[13] RFR (XS): 8202952: C2: Unexpected dead nodes after matching

[Vladimir Ivanov]

On 22/01/2019 11:54, Vladimir Kozlov wrote:
> The fix is different from what we discussed.
> Can you explain how it helps?

We discussed adding AddP case to _shared_nodes.

Proposed fix achieves similar result with a different approach:

   * Matcher::clone_address_expressions() marks problematic AddP as 
shared (based on constant value);

   * DFA() doesn't construct duplicated State for inner AddP (since it's 
marked as shared);

   * Matcher doesn't need to materialize duplicated mach nodes, since it 
matches inner AddP separately;

Best regards,
Vladimir Ivanov

> On 1/22/19 11:05 AM, Vladimir Ivanov wrote:
>> http://cr.openjdk.java.net/~vlivanov/8202952/webrev.00/
>> https://bugs.openjdk.java.net/browse/JDK-8202952
>>
>> The crash happens when PhaseCFG encounters a dead MachNode in the graph.
>> The problematic node is a leftover from matching of an instruction 
>> with a duplicated memory operand (sarI_mem_CL [1] in that particular 
>> case).
>>
>> Address has the following shape [2]:
>>    AddP (AddP DecodeN (LShiftL ConvI2L ConI)) ConL
>>
>> It could be subsumed into complex addressing expression, but the 
>> constant is too large (doesn't fit into immL32). So, matcher has to 
>> compute inner address expression separately and put it into a register.
>>
>> Since memory operand is duplicated, 2 copies are materialized during 
>> matching, but as part of ::Expand() one of the copies is eliminated, 
>> thus leaving a dead mach node in the IR (for the address expression).
>>
>> The fix is to adjust Matcher::clone_address_expressions() to avoid 
>> cloning inner AddP when constant offset is too large.
>>
>> Testing: hs-precheckin-comp, hs-tier1, hs-tier2
>>
>> Best regards,
>> Vladimir Ivanov
>>
>> [1] instruct sarI_mem_CL(memory dst, rcx_RegI shift, rFlagsReg cr)
>> %{
>>    match(Set dst (StoreI dst (RShiftI (LoadI dst) shift)));
>>
>>
>> [2]
>>   o347 AddP  === _ o2181 o1768 o1769  [[o349 o371 ]]
>>      o1768 AddP  === _ o2181 o2181 o1765  [[o347 ]]
>>          o2181 DecodeN === _ o287  [[o1768 o1768 o327 o347 o327 ]] 
>> #int[int:>=0]:NotNull:exact *
>>          o1765 LShiftL === _ o1761 o60  [[o1768 ]]
>>              o1761 ConvI2L === _ o1741  [[o1765 ]] 
>> #long:maxint-51..maxint-48
>>              o60   ConI  === o0  [[o61 o1765 o1434 o2013 o1631 o2017 
>> o1808  60 ]]  #int:2
>>      o1769 ConL  === o0  [[o347 ]]  #long:-8589932784