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

Tobias Hartmann tobias.hartmann at oracle.com
Wed Nov 5 12:12:53 UTC 2014


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 [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.

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.

- JPRT including failing whitebox test


[1] See implementation of 'SimpleThresholdPolicy::is_trivial(..)'
[2] First compilation request:
            [is_trivial() returns false because no nmethod available]
            [next_level = CompLevel_Full_Optimization]
  [continue execution in interpreter]

[3] Second compilation request:
      [First compilation finishes here -> !CompileBroker::compilation_is_in_queue()]
          [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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