r236075 - Add -Wpessimizing-move and -Wredundant-move warnings.

[Richard Trieu]

On Mon, May 4, 2015 at 10:38 AM, David Blaikie <dblaikie at gmail.com> wrote:

>
>
> On Sat, May 2, 2015 at 11:47 AM, Nico Weber <thakis at chromium.org> wrote:
>
>> On Wed, Apr 29, 2015 at 2:41 AM, Benjamin Kramer <benny.kra at gmail.com>
>> wrote:
>>
>>> On Wed, Apr 29, 2015 at 5:59 AM, Richard Trieu <rtrieu at google.com>
>>> wrote:
>>> > Since this warning did not exist when std::move was first introduced,
>>> and
>>> > many people are not familiar with copy elision rules, there have been
>>> a lot
>>> > of uses of the pessimizing move pattern.  In relative numbers, the
>>> > pessimizing move warning is over 5x more common than either the range
>>> loop
>>> > warning (-Wrange-loop-analysis) or the proposed comma warning
>>> (-Wcomma).
>>> > With such a high occurrence, it seemed better not to have it on by
>>> default.
>>>
>>> And every single occurence is a potential performance issue and easy
>>> to fix. While this may be too noisy for default, it's definitely a
>>> candidate for -Wall imho.
>>>
>>
>> +1. As-is, nobody will find this warning. And in general, the bar for
>> warnings is "useful enough to be in -Wall, or it shouldn't be there",
>> right? (Which this warning definitely is.)
>>
>
> I think the bar at least used to be "useful enough to be on by default or
> it shouldn't be there" (the big exception in any analysis based warning
> because building the CFG costs some non-trivial % - so we keep /all/ of
> those off by default). Doug particularly espoused this notion & I've mostly
> been promoting it since then on the assumption that that was still the goal
> - though I don't agree with it entirely myself.
>
> - David
>
>
I created a patch (http://reviews.llvm.org/D9493) to place the std::move
warnings into -Wmove and put that warning group into -Wmost, which gets
pulled into -Wall.
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