Small doc update for the USE rtx

[Bernd Schmidt <bernds at redhat dot co dot uk>]

This patch came out of a discussion at Red Hat after I noticed that one of
our ports was doing something dodgy wrt. USEs.  Jim Wilson OK'ed an earlier
version of this, I've only incorporated some of his suggestions in this
newer version, which I've installed.

Bernd

	* rtl.texi (description of USE): Add note about possible pitfalls
	with this rtx.
 
Index: rtl.texi
===================================================================
RCS file: /cvs/gcc/egcs/gcc/rtl.texi,v
retrieving revision 1.28
diff -u -p -r1.28 rtl.texi
--- rtl.texi	2000/07/28 17:46:17	1.28
+++ rtl.texi	2000/09/08 09:51:23
@@ -2057,6 +2057,34 @@ it may not be apparent why this is so.  
 not attempt to delete previous instructions whose only effect is to
 store a value in @var{x}.  @var{x} must be a @code{reg} expression.
 
+In some situations, it may be tempting to add a @code{use} of a
+register in a @code{parallel} to describe a situation where the value
+of a special register will modify the behaviour of the instruction.
+An hypothetical example might be a pattern for an addition that can
+either wrap around or use saturating addition depending on the value
+of a special control register:
+
+@example
+(parallel [(set (reg:SI 2) (unspec:SI [(reg:SI 3) (reg:SI 4)] 0))
+           (use (reg:SI 1))])
+@end example
+
+@noindent
+
+This will not work, several of the optimizers only look at expressions
+locally; it is very likely that if you have multiple insns with
+identical inputs to the @code{unspec}, they will be optimized away even
+if register 1 changes in between.
+
+This means that @code{use} can @emph{only} be used to describe
+that the register is live.  You should think twice before adding
+@code{use} statements, more often you will want to use @code{unspec}
+instead.  The @code{use} RTX is most commonly useful to describe that
+a fixed register is implicitly used in an insn.  It is also safe to use
+in patterns where the compiler knows for other reasons that the result
+of the whole pattern is variable, such as @samp{movstr@var{m}} or
+@samp{call} patterns.
+
 During the reload phase, an insn that has a @code{use} as pattern
 can carry a reg_equal note.  These @code{use} insns will be deleted
 before the reload phase exits.