Download Java Virtual Machine and Jikes RVM

Java Virtual Machine and Jikes RVM
Topics…
- Virtual Machine
- Performance
- Jikes RVM
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Virtual machine (Abstract computer)
• JVM (Java Virtual Machine)
Collection of libraries that runs java code
• CLR (Common Language Runtime)
Similar to JVM but more target languages
• VMM (Virtual Machine Monitor)
• PVM (Parallel Virtual Machine)
Network computers-like
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Background
• JVM in Fig1 (layered abstraction)
• Bytecode in Fig 2 (machine language of JVM)
Stream of bytecode is a sequence of inst, each
has a 1-byte opcode, zero or more operand
• Mnemonic: assembly language in JVM
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Fig 1
Fig 2
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Bytecode example
class count {
public static void main(String[] s) {
int x=2, y=0, i;
for (i=0; i < 5; i++)
y = x + i;
System.out.println( y );
}
}
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Method count()
0 aload_0
1 invokespecial #1 <Method java.lang.Object()>
4 return
Method void main(java.lang.String[])
0 iconst_2
1 istore_1
2 iconst_0
3 istore_2
4 iconst_0
5 istore_3
6 goto 16
9 iload_1
10 iload_3
11 iadd
12 istore_2
13 iinc 3 1
16 iload_3
17 iconst_5
18 if_icmplt 9
21 getstatic #2 <Field java.io.PrintStream out>
24 iload_2
25 invokevirtual #3 <Method void println(int)>
28 return
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JVM Facts
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Stack machine
Object oriented
Method invocation (static, virtual, interface)
Exceptions
Type checking
Multithreading
Memory management
Class loading
…
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Virtual Hardware
• Four basic parts
– register, stack, garbage-collected heap,
method area
• 32-bit address, 4GB addressable
memory, integer is 32-bit long
• Stack machine (vs. register machine)
Registers: PC, optop, frame, and vars
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JVM performance
• “Do not expect Java compiler/Jikes to perform
many clever optimization due to Java’s rather
strict sequencing and thread semantics, there
is a little the compiler can safely do, in
contrast to compilers for less strictly defined
languages such C/Fortran.”
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JVM Runtime state
• Method area
– like .text, store bytecode (pc)
runtime mem
• Stack
– stack frame storing params and result
– 3 sections: local var, execution env,
operand stack
vars
Frames
optop
Stack
• Heap
created dynamically by new
operator, runtime env tracks it
PC
Method
Heap
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Local var,
operand
Frame data
bytecode,
static var…
Objects
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Stack frame
Frame for foo(2)
Frame for foo(1)
Frame for main()
Bottom of
frame stack
Local variables,
Env variables,
Local evaluation
stack, PC, …
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JVM Instruction-set family
• Load and Store
load, store, push…
• Arithmetic and Logic
add, sub, mul, div, rem, neg, shl, shr, or, xor…
• Conversions
i2l, i2f, i2b, i2c, i2s…
• Objects
new, newarray, getfield, putfield, aload, astore,
instanceof
• Stack management
pop, dup, dup_x, swap…
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• Control transfer
if_icmpeq, ifreq, iflt, ifnull, tableswitch, cmp, goto,
jsr, ret, athrow
• Method invocation
invokevirtual, invokeinterface, invokespecial,
invokestatic…
• Multiple encodings
iconst_m1, iconst_0, bipush, sipush, ldc…
• Constant pool
Utf8, Inteter, Float, Class, NameAndType,
Fieldref, MethodRef, InterfaceMethodref
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Jikes RVM Introduction
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Formerly called Jalapeho project at IBM
Based on JIT (Just-in-time) compilation
Not a full VM, meant for research
Nothing to do with source to bytecode
compiler
• Implemented in Java (203k Java, Feb ‘02)
with GNU’s CLASSPATH
• Runs on AIX/PowerPC and Linux/IA32
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Jikes RVM subsystem
• Two compilers (baseline and optimizing
compiler) and Adaptive Optimization
System (AOS).
• Major java features
Bootstrapping, Method invocation, Method
dispatch, interface method invocation,
Exception, Dynamic type checking, Memory
memory management, Garbage collection,
Threads, Class loading… etc
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Baseline Compiler
• Based on Just-In-Time compiler
• Two main components
– Generate GC map for safe point
– Generate executable (209 switch
statement)
• Generate machine code
• Emulate stack machine
• Faster in compilation but much slower
(5+) than optimizing compiler
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Optimizing compiler
• Translating code into IR (Intermediate Representation)
and perform optimization.
• Three levels of IRs, multiple optimization
levels (many classical, some novel), ~100K.
• Template driven, instruction selection, IA32
assembler…
• Three groups: local (basic block), global
(method level) and inter-procedural (across
methods).
• Interface to adaptive optimization system.
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Transformation
• Four transformations
- HIR (High IR)
transform
- LIR (Low IR)
- MIR (Machine IR)
- Final assembly to
machine code
• Transformation
- Preserve original meaning
- Measurable speedup
- Worth effort
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Bytecode to IRs
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• High level IR (HIR)
- Recognize arch independent
bytecode and resemble them
- Use register transfer language
(JVM stack abstraction)
- Array bound check
- Inline of bytecode subroutine
- Devirtualization
• Low level IR (LIR)
- Arch independent, Complex HIR operators (new, call
virtual) are expanded
• Machine Level IR (MIR)
– Arch specific, defining MIR file (29 format)
– Inst selection, register allocation
– One-to-one mapping between operators and target ISA21
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• Common subexpression
for (i=0…) { x=a[i]; …; a[i]=a[j] }
t6=4*i;
x=a[t6];
…
t7=4*i;
a[t7]=t9;
t6 = 4*i;
x = a[t6];
…
a[t6] = t9
• Dead-code
if (debug) print
• Loop optimization (induction variables)
while (i<limit-2)
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t = limit – 2;
while (i<t)
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HIR (BC2IR)
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Transform to HIR
Abstract interpretation of bytecode
Bytecode subroutines inlined
Translate stack inst to 3-address inst
On-the-fly dataflow optimizations
(const folding, branch optimization, unreachable code
elimination, const and type propagation, …)
• Build FCFG
Ref: ACM Java Grande 99
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HIR Example
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FCFG example
• Reduced # of nodes
• Improve basic block analysis
• Adapt forward/backward analysis
…
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MIR (HIR2LIR)
• Convert to LIR
• Introduce Jikes RVM detail into IR
• VM services
(allocation, locks, type, checks, write barriers)
• Object model
(field, array, static layout, method invocation)
• Other low-level expansion
(switch, compare/branch, exception checks)
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LIR Example
t1 = new java.lang.Object
return t1
public static Object quickNewScalar(int size, Object[] tib,
boolean hasFinalizer) throws OutOfMemoryError {
Object rrt = VM_Allocator.allocateScalar(size, tib);
if ( hasFinalizer ) VM_Finalizer.addElement(ret);
return ret;
}
public static Object allocateScalar(int size, Object[] tib)
throws OutOfMemoryError {
VM_Address region=getHeapSpaceFast(size);
Object newObj = VM_ObjectMode.initializeScalar(
region, tob, size);
return newObj;
}
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MIR (LIR2MIR)
• Convert to MIR
• Instruction selection rules for each
architecture
• Each rule in LIR2MIR.rules is defined
by a four-line record (PRODUCTION, COST,
FLAGS, TEMPLATE)
Ref: Bottom-up, rewrite system (Fraser ’92,
Sarkar 01)
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Code optimization techniques
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Three address code
Keep heavily used var in registers
Control-flow improvement
Common subexpression
Dead-code elimination
Constant folding
Loop optimization
Basic block optimization (flow graph)
Data Flow analysis and flow graph
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More…
• Flow insensitive optimizations (register list)
• Inlining any method with on-the-fly optimization
(guarded inlining)
• Load Elimination
• Loop Normalization
• Loop Unrolling
• Global code placement
• Commoning (separate exeception from computation)
• Heap Array SSA
• FCFG forward analysis
• Simple Escape analysis (use register list to identify
object)
…
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Final Assembly
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Machine code generation
GC maps
Exception tables
Machine code info for
– Online profiling, stackframe inspection, debugging,
dynamic linking, lazy compilation
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Other interesting work
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GUN Kissme JVM
Porting the Jikes RVM to Linux/IA32
Cache performance in JVM
What missed: Java Memory Management,
Jikes specific optimization.
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