RFR(M): 8136725 Provide utility for creation a counted loop reserve copy (clone)

[Vladimir Kozlov]

in loopUnswitch.cpp debug output should be under nonproduct flag (could 
be TraceLoopOpts or separate new flag). Use #ifndef PRODUCT around print 
code instead of repeated NOT_PRODUCT() macro. And I don't think you need 
all output you added in final version.

All constants are IGVNed so you don't need to pre-generate const_0. 
Since it is not connected to graph it could be eliminated.
And I don't think you need next for the same reason:

  311   lk->set_const_0(const_0);
  312   lk->set_const_1(const_1);

In ~CountedLoopReserveKit() and switch_to_reserved() you can make new 
const_0 since you have pointer to IGVN:

_phase->_igvn.intcon(0);

You can't use subsume() since it will replace all users of const_1:

+    //TODO bug? could not use here _const_1->subsume_by(_const_0, 
_phase->C);

It is correct to use set_req() for constants here.

superword.cpp I don't think you need to keep _CountedLoopReserveKit_test 
code in final version after you verified it works during development:

+  //!///    Testing that switching to reserved copy works

Thanks,
Vladimir

On 9/19/15 1:05 AM, Civlin, Jan wrote:
> As I was working on modification of SuperWord::output - and this is a function where you modify the ideal graph on the fly and cannot exit on error - I was wondering how can I protect my algorithm in a case some wrong decision was made on the previous stage of the SuperWord but discovered only at the final stage, when the graph is already half modified.
>
> So here is my solution:
> - the graph to be modified is cloned.
> - five nodes are added: intcon(0) is disconnected, intcon(1)->If-> IfTrue->orig_loop: IfFalse->clone_loop.
>
> Then any optimization is executed on the original loop, and if it finishes ok, nothing else need to be done, the clone_loop will be removed as dead in the consecutive stages of compiler.
> At any moment, if an error occurs, the node intcon(1) may be subsumed by node intcon(0), therefore  the clone_loop becomes "active" and the original "unfinished in modification" loop will be removed later.
>
> This utility is implemented in class CountedLoopReserveKit.
> The loop cloning and 5 nodes adding is realized in the ctor and possible subsuming intcon(1) by intcon(0) in dtor, so a simple return from the modifying graph function will do graph correction (switching in choice) of the graph.
> Basically, it is sufficient the create a local object of CountedLoopReserveKit class, the scoped dtor makes the choice: modified or original loop.
>
> SuperWord::output in this submission is included only to illustrate how to use the CountedLoopReserveKit.
> The actual new SuperWord::output where CountedLoopReserveKit  is indeed substantial is coming in the next patch.
>
> This reserve loop cloning in SuperWord may be disabled by the global flag DoReserveCopyInSuperWord.
>
> -----Original Message-----
> From: Igor Veresov [mailto:igor.veresov at oracle.com]
> Sent: Thursday, September 17, 2015 2:31 PM
> To: hotspot compiler
> Cc: Civlin, Jan
> Subject: RFR(M): 8136725 Provide utility for creation a counted loop reserve copy (clone)
>
> Provide utility for creation a counted loop reserve copy (clone).
> May be used in any graph optimization for simple roll back to the original loop, in partially will be used in SuperWord, where the loop modification goes on-the-fly and potentially may not finish correctly (the patch for SuperWord is coming soon).
>
> This is contributed by Jan Civlin <jan.civlin at intel.com>
>
> Webrev: http://cr.openjdk.java.net/~iveresov/loop-reverse/webrev-091515/
>
> igor
>