[Bug ipa/65076] [5 Regression] 16% tramp3d-v4.cpp compile time regression

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65076

--- Comment #18 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 25 Mar 2015, hubicka at ucw dot cz wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65076
> 
> --- Comment #17 from Jan Hubicka <hubicka at ucw dot cz> ---
> > > Even though the inline decisions does not seem to be changed considerably
> > > (at least on tramp3d).
> > 
> > Yeah, clobbers don't account for anything for size/inline estimates
> > (well, I hope so!).
> 
> Yep, they are ignored.

Btw, and we indeed generate more clobbers with GCC 5:

> grep CLOBBER a/tramp3d-v4.ii.015t.ssa | wc -l
8937
> grep CLOBBER b/tramp3d-v4.ii.018t.ssa | wc -l
12636

which is odd as we now have new code that replaces clobbers with
SSA default defs...

> grep CLOBBER a/tramp3d-v4.ii.010t.eh | wc -l
13012
> grep CLOBBER b/tramp3d-v4.ii.010t.eh | wc -l
13012
> grep CLOBBER a/tramp3d-v4.ii.011t.cfg | wc -l
12827
> grep CLOBBER b/tramp3d-v4.ii.011t.cfg | wc -l
12827

so we manage to drop a lot of them during into-SSA with GCC 4.9 but
fail to do that with GCC 5.  Which would mean we write less vars
into SSA form(?)

> grep '_[0-9]* =' a/tramp3d-v4.ii.015t.ssa | wc -l
46055
> grep '_[0-9]*(D)' a/tramp3d-v4.ii.015t.ssa | wc -l
30839

vs.

> grep '_[0-9]* =' b/tramp3d-v4.ii.018t.ssa | wc -l
45543
> grep '_[0-9]*(D)' b/tramp3d-v4.ii.018t.ssa | wc -l
33782

eventually it might be that in 4.9 local_pure_const runs for a functions
callees before we fixup its CFG and write it into SSA form.  That might
cause a significant cleanup of EH before entering into-SSA for 4.9
(as callees might become nothrow).  At least that's now my theory.

I wonder if it makes sense to run local + IPA pure-const as we now
do "IPA into-SSA".  So sth like

Index: gcc/passes.def
===================================================================
--- gcc/passes.def      (revision 221633)
+++ gcc/passes.def      (working copy)
@@ -58,7 +58,9 @@ along with GCC; see the file COPYING3.
       NEXT_PASS (pass_build_ssa);
       NEXT_PASS (pass_ubsan);
       NEXT_PASS (pass_early_warn_uninitialized);
+         NEXT_PASS (pass_local_pure_const);
   POP_INSERT_PASSES ()
+  NEXT_PASS (pass_ipa_pure_const);

   NEXT_PASS (pass_chkp_instrumentation_passes);
   PUSH_INSERT_PASSES_WITHIN (pass_chkp_instrumentation_passes)

(which likely doesn't work exactly like that, of course).  Or
keep doing local_pure_const only and make sure to process
pass_build_ssa_passes in a proper order.  Of course the real
power of local-pure-const only arrives when done after local
optimizations...

But I'd say the compile-time effect is clearly going into-SSA
before cleaning up EH significantly.

> > And yes, doing DSE early is quite an old idea...  we should revisit it
> > next stage1.
> 
> I also run into it several time.  We should not forget this time ;)

;)