Re: [PATCH v3 8/8] meson: precompile "git-compat-util.h"

["SZEDER Gábor" <szeder.dev@gmail.com>]

On Thu, Mar 19, 2026 at 06:33:27AM +0100, 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 causes Meson to implicitly include the precompiled
> header in all compilation units. With GCC and Clang for example this is
> done via the "-include" statement [1].
> 
> 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~)
> 
> [1]: https://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html
> 
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  meson.build         | 2 ++
>  tools/precompiled.h | 1 +
>  2 files changed, 3 insertions(+)
> 
> diff --git a/meson.build b/meson.build
> index cd00be1c23..2002f4795e 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,
>      ),

Well, I don't do meson, but...

If I understand this right, this section here compiles all the source
files listed in "libgit_sources" using our new precompiled header.
But "libgit_sources" contains all source files under "reftable/",
which, with the sole exception of "reftable/system.c", don't include
"git-compat-util.h".

Now, building the reftable sources with "git-compat-util.h" included
through the precompiled header apparently didn't cause any compilation
errors...
But I think that's just accidental, and if a source file doesn't
include "git-compat-util.h", then it shouldn't be compiled with the
precompiled header.


> @@ -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"
> 
> -- 
> 2.53.0.959.g497ff81fa9.dirty
>