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

[Toon Claes <toon@iotcl.com>]

Patrick Steinhardt <ps@pks.im> writes:

> 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~)
>

Quite cool! The only nit I would have about this, this optimization is
not applied to building with Makefiles. While that isn't an issue, I
wouldn't have hurt if it was mentioned in the commit message.

Anyway, overall I got nothing that's holding back this series from
merging. Looks good!

-- 
Cheers,
Toon