[9] RFR(S): 8056071: compiler/whitebox/IsMethodCompilableTest.java fails with 'method() is not compilable after 3 iterations'

[Tobias Hartmann]

Hi Vladimir,

thanks for the review.

On 06.11.2014 01:56, Vladimir Kozlov wrote:
> So the fix is add -XX:-TieredCompilation.
> The rest is fix problems found due to these failure.

Yes, exactly.

> May be we should add a new test (other RFE) which would test compilation of
> trivial methods in Tiered mode. Current failure shows that we don't test it.

I agree. I filed RFE JDK-8056071 and will take care of it.

> I am not comfortable to rely on data which is set only by C1. This information
> is accessed in non Tiered mode too.

'is_trivial' is only used with the Simple and AdvancedThresholdPolicy, i.e., if
TieredCompilation is enabled. Where do you think it is used in non-tiered mode?

> Why not use method->has_loops(), for example?

We also decide based on the number of blocks and 'MethodData::would_profile'
which is only available if we have a C1 compiled version.

Thanks,
Tobias

> 
> Thanks,
> Vladimir
> 
> On 11/5/14 4:12 AM, Tobias Hartmann wrote:
>> Hi,
>>
>> please review the following patch.
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8056071
>> Webrev: http://cr.openjdk.java.net/~thartmann/8056071/webrev.00/
>>
>> Problem:
>> The test compiles and deoptimizes a method m multiple times. After each
>> deoptimization the test checks if m was compiled with C2 and if so increments a
>> counter. The test assumes that m is compilable until the counter reaches the
>> 'PerMethodRecompilationCutoff'. This does not hold in the following case:
>> - The first compilation request for m is added to the compile queue. Although we
>> profiled m and the MDO suggests that m is trivial, we do not use this
>> information because due to deoptimization there is no corresponding nmethod [1].
>> We compile at level 4 and execution continues in the interpreter [2].
>> - A second compilation request for m is issued and while it is being processed
>> the first compilation request finishes [3]. Because a nmethod now exists, we use
>> the MDO and decide to compile at level 1 since m is trivial.
>> - The level 4 compiled nmethod is replaced by the new level 1 nmethod [4].
>>
>> After the following deoptimization the counter is not incremented because the
>> nmethod was compiled at level 1. As a result m is set to not compilable before
>> the counter reaches the 'PerMethodRecompilationCutoff'. The test fails.
>>
>> Problems with the current implementation:
>> (1) We should always use the MDO if it is valid, even if there is no
>> corresponding nmethod. Otherwise we compile trivial methods at level 4 or start
>> profiling again.
>> (2) If there is a nmethod the MDO may still be invalid: We can compile a method
>> immediately at C2, deoptimize, and the MDO is uninitialized. In this case we
>> wrongly assume that the method is trivial and re-compile it with C1.
>> (3) To avoid a level 2 or 3 compilation and profiling we should mark simple
>> constant getter methods, like those used by the test, as trivial.
>> (4) We should add verification code to avoid/catch such bad tiered level
>> transitions in the future.
>>
>> Solution:
>> To fix (1) and (2) I added a field '_stats_valid' to MethodData that determines
>> if '_num_loops' and '_num_blocks' are set (i.e. the method was previously C1
>> compiled). Instead of checking for a nmethod, 'is_trivial()' now checks if the
>> MDO is valid. I added -XX:-TieredCompilation to the test because otherwise we
>> always compile the (trivial) methods at level 1 and the test fails.
>>
>> To fix (3) I added the method 'Method::is_constant_getter()' that determines if
>> a method consists of a constant push and a return statement only. If so, we
>> treat the method as trivial. See trivial.log [5] and trivial_fixed.log [6].
>>
>> I filed JDK-8062913 for (4) because the verification code I added still fails
>> with the current implementation due to 2->2 and 3->2 transitions.
>>
>> Testing:
>> - JPRT including failing whitebox test
>>
>> Thanks,
>> Tobias
>>
>>
>> [1] See implementation of 'SimpleThresholdPolicy::is_trivial(..)'
>> [2] First compilation request:
>> InterpreterRuntime::frequency_counter_overflow
>>    InterpreterRuntime::frequency_counter_overflow_inner
>>      SimpleThresholdPolicy::event
>>        AdvancedThresholdPolicy::method_invocation_event
>>          AdvancedThresholdPolicy::call_event
>>            AdvancedThresholdPolicy::common
>>              [is_trivial() returns false because no nmethod available]
>>              [next_level = CompLevel_Full_Optimization]
>>          SimpleThresholdPolicy::compile
>>            SimpleThresholdPolicy::submit_compile
>>              CompileBroker::compile_method
>>                CompileBroker::compile_method_base
>>                  CompileBroker::create_compile_task
>>    [continue execution in interpreter]
>>
>> [3] Second compilation request:
>> InterpreterRuntime::frequency_counter_overflow
>>    InterpreterRuntime::frequency_counter_overflow_inner
>>      SimpleThresholdPolicy::event
>>        AdvancedThresholdPolicy::method_invocation_event
>>        [First compilation finishes here ->
>> !CompileBroker::compilation_is_in_queue()]
>>        AdvancedThresholdPolicy::call_event
>>          AdvancedThresholdPolicy::common
>>            [is_trivial() returns true because nmethod/mdo now available]
>>            [next_level = CompLevel_Simple]
>>    [CompLevel_Simple nmethod replaced CompLevel_Full_Optimization method]
>>
>> [4] https://bugs.openjdk.java.net/secure/attachment/23321/PrintCompilation.log
>> [5] https://bugs.openjdk.java.net/secure/attachment/23327/trivial.log
>> [6] https://bugs.openjdk.java.net/secure/attachment/23328/trivial_fixed.log
>>