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

[Phillip Wood <phillip.wood123@gmail.com>]

On 10/04/2026 16:36, SZEDER Gábor wrote:
> 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.

FWIW I agree it would be better to keep the same includes when building 
with precompiled headers. There is some discussion about this starting 
at 
https://lore.kernel.org/git/2fe87868-dff5-4b3a-95e3-d4b6376b59ed@gmail.com/

Thanks

Phillip

> 
>> @@ -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
>>