Re: [PATCH] PR tree-optimization/109417 - Check if dependency is valid before using in may_recompute_p.

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

On Mon, Apr 24, 2023 at 3:51 PM Andrew MacLeod <amacleod@redhat.com> wrote:
>
>
> On 4/11/23 05:21, Richard Biener via Gcc-patches wrote:
> > On Wed, Apr 5, 2023 at 11:53 PM Jeff Law via Gcc-patches
> > <gcc-patches@gcc.gnu.org> wrote:
> >>
> >>
> >> On 4/5/23 14:10, Andrew MacLeod via Gcc-patches wrote:
> >>> When a statement is first processed, any SSA_NAMEs that are dependencies
> >>> are cached for quick future access.
> >>>
> >>> if we ;later rewrite the statement (say propagate a constant into it),
> >>> its possible the ssa-name in this cache is no longer active.   Normally
> >>> this is not a problem, but the changed to may_recompute_p forgot to take
> >>> that into account, and was checking a dependency from the cache that was
> >>> in the SSA_NAME_FREE_LIST. It thus had no SSA_NAME_DEF_STMT when we were
> >>> expecting one.
> >>>
> >>> This patch simply rejects dependencies from consideration if they are in
> >>> the free list.
> >>>
> >>> Bootstrapping on x86_64-pc-linux-gnu  and presuming no regressio0ns, OK
> >>> for trunk?
> >> eek.  So you've got a released name in the cache?  What happens if the
> >> name gets released, then re-used?  Aren't you going to get bogus results
> >> in that case?
>
> Its not a real cache..  its merely a statement shortcut in dependency
> analysis to avoid re-parsing statements every time we look at them for
> dependency analysis
>
> It is not suppose to be used for anything other than dependency
> checking.   ie, if an SSA_NAME changes, we can check if it matches
> either of the 2 "cached" names on this DEF, and if so, we know this name
> is stale.  we are never actually suppose to use the dependency cached
> values to drive anything, merely respond to the question if either
> matches a given name.   So it doesnt matter if the name here has been freed
>
>
> > We never re-use SSA names from within the pass releasing it.  But if
> > the ranger cache
> > persists across passes this could be a problem.  See
>
>
> This particular valueswould never persist beyond a current pass.. its
> just the dependency chains and they would get rebuilt every time because
> the IL has changed.
>
>
> > flush_ssaname_freelist which
> > for example resets the SCEV hash table which otherwise would have the
> > same issue.
> >
> > IIRC ranger never outlives a pass so the patch should be OK.
> >
> > _But_ I wonder how ranger gets at the tree SSA name in the first place - usually
> > caches are indexed by SSA_NAME_VERSION (because that's cheaper and
>
>
> Its stored when we process a statement the first time when building
> dependency chains.  All comparisons down the road for
> staleness/dependency chain existence are against a pointer.. but we
> could simple change it to be an "unsigned int",  we'd then just have to
> compare again SSA_NAME_VERSION(name) instead..
>
>
> > better than a pointer to the tree) and ssa_name (ver) will return NULL
> > for released
> > SSA names.  So range_def_chain::rdc could be just
> >
> >    struct rdc {
> >     int ssa1;           // First direct dependency
> >     int ssa2;           // Second direct dependency
> >     bitmap bm;           // All dependencies
> >     bitmap m_import;
> >    };
> >
> > and ::depend1 return ssa_name (m_def_chain[v].ssa1) and everything woul
> if the ssa-name is no longer in existence, does ssa_name (x) it return
> NULL?

Yes.

> > work automatically (and save 8 bytes of storage).
> >
> > Richard.
>
> >> jeff
>