re PR c/26366 (__builtin_expect needs better documentation)

2018-11-11  Sandra Loosemore  <sandra@codesourcery.com>

	PR c/26366

	gcc/
	* doc/extend.texi (Other Builtins): Document probability associated
	with __builtin_expect.

From-SVN: r266017

gcc/ChangeLog

+2018-11-11  Sandra Loosemore  <sandra@codesourcery.com>
+
+	PR c/26366
+	* doc/extend.texi (Other Builtins): Document probability associated
+	with __builtin_expect.
+
 2018-11-11  Uros Bizjak  <ubizjak@gmail.com>
 
 	PR target/87928

gcc/doc/extend.texi

@@ -12121,6 +12121,12 @@ if (__builtin_expect (ptr != NULL, 1))
 
 @noindent
 when testing pointer or floating-point values.
+
+For the purposes of branch prediction optimizations, the probability that
+a @code{__builtin_expect} expression is true is controlled by GCC's
+@code{builtin-expect-probability} parameter, which defaults to 90%.  
+You can also use @code{__builtin_expect_with_probability} to explicitly 
+assign a probability value to individual expressions.
 @end deftypefn
 
 @deftypefn {Built-in Function} long __builtin_expect_with_probability