Re: Gcc 3.1 performance regressions with respect to 2.95.3

[law at redhat dot com]

 In message <200203252318.SAA27178@makai.watson.ibm.com>, David Edelsohn 
writes:
 > >>>>> Jeff Law writes:
 > 	The dependence code computes the correct dependence relationship,
Right.  Without it the clobbers could move to a point after the use/set,
which would generate incorrect code.

 > but all of the register pressure code in the final version of the Haifa
 > Scheduler deliverable from IBM is not present in the FSF tree.
Err, the register pressure code from IBM Haifa was junk.  I know because I
sat down and evaluated the later drops.  It was total junk.

 > 	The main affect on the scheduler moving the clobbers far away from
 > the use/def sites is the issue rate. 
So that's were we need to concentrate our focus.

When we schedule a clobber, we should not decrement can_issue_more for
the current cycle.  Additionally, any instructions which have their final
dependency resolved due to issuance of the clobber should go into the
ready queue with a cost independent of the anti dependency relative to
the clobber.  [ And priority computation should ignore clobbers. ]

I believe if you do those things your problem will be solved.


 > 	IMHO, no-unit should not be counted against the issue rate and
 > some other mechanism needs to keep those instructions from issuing early. 
Possibly.  Though that may have other undesirable effects depending on how
the machine description works.  Instead I would make the special handling
dependent on PATTERN (insn) == CLOBBER.