RFR(M): 8181211: C2: Use profiling data to optimize on/off heap unsafe accesses

Roland Westrelin rwestrel at redhat.com
Tue May 30 14:07:27 UTC 2017


The not yet pushed 8176506 makes code emitted for unsafe accesses a lot
more conservative and harder for c2 to optimize when an access is not
known to be on or off heap. This change proposes that profile data be
used to detect on or off heap accesses allowing c2 to better optimize
sequences of unsafe accesses (at the cost of an extra null

With this change:

1- argument profiling is enabled at unsafe put* and get* call site

2- I found that enabling argument profiling but not return value
  profiling is broken. That change fixes it.

3- C2 needs to be able to tell, from profile data, whether a pointer is
  always null, never null or sometimes null. So far, whether a pointer
  is always null wasn't made available to c2.

4- the C2 unsafe code now checks profile data when it cannot tell
  whether an access is on or off heap and emits a guard.

5- load from non null objects at an offset guaranteed to fall in the
object are not pinned anymore.

With this change, Paul's benchmarks perform as well with 8176506+8181211
as they do without (actually better because of 5). Andrew's byte buffer
tests generate a main loop with 8176506+8181211 that's identical to what
is generated currently.


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