Unfortunately that change causes the emacs-26 build to fail for me:

make[1]: Entering directory '/home/eggert/src/gnu/emacs/emacs-26-sc/src'
   CC       character.o
In file included from character.c:34:0:
character.c: In function ‘char_width’:
lisp.h:1682:11: error: ‘c’ may be used uninitialized in this function 
[-Werror=maybe-uninitialized]
     ? ((a) + (unsigned) 0) op ((b) + (unsigned) 0)   \
            ^
character.c:292:10: note: ‘c’ was declared here
       int c;
           ^
cc1: all warnings being treated as errors
Makefile:376: recipe for target 'character.o' failed


Although the attached patch fixes this and presumably speeds up the code a bit 
when optimized, is this the right thing to do? I have not looked into the code 
carefully.