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

[Igor Veresov]

I didn’t add an entry in _trap_hist for Reason_tenured because it’s tracked by a different counter. Perhaps Reason_unstable_if can be moved before Reason_tenured so that _trap_hist_limit can be 21.

igor

On Jun 3, 2014, at 3:42 AM, Vladimir Ivanov <vladimir.x.ivanov at oracle.com> 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
>>>