Document Fortran header directive.

2019-02-20  Martin Liska  <mliska@suse.cz>

	* gfortran.texi: Document Fortran header directive.

From-SVN: r269035

gcc/fortran/ChangeLog

+2019-02-20  Martin Liska  <mliska@suse.cz>
+
+	* gfortran.texi: Document Fortran header directive.
+
 2019-02-19  Thomas Koenig  <tkoenig@gcc.gnu.org>
 
 	PR fortran/89384

gcc/fortran/gfortran.texi

@@ -3505,6 +3505,7 @@ as this requires the new array descriptor.
 @menu
 * ATTRIBUTES directive::
 * UNROLL directive::
+* BUILTIN directive::
 @end menu
 
 @node ATTRIBUTES directive
@@ -3596,6 +3597,25 @@ loop that follows.  N is an integer constant specifying the unrolling factor.
 The values of 0 and 1 block any unrolling of the loop.
 
 
+@node BUILTIN directive
+@subsection BUILTIN directive
+
+The syntax of the directive is
+
+@code{!GCC$ BUILTIN (B) attributes simd FLAGS IF('target')}
+
+You can use this directive to define which middle-end built-ins provide vector
+implementations.  @code{B} is name of the middle-end built-in.  @code{FLAGS}
+are optional and must be either "(inbranch)" or "(notinbranch)".
+@code{IF} statement is optional and is used to filter multilib ABIs
+for the built-in that should be vectorized.  Example usage:
+
+@smallexample
+!GCC$ builtin (sinf) attributes simd (notinbranch) if('x86_64')
+@end smallexample
+
+The purpose of the directive is to provide an API among the GCC compiler and
+the GNU C Library which would define vector implementation of math routines.
 
 @node Non-Fortran Main Program
 @section Non-Fortran Main Program