[llvm-dev] wasteful cmake defaults

Tue Nov 17 19:15:52 PST 2020

Hi,

The CMake default is an empty build type, which yield a non-optimized build
without debug information and without assertions, it isn't clear that it is
really good for most (developers likely want debug info and assertions in
non-optimized builds, non developers may want a release build).
See also the revision that introduced the current behavior:
http://reviews.llvm.org/D7360

We could make Release the default I guess, the main drawback I see right
now is that it could affect bots that are relying on the default and
wouldn't get assertions anymore?
That said our upstream buildbots infra seems to have explicit defaults:
https://github.com/llvm/llvm-zorg/blob/master/zorg/buildbot/builders/UnifiedTreeBuilder.py#L125-L127

For LLD, I don't think we have auto-detection, but I'd be on-board with
auto-detecting it and have -DLLVM_ENABLE_LLD default to the auto-detection
when empty. But that's another topic...
-- 
Mehdi

On Tue, Nov 17, 2020 at 5:50 PM David Blaikie via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Yeah, that's one I'd be in favor of fixing, if it's still the case.
>
> On Tue, Nov 17, 2020 at 5:38 PM Sean Silva via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > I'm curious how much of this is due to not using LLD? Last I checked,
> using ld.bfd instead of LLD (or gold) was one of my major build time
> issues, and somehow our default configuration would use ld.bfd.
> >
> > -- Sean Silva
> >
> > On Tue, Nov 17, 2020 at 10:25 AM Luke Drummond via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>
> >> Hi all
> >>
> >> I wanted to do a quick build of a clean branch yesterday and noticed
> >> something surprising in the configure log:
> >>
> >> > -- No build type selected, default to Debug
> >>
> >> It appears that llvm's configuration forces Debug builds if the user
> >> does not specify the build type.
> >>
> >>
> https://github.com/llvm/llvm-project/blob/9218ff50f93085d0a16a974db28ca8f14bc66f64/llvm/CMakeLists.txt#L57-L60
> >>
> >> I've just done a build of llvm and clang 10 in debug mode for X86 and
> >> ARM targets and it weighs in at a whopping 75GiB. I feel that forcing
> >> Debug builds in the absence of an option to be a wasteful default. It is
> >> a valid and useful thing to call cmake without specifying a build type;
> >> absence of a command line switch does not always imply absence of a
> >> choice.
> >>
> >> I took some measurements. The machine under test is an otherwise
> >> unloaded Ryzen 2 server with 6 cores / 12 threads and fast NVME SSDs
> >> running Debian sid with gcc-10.2 as the host toolchain.
> >>
> >>     $ cmake ../llvm -DCMAKE_BUILD_TYPE=Debug -GNinja \
> >>         -DLLVM_TARGETS_TO_BUILD=X86 -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
> >>     ...
> >>     $ time ninja llc
> >>     ...
> >>     [1199/1199] Linking CXX executable bin/llc
> >>     ninja llc  6858.95s user 218.53s system 1095% cpu 10:45.78 total
> >>
> >> and then:
> >>
> >>     $ git apply <<'EOF'
> >>     diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
> >>     index 21563e15118..3f5821351a1 100644
> >>     --- a/llvm/CMakeLists.txt
> >>     +++ b/llvm/CMakeLists.txt
> >>     @@ -55,8 +55,8 @@ set(CMAKE_CXX_STANDARD_REQUIRED YES)
> >>      set(CMAKE_CXX_EXTENSIONS NO)
> >>
> >>       if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
> >>       -  message(STATUS "No build type selected, default to Debug")
> >>       -  set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default
> Debug)" FORCE)
> >>       +  # message(STATUS "No build type selected, default to Debug")
> >>       +  # set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type
> (default Debug)" FORCE)
> >>        endif()
> >>
> >>         # Side-by-side subprojects layout: automatically set the
> >>     EOF
> >>
> >>     $ cmake ../llvm -DCMAKE_BUILD_TYPE= -GNinja
> -DLLVM_TARGETS_TO_BUILD=X86 \
> >>         -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
> >>     ...
> >>     $ time ninja llc
> >>     [1199/1199] Linking CXX executable bin/llc
> >>     ninja llc  5714.52s user 159.31s system 1125% cpu 8:41.99 total
> >>
> >> There's two minutes difference just for an X86 capable `llc` target on
> >> this machine. I imagine there are plenty of configurations in which the
> >> ratio is *much* larger e.g. machines with slow disks and lots of cores.
> >>
> >> I think this speedup will be especially useful in CI systems where
> >> getting a compiler build quickly is more important than getting a quick
> >> compiler. I don't imagine we ever use the debuginfo generated by the
> >> host toolchain in the buildbots, either.
> >>
> >> Is there anything I'm missing, or can this override be safely deleted?
> >> Would a patch to Zorg be needed / appropriate for the fast buildbots?
> >>
> >> All the Best
> >>
> >> Luke
> >>
> >> --
> >> Codeplay Software Ltd.
> >> Company registered in England and Wales, number: 04567874
> >> Registered office: Regent House, 316 Beulah Hill, London, SE19 3HF
> >> _______________________________________________
> >> 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
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20201117/21e683a9/attachment.html>