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Re: x86: -Os -msse2 needs -maccumulate-outgoing-args

On Jan 13, 2004, at 12:51 PM, Jan Hubicka wrote:

This testcase

Hi, this is not -maccumulate-outgoing-args that breaks, it is -mpreffered-stack-boundary=2 implied by -Os. We still don't have the dynamic stack alignment code merged in, unfortunately.

I looked at this, and it's not clear to me how it would help. Here's the thread:

Also, given the unique semantics of the Darwin linker, I'm not sure this could work on Darwin today (our linker "knows" too much about entry points). I think this would be implementable with a future Darwin linker.

It would be probably sane to give some warning or hard error when SSE
register is put onto stack, but I don't see easy way to do this, as we
should not output such a warning for long doubles that have same
alignment properties.

I'm sorry, I didn't fully understand this.

If a long double has the same alignment properties as a vector, then misaligned loads of that long double should fault just like a misaligned vector load (?). If the long double prefers a high alignment, but will work with lesser alignment, then its alignment requirements aren't the same as a vector (??).


Here's another approach:

If main() is part of a module compiled with -mpreferred-stack-boundary=2, and main() wants to pass a vector argument to function foo_v() in another module, main() knows that A) main()s own stackframe is not aligned, and B) since foo_v() is expecting a vector parameter, foo_v() must have been compiled with a suitable stack alignment. (O.K., it's a weak argument.)

Instead of generating a warning, howabout forcing the stack into alignment just for the call to foo_v() ? Sort of a per-call variant of FORCE_PREFERRED_STACK_BOUNDARY_IN_MAIN (see function.c:expand_main_function()).

To insure safety, any function that accepts vector arguments would insist upon a suitable stack alignment at compile time (hard error). The problem I see here is what to do when a function accepts variable arguments...

Ack! I just suggested an additional check to calls.c; it is already much too complicated. :-(

stuart hastings
Apple Computer

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