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JIT and method call devirtualization Slide from meet up, 18 Apr 2013, Moscow
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Old good C++
010110010010101010100110101010100101010101001010101010101010101010101010101010101010101010100010
00: methodA
02: methodC
03: methodD
CODEOBJECT
VTABLE
01: methodB
Simple case
Old good C++
010110010010101010100110101010100101010101001010101010101010101010101010101010101010101010100010111010100100011110000010101001010100
00: methodA
02: methodC
03: methodD
CODEOBJECT
VTABLE
01: methodB
00: methodX
02: methodZ
01: methodY
VTABLE
Multiple inheritance
Old good C++Multiple inheritance – more fun
A
B C
D D
A
B C
D
Vs
Problems
Two memory reads before jump Memory access is serialized instruction pipeline is blocked
Dynamic languages
Every call is virtual No predefined classes Multiple inheritance
Nightmare for super scalar CPU
Call site polymorphism
A particular call sitein most case
would be callingexactly one method instance
Call sites are mostly monomorthic!
Exploiting monomorthism
Tracing JIT JavaScript – Mozila TraceMonkey Python – PyPy
Profiling for whole method JIT JVM
Tracing JIT
Interpretation phase Record actions, record branch conditions
Compiling trace Actions compiled into branchless machine code Guards added for conditions If guard is broken – fall back to interpretation
Tracing JIT
Mozila’s TraceMonkey1. Incremental Dynamic Code Generation with Trace Trees
http://www.ics.uci.edu/~franz/Site/pubs-pdf/ICS-TR-06-16.pdf
PyPy RPython – restrict python dialect compiling to C Rpython based interpreter trace JITed by runtime1. http://tratt.net/laurie/research/pubs/papers/bolz_tratt__the_impact_of_metatrac
ing_on_vm_design_and_implementation.pdf
Tracing JIT
Limitation Trace is EXPENSIVE Falling out of path is EXPENSIVE A lot of code is executed just once
– wasting resources on tracing
Whole method JIT
JVM Classic whole method JIT Profile “morphism” of call sites Multi tier compiler Dynamics recompilation / decompilation
Whole method JIT
Call sites Monomorphic Bimorphic Megamorphic
Invoke dynamic
Problem Interpreters tends to produce
megamorphic sites
Invoke dynamic Detach call site into manageable object
More JVM back magic
On stack replacement Code for method could be replaced without
reentering to a method
Scalar replacement Object not leaving method scope
can be reduced to a number of local variables
