[Bug libstdc++/108030] `std::experimental::simd` not inlined

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108030

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Matthias Kretz (Vir) from comment #3)
> (In reply to Jakub Jelinek from comment #2)
> > I bet by adding too many always_inline functions that call normal inlines
> > that is what is bound to happen, one runs into inline growth limits.  It is
> > better to use always_inline on the leaf functions rather than on what calls
> > them.
> 
> How is the inline growth limit determined? I mean, in the cases where it
> really hurts, the resulting function compiles down to a single instruction
> (plus parameter passing boilerplate). The optimizer cannot know about the
> number of instructions, so what is the measure it uses?

I've CCed Honza, who should know the answers.
The inliner can't know if say some builtin will fold into a single instruction
or not,
it uses some heuristics on GIMPLE IL sizes.  Bet -fdump-ipa-inline-details
contain
all the reasons, but at least for me those dumps are hard to read.