevant options
>> should be OK, but the function you call first in the thread must not
>> have any sse, if this is the problem.
>> I guess I don't understand where you got your function to start a thread.
>>     
>
> I found -mpreferred-stack-boundary and -falign-functions options, I
> think they both default to 4, so it's not relevant to set these (I'm not
> using any optimizations).
>
> I'll take a look at the assembly code and see if anything is different
> (between gcc and msvc) for the thread function.  It looks like gcc is
> getting the stack four bytes off alignment in the thread function for
> some reason.
>
>
>
>
>   
If your function gets a mis-aligned stack, that is not the fault of 
gcc.  However, if your gcc function passes a mis-aligned stack, when you 
use the correct options, that is a bug.
gcc is more idealistic than commercial compilers which incur the 
additional overhead of adjusting the stack everywhere it is needed.
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