rs6000.md (movdf_hardfloat32): Add a comment explaining the register ordering preferences.

2012-11-20  Michael Meissner  <meissner@linux.vnet.ibm.com>

	* config/rs6000/rs6000.md (movdf_hardfloat32): Add a comment
	explaining the register ordering preferences.

From-SVN: r193673

gcc/ChangeLog

+2012-11-20  Michael Meissner  <meissner@linux.vnet.ibm.com>
+
+	* config/rs6000/rs6000.md (movdf_hardfloat32): Add a comment
+	explaining the register ordering preferences.
+
 2012-11-20  Aldy Hernandez  <aldyh@redhat.com>
 
 	PR tree-optimization/55350

gcc/config/rs6000/rs6000.md

@@ -8018,6 +8018,14 @@
 ;; Don't have reload use general registers to load a constant.  It is
 ;; less efficient than loading the constant into an FP register, since
 ;; it will probably be used there.
+
+;; The move constraints are ordered to prefer floating point registers before
+;; general purpose registers to avoid doing a store and a load to get the value
+;; into a floating point register when it is needed for a floating point
+;; operation.  Prefer traditional floating point registers over VSX registers,
+;; since the D-form version of the memory instructions does not need a GPR for
+;; reloading.
+
 (define_insn "*movdf_hardfloat32"
   [(set (match_operand:DF 0 "nonimmediate_operand" "=m,d,d,ws,?wa,Z,?Z,ws,?wa,wa,Y,r,!r,!r,!r,!r")
 	(match_operand:DF 1 "input_operand" "d,m,d,Z,Z,ws,wa,ws,wa,j,r,Y,r,G,H,F"))]