Re: The Emacs master is much slower than the emacs-27 branch.

[Gregory Heytings]

> > > I just rebuilt emacs-27 with "make bootstrap", reran the benchmark, 
> > > and it took ~60 seconds, about a second faster than the previous 
> > > non-bootstrapped build.  The profile is similar to the previous one 
> > > from emacs-27, but I've attached for your perusal anyway.  I'll 
> > > bootstrap master too and run the benchmark there again.
> >
> > On freshly bootstrapped master the benchmark took the same time as 
> > before, ~21 seconds, and the profile looks pretty much the same, so I 
> > haven't attached it, but will send it if you want to see it.
>
> Thanks.  This is very strange.  What are the build options and the 
> features in each build? are they identical?

Indeed with CFLAGS='-Og -g3', on my computer (Debian GNU/Linux, GCC 
10.2.0) with 28.0.50 this benchmark takes 25.2 seconds, whereas with 
27.1.50 it takes 67.6 seconds.

I bisected this, and the origin of the regression is commit 
365dad197bac5deec9244fd9c189d23c46c99b31, which changes the default to 
"use 'static' instead of 'extern inline' [in C headers] because 'static' 
typically has better performance for Emacs".  With -Og or -O0, the 
benchmark takes 67 seconds, with -O1 or above (-O2 is the default) it 
takes 25 seconds.

Emacs 28.0.50 uses 'static inline' instead of 'extern inline' again, 
"because 'static inline' has much better performance for Emacs when 
compiled with 'gcc -Og'" (see src/conf_post.h).