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. (Like Stefan, I'm wary of marking functions 'hot' so I didn't do that.)

This patch also adds a convenience macro AVOID for the now-common pattern 
'_Noreturn ATTRIBUTE_COLD void'.