Re: [PATCH 4/6]: Ping: Merge from Stack Branch - i386 backend changes

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, May 5, 2008 at 8:12 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Mon, May 5, 2008 at 5:57 PM, Ye, Joey <joey.ye@intel.com> wrote:
>  > Jan,
>  >
>  > > You can certainly track this with callgraph.  I.e. use case 0 by
>  >  > default, but case 2 if you know bar is being called only from foos
>  >  that
>  >  > will align anyway.
>  >  > Since we compile in topological order, it all depends on how well you
>  >  > are able to decide on alignments at tree level or if you want to go
>  >  down
>  >  > the road of alternate entry point (so foo can skip alignment prologue
>  >  if
>  >  > called from function with proper alignment guaranteed).
>  >  How about bar is called from other compilation units? Call callgraph
>  >  track that?
>  >
>
>  As far as I can tell, preferred_incoming_stack_boundary tracked by callgraph
>  is a best effort approach. There is no guarantee that all callers will satisfy
>  preferred_incoming_stack_boundary needed by callee. If callee needs strict
>  alignment, it has to align the stack itself.
>
>  With automatic stack alignment, we don't need to collect
>  preferred_incoming_stack_boundary
>  in callgraph.  Without automatic stack alignment, we align outgoing stack
>  to PREFERRED_STACK_BOUNDARY, no more and no less. If
>  preferred_incoming_stack_boundary > PREFERRED_STACK_BOUNDARY,
>  we still can't align outgoing stack to preferred_incoming_stack_boundary.
>

Hi Joey,

If we know all incoming stack alignments of functions we call, can we optimize
stack for caller? That is for

foo ()
{
  bar1 ();
...
  bar2 ();
}

the required stack boundary of foo will be

MAX (LOCAL, MIN (PREFERRED, MAX (INCOMING (bar1), INCOMING (bar2))))

We may be able to use less stack in foo if the required stack boundary
is smaller.
But it won't affect stack alignment in foo since we won't align
outgoing stack beyond
PREFERRED unless we need to put something on stack.


H.J.