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

[Richard Trieu]

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.

>
>
>> 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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>>>>
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