[llvm-dev] wasteful cmake defaults

[Min-Yih Hsu via llvm-dev]

Just trying to understand: Are you suggesting a way to have a “-O0” build without all the debug information?

-Min

> On 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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