Re: [PATCH] PR91195: fix -Wmaybe-uninitialized warning for conditional store optimization

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Jul 24, 2019 at 12:07:36PM -0600, Martin Sebor wrote:
> > > There are a number of existing instances of setting TREE_NO_WARNING
> > > to suppress -Wuninitialized, and some are the cause of known problems.
> > > Bugs 51545, 58950, 74762, 74765 or 89697 are examples.  They all boil
> > > down to the fact that there's just one bit for all warnings.  Jakub
> > > mentioned adding a hash-map for this.  That seems like a simple and
> > > good solution.
> > I'm not sure how that really helps here.  We marking the MEM on the LHS
> > and that's not a shared object.  I don't see how it's going to be
> > significantly different using a hash map vs the bit in this circumstance.
> 
> I don't know what Jakub had in mind but the mapping I envision is
> one like hash_map<tree, bitmap> that would make it possible to set
> a bit for each distinct warning for every tree node.  It would let
> us set a bit for -Wuninitialized while leaving the bit for
> -Warray-bounds (and all other warnings) clear.

I had in mind something like a hash_map<tree, const_bitmap> / hash_map<gimple *,
const_bitmap> (or just make it a <void *, const_bitmap>) where possibly the
bitmaps would be shared, so have a hash table in which one would look up existing
bitmaps containing particular sets of warnings and if not create a new one (and
where the bitmaps would be const after creation).
The TREE_NO_WARNING or gimple_no_warning bits would be a flag that one
should look up the hash_map if needed, those bits clear would imply empty
bitmap.  Routines that copy trees or gimple stmts, like copy_node or
gimple_copy would take care of adding the new tree/gimple * into the
hash_map if needed too.
And, because we have a lot of warnings that are only emitted in the FEs, or
say before IPA and not afterwards, we could also have spots where we'd
replace the bitmaps with ones that don't have any of the no longer
interesting warning bits.
Say during gimplification we could drop TREE_NO_WARNING bit or not set
gimple_no_warning if the bitmap only contains FE warning bits, or (perhaps
more questionable whether worth it) replace with a const_bitmap that doesn't
have those).

	Jakub