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Bytecode

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Bytecode (also called portable code or p-code) is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable<ref name="Dynamic_Machine_Code"/> source code, bytecodes are compact numeric codes, constants, and references (normally numeric addresses) that encode the result of compiler parsing and performing semantic analysis of things like type, scope, and nesting depths of program objects.

The name bytecode stems from instruction sets that have one-byte opcodes followed by optional parameters. Intermediate representations such as bytecode may be output by programming language implementations to ease interpretation, or it may be used to reduce hardware and operating system dependence by allowing the same code to run cross-platform, on different devices. Bytecode may often be either directly executed on a virtual machine (a p-code machine, i.e., interpreter), or it may be further compiled into machine code for better performance.

Since bytecode instructions are processed by software, they may be arbitrarily complex, but are nonetheless often akin to traditional hardware instructions: virtual stack machines are the most common, but virtual register machines have been built also.<ref name="Jucs_Lua"/><ref name="Dalvik"/> Different parts may often be stored in separate files, similar to object modules, but dynamically loaded during execution.

ExecutionEdit

A bytecode program may be executed by parsing and directly executing the instructions, one at a time. This kind of bytecode interpreter is very portable. Some systems, called dynamic translators, or just-in-time (JIT) compilers, translate bytecode into machine code as necessary at runtime. This makes the virtual machine hardware-specific but does not lose the portability of the bytecode. For example, Java and Smalltalk code is typically stored in bytecode format, which is typically then JIT compiled to translate the bytecode to machine code before execution. This introduces a delay before a program is run, when the bytecode is compiled to native machine code, but improves execution speed considerably compared to interpreting source code directly, normally by around an order of magnitude (10x).<ref name="Byte_Machine"/>

Because of its performance advantage, today many language implementations execute a program in two phases, first compiling the source code into bytecode, and then passing the bytecode to the virtual machine. There are bytecode based virtual machines of this sort for Java, Raku, Python, PHP,Template:Efn Tcl, mawk and Forth (however, Forth is seldom compiled via bytecodes in this way, and its virtual machine is more generic instead). The implementation of Perl and Ruby 1.8 instead work by walking an abstract syntax tree representation derived from the source code.

More recently, the authors of V8<ref name="Dynamic_Machine_Code"/> and Dart<ref name="Loitsch_Bytecode"/> have challenged the notion that intermediate bytecode is needed for fast and efficient VM implementation. Both of these language implementations currently do direct JIT compiling from source code to machine code with no bytecode intermediary.<ref name="Javascript"/>

ExamplesEdit

  • ActionScript executes in the ActionScript Virtual Machine (AVM), which is part of Flash Player and AIR. ActionScript code is typically transformed into bytecode format by a compiler. Examples of compilers include one built into Adobe Flash Professional and one built into Adobe Flash Builder and available in the Adobe Flex SDK.
  • Adobe Flash objects
  • BANCStar, originally bytecode for an interface-building tool but used also as a language
  • Berkeley Packet Filter
  • EBPF
  • Berkeley Pascal<ref>{{#invoke:citation/CS1|citation

|CitationClass=web }}</ref>

|CitationClass=web }}</ref> which prints to the standard output the underlying code of a specified function. The result is implementation-dependent and may or may not resolve to bytecode. Its inspection can be utilized for debugging and optimization purposes.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Steel Bank Common Lisp, for instance, produces:

<syntaxhighlight lang="lisp">

(disassemble '(lambda (x) (print x)))

disassembly for (LAMBDA (X))
2436F6DF
850500000F22 TEST EAX, [#x220F0000]  ; no-arg-parsing entry point
E5
8BD6 MOV EDX, ESI
E7
8B05A8F63624 MOV EAX, [#x2436F6A8]  ; #<FDEFINITION object for PRINT>
ED
B904000000 MOV ECX, 4
F2
FF7504 PUSH DWORD PTR [EBP+4]
F5
FF6005 JMP DWORD PTR [EAX+5]
F8
CC0A BREAK 10  ; error trap
FA
02 BYTE #X02
FB
18 BYTE #X18  ; INVALID-ARG-COUNT-ERROR
FC
4F BYTE #X4F  ; ECX

</syntaxhighlight>

Compiled code can be analysed and investigated using a built-in tool for debugging the low-level bytecode. The tool can be initialized from the shell, for example:
<syntaxhighlight lang="pycon">

>>> import dis # "dis" - Disassembler of Python byte code into mnemonics. >>> dis.dis('print("Hello, World!")') 

 1           0 LOAD_NAME                0 (print)
             2 LOAD_CONST               0 ('Hello, World!')
             4 CALL_FUNCTION            1
             6 RETURN_VALUE

</syntaxhighlight>

See alsoEdit

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NotesEdit

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ReferencesEdit

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