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

[Jan Voung via llvm-dev]

The original thread w.r.t. fast isel: http://lists.llvm.org/pi
permail/llvm-commits/Week-of-Mon-20130225/166934.html. Nadav cited 5% of
specific part of isel, but I don't know which benchmark. On my side, we
were testing build times for PNaCl
<https://codereview.chromium.org/20162004/diff/1/tools/nacl_perf_expectations/nacl_perf_expectations.json>
apps.
The setup would have been an LTO-like build, timing just the backend -- an
odd mix of getting optimized bitcode then running LLC under -O0. A minor
point, it also made the toolchain binaries a bit smaller (e.g., maybe 500KB
out of 12MB?). But, that's not a use case you'd need to worry about.


On Wed, May 31, 2017 at 11:17 AM, Daniel Berlin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
>
> 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?
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>
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