RFR: 8284848: C2: Compiler blackhole arguments should be treated as globally escaping

[Aleksey Shipilev]

On Wed, 13 Apr 2022 17:33:27 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> WIP so far, but I would appreciate early review of someone savvy in C2 EA code. I'll try to whip up the test with IR Framework too.
>> 
>> See more discussion in the bug.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 fastdebug `tier1`
>>  - [x] Linux x86_64 fastdebug `tier2`
>>  - [x] OpenJDK microbenchmark corpus sanity run
>
> src/hotspot/share/opto/escape.cpp line 825:
> 
>> 823:           set_escape_state(ptn, PointsToNode::GlobalEscape NOT_PRODUCT(COMMA "blackhole"));
>> 824:         }
>> 825:         add_edge(n_ptn, ptn);
> 
> Why not use `add_local_var_and_edge()` here?

Because the input for the node might not be a `LocalVar` already, but rather `Field`, `Arraycopy`, etc.  `add_local_var_and_edge` checks this and fails on asserts. AFAICS, this is only safe to do for the node "output", but here we handle the node inputs. Maybe I should instead do what `Op_Phi` does?

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PR: https://git.openjdk.java.net/jdk/pull/8228