Re: [PR target/20126, RFC] loop DEST_ADDR biv replacement may fail

[Alexandre Oliva <aoliva at redhat dot com>]

On Apr 15, 2005, Roger Sayle <roger@eyesopen.com> wrote:

> I agree with your proposed game plan of keeping the hard failure in
> place temporarily, to discover whether there are any other "fallback"
> strategies that would be useful.  Ultimately though, I don't think we
> should close PR20126 until a "soft failure" is implemented on mainline,
> like we've (Jakub has) done on the gcc-4_0-branch (such as the
> mainline code proposed in comment #30).

But see, the problem with the soft failure mode is that, if it is ever
legitimate to leave the giv alone and not make sure we set whatever
register appears in it to the right value, then can't we always do it,
removing all of the (thus useless) workarounds?

And if there's any case in which it is not legitimate to do so, then
the soft failure mode would be a disservice to the user, that would
silently get a miscompiled program.  We should probably at least warn
in this case.

Anyhow, here's a patch that was tested with bootstrap and regtest on
amd64-linux-gnu on mainline, that brings in the soft failure mode from
the 4.0 branch to mainline, without removing the potentially-useless
workarounds.

Index: gcc/loop.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/loop.c,v
retrieving revision 1.526
diff -u -p -r1.526 loop.c
--- gcc/loop.c 10 Apr 2005 04:00:45 -0000 1.526
+++ gcc/loop.c 16 Apr 2005 21:37:45 -0000
@@ -5494,11 +5494,23 @@ loop_givs_rescan (struct loop *loop, str
 	      rtx reg, seq;
 	      start_sequence ();
 	      reg = force_reg (v->mode, *v->location);
-	      seq = get_insns ();
-	      end_sequence ();
-	      loop_insn_emit_before (loop, 0, v->insn, seq);
-	      if (!validate_change_maybe_volatile (v->insn, v->location, reg))
-		gcc_unreachable ();
+	      if (validate_change_maybe_volatile (v->insn, v->location, reg))
+		{
+		  seq = get_insns ();
+		  end_sequence ();
+		  loop_insn_emit_before (loop, 0, v->insn, seq);
+		}
+	      else
+		{
+		  end_sequence ();
+		  if (loop_dump_stream)
+		    fprintf (loop_dump_stream,
+			     "unable to reduce iv in insn %d\n",
+			     INSN_UID (v->insn));
+		  bl->all_reduced = 0;
+		  v->ignore = 1;
+		  continue;
+		}
 	    }
 	}
       else if (v->replaceable)

-- 
Alexandre Oliva             http://www.ic.unicamp.br/~oliva/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}