review request: 7147464: Java crashed while executing method with over 8k of dneg operations

Vladimir Kozlov vladimir.kozlov at
Fri Jul 13 12:05:13 PDT 2012

I would call states PROCESS_INPUTS, PROCESS_OUTPUTS instead.

Next lines could be moved just after progress_state check

+    if (progress_state == PROCESS_INPUTS) {
+      // After following inputs, continue to outputs
+      _stack.set_index(PROCESS_OUTPUTS);

that allow you to remove setting index in output processing code.


Dean Long wrote:
> OK.  The new webrev is here:
> dl
> On 07/12/2012 12:57 PM, Vladimir Kozlov wrote:
>> Dean,
>> Use enum values with meaningful names instead of 0/1 for progress_state.
>> You don't need to pop/push dead node to change progress state - use 
>> set_index(i) method. Also there is is_nonempty() method to use instead 
>> of !is_empty().
>> Vladimir
>> Dean Long wrote:
>>> Summary of changes: 77 lines changed: 38 ins; 7 del; 32 mod; 2264 unchg
>>> Deep recursion in PhaseIterGVN::remove_globally_dead_node() can cause 
>>> a stack overflow crash.  The test in the bug report causes recursion 
>>> that is 10000 levels deep.  The solution is to make the method 
>>> iterative with an explicit stack.
>>> The new version does not follow the recursive version exactly. It 
>>> does the recursive step of following a dead input only after all the 
>>> inputs have been looked at and some may have been pushed to the 
>>> worklist.  This could potentially cause a little more work if those 
>>> inputs are later found to be dead and have to be removed from the 
>>> worklist.  But in practice this almost never happens.  Out of 
>>> 93319791 calls to remove_globally_dead_node, it found a dead node 
>>> that was pushed to the worklist only 1181 times, so I don't think 
>>> following the original algorithm is worth the added complexity.  By 
>>> the way the original algorithm has the same flaw but to a lesser 
>>> degree, because it follows dead inputs in the order they are seen. If 
>>> this was truly a performance problem, then dead inputs should be 
>>> followed first.
>>> Tested with CTW and the CVM dneg test from bug report.  In CTW 
>>> testing the average depth of the explicit stack was 1.53939 and the 
>>> maximum depth was 541.  99.957% of the time the depth was 16 or less.
>>> Thanks to Vladimir Kozlov for implementation and testing suggestions 
>>> (but any bugs are mine).
>>> dl

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