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Re: RFA: non-const libcalls

On May 13, 2001, Alexandre Oliva <> wrote:

> On May 12, 2001, Richard Henderson <> wrote:
>> On Sat, May 12, 2001 at 03:21:56PM -0300, Alexandre Oliva wrote:
>>> I'm pretty sure there was no problem in case the stores came before
>>> a regular call because this thing operated only within basic blocks,

>> No.  There was no problem on regular calls because they are 
>> assumed to read and write all memory.

>> /* Non-constant calls clobber memory.  */
>> if (! CONST_CALL_P (insn))

> Hmm...  So we probably still have a problem with regular function
> calls declared with attribute const, don't we?  We should probably be
> emitting CALL_INSN_FUNCTION_USAGE notes for arguments passed by
> reference to non-libcall const functions, shouldn't we?

Anyway, ok to install this so far?

Index: gcc/ChangeLog
from  Alexandre Oliva  <>

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

Index: gcc/rtl.texi
RCS file: /cvs/gcc/egcs/gcc/rtl.texi,v
retrieving revision 1.38
diff -u -p -r1.38 rtl.texi
--- gcc/rtl.texi 2001/04/19 19:44:12 1.38
+++ gcc/rtl.texi 2001/05/13 22:42:40
@@ -2523,12 +2523,25 @@ 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

Alexandre Oliva   Enjoy Guarana', see
Red Hat GCC Developer                  aoliva@{,}
CS PhD student at IC-Unicamp        oliva@{,}
Free Software Evangelist    *Please* write to mailing lists, not to me

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