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


On 7/30/19 8:42 AM, Martin Sebor wrote:
> On 7/30/19 2:44 AM, Richard Biener wrote:
>> On Tue, Jul 30, 2019 at 10:35 AM Jakub Jelinek <jakub@redhat.com> wrote:
>>>
>>> On Tue, Jul 30, 2019 at 10:21:15AM +0200, Richard Biener wrote:
>>>> Would you need to LTO stream & merge the bitmaps / maps somehow?
>>>
>>> Yes.  And if we do not throw unneeded warnings from the sets
>>> normally, LTO
>>> streaming might be a good time to do that, so that we merge in only
>>> warnings
>>> that will be tested during/post IPA.
>>>
>>>> (maybe "unmap" to sth else when streaming individual stmts)  Not
>>>> sure if
>>>> a bitmap is really necessary, we could simply have a tree/gimple *
>>>> -> vec<>
>>>> mapping with records about emitted (or queued) diagnostics, like
>>>> OPT_, message pairs or so.
>>>
>>> Right now we use it both for the case we've already emitted some
>>> diagnostics
>>> and don't want to emit it again later, or for the case where we insert
>>> something in the IL that a warning could be diagnosed on but we know we
>>> don't want that.  The latter is the case e.g. for when we add
>>> TREE_NO_WARNING on the last value in a statement expression so that
>>> we don't
>>> diagnose it as unused, or the case we are discussing here.
>>> If we need queued diagnostics, sure, we could add it in, but I don't
>>> see a
>>> need for that right now.  vecs compared to bitmap might be larger and
>>> harder
>>> to avoid duplicates.  I guess we'd need to do some analysis though,
>>> and e.g.
>>> if 99% of cases we need to disable just one warning and not multiple,
>>> perhaps optimize for that case.
>>
>> I was thinking we may want to use the same facility to record warnings we
>> want to not emit if we later figure a stmt was in an unreachable
>> region.  For
>> this we need to record the actual warning, not only the warning kind.
> 
> I was thinking of introducing a __builtin_warning() for this and
> issuing it if it's not eliminated just before RTL expansion.  That
> would let programs like libstdc++ use it too.
I've stated before, I think this is a *great* idea and would encourage
you to prototype it out to see if there's any significant gotchas.
There's several places I think we could use it.

> 
> The downside of these solutions is that warnings may be issued out
> of order as code moves around.  To have the issued in the order of
> increasing source lines they would need to be queued.
I think David is out right now, but I think he'd claim that emitting
diagnostics strictly in line number order isn't best anyway.  Building a
queuing mechanism with an eye towards a more complex comparison to
facilitate priority ordering of diagnostics would be a good thing.  He
may already have some prototype code, not sure on that.

jeff


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