RFA: new pass to warn on questionable uses of alloca() and VLAs

[Martin Sebor]

On 07/11/2016 09:40 AM, Aldy Hernandez wrote:
> On 07/11/2016 11:08 AM, Martin Sebor wrote:
>>>
>>> Hey, I'm all for different options.  It would definitely be easier for
>>> me :).  I wast trying really hard to use -Wvla= and keep the current
>>> -Wvla meaning the same.  Though I see your point about using -Wvla* but
>>> using different variables for representing -Wvla and -Wvla=blah.
>>>
>>> The easiest thing would be:
>>>
>>> -Wvla: Current behavior (warn on everything).
>>>
>>> -Wvla-larger-than=xxxx: Warn on unbounded VLA and bounded VLAs > xxxx.
>>>
>>> -Walloca: Warn on every use of alloca (analogous to -Wvla).
>>>
>>> -Walloca-larger-than=xxxx: Warn on unbounded alloca and bounded allocas
>>>  > xxxx.
>>>
>>> This seems straightforward and avoids all the overloading you see. Would
>>> this be ok with everyone?
>>
>> I like the consistency.
>>
>> Given the common root with -Wlarger-than=N, what do envision the effect
>> of setting -Wlarger-than=N to be on the two new options?
>>
>> FWIW, I tend to view -Wlarger-than= and certainly -Wframe-larger-than=
>> not warning on alloca larger than N as a defect.  It could be fixed by
>> simply hooking -Walloca-larger-than= up to it.
>
> I'm not touching any of these independent options.  -Wvla-larger-than=
> and -Walloca-larger-than= will be implemented independently of whatever
> -Wlarger-than= and -Wframe-larger-than=.  Any problems with these last
> two options can be treated indpendently (PRs).

Strictly speaking there is no defect in -Wframe-larger-than= because
it's documented not to warn for alloca or VLAs.  I asked because it
seems like an unnecessary (and IMO undesirable) limitation that could
be easily removed as part of this patch.  Not removing it, OTOH, and
providing a separate solution, feels like highlighting the limitation.
With the new options, users interested in detecting all forms of
excessive stack allocation will be able to do so but not using
a single option.  Instead, they will need to use all three.

Martin