Re: Question about -mpreferred-stack-boundary

[Andreas Jaeger <aj at suse dot de>]

"Geert Bosch" <bosch@gnat.com> writes:

> On 03 Feb 2001 14:56:06 +0100, Andreas Jaeger wrote:
> 
>   I should have descripted the problem in more detail: glibc's startup
>   code aligns the stack to 32 bytes - and then calls a function.  Now,
>   if you tell me that the stack has to be aligned (e.g. to 32 bytes)
>   *after* the push of the return address, then glibc is wrong - but if
>   it has to be aligned *before* the push of the return address, we might
>   have a bug in GCC.
> 
> I was going to raise this issue, but decided not to make my previous
> mail more confusing than necessary :-) As I described, the main reason
> for using the preferred stack boundary is to (try to) have arguments
> that are passed by value on the stack properly aligned.
> 
> Consider a function f that takes two long float values as arguments 
> x and y. Long floats are passed by value and reading them from addresses 
> that are not 8-byte aligned is costly. The i386 ABI requires the stack
> to look as follows when control is transferred to f:
> 
>                ....
> 16(%esp)        y (hi)
> 12(%esp)        y (lo)
>  8(%esp)        x (hi)
>  4(%esp)        x (lo)
>  0(%esp)     [return address]
> 
> So you'd like 4(%esp) to be 8-byte aligned. So %esp should be 8-byte
> aligned before calling the function.

Ok, then glibc is correct and it seems we've found a bug in GCC.  I'm
currently bootstrapping GCC with a patch that might solve this.

Btw. glibc only aligns the code to 8 bytes, I'll propose to change
this.

Thanks,
Andreas
-- 
 Andreas Jaeger
  SuSE Labs aj@suse.de
   private aj@arthur.inka.de
    http://www.suse.de/~aj