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

[Vladimir Kozlov]

The fix is different from what we discussed.
Can you explain how it helps?

Thanks,
Vladimir K

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