Bruno Haible wrote:

> +   If you want the use of this macro to improve, not deteriorate,
> +   performance, R should not contain function calls except to functions
> +   that are declared 'inline __attribute__((__always_inline__))'.  */

A reader of that might incorrectly conclude that using such functions will 
always improve performance, compared to using functions not declared that way. 
And I'm leery of putting such GCC-specific info into the documentation of a 
generic macro.

Also, given Pip Cet's misunderstanding it'd be helpful to add a word or two 
about the intent of 'assume (R)'.

How about the attached patch?

 > -   evaluated.  Behavior is undefined if R is false.  */
 > +   evaluated.  The behavior is undefined if R is false.

This is a nit, but to my ears the shorter version is better, and has a 
more-accurate connotation as "the behavior" connotes "the behavior of 'assume 
(R)'", whereas behavior is also undefined for code before or after a call to an 
incorrect 'assume (R)'.