[Bug 2072478] Re: g++-14 performance regression - built with debug checks

[Matthias Klose]

14.2.0 is now in 24.04 LTS


** Changed in: gcc-14 (Ubuntu)
       Status: Confirmed => Fix Released

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Title:
  g++-14 performance regression - built with debug checks

Status in gcc-14 package in Ubuntu:
  Fix Released

Bug description:
  System information (Docker container for ubuntu:24.04):

  ```
  $> lsb_release -rd
  No LSB modules are available.
  Description:    Ubuntu 24.04 LTS
  Release:        24.04
  ```

  Package information:

  ```
  $> apt-cache policy g++-14 
  g++-14:
    Installed: 14-20240412-0ubuntu1
    Candidate: 14-20240412-0ubuntu1
    Version table:
   *** 14-20240412-0ubuntu1 500
          500 http://archive.ubuntu.com/ubuntu noble/universe amd64 Packages
          100 /var/lib/dpkg/status
  ```

  g++-14 in Ubuntu 24.04 builds extremely slow. At Boost, we've noticed
  increases of around x2 in CI build times.

  This thread by some of my colleagues at Boost summarizes the problems
  we've found:
  https://www.reddit.com/r/cpp/comments/1cfzydb/gcc_14_twice_as_slow_as_gcc_13/

  It looks like Ubuntu 24.04 g++-14 is indeed a debug build, and hence
  the slowdown. Running in an `ubuntu:24.04` Docker container the
  following:

  
  ```
  apt update
  apt install -y --no-install-recommends g++-14
  g++-14 -v
  ```

  Yields the following:

  ```
  g++-14 -v
  Using built-in specs.
  COLLECT_GCC=g++-14
  COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/14/lto-wrapper
  OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
  OFFLOAD_TARGET_DEFAULT=1
  Target: x86_64-linux-gnu
  Configured with: ../src/configure -v --with-pkgversion='Ubuntu 14-20240412-0ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-14/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2,rust --prefix=/usr --with-gcc-major-version-only --program-suffix=-14 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-libstdcxx-backtrace --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-14-OQFzmN/gcc-14-14-20240412/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-14-OQFzmN/gcc-14-14-20240412/debian/tmp-gcn/usr --enable-offload-defaulted --without-cuda-driver --enable-checking=yes,extra,rtl --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
  Thread model: posix
  Supported LTO compression algorithms: zlib zstd
  gcc version 14.0.1 20240412 (experimental) [master r14-9935-g67e1433a94f] (Ubuntu 14-20240412-0ubuntu1)
  ```

  The problem is `--enable-checking=...` - as per the GCC configure
  reference page (https://gcc.gnu.org/install/configure.html), it should
  be set to `--enable-checking=release`.

  Reading gcc's configure page, I think this happened because gcc was
  built from the master branch (as the release branch wasn't out at the
  time Ubuntu 24.04 was published).

  Now that gcc 14 is officially out, it would be great if we could get
  an updated package with gcc built in release mode.

  Thanks,
  Ruben.

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