re PR target/66474 (Document the use of %x in powerpc asm statements)

2015-06-10  Michael Meissner  <meissner@linux.vnet.ibm.com>

	PR target/66474
	* doc/md.texi (Machine Constraints): Document that on the PowerPC
	if you use a constraint that targets a VSX register, you must use
	%x<n> in the template.

From-SVN: r224332

gcc/ChangeLog

+2015-06-10  Michael Meissner  <meissner@linux.vnet.ibm.com>
+
+	PR target/66474
+	* doc/md.texi (Machine Constraints): Document that on the PowerPC
+	if you use a constraint that targets a VSX register, you must use
+	%x<n> in the template.
+
 2015-06-10  Max Filippov  <jcmvbkbc@gmail.com>
 
 	* config/xtensa/xtensa.h (TARGET_DEBUG): New definition.

gcc/doc/md.texi

@@ -3070,6 +3070,27 @@ Altivec vector register
 @item wa
 Any VSX register if the -mvsx option was used or NO_REGS.
 
+When using any of the register constraints (@code{wa}, @code{wd},
+@code{wf}, @code{wg}, @code{wh}, @code{wi}, @code{wj}, @code{wk},
+@code{wl}, @code{wm}, @code{ws}, @code{wt}, @code{wu}, @code{wv},
+@code{ww}, or @code{wy}) that take VSX registers, you must use
+@code{%x<n>} in the template so that the correct register is used.
+Otherwise the register number output in the assembly file will be
+incorrect if an Altivec register is an operand of a VSX instruction
+that expects VSX register numbering.
+
+@smallexample
+asm ("xvadddp %x0,%x1,%x2" : "=wa" (v1) : "wa" (v2), "wa" (v3));
+@end smallexample
+
+is correct, but:
+
+@smallexample
+asm ("xvadddp %0,%1,%2" : "=wa" (v1) : "wa" (v2), "wa" (v3));
+@end smallexample
+
+is not correct.
+
 @item wd
 VSX vector register to hold vector double data or NO_REGS.