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[COMMITTED] Update earlyclobber documentation

Message ID 53B6B0A4.2010708@mentor.com
State New
Headers show

Commit Message

Tom de Vries July 4, 2014, 1:48 p.m. UTC 
[ was: Re: combination of read/write and earlyclobber constraint modifier on gcc@ ]
On 03-07-14 00:42, Jeff Law wrote:
> Based on various followups (public & private), let's go with the clarification
> above.  Richard E. explicitly added support for this in the mid/late 90s as an
> optimization for the ARM.

Jeff,

Committed as attached.

Thanks to all for the helpful comments.

- Tom
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Patch

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

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

diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
index 539865e..fde67d7 100644
--- a/gcc/doc/md.texi
+++ b/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 %
-- 
1.9.1