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

[Rickard Bäckman]

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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