Re: Using __builtin_expect (likely/unlikely macros)

[Konstantin Kharlamov]

On Пт, Apr 19, 2019 at 07:45, Alex Gramiak <address@hidden> wrote:
> Paul Eggert <address@hidden> writes:
>
> >  Konstantin Kharlamov wrote:
> > >  I was told that e.g. "cold" attribute can sometimes produce 
> > > unbearably slow
> > >  code https://gcc.gnu.org/ml/gcc-help/2019-01/msg00035.html
> >
> >  Although cold functions can be slow, it appears that overall it's a 
> > win for
> >  Emacs to mark _Noreturn error function declarations as cold: on my 
> > platform,
> >  'make compile-always' ran about 1.3% faster. So I installed the 
> > attached patch
> >  into master.
>
> Thanks. What do you think about marking a few bytecode cases as
> cold/unused? I've attached a diff below that does that. Does it make a
> difference for you on your setup? It seems to slow Emacs down a slight
> bit for me, but I was hoping you might know why it would do so. Since
> the newly cold attributes should be unused, is this perhaps a GCC bug?

The slowdown is to be expected, given that GCC devs said that "cold" 
produces very inefficient code.

It worked in Paul's case because he marked as "cold" functions that 
never return.