Improve documentation of earlyclobber

2014-07-04  Tom de Vries  <tom@codesourcery.com>

	* doc/md.texi (@subsection Constraint Modifier Characters): Clarify
	combination of earlyclobber and read/write modifiers.

From-SVN: r212297

gcc/ChangeLog

 2014-07-04  Tom de Vries  <tom@codesourcery.com>
 
+	* doc/md.texi (@subsection Constraint Modifier Characters): Clarify
+	combination of earlyclobber and read/write modifiers.
+
+2014-07-04  Tom de Vries  <tom@codesourcery.com>
+
 	* config/aarch64/aarch64-simd.md
 	(define_insn "vec_unpack_trunc_<mode>"): Fix constraint.
 

gcc/doc/md.texi

@@ -1582,7 +1582,10 @@ alternatives of this form often allows GCC to produce better code
 when only some of the inputs can be affected by the earlyclobber.
 See, for example, the @samp{mulsi3} insn of the ARM@.
 
-@samp{&} does not obviate the need to write @samp{=}.
+Furthermore, if the @dfn{earlyclobber} operand is also read/write operand, then
+that operand is modified only after it's used.
+
+@samp{&} does not obviate the need to write @samp{=} or @samp{+}.
 
 @cindex @samp{%} in constraint
 @item %