RFR(S): 8056071: compiler/whitebox/IsMethodCompilableTest.java fails with 'method() is not compilable after 3 iterations'
igor.ignatyev at oracle.com
Thu Nov 6 19:47:44 UTC 2014
On 11/06/2014 01:25 PM, 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.
you posted wrong cr.
actually, PavelP is working on such the tests, it's tracked by 8059575
>> 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.
>> On 11/5/14 4:12 AM, Tobias Hartmann wrote:
>>> please review the following patch.
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8056071
>>> Webrev: http://cr.openjdk.java.net/~thartmann/8056071/webrev.00/
>>> 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 .
>>> We compile at level 4 and execution continues in the interpreter .
>>> - A second compilation request for m is issued and while it is being processed
>>> the first compilation request finishes . 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 .
>>> 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.
>>> 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  and trivial_fixed.log .
>>> 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.
>>> - JPRT including failing whitebox test
>>>  See implementation of 'SimpleThresholdPolicy::is_trivial(..)'
>>>  First compilation request:
>>> [is_trivial() returns false because no nmethod available]
>>> [next_level = CompLevel_Full_Optimization]
>>> [continue execution in interpreter]
>>>  Second compilation request:
>>> [First compilation finishes here ->
>>> [is_trivial() returns true because nmethod/mdo now available]
>>> [next_level = CompLevel_Simple]
>>> [CompLevel_Simple nmethod replaced CompLevel_Full_Optimization method]
>>>  https://bugs.openjdk.java.net/secure/attachment/23321/PrintCompilation.log
>>>  https://bugs.openjdk.java.net/secure/attachment/23327/trivial.log
>>>  https://bugs.openjdk.java.net/secure/attachment/23328/trivial_fixed.log
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