RFR: 8201543: Modularize C1 GC barriers

Erik Österlund erik.osterlund at oracle.com
Fri Apr 13 15:11:26 UTC 2018


The GC barriers for C1 are not as modular as they could be. It currently 
uses switch statements to check which GC barrier set is being used, and 
calls one or another barrier based on that, in a way that it can only be 
used for write barriers.

The solution I propose is to add the same facilities that have been 
added in runtime and the interpreter already: a barrier set backend for 
C1. I call it BarrierSetC1, and it helps us generate decorated accesses 
that give the GC control over the details how to generate this access. 
It recognizes the same decorators (accessDecorators.hpp) that the other 
parts of the VM recognize. Each concrete barrier set has its own 
backend. For now, these are CardTableBarrierSetC1 and G1BarrierSetC1, 
but this should pave way for upcoming concurrently compacting GCs as well.

Two decorators were added for C1 specifically (found in c1_Decorators.hpp):
C1_NEEDS_PATCHING for accesses where the index is not yet load because 
the class has yet to be loaded, and
C1_MASK_BOOLEAN for accesses that need to mask untrusted boolean values.

LIRGenerator calls a wrapper called access_store_at, access_load_at, etc 
(there are variants for cpmxchg, xchg and atomic add as well).
The access call calls straight into the BarrierSetC1 hierarchy using 
virtual calls. It is structured in a way very similar to 

BarrierSetC1 can also be called during initialization to generate stubs 
and runtime methods required by C1. For G1BarrierSetC1, this results in 
calling the BarrierSetAssembler for the platform specific code. This 
way, the BarrierSetC1 hierarchy has been carefully kept in shared code, 
and the switch statements for generating G1 code have been removed. Some 
code that used to be platform specific (like unsafe get/set and array 
store) have been broken out to shared code, with the actual platform 
specific details (some register allocation for store check and atomics) 
broken out to platform specific methods. This way, calls to access are 
kept in platform specific code.

As usual, big thanks go to Martin Doerr for helping out with S390 and 
PPC, and Roman for taking care of AArch64.




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