Subject: Re: [PATCH 8/8] meson: precompile "git-compat-util.h"

From: Phillip Wood <[email-masked]>
Date: Wed, 11 Mar 2026 14:32:36 +0000

On 10/03/2026 17:52, Patrick Steinhardt wrote:
> Every compilation unit in Git is expected to include "git-compat-util.h"
> first, either directly or indirectly via "builtin.h". This header papers
> over differences between platforms so that we can expect the typical
> POSIX functions to exist. Furthermore, it provides functionality that we
> end up using everywhere.
> 
> This header is thus quite heavy as a consequence. Preprocessing it as a
> standalone unit via `clang -E git-compat-util.h` yields over 23,000
> lines of code overall. Naturally, it takes quite some time to compile
> all of this.
> 
> Luckily, this is exactly the kind of use case that precompiled headers
> aim to solve: instead of recompiling it every single time, we compile it
> once and then link the result into the executable. If include guards are
> set up properly it means that the file won't need to be reprocessed.
> 
> Set up such a precompiled header for "git-compat-util.h" and wire it up
> via Meson. This leads to a significant speedup when performing full
> builds:
> 
>    Benchmark 1: ninja (rev = HEAD~)
>    Time (mean ± σ):     14.467 s ±  0.126 s    [User: 248.133 s, System: 31.298 s]
>    Range (min … max):   14.195 s … 14.633 s    10 runs
> 
>    Benchmark 2: ninja (rev = HEAD)
>      Time (mean ± σ):     10.307 s ±  0.111 s    [User: 173.290 s, System: 23.998 s]
>      Range (min … max):   10.030 s … 10.433 s    10 runs
> 
>    Summary
>      ninja (rev = HEAD) ran
>        1.40 ± 0.02 times faster than ninja (rev = HEAD~)

This is a nice speedup for a full build. I'm not sure about dumping the 
precompiled header in tools/ though, it seems like an odd location for a 
header file. It is a requirement of meson that the source for the 
precompiled header lives in a separate directory to the rest of the 
sources but it might be better to adopt the suggestion in the 
documentation of a "pch" (or maybe "precompiled"?) directory rather than 
mixing it in with our build scripts.

Thanks

Phillip

> Signed-off-by: Patrick Steinhardt <[email-masked]>
> ---
>   meson.build         | 2 ++
>   tools/precompiled.h | 1 +
>   2 files changed, 3 insertions(+)
> 
> diff --git a/meson.build b/meson.build
> index cd00be1c23..4b3fd47061 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -1760,6 +1760,7 @@ libgit = declare_dependency(
>         c_args: libgit_c_args + [
>           '-DGIT_VERSION_H="' + version_def_h.full_path() + '"',
>         ],
> +      c_pch: [ 'tools/precompiled.h' ],
>         dependencies: libgit_dependencies,
>         include_directories: libgit_include_directories,
>       ),
> @@ -1820,6 +1821,7 @@ test_dependencies = [ ]
>   
>   git_builtin = executable('git',
>     sources: builtin_sources + 'git.c',
> +  c_pch: [ 'tools/precompiled.h' ],
>     dependencies: [libgit_commonmain],
>     install: true,
>     install_dir: git_exec_path,
> diff --git a/tools/precompiled.h b/tools/precompiled.h
> new file mode 100644
> index 0000000000..b2bec0d2b4
> --- /dev/null
> +++ b/tools/precompiled.h
> @@ -0,0 +1 @@
> +#include "git-compat-util.h"
>