[Bug middle-end/38474] slow compilation at -O0 due to expand's temp slot goo

["jamborm at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38474

--- Comment #61 from Martin Jambor <jamborm at gcc dot gnu.org> 2012-06-26 14:26:34 UTC ---
(In reply to comment #57)
> 
> I will, on Monday.

And by Monday I obviously meant yesterday ;-)

Anyway, on the machine where are debugged this, compilation at -O3
took over 16 seconds which dropped to about 13.5 seconds when I also
added -fno-devirtualize (-ftime-report showed that alias stmt walking
dropped from 82% to 75%).  This is mainly due to calls to
detect_type_change from compute_known_type_jump_func, there are 36454
of them and all are of course completely pointless because we do not
devirtualize in Fortran.

Looking into the code, it is apparent that I even attempted to avoid
such situations but somehow was not paying enough attention.  The
rather obvious patch below fixes that.  With it, the compile time at
-O3 drops to 13.5 without any additional options (~50 calls to
detect_type_change_ssa and detect_type change from other places remain
but those are not a big problem here, they are not so easy to get rid
of and I hope to eventually remove the type detection machinery from
IPA altogether so I'll keep those for later).

I'll bootstrap and test the patch and post it to the mailing list
soon.

Index: gcc/ipa-prop.c
===================================================================
--- gcc/ipa-prop.c      (revision 188931)
+++ gcc/ipa-prop.c      (working copy)
@@ -912,8 +912,8 @@ compute_known_type_jump_func (tree op, s
       || is_global_var (base))
     return;

-  if (detect_type_change (op, base, call, jfunc, offset)
-      || !TYPE_BINFO (TREE_TYPE (base)))
+  if (!TYPE_BINFO (TREE_TYPE (base))
+      || detect_type_change (op, base, call, jfunc, offset))
     return;