[9] RFR(S): 8056071: compiler/whitebox/IsMethodCompilableTest.java fails with 'method() is not compilable after 3 iterations'
Vladimir Kozlov
vladimir.kozlov at oracle.com
Fri Nov 7 16:21:20 UTC 2014
You may change behavior in other places where would_profile() is used. Please, verify. Otherwise looks good.
Thanks,
Vladimir
On 11/7/14 1:58 AM, Tobias Hartmann wrote:
> Hi Vladimir,
>
> thanks for the review.
>
> On 07.11.2014 01:50, Vladimir Kozlov wrote:
>> On 11/6/14 2:25 AM, Tobias Hartmann wrote:
>>> 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?
>>
>> You are right. NonTieredCompPolicy::is_mature() does not use it.
>>
>>>
>>>> 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.
>>
>> Yes, but next code will do the same and only checking MDO in the last case:
>>
>> bool SimpleThresholdPolicy::is_trivial(Method* method) {
>> if (method->is_accessor() ||
>> method->is_constant_getter()) {
>> return true;
>> }
>> if (method->has_loops() || (method->code_size() >= 15)) {
>> return false;
>> }
>> MethodData* mdo = method->method_data();
>> if (mdo != NULL && mdo->stats_valid() && !mdo->would_profile() &&
>> (method->code_size() < 5 || (mdo->num_blocks() < 4))) {
>> return true;
>> }
>> return false;
>> }
>
> Okay, I changed the code accordingly.
>
>> Also mdo->_would_profile will not be set if C1 compilation is bailed out
>> (Compilation() in c1_Compilation.cpp). So we can't trust it too.
>> May be mdo->_would_profile should be enum {unknown, no_profile, profile} and we
>> use it instead of mdo->stats_valid().
>
> I agree. Like this, we also save the additional '_stats_valid' field in
> MethodData. I added an enum and changed the implementation of 'stats_valid()'.
> It now returns true if '_would_profile' is not initialized or set to true.
>
> New webrev: http://cr.openjdk.java.net/~thartmann/8056071/webrev.01/
>
> Thanks,
> Tobias
>
>> Thanks,
>> Vladimir
>>
>>>
>>> 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
>>>>>
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