Re: Add a param to decide stack slot sharing at -O0

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Oct 8, 2013 at 11:04 PM, Easwaran Raman <eraman@google.com> wrote:
> In cfgexpand.c, variables in non-overlapping lexical scopes are
> assigned same stack locations at -O1 and above. At -O0, this is
> attempted only if the size of the stack objects is above a threshold
> (32). The rationale is at -O0, more variables are going to be in the
> stack and the O(n^2) stack slot sharing algorithm will increase the
> compilation time. This patch replaces the constant with a param which
> is set to 32 by default. We ran into a case where the presence of
> always_inline attribute triggered Wframe-larger-than warnings at -O0
> but not at -O2 since the different inlined copies share the stack. We
> are ok with a slight increase in compilation time to get smaller stack
> frames even at -O0 and this patch would allow us do that easily.
>
> Bootstraps on x86_64/linux. Is this ok for trunk?

Ok with

+DEFPARAM (PARAM_MIN_SIZE_FOR_STACK_SHARING,
+         "min-size-for-stack-sharing",
+         "Attempt to share stack slots among variables in different
lexical blocks "
+         "at O0 only if their sizes exceed this value",
+         32, 0, 0)

changed to

           "The minimum size of variables taking part in stack slot sharing "
           "when not optimizing"
           32, 0, 0)

And with adding documentation for that param in doc/invoke.texi.

Btw, I'm not sure the sharing algorithm is still quadratic - can you
investigate on that?

Thanks,
Richard.

> Thanks,
> Easwaran
>
>
> 2013-10-08  Easwaran Raman <eraman@google.com>
>
>         * params.def (PARAM_MIN_SIZE_FOR_STACK_SHARING): New param...
>         * cfgexpand.c (defer_stack_allocation): ...use here