Re: [PATCH] -Walloca-larger-than with constant sizes at -O0 (PR 100425)

[Richard Biener <richard.guenther@gmail.com>]

On Wed, May 5, 2021 at 7:29 PM Martin Sebor <msebor@gmail.com> wrote:
>
> On 5/5/21 1:32 AM, Richard Biener wrote:
> > On Wed, May 5, 2021 at 4:20 AM Martin Sebor via Gcc-patches
> > <gcc-patches@gcc.gnu.org> wrote:
> >>
> >> Even when explicitly enabled, -Walloca-larger-than doesn't run
> >> unless optimization is enabled as well.  This prevents diagnosing
> >> alloca calls with constant arguments in excess of the limit that
> >> could otherwise be flagged even at -O0, making the warning less
> >> consistent and less useful than is possible.
> >>
> >> The attached patch enables -Walloca-larger-than for calls with
> >> constant arguments in excess of the limit even at -O0 (variable
> >> arguments are only handled with optimization, when VRP runs).
> >
> > Hmm, but then the pass runs even without -Walloca or -Walloca-larger-than
> > or -Wvla[-larger-than].  It performs an IL walk we should avoid in those
> > cases.
> >
> > So the patch is OK but can you please come up with a gate that disables
> > the pass when all of the warnings it handles won't fire anyway?
>
> -W{alloca,vla}-larger-than=PTRDIFF_MAX are enabled by default so
> the pass needs to do walk.
>
> FWIW, it would make sense to me to consolidate all the checking of
> calls for arguments with excessive sizes/values into the same pass
> and single walk (with code still in separate source files).  As it
> is, some are done in their own passes (like alloca and sprintf),
> and others during expansion (-Wstringop-overflow), and others in
> calls.c (-Walloc-size-larger-than).  That leads to repetitive code
> and inconsistent approaches and inconsistent false positives and
> negatives (because some are done at -O0 but others require
> optimization).

True - that would be a nice cleanup (and speedup as well).

Richard.

> Martin
>
> >
> > Thanks,
> > Richard.
> >
> >> Martin
>