[GCC][AArch64] [middle-end][docs] Document the xorsign optab

[Tamar Christina]

Hi All,

This patch just adds documentation for the xorsign optab that was added a while 
ago.

Ok for trunk?

Thanks,
Tamar

gcc/ChangeLog:

2018-11-11  Tamar Christina  <tamar.christ...@arm.com>

        * doc/md.texi (xorsign): Document it.

-- 

diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
index 360b36b862f7eb13964e60ff53b04e1274f89fe4..510321ef12a167f79875cd13a8683271ee590ebe 100644
--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -5997,6 +5997,15 @@ vector floating-point mode.
 
 This pattern is not allowed to @code{FAIL}.
 
+@cindex @code{xorsign@var{m}3} instruction pattern
+@item @samp{xorsign@var{m}3}
+Target suppports an efficient expansion of x * copysign (1.0, y)
+as xorsign (x, y).  Store a value with the magnitude of operand 1
+and the sign of operand 2 into operand 0.  All operands have mode
+@var{m}, which is a scalar or vector floating-point mode.
+
+This pattern is not allowed to @code{FAIL}.
+
 @cindex @code{ffs@var{m}2} instruction pattern
 @item @samp{ffs@var{m}2}
 Store into operand 0 one plus the index of the least significant 1-bit