Deoptimize on putfield CallSite.target?

[Krystal Mok]

Hi everyone,

I'd like to ask a question on the handling of setting CallSite.target in
compiled code.
Specifically, is the following piece of code in C2's
Parse::do_field_access() [1] necessary?

  // Deoptimize on putfield writes to call site target field.
  if (!is_get && field->is_call_site_target()) {
    uncommon_trap(Deoptimization::Reason_unhandled,
                  Deoptimization::Action_reinterpret,
                  NULL, "put to call site target field");
    return;
  }

If it is necessary, then why isn't it mirrored in C1's
GraphBuilder::access_field() [2] ?

My take is that the C2 deoptimization is unnecessary here, since a
dependency is added when the compilers speculatively take a CallSite.target
as constant.

There's no internal Java code that would do a putfield on a CallSite.target
anyway, and all external requests to change the target would have gone
through CallSite.setTarget(), which is appropriately routed to the runtime
already.

I'm okay with the "it won't hurt to leave the code there" idea. Just want
to make sure I understood things correctly.

Thanks,
Kris

[1]:
http://hg.openjdk.java.net/jdk9/jdk9/hotspot/file/38cb4fbd47e3/src/share/vm/opto/parse3.cpp#l104
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