[llvm-dev] LLVM build performance with LLVM

[Itaru Kitayama via llvm-dev]

At least, to obtain enough information from libomptarget while running my
offloading app on GPU capable environment, I have to build it with an LLVM
which was built in Debug mode.

On Sat, Jan 4, 2020 at 4:34 David Blaikie <dblaikie at gmail.com> wrote:

>
>
> 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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