[llvm-dev] LLVM build performance with LLVM

[David Blaikie via llvm-dev]

On Thu, Jan 2, 2020 at 10:55 PM Itaru Kitayama <itaru.kitayama at gmail.com>
wrote:

> As I am facing the serious super slow down:
> https://bugs.llvm.org/show_bug.cgi?id=44407 , I'd like to
> learn more about how it can be minimized. When debugging an app, there are
> times building it with a
> Debug build LLVM can not be avoided, but a situation like 40 times time
> increase (40 minutes) is hard
> to work with.
>

Not sure I follow - you shouldn't need a debug build of clang to debug an
application built with clang. You'd use a release build of clang to build
your application with debug info (by passing -g).


>
>
> On Fri, Jan 3, 2020 at 11:14 AM David Blaikie <dblaikie at gmail.com> wrote:
>
>>
>>
>> On Thu, Jan 2, 2020 at 6:09 PM Itaru Kitayama <itaru.kitayama at gmail.com>
>> wrote:
>>
>>> David,
>>> Yes, I was indeed trying to build LLVM with a Debug build. I'll stop
>>> doing that from now on.
>>>
>>
>> I mean, you can - it's just going to be super slow & mostly not what you
>> want, unless you're trying to debug that building process. Usually you'd
>> want to isolate one particular test case, maybe even reduce it, before
>> running it with a debug build of LLVM which will be quite slow (because
>> it's not optimized at all).
>>
>>
>>> I am on JSC's JURON machine which has 251 GB of memory on the login
>>> node, that's more than sufficient
>>> to do a build, I suppose, and the linker is LLD as LLVM has a CMake
>>> variable to select the linker.
>>>
>>
>> Yep, sounds like you're just tripping over the fact that an
>> unoptimized/debug build of LLVM is very slow. *thumbs up*
>>
>>
>>>
>>> On Fri, Jan 3, 2020 at 11:02 AM David Blaikie <dblaikie at gmail.com>
>>> wrote:
>>>
>>>> Were you trying to use a Debug build to build LLVM? Yes, that would be
>>>> very slow.
>>>>
>>>> If you mean you were using a release build of LLVM to build a Debug
>>>> build of LLVM - yeah, that's generally going to be recommended. Did this
>>>> get slower/change significantly in performance? Many people have trouble
>>>> with building Debug builds (no matter the host compiler) especially if
>>>> they're using bfd-ld, since it's quite slow/uses a lot of memory. There are
>>>> a few other issues to do with memory usage (do you have less than about a
>>>> GB of RAM per CPU? Then you'll probably hit swapping by default & have a
>>>> bad time - there are ways around that)
>>>>
>>>>
>>>> On Thu, Jan 2, 2020 at 5:42 PM Itaru Kitayama via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hi,
>>>>> When building LLVM, is it always recommended to do it with the
>>>>> latest official release, currently, it is 9.0.1? I ask because when I
>>>>> tried
>>>>> it with a Debug build, it took an enormous amount of time on POWER8.
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>
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