On Wed, Feb 04, 2026 at 11:35:50AM -0500, Patrick Palka wrote:
> I didn't know PCH'd stdc++.h is so big! I'll continue using
> --disable-libstdcxx-pch during development to avoid wearing down my
> SSD (and avoid the slowdown of make having to rebuild the PCH after
> every library header change).
>
> I wonder whether using PCH'd stdc++.h is still faster anymore given how
> big the standard library is nowadays. Is the overhead of deserializing
> the entire library AST, keeping it in memory, slowing down parts of the
> front end that are >=linear in the size of the TU (including GC which is
> frequent in checking mode) less than the overhead of not using PCH
> and just parsing the subset of the library that's actually needed, for
> our average libstdc++ testcase?
I think for x86_64 PCH is not used anyway due to some CET related mismatch
during testing but I could be wrong.
If you don't GC, then just mapping it in should be quite fast and even
if there is GC, while the first GC will take longer, it should kill
everything not needed and get back to the normal size.
Anyway, I wonder why we build the -O2ggnu++0x.gch files these days:
-rw-r--r--. 1 jakub jakub 213581873 Feb 3 23:07
./x86_64-pc-linux-gnu/32/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/extc++.h.gch/O2g.gch
-rw-r--r--. 1 jakub jakub 166490263 Feb 3 23:06
./x86_64-pc-linux-gnu/32/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/stdc++.h.gch/O2g.gch
-rw-r--r--. 1 jakub jakub 101706811 Feb 3 23:06
./x86_64-pc-linux-gnu/32/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/stdc++.h.gch/O2ggnu++0x.gch
-rw-r--r--. 1 jakub jakub 172619402 Feb 3 23:07
./x86_64-pc-linux-gnu/32/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/stdtr1c++.h.gch/O2g.gch
-rw-r--r--. 1 jakub jakub 214381431 Feb 3 23:05
./x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/extc++.h.gch/O2g.gch
-rw-r--r--. 1 jakub jakub 167179995 Feb 3 23:04
./x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/stdc++.h.gch/O2g.gch
-rw-r--r--. 1 jakub jakub 102411920 Feb 3 23:04
./x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/stdc++.h.gch/O2ggnu++0x.gch
-rw-r--r--. 1 jakub jakub 173411402 Feb 3 23:05
./x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/stdtr1c++.h.gch/O2g.gch
That made sense when -std=gnu++98 has been the default, when -std=gnu++11
has been the default no sense at all and even now, I think if we want to
include something other than the default -std=gnu++20, it would be
-std=gnu++17, not -std=gnu++0x, I hope most of C++ projects aren't stuck
in C++11 days.
Jakub
