[llvm-dev] wasteful cmake defaults

[Sean Silva via llvm-dev]

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
>
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