On Wed, Feb 04, 2026 at 03:48:36PM -0500, Patrick Palka wrote:
> If the PCH file is for some reason unusable would that mean every
> libstdc++ test falls back to parsing bits/stdc++.h instead of
> mapping it in?
>
> Out of curiosity I just timed running a subset of the libstdc++ tests
> with and without --disable-libstdcxx-pch (with checking + non-bootstrap
> on x86_64) and it seems the PCH stuff slows things down! The vector
> tests finish in roughly half the time without PCH, and the ranges +
> regex tests in roughly 70% of the time.
>
> IIUC we use PCH in the libstdc++ testsuite to speed it up, but according
> to this experiment it's having the opposite effect nowadays.
Is PCH actually mapped or is the slow down due to parsing of bits/stdc++.h ?
I think the PCH file is created just for the default options (so
-std=gnu++20), so all tests using -std=gnu++2{3,6} or -std=c++* or
-std=gnu++[19]* will be slower if -include bits/stdc++.h is used when
compiling them.
Jakub
