[PATCH] D140860: [Diagnostics][NFC] Fix -Wlogical-op-parentheses warning inconsistency for const and constexpr values

[David Blaikie via Phabricator via cfe-commits]

dblaikie accepted this revision.
dblaikie added a comment.
This revision is now accepted and ready to land.

In D140860#4047632 <https://reviews.llvm.org/D140860#4047632>, @hazohelet wrote:

> In D140860#4047534 <https://reviews.llvm.org/D140860#4047534>, @aaron.ballman wrote:
>
>> In D140860#4045224 <https://reviews.llvm.org/D140860#4045224>, @dblaikie wrote:
>>
>>> In D140860#4044937 <https://reviews.llvm.org/D140860#4044937>, @aaron.ballman wrote:
>>>
>>>> In D140860#4031872 <https://reviews.llvm.org/D140860#4031872>, @dblaikie wrote:
>>>>
>>>>> The risk now is that this might significantly regress/add new findings for this warning that may not be sufficiently bug-finding to be worth immediate cleanup, causing users to have to choose between extensive lower-value cleanup and disabling the warning entirely.
>>>>>
>>>>> Have you/could you run this over a significant codebase to see what sort of new findings the modified warning finds, to see if they're high quality bug finding, or mostly noise/check for whether this starts to detect certain idioms we want to handle differently?
>>>>>
>>>>> It might be hard to find a candidate codebase that isn't already warning-clean with GCC (at least Clang/LLVM wouldn't be a good candidate because of this) & maybe that's sufficient justification to not worry too much about this outcome...
>>>>>
>>>>> @aaron.ballman curious what your take on this might be
>>>>
>>>> Thank you for the ping (and the patience waiting on my response)!
>>>>
>>>> I think there's a design here that could make sense to me.
>>>>
>>>> Issuing the diagnostic when there is a literal is silly because the literal value is never going to change. However, with a constant expression, the value could change depending on configuration. This begs the question of: what do we do with literals that are expanded from a macro? It looks like we elide the diagnostic in that case, but macros also imply potential configurability. So I think the design that would make sense to me is to treat macro expansions and constant expressions the same way (diagnose) and only elide the diagnostic when there's a (possibly string) literal. WDYT?
>>>
>>> Yeah, I'm OK with that - though I also wouldn't feel strongly about ensuring we warn on the macro case too - if the incremental improvement to do constexpr values is enough for now and a note is left to let someone know they could expand it to handle macros.
>>>
>>> But equally it's probably not super difficult to check if the literal is from a macro source location that differs from the source location of either of the operators, I guess? (I guess that check would be needed, so it doesn't warn when the macro is literally 'x && y || true' or the like.
>>
>> I mostly don't want to insist on dealing with macros in this patch, but it does leave the diagnostic behavior somewhat inconsistent to my mind. I think I can live without the macro functionality though, as this is still forward progress. And yes, you'd need to check the macro location against the operator location, I believe. Testing for a macro expansion is done with `SourceLocation::isMacroID()`, in case @hazohelet wants to try to implement that functionality as well.
>
> I ran the diagnostic over `microsoft/lightgbm`, `oneapi-src/oneTBB`, and `rui314/mold` builds. As a result, I found no new warnings from this patch.
>
> To my surprise, both unpatched/patched clang does not issue the `-Wlogical-op-parentheses` warning for the following code I mentioned in the previous comment.
>
>> https://github.com/oneapi-src/oneTBB/blob/e6e493f96ec8b7e2e2b4d048ed49356eb54ec2a0/src/tbbmalloc/frontend.cpp#L1266
>
> It is because clang does not issue warnings on `x || y && z` and `x && y || z` in the result of macro expansions as of now.
> I found an issue on GitHub:
> https://github.com/llvm/llvm-project/issues/19345

Ah, thanks for doing the archaeology - and looks like there's an abandoned patch there that might be relevant to addressing some of those macro issues.

>> And yes, you'd need to check the macro location against the operator location, I believe. Testing for a macro expansion is done with `SourceLocation::isMacroID()`, in case @hazohelet wants to try to implement that functionality as well.
>
> Thanks for your help. I think testing macro location against the operator is already handled in `DiagnoseBinOpPrecedence`, and is somewhat relevant to the issue above.
>
> Anyway, I confirm no new instances of parentheses warning in the three repositories above.

Awesome :)

Seems reasonable to me, then - how about you, @aaron.ballman  ?


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