rtl.texi (CALL_INSN_FUNCTION_USAGE): Note that (and when) it may contain MEMs.

* rtl.texi (CALL_INSN_FUNCTION_USAGE): Note that (and when) it may
contain MEMs.  Remove useless distinction about clobbering
registers.

From-SVN: r42060

gcc/ChangeLog

+2001-05-14  Alexandre Oliva  <aoliva@redhat.com>
+
+	* rtl.texi (CALL_INSN_FUNCTION_USAGE): Note that (and when) it may
+	contain MEMs.  Remove useless distinction about clobbering
+	registers.
+
 2001-05-13  Geoff Keating  <geoffk@redhat.com>
 
 	* config/rs6000/rs6000.h (EXCEPTION_SECTION): Move to...

gcc/rtl.texi

@@ -2530,12 +2530,25 @@ unpredictably.
 accessed in the same way and in addition contain a field
 @code{CALL_INSN_FUNCTION_USAGE}, which contains a list (chain of
 @code{expr_list} expressions) containing @code{use} and @code{clobber}
-expressions that denote hard registers used or clobbered by the called
-function.  A register specified in a @code{clobber} in this list is
-modified @emph{after} the execution of the @code{call_insn}, while a
-register in a @code{clobber} in the body of the @code{call_insn} is
-clobbered before the insn completes execution.  @code{clobber}
-expressions in this list augment registers specified in
+expressions that denote hard registers and @code{MEM}s used or
+clobbered by the called function.
+
+A @code{MEM} generally points to a stack slots in which arguments passed
+to the libcall by reference (@pxref{Register Arguments,
+FUNCTION_ARG_PASS_BY_REFERENCE}) are stored.  If the argument is
+caller-copied (@pxref{Register Arguments, FUNCTION_ARG_CALLEE_COPIES}),
+the stack slot will be mentioned in @code{CLOBBER} and @code{USE}
+entries; if it's callee-copied, only a @code{USE} will appear, and the
+@code{MEM} may point to addresses that are not stack slots.  These
+@code{MEM}s are used only in libcalls, because, unlike regular function
+calls, @code{CONST_CALL}s (which libcalls generally are, @pxref{Flags,
+CONST_CALL_P}) aren't assumed to read and write all memory, so flow
+would consider the stores dead and remove them.  Note that, since a
+libcall must never return values in memory (@pxref{Aggregate Return,
+RETURN_IN_MEMORY}), there will never be a @code{CLOBBER} for a memory
+address holding a return value.
+
+@code{CLOBBER}ed registers in this list augment registers specified in
 @code{CALL_USED_REGISTERS} (@pxref{Register Basics}).
 
 @findex code_label