Re: libstdc++ ABI update rule? (baseline_symbols.txt)

[YumeYao <yumeyao@gmail.com>]

On Wed, Aug 10, 2022 at 9:56 PM Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
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> On Tue, 9 Aug 2022, 17:10 YumeYao via Libstdc++, <libstdc++@gcc.gnu.org> wrote:
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>> 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.
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> What errors do you get?

added symbols / incompatible symbols. For example I got some
std::basic_string<_Char>::function instantiations, because I touched
string headers.

>>
>> Specifically, I met most of such issues when optimizing basic_string.
>>
>> Then I tried fix the ABI changes by templatizing the functions,
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> 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.

>> adding
>> __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:
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>> 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.
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> I don't understand how.
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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.

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>> 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:
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>> 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
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> See https://gcc.gnu.org/onlinedocs/libstdc++/manual/appendix_porting.html#build_hacking.configure.version
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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...