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

[Vladimir Kozlov]

Looks good.

Thanks,
Vladimir

On 3/17/15 8:59 AM, Vladimir Ivanov wrote:
> 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
>>>>>> _______________________________________________
>>>>>> mlvm-dev mailing list
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>>>>>> http://mail.openjdk.java.net/mailman/listinfo/mlvm-dev