[llvm-dev] Enable STATISTIC all the time again?

[Daniel Berlin via llvm-dev]

On Wed, May 31, 2017 at 7:48 AM, Reid Kleckner <rnk at google.com> wrote:

> I think adding an option to cmake is fine, but it should definitely be off
> by default in release builds.
>

Why?


> If we make STATISTIC useful, then people will eventually put it on some
> hot path, and that will kill multi-threaded performance for users like
> ThinLTO, because multiple threads doing read-modify-write on the same
> memory is just slow.
>

With no offense meant: Can we please be driven by data?
This argument is "people will eventually do something silly and it will
make things slow for others". That is something that can be said about just
about anything in LLVM :).  This is why we have code review, etc.
If your concern is the atomic increments, then is there a reason to not
have it do the counting as a runtime option instead of a compile time one?

>
>

> On Wed, May 31, 2017 at 7:39 AM, Daniel Berlin via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Right now, the LLVM Statistic class does not increment values if we are
>> in non-debug mode, unless you define LLVM_ENABLE_STATS (which, sadly, is
>> also not available from cmake).
>>
>> Before i go and add it to cmake, i ran numbers.
>>
>> I can't find a single testcase, large or small, where enabling statistics
>> all the time isn't completely noise.
>>
>> It looks like it was disabled in march  2013.
>> https://reviews.llvm.org/D486
>>
>> The only discussion i can find in the same time period is around fastisel
>> using a lot of per-instruction stats, and it causing some slow-down.
>>
>> However, I can't find any actual data/testcases in this discussion at
>> all, and disabling stats globally is actually fairly annoying for
>> performance work.
>> :(
>>
>> Does anyone have any testcases where it is actually slow that i can look
>> at?
>>
>>
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>>
>>
>
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