Re: [PATCH] PR tree-optimization/107570 - Reset SCEV after folding in VRP.

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

On Wed, Feb 1, 2023 at 7:12 PM Andrew MacLeod via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> We can reset SCEV after we fold, then SCEVs cache shouldn't have
> anything in it when we go to remove ssa-names in remove_unreachable().
>
> We were resetting it later sometimes if we were processing the array
> bounds warning, so I removed that call and just always reset it now.
>
> Bootstraps on x86_64-pc-linux-gnu. Testing running. Assuming no
> regressions,  OK for trunk?

+
+  // SCEV needs to be reset for array bounds, and we do not wish to trigger
+  // any SCEV lookups when removing unreachable globals, so reset it here.
+  scev_reset ();

the comment suggests that SCEV queries (aka analyze_scalar_evolution)
won't return anything after a scev_reset ().  That's not true - instead what
it does is nuke the SCEV cache.  That's necessary when you
release SSA names or alter the CFG and you want to avoid followup
SCEV queries to pick up stale data.

So if remove_and_update_globals performs SCEV queries and eventually
releases SSA names you cannot remove the second call to scev_reset.

But yes, it's probably substitute_and_fold_engine::substitute_and_fold
itself that should do a

  if (scev_initialized_p ())
    scev_reset ();

possibly only in the case it released an SSA name, or removed an
edge (but that's maybe premature optimization).

Richard.

>
> Andrew