[llvm-dev] LLVM build performance with LLVM

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 3 12:03:17 PST 2020

I'm still confused by that - whether or not the LLVM you built has debug
info in it shouldn't at all change what goes into the binaries that LLVM
produces (modulo a few bugs, but nothing that'd produce drastic performance
swings). You mention in the bug that it's
specifically LIBOMPTARGET_ENABLE_DEBUG that is slowing down your
compilation time? Not the use of a Debug build of LLVM? Have you tried a
Release build of LLVM but with LIBOMPTARGET_ENABLE_DEBUG enabled?

On Fri, Jan 3, 2020 at 11:58 AM Itaru Kitayama <itaru.kitayama at gmail.com>

> 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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