[llvm-dev] LLVM build performance with LLVM

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Thu Jan 2 18:13:53 PST 2020

On Thu, Jan 2, 2020 at 6:09 PM Itaru Kitayama <itaru.kitayama at gmail.com>

> 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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