Speeding up (format "%s" STRING)

[Paul Eggert <eggert@cs.ucla.edu>]

In Bug#28625 I propose changing the documentation for the 'format' 
function so that it no longer states that 'format' returns a 
newly-allocated string. This statement is already incorrect for cases 
like (format "%s" ""), and removing the statement lets us improve the 
performance for other cases such as (format "foo") and (format "%s" 
"foo"). Although I doubt whether any code depends on 'format' always 
returning a newly-allocated string for these cases, the proposed patch 
does change the semantics of 'format' slightly, so I'm sending this 
heads-up to emacs-devel to ask if anyone sees problems with it.

As noted in Bug#28625, on my old desktop (AMD Phenom II X4 910e), the 
proposed patch speeds up (format "%s" "Hello, world!") by a factor of 
about 9. It also causes Emacs to use less RAM (harder to measure) and 
simplifies the Emacs source code.