[llvm-dev] LLVM build performance with LLVM

Sun Jan 5 15:27:57 PST 2020

I guess I like what you described, but that's also beyond my toolchain
build skills.

All,
Is it thought as acceptable for a user seeing 40 minutes of app build time
if he is using
a "Debug" Clang? While, with a "Release" Clang, it just finishes within a
minute.

On Mon, Jan 6, 2020 at 5:15 AM David Blaikie <dblaikie at gmail.com> wrote:

> Ah, OK, sounds lik LIBOMPTARGET_ENABLE_DEBUG doesn't have anything to do
> with your problem, then. Bit of a red herring.
>
> If the LLVM build doesn't support /just/ building the runtime with debug
> info (if this is a normal/common scenario, to need to debug into the
> runtime library, perhaps it should - but I Have no idea if that's a normal
> use case), then you can probably mix-and-match between two builds. Build
> the runtime with debug info and drop it into an installation of the
> compiler built in release mode?
>
> On Sun, Jan 5, 2020 at 11:39 AM Itaru Kitayama <itaru.kitayama at gmail.com>
> wrote:
>
>> Yes, exactly.
>>
>> On Sun, Jan 5, 2020 at 23:43 Mehdi AMINI <joker.eph at gmail.com> wrote:
>>
>>>
>>>
>>> On Fri, Jan 3, 2020 at 8:58 PM Itaru Kitayama via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> I have just tried a Release build of LLVM but
>>>> with
>>>> LIBOMPTARGET_ENABLE_DEBUG enabled. The app build became super fast, but
>>>> I can not obtain debugging information that I need at runtime.
>>>>
>>>
>>> The description for this flag seems to be: LIBOMPTARGET_ENABLE_DEBUG
>>> "Allow debug output with the environment variable LIBOMPTARGET_DEBUG=1"
>>>
>>> Is this what you are looking for? Or are you looking into getting debug
>>> info (Dwarf) for the OpenMP runtime itself?
>>>
>>> --
>>> Mehdi
>>>
>>>
>>>
>>>>
>>>> On Sat, Jan 4, 2020 at 5:03 David Blaikie <dblaikie at gmail.com> wrote:
>>>>
>>>>> 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> wrote:
>>>>>
>>>>>> 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
>>>>>>>>>>>>
>>>>>>>>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
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>>>>
>>>
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