Re: Arranging for -Os to imply -mpreferred-stack-boundary=2 on x86

[Alexandre Oliva <aoliva at redhat dot com>]

On Oct 23, 2001, Jan Hubicka <jh@suse.cz> wrote:

> Also note that I do have patches somewhere to compute alignment required
> by given function after reload and use the value when compiling later
> calls to that function.

So we're talking about two different things.  The attribute I
suggested would get a function to ensure its stack pointer is aligned
to a certain boundary, so that its automatic variables are properly
aligned.  But now I realize this would require an additional register,
because %ebp can't be re-aligned otherwise you can't get to the
arguments.

Getting the caller to align the stack would also have the advantage
that sometimes the caller may be able to guarantee that the stack is
already properly aligned, thus avoiding the overhead in the callee of
introducing additional stack alignment, and the arguments, when passed
in the stack, would also be aligned correctly.

So I guess this is the way to go.

The problem that remains is that the preferred alignment of a function
won't be enforced when it's called from another translation unit,
unless the alignment requirements are explicitly stated in a function
declaration visible for the caller.  But then, when performance
matters so much, one may always make the actual hotspot into a static
function called by an exported wrapper function that introduces the
alignment and calls the other function.  But figuring out how this
interacts with tail calls and inlining is left as an exercise to the
reader :-)

-- 
Alexandre Oliva   Enjoy Guarana', see http://www.ic.unicamp.br/~oliva/
Red Hat GCC Developer                  aoliva@{cygnus.com, redhat.com}
CS PhD student at IC-Unicamp        oliva@{lsd.ic.unicamp.br, gnu.org}
Free Software Evangelist    *Please* write to mailing lists, not to me