[9] RFR (S): 8074548: Never-taken branches cause repeated deopts in MHs.GWT case

[Vladimir Ivanov]

Updated webrev in place:
   http://cr.openjdk.java.net/~vlivanov/8074548/webrev.00/

On 3/17/15 4:13 AM, Vladimir Kozlov wrote:
> Reading an other comment in this method helped to understand what is
> going on:
>
> // Profile is int[2] where [0] and [1] correspond to false and true
> value occurrences respectively.
>
> May be add comment before next line that result is 0 or 1 and profile is
> number of each value occurrences:
>
> Node* result = argument(0);
Done.

> I think you need to changes PROB_NEVER to PROB_ALWAYS since you swapped
> condition. But please, verify generated code.
You are right. Generated code looks good.

Best regards,
Vladimir Ivanov

>
> Thanks,
> Vladimir
>
>
> On 3/16/15 4:42 PM, Vladimir Ivanov wrote:
>>>>> There is no explanation in changes why false_count never seen (==
>>>>> 0) is
>>>>> more important than true_count == 0. The comment says "one of the
>>>>> values".
>>>> The code is the following:
>>>>      if (false_cnt == 0 || true_cnt == 0) {
>>>>        // According to profile, one value has been never seen.
>>>
>>> That is what confuse me "one value never seen" but true_cnt > 0. What
>>> does it mean?
>> By "value" I mean boolean value here: "true" or "false".
>>
>> If (false_cnt == 0) or (true_cnt == 0), but (false_cnt + true_cnt) > 0
>> (earlier check), then either the value is always "true" or "false"
>> correspondingly. In other words, "false" or "true" (but not both!) has
>> been never observed during profiling. That's what the comment
>> communicates.
>>
>> Best regards,
>> Vladimir Ivanov
>>
>>>
>>> Vladimir K
>>>
>>>>        int expected_val = (false_cnt == 0) ? 1 : 0;
>>>>
>>>> (false_cnt != 0) => (true_cnt == 0).
>>>> Do you want me to add additional comment about that?
>>>>
>>>> I chose (false_cnt == 0) because it refers to "always true" case.
>>>>
>>>> Best regards,
>>>> Vladimir Ivanov
>>>>
>>>>>
>>>>> Thanks,
>>>>> Vladimir
>>>>>
>>>>> On 3/16/15 11:26 AM, Vladimir Ivanov wrote:
>>>>>> http://cr.openjdk.java.net/~vlivanov/8074548/webrev.00/
>>>>>> https://bugs.openjdk.java.net/browse/JDK-8074548
>>>>>>
>>>>>> MethodHandleImpl::profileBoolean doesn't update never-taken branch
>>>>>> count
>>>>>> when hitting a deopt on it. As a result, for rarely taken branches
>>>>>> consequent compilations consider them as never-taken and prune them
>>>>>> again causing repeated deopts. It severely affects peak performance.
>>>>>>
>>>>>> The fix is to update MHI::profileBoolean intrinsic to insert a guard
>>>>>> and
>>>>>> uncommon trap w/ reexecute bit set for never-seen value. Once
>>>>>> previously
>>>>>> never seen value is encountered, the execution resumes after deopt in
>>>>>> MHI::profileBoolean and corresponding count becomes non-zero.
>>>>>>
>>>>>> The guard doesn't add any additional overhead, since it dominates all
>>>>>> value usages and all branches on the same value are eliminated.
>>>>>>
>>>>>> Testing: java/lang/invoke, nashorn, octane
>>>>>>
>>>>>> Best regards,
>>>>>> Vladimir Ivanov
>>>>>> _______________________________________________
>>>>>> mlvm-dev mailing list
>>>>>> mlvm-dev at openjdk.java.net
>>>>>> http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev
>>>>> _______________________________________________
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