From: YumeYao <firstname.lastname@example.org>
To: Jonathan Wakely <email@example.com>
Cc: "libstdc++" <firstname.lastname@example.org>
Subject: Re: libstdc++ ABI update rule? (baseline_symbols.txt)
Date: Sat, 13 Aug 2022 04:44:01 +0800 [thread overview]
Message-ID: <CALBhkEfHScpE09ig+S9VgLo5y=GtUkPCZAEoLNv030irmtrbFA@mail.gmail.com> (raw)
On Wed, Aug 10, 2022 at 9:56 PM Jonathan Wakely <email@example.com> wrote:
> On Tue, 9 Aug 2022, 17:10 YumeYao via Libstdc++, <firstname.lastname@example.org> wrote:
>> Hi All,
>> I'm trying to do some experimental optimization to libstdc++, which
>> involves heavy use of __builtin_constant_p(), this, however,
>> eventually impacts how gcc calculate inlining or not, hence I got
>> error when testing libstdc++ about ABI changes.
> What errors do you get?
added symbols / incompatible symbols. For example I got some
std::basic_string<_Char>::function instantiations, because I touched
>> Specifically, I met most of such issues when optimizing basic_string.
>> Then I tried fix the ABI changes by templatizing the functions,
> That certainly isn't going to work, that's explicitly introducing an ABI change which seems a strange way to try to fix ABI breakage.
Actually it did work. I introduced some helper functions in
std::basic_string, and somehow some of them didn't get inlined and
appeared in the ABI check as added symbols as a result of template
class member function instantiation.
I then changed the helper function into a (template class member)
template function and it didn't appear in the ABI check any more.
>> __attribute__((always_inline)) and even gnu_inline(sometimes adding
>> only always_inline breaks the optimization, I noticed this issue very
>> long ago...) to the failed parts, which in turn makes other existing
>> functions in libstdc++ got uninlined and present in libstdc++.so as
>> ABI changes of added symbols/functions.
>> What really confuses me is the following fact:
>> without any attribute notation and only by templatizing functions, I
>> managed to get a version working on some old gcc version without any
>> abi change.
> I don't understand how.
I don't understand either.
Initially on gcc7 I didn't use template class member template
function, then ABI check complained about added symbols.
Then like I said above, I turned them into template class member
template functions and got ABI fixed.
Then I migrated the code change to gcc8, only getting the issue again.
So I manually fixed the ABI break by always_inline'ing the functions
corresponding to the added symbols. Initially it resulted the outer
(caller) functions got un-inlined, resulting in the outer functions
appearing as added symbols in ABI check, so I had to add always_inline
level by level until I met a caller function that was in the existing
ABI list already.
>> But it then failed when I migrated it to a newer version.
>> I have some basic knowledge about extern template and inlining and
>> symbols, etc. and I tried to find the answers from gcc source, but
>> obviously there's something beyond the source.
>> I know inline decision is calculated by pseudo "cost", therefore not
>> only the change in library could make an ABI change, but also the
>> logic change in gcc inline calculator could make such change, even
>> more, how the user uses the code can also make a difference on inline
>> decision because the times or frequency of using a library function
>> can also impact the inline decision.
>> So I want to ask:
>> 1. how the ABI of libstdc++ is maintained in gcc, just update
>> baseline_symbols.txt unconditionally when preparing for a release?
>> 2. How does gcc decide to put which symbol to libstdc++.so? I can see
>> files named xxxx-inst(antiation).cc but obviously some symbols not
>> belonging to it get instantiated and put in libstdc++.so
> See https://gcc.gnu.org/onlinedocs/libstdc++/manual/appendix_porting.html#build_hacking.configure.version
This seems to only explain ABI versioning, which is something I'm
already (somewhat) aware of. But here in question 2 I didn't mean gcc
community, I actually meant the compiler itself.
Well, to me, the Makefiles in gcc are just some dinosaurs I don't want
to read over. So... just let me guess.
I guess it's when compiling some source files in libstdc++ the default
visibility is on for that compile unit (for exporting symbols). So
when some functions don't get inlined while compiling such files,
these functions appear in libstdc++.so as added symbols.
Fix me if it totally doesn't work in this way...
prev parent reply other threads:[~2022-08-12 20:44 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-09 15:09 YumeYao
2022-08-10 13:55 ` Jonathan Wakely
2022-08-12 20:44 ` YumeYao [this message]
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