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

[YumeYao <yumeyao@gmail.com>]

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.

Specifically, I met most of such issues when optimizing basic_string.

Then I tried fix the ABI changes by templatizing the functions, 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:

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. 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

Thanks in advance,
yumeyao