RFR(XS): 8030976: untaken paths should be more vigorously pruned at highest optimization level

[Rickard Bäckman]

1) Like Igor said, one was missing. I'll move Reason_unstable_if above
Reason_tenured and 21 should be enough.

2) We only record when an uncommon trap is taken for the first 7
reasons. So I map unstable_if to intrinsic because intrinsic should
probably never happen on the bci of an if.

/R

On 06/03, Vladimir Ivanov wrote:
> 2 questions:
>   - why do you set _trap_hist_limit = 22 and not 21?
> 
>   - why do you map Reason_unstable_if to Reason_intrinsic in
> reason_recorded_per_bytecode_if_any?
> +     else if (reason == Reason_unstable_if)
> +       return Reason_intrinsic;
> 
> 
> Best regards,
> Vladimir Ivanov
> 
> On 6/3/14 5:07 PM, Rickard Bäckman wrote:
> >Updated: http://cr.openjdk.java.net/~rbackman/8030976.2/
> >
> >Thanks
> >/R
> >
> >On 06/03, Roland Westrelin wrote:
> >>Hi Rickard,
> >>
> >>>Updated webrev: http://cr.openjdk.java.net/~rbackman/8030976.1/
> >>
> >>You need to pass Deoptimization::Reason_unstable_if to uncommon_trap() in Parse::adjust_map_after_if(). Also the code below in Parse::adjust_map_after_if() is no longer needed AFAICT.
> >>
> >>Roland.
> >>
> >>1185     // If this might possibly turn into an implicit null check,
> >>1186     // and the null has never yet been seen, we need to generate
> >>1187     // an uncommon trap, so as to recompile instead of suffering
> >>1188     // with very slow branches.  (We'll get the slow branches if
> >>1189     // the program ever changes phase and starts seeing nulls here.)
> >>1190     //
> >>1191     // We do not inspect for a null constant, since a node may
> >>1192     // optimize to 'null' later on.
> >>1193     //
> >>1194     // Null checks, and other tests which expect inequality,
> >>1195     // show btest == BoolTest::eq along the non-taken branch.
> >>1196     // On the other hand, type tests, must-be-null tests,
> >>1197     // and other tests which expect pointer equality,
> >>1198     // show btest == BoolTest::ne along the non-taken branch.
> >>1199     // We prune both types of branches if they look unused.
> >>1200     repush_if_args();
> >>1201     // We need to mark this branch as taken so that if we recompile we will
> >>1202     // see that it is possible. In the tiered system the interpreter doesn't
> >>1203     // do profiling and by the time we get to the lower tier from the interpreter
> >>1204     // the path may be cold again. Make sure it doesn't look untaken
> >>1205     if (is_fallthrough) {
> >>1206       profile_not_taken_branch(!ProfileInterpreter);
> >>1207     } else {
> >>1208       profile_taken_branch(iter().get_dest(), !ProfileInterpreter);
> >>1209     }
> >>
> >>>
> >>>Thanks
> >>>/R
> >>>
> >>>On 05/22, Rickard Bäckman wrote:
> >>>>Roland pointed out a problem with the Reason used. New webrev coming
> >>>>shortly.
> >>>>
> >>>>Thanks Roland.
> >>>>
> >>>>On 05/22, Rickard Bäckman wrote:
> >>>>>Hi all,
> >>>>>
> >>>>>can I please have reviews for this change.
> >>>>>The patch makes C2 place uncommon traps on previously untaken branches
> >>>>>much more aggressively (we are simply trusting the profiling more).
> >>>>>This improves performance for a couple of different patterns.
> >>>>>
> >>>>>Example:
> >>>>>
> >>>>>class Test {
> >>>>>  public int[] array = new int[] = { 1, 1, 2, 3, 5, 8, 13 };
> >>>>>
> >>>>>  public void some_method() {
> >>>>>    for (int i = 0; i < array.length; i++) {
> >>>>>      if (array[i] < 255) {
> >>>>>        some_call();
> >>>>>      } else {
> >>>>>        some_other_call();
> >>>>>      }
> >>>>>    }
> >>>>>  }
> >>>>>}
> >>>>>
> >>>>>Where we previously if the else branch had never been taken rarely would
> >>>>>inline the some_other_call and when array escapes we can't make
> >>>>>assumptions on non-changing lengths, call killing registers, etc.
> >>>>>
> >>>>>On some of the Nashorn benchmark this patch increases score by 35%,
> >>>>>others don't see any change at all. No difference on SpecJBB 2005.
> >>>>>More performance numbers / microbenchmark in the comments of the bug.
> >>>>>
> >>>>>Webrev: http://cr.openjdk.java.net/~rbackman/8030976/
> >>>>>Bug: https://bugs.openjdk.java.net/browse/JDK-8030976
> >>>>>
> >>>>>Thanks
> >>>>>/R
> >>>>
> >>>>
> >>>>/R
> >>
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