[Bug libstdc++/60621] std::vector::emplace_back generates massively more code than push_back

[marc at kdab dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60621

--- Comment #3 from marc at kdab dot com ---
Now, what is _really_ weird is that push_back(T&&) _calls_ emplace_back(). I
also tried the magic incantation

   g++ --param large-unit-insns=100000000 \
       --param inline-unit-growth=100000000 \
       --param max-inline-insns-single=100000000 \
       --param large-function-growth=100000000 \
       --param large-function-insns=100000000 -O2

to no avail. I can get the two version to within 80 bytes of text of each other
by adding -fno-exceptions, so it's probably related to that. The (implicit)
move ctor of S cannot throw, but the std::string(const char*) ctor can. Ie. in
the rvalue-push_back case, emplace_back only dabbles in noexcept operations,
and in the 3xconst char* case, it needs to deal with three throwing ctors.

I can reduce the text size to within a few hundreds of bytes by marking both
emplace_back and _M_emplace_back_aux as __attribute__((always_inline)), so
something prevents gcc from inlining even when turning the inlining paramters
all the way up.

I can also reduce the text size by passing std::strings instead of conat
char*s:

   text    data     bss     dec     hex filename
   5628     672      40    6340    18c4 emplace-vs-push_back.eb
   4991     672      40    5703    1647 emplace-vs-push_back.nt
   4516     648      40    5204    1454 emplace-vs-push_back.pb

(where .nt is EMPLACE_BACK_NOTHROW). Still a large gap...

Have we accepted another auto_ptr into the standard? :)