[llvm-dev] wasteful cmake defaults

[David Blaikie via llvm-dev]

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.
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