[PATCH] store VLA bounds in attribute access as strings (PR 97172)

[Richard Biener]

On Sun, Dec 20, 2020 at 6:43 PM Martin Sebor <msebor@gmail.com> wrote:
>
> On 12/19/20 12:48 AM, Richard Biener via Gcc-patches wrote:
> > On December 19, 2020 1:55:02 AM GMT+01:00, Martin Sebor via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> >> To keep tree expressions stored by the front end in attribute
> >> access for nontrivial VLA bounds from getting corrupted during
> >> Gimplification and to avoid breaking the preconditions verified
> >> by the LTO streamer that no such trees exist in the IL,
> >> the attached patch replaces those bounds with a string
> >> representation of those expressions (as STRING_CST).  It also
> >> tweaks the pretty-printer to improve the formatting of the VLA
> >> bounds and avoid inserting spurious spaces in some cases.
> >>
> >> The strings are only used by the front end to verify that
> >> redeclarations of the same function match in the form and bounds
> >> of their VLA arguments, and they're not needed in the middle end.
> >> I considered removing them just before the front end finishes but
> >> I couldn't find an efficient way to do that.  Is there some data
> >> structure that stores all function declarations in a translation
> >> unit?  If there is, then traversing it and removing the attribute
> >> arguments might also be an option, either in addition to this
> >> change or in lieu of it.
> >
> > There is the free lang data pass in tree.c which walks all reachable tree nodes.
>
> You said in response to Honza's suggestion in pr97172 to do that:
>
>    The frontend needs to make sure no frontend specific tree codes
>    leak into GENERIC so GENERICization is the place where the FE
>    should clear those.  FLD is too late and doing it there would
>    be a hack.
>
> Are you now saying you've had a change of heart and that doing it
> there isn't a hack after all?

Well, removing a FE specific attribute [when having non-constant arg] might
be OK there.  But note you asked for a "pass over all tree nodes" thing
and I just pointed out free-lang-data.

Doing the STRING_CST is certainly less fragile since the SSA names
created at gimplification time could even be ggc_freed when no longer
used in the IL.

Richard.

> Martin
>
> >> The patch was tested on x86_64-linux.
> >>
> >> Martin
> >
>