Bytecode

Source: Wikipedia, the free encyclopedia.
"Portable code" and "P-code" redirect here. For other uses, see software portability and P-Code (disambiguation).

Program execution
General concepts
Types of code
Compilation strategies
Notable runtimes
Notable compilers & toolchains

Bytecode (also called portable code or p-code[

human-readable[1] 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-

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.[2][3] Different parts may often be stored in separate files, similar to object modules, but dynamically loaded during execution.

Execution

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

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).[4]

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,[a] 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[1] and Dart[7] 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.[8]

Examples

(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
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:
>>> 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

See also

Look up bytecode in Wiktionary, the free dictionary.

Notes

  1. ^ PHP has just-in-time compilation in PHP 8,[5][6] and before while not on in the default version, had options like HHVM. For older versions of PHP: Although PHP opcodes are generated each time the program is launched, and are always interpreted and not just-in-time compiled.

References

  1. ^ a b "Dynamic Machine Code Generation". Google Inc. Archived from the original on 2017-03-05. Retrieved 2023-02-21.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
  2. ^ "The Implementation of Lua 5.0". (NB. This involves a register-based virtual machine.)
  3. ^ "Dalvik VM". Archived from the original on 2013-05-18. Retrieved 2012-10-29. (NB. This VM is register based.)
  4. ^ "Byte Code Vs Machine Code". www.allaboutcomputing.net. Retrieved 2017-10-23.
  5. ^ O’Phinney, Matthew Weier. "Exploring the New PHP JIT Compiler". Zend by Perforce. Retrieved 2021-02-19.
  6. ^ "PHP 8: The JIT - stitcher.io". stitcher.io. Retrieved 2021-02-19.
  7. ^ Loitsch, Florian. "Why Not a Bytecode VM?". Google. Archived from the original on 2013-05-12.
  8. ^ "JavaScript myth: JavaScript needs a standard bytecode". 2ality.com.
  9. ^ G., Adam Y. (2022-07-11). "Berkeley Pascal". GitHub. Retrieved 2022-01-08.
  10. ^ "CLHS: Function DISASSEMBLE". www.lispworks.com.
  11. ^ "Performance Tuning and Tips". lispcookbook.github.io.
  12. ^ "The Implementation of the Icon Programming Language" (PDF). Archived from the original (PDF) on 2016-03-05. Retrieved 2011-09-09.
  13. ^ "The Implementation of Icon and Unicon a Compendium" (PDF). Archived (PDF) from the original on 2022-10-09.
  14. KEYB
    driver has such a huge memory footprint compared to table-driven keyboard drivers which can be done in 3 - 4 Kb getting the same level of function except for the interpreter. […]
  15. KEYBOARD.SYS
    instead of mixing Microsoft and IBM versions […] (NB. What is meant by "procedures" here are some additional bytecodes in the IBM KEYBOARD.SYS file not supported by the Microsoft version of the KEYB driver.)
  16. ^ "United States Patent 6,973,644". Archived from the original on 2017-03-05. Retrieved 2009-05-21.
  17. floating point format to calculate on, and an external (standard) format, which was binary coded decimal
    (BCD). The PACK and UNPACK instructions converted between the two.
  18. ^ "R Installation and Administration". cran.r-project.org.
  19. ^ "The SQLite Bytecode Engine". Archived from the original on 2017-04-14. Retrieved 2016-08-29.
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