[PATCH] Add new warning to Clang to detect when all code paths in a function has a call back to the function.

[Richard Smith]

On Tue, Nov 12, 2013 at 10:37 PM, David Blaikie <dblaikie at gmail.com> wrote:

> On Tue, Nov 12, 2013 at 10:30 PM, Richard Trieu <rtrieu at google.com> wrote:
>
>> On Tue, Nov 12, 2013 at 9:56 PM, Richard Smith <richard at metafoo.co.uk>wrote:
>>
>>> On Tue, Nov 12, 2013 at 9:44 PM, Richard Trieu <rtrieu at google.com>wrote:
>>>
>>>> Timing details:
>>>>
>>>> 21.03s for Clang
>>>> 21.27s for Patched Clang with warning
>>>>
>>>> This is a difference of .24s or less than 1.5% slowdown.
>>>>
>>>> Times are averaged over 20 runs.  Input is the same pre-processed file.
>>>>  Both Clang binaries were built from the same revision.  -fsyntax-only was
>>>> also used.
>>>>
>>>
>>> What's your test case?
>>>
>> Portion of Clang driver code.
>>
>>> Is this some randomly-chosen file or is it selected to be the worst case
>>> for this warning somehow?
>>>
>> Randomly chosen.
>>
>>> A 1.5% slowdown on average seems like far too much for this. Was this in
>>> a setup where the CFG was being built regardless (for instance, with
>>> -Wuninitialized enabled)?
>>>
>> I used the standard Clang invocation.  I am not sure if that constructs
>> the CFG.  Turning -Wuninitialized on and off produces similar timing
>> differences to this warning.
>>
>
> The important question is what's the timing difference between:
> -Wuninitialized -Wyour-new-warning and just -Wuninitialized
>
> That way you won't be comparing the timing of CFG building and no CFG
> building, but CFG building with your warning and CFG building without your
> warning.
>

Right. If this warning doesn't add measurable compile time in a build
that's already constructing CFGs, then that's OK, but it needs to be off by
default, like the other CFG-based warnings.

On Fri, Nov 8, 2013 at 10:47 PM, Sean Silva <silvas at purdue.edu> wrote:
>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Fri, Nov 8, 2013 at 8:52 PM, Chandler Carruth <chandlerc at google.com
>>>>> > wrote:
>>>>>
>>>>>> Random thoughts...
>>>>>>
>>>>>> On Fri, Nov 8, 2013 at 3:20 PM, Sean Silva <silvas at purdue.edu> wrote:
>>>>>>
>>>>>>> At a higher level, is this really needed as a compiler warning? I
>>>>>>> mean, it's nice and all to detect these things statically, but is this
>>>>>>> really something that needs to be happening on every build?
>>>>>>
>>>>>>
>>>>>> Note that if this is a significant compile time issue, that's
>>>>>> relevant. Everything else is assuming that it isn't. =]
>>>>>>
>>>>>> Could we just do this in the static analyzer? In practice, I can't
>>>>>>> imagine any of these being hard to debug "while developing", since they
>>>>>>> will always result in a stack overflow the second they are called (and then
>>>>>>> you just look at the core file (or the debugger that your IDE attached) to
>>>>>>> see which function it was)
>>>>>>
>>>>>>
>>>>>> Your description alone is the evidence for why developers should have
>>>>>> a warning. ;] First off, stack overflows are notoriously annoying to debug.
>>>>>> Core files are often missing. Running under the debugger is yet another
>>>>>> step to do. I would much rather the compiler just tell me about it.
>>>>>>
>>>>>
>>>>> Absolutely; a static warning is always preferable. All my comments
>>>>> were in light of a perception that this was maybe not cheap enough to
>>>>> justify running at every compile (of course, need to wait for Richard to
>>>>> respond with some real measurements of the compile time impact).
>>>>>
>>>>> -- Sean Silva
>>>>>
>>>>>
>>>>>>
>>>>>> Almost all of our warnings "aren't needed" because they could be
>>>>>> tested and debugged. That doesn't remove their value.
>>>>>>
>>>>>>
>>>>>>> So essentially it seems like this is finding bugs in code that has
>>>>>>> no test coverage and has never been executed in practice; that kind of
>>>>>>> "cleaning out crusty unused parts of the codebase" seems like it would be
>>>>>>> better left to the static analyzer.
>>>>>>
>>>>>>
>>>>>> No, it's shortening the cycle time for developers that hit this bug.
>>>>>>
>>>>>> It also is cleaning out bugs in crufty code, which is always nice.
>>>>>> Very few people will run the static analyzer over all of their crufty code
>>>>>> because if they aren't changing it and it is working in production, they
>>>>>> aren't going to want to sift through the false positives of "maybe that's a
>>>>>> bug". Static analyzers run much more over code under active development.
>>>>>>
>>>>>> And fundamentally, we *routinely* do all of the static analysis that
>>>>>> is sufficiently inexpensive and has sufficiently rare false positives at
>>>>>> compile time. So I think we should focus on those two criteria, the latter
>>>>>> one being the most interesting here.
>>>>>>
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