alignment issues for sse

Eljay Love-Jensen
Wed Feb 16 06:05:00 GMT 2005

Hi Brian,

As I understand it (in this alignment situation), the __attribute__ is 
applicable to a variable, not to a type.

Also, the alignment of the variable is constrained by whatever alignment 
support is provided by the linker.  For example, if you need 256 byte 
alignment for a PMMU page, many linkers may not support such a big 
alignment constraint and will only honor the request to the best of their 

So for certain things, such as PMMU swap pages, the alignment burden often 
falls on the shoulders of the software engineer handling PMMU swap 
pages.  (It is MOST unfortunate that the malloc routine doesn't have an 
extra optional parameter for alignment requirements.  Alas and alack.)

Let's hope that's not the issue you are running into.

I don't know if __mode__(__V4SF__) entails the alignment request.  You'll 
have to double-check the documentation.

Regardless, I think you should replace your typedef with a struct.

struct __m128 { int m __attribute((__mode__(__V4SF__))); };

And if the __mode__(__V4SF__) doesn't entail the alignment constrained, add 
in the aligned(16) as well.

And if your linker doesn't honor the aligned(16) request, you'll have to 
take some extra-ordinary steps -- let's hope you don't have to go there.


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