Re: ix86 double alignment (was Re: egcs-1.1 release schedule)

[john@feith.com (John Wehle)]

> It's an interesting question to think about.  HP recommends a 64byte
> alignment for the stack on PAs.  It has some *really* nice benefits
> as far as the dcache is concerned.  And until about a year ago we
> actually followed that guideline -- by setting STACK_BOUNDARY appropriately :-)
> 
> That's how I know about the problems that combine will cause if you
> end up with a mis-aligned stack pointer relative to STACK_BOUNDARY.
> It turned out the crt0 code on hpux10 only provided 8 byte alignment
> for the stack pointer.  Opps.

What about defining PREFERRED_STACK_BOUNDARY to mean the optimal stack
alignment and having it default to STACK_BOUNDARY?  Then change the
places which align the stack based on STACK_BOUNDARY to use
PREFERRED_STACK_BOUNDARY.  Leave code which implements optimizations
(and records the stack alignment) based on STACK_BOUNDARY alone.  This
way gcc will attempt to align the stack based on PREFERRED_STACK_BOUNDARY
and assume STACK_BOUNDARY when implementing optimizations which should
be safe (assuming that PREFERRED_STACK_BOUNDARY >= STACK_BOUNDARY is
enforced).

I known ... I've probably oversimplified the issue. :-)

-- John
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