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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.

Yes, it is tricky, but it would allow us to simply align frame when we
know we need it aligned.
> >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 

No, i386 is trickier.  Originally it was designed as architecture that
allow missaligned memory accesses everywhere and just recently this
broke with SSE.  Everything earlier (even MMX) allows missaligned
loads/stores, so we don't lose when at -Os we don't align stack frame to
16 byte boundary.
> alignment, but will work with lesser alignment, then its alignment 
> requirements aren't the same as a vector (??).
> >Ideas?
> 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 
> function.c:expand_main_function()).

The problem with this is that function don't need to have SSE vector
argument and still can use SSE internally, so this scheme would miss it.
> To insure safety, any function that accepts vector arguments would 
> insist upon a suitable stack alignment at compile time (hard error). 
Outputting such hard errors when compiling function that accepts vectors
can be implementable and can tell user that -Os without
-mpreferred-stack-boundary is no-go, so perhaps this is pretty good
idea.  WHat other thinks?
> 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.  :-(

You can do such tricks in init_cumulative_args code.

> stuart hastings
> Apple Computer

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