Perl virtual machine: Difference between revisions

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The '''Perl virtual machine''' is a [[Stack machine|stack-based]] [[Application virtual machine|process virtual machine]] implemented as an [[opcode|opcodes]]s [[Interpreter (computing)|interpreter]] which runs a previously compiled programs written in the [[Perl]] programslanguage. OpcodesThe opcodes interpreter is is a part of the Perl interpreter, which also contains alsoa [[Compiler|compiler]] ([[Lexical analysis|lexer]], [[Parsing|parser]] and [[Compiler optimization|optimizer]]) in one executable file, commonly /usr/bin/perl on various [[Unix-like]] systems or perl.exe on [[Microsoft Windows]] systems.

{{Expert-subject|Free Software}}

The '''Perl virtual machine''' is a [[Stack machine|stack-based]] [[Application virtual machine|process virtual machine]] implemented as [[opcode|opcodes]] [[Interpreter (computing)|interpreter]] which runs a previously compiled [[Perl]] programs. Opcodes interpreter is is a part of Perl interpreter which contains also [[Compiler|compiler]] ([[Lexical analysis|lexer]], [[Parsing|parser]] and [[Compiler optimization|optimizer]]) in one executable file, commonly /usr/bin/perl on various [[Unix-like]] systems or perl.exe on [[Microsoft Windows]] systems.

==Implementation==

===Opcodes===

The Perl compiler outputs a compiled program into memory as an internal structure which can be represented as a tree graph in which each node represents an opcode. Opcodes are represented internally by [[typedef]]s. Each opcode has '''next''' node/ ''other'' and ''first'sibling' / ''sibling'' nodepointers, so the opcode tree can be draweddrawn as a basic OP tree starting from root node or as flat OP list in the order they would normally execute from start node. Opcodes tree can be mapped to the actual source code, so it is possible to [[Decompiler|decompile]] to high-level source code.<ref>{{cite web | url=httphttps://perldoc.perl.org/B/Deparse.html | title=B::Deparse - Perl compiler backend to produce perl code}}</ref>

Perl's opcodes interpreter is implemented as a tree walker which travels bythe opcode tree in executeexecution order from the start node, following the ''next'' or ''other'' pointers. Each opcode ishas assigneda withfunction internalpointer to a pp_''opname'' function, i.e. the ''say'' opcode calls the ''pp_say'' function of internal Perl API.

The phase of compiling thea Perl program is hidden forfrom the end user, but it can be exposed with variousthe B:: Perl modulesmodule<ref whichname="B">{{cite providesweb an| accessurl=https://metacpan.org/module/B to| internaltitle=B API- ofThe compilatorPerl andCompiler opcodeBackend}}</ref> or other specialized modules, as the B::Concise Perl walkermodule.<ref>{{cite web | url=httphttps://perldoc.perl.org/B/Concise.html | title=B::Concise - Walk Perl syntax tree, printing concise info about ops}}</ref>

An example of compileda simple compiled [["Hello, World!" program|Hello world]] program withdumped ain helpexecute oforder (with the B::Concise Perl module, dumped in execute order):

<sourcesyntaxhighlight lang="bashconsole">

$ perl -MO=Concise,-exec -E 'say "Hello, world!"'

1 <0> enter

2 <;> nextstate(main 46 -e:1) v:%,{

3 <0> pushmark s

Line 20:

5 <@> say vK

6 <@> leave[1 ref] vKP/REFC

</syntaxhighlight>

</source>

Some opcodes (entereval, dofile, require) call Perl compiler functions which in turn generate anotherother opcodes in the same Perl virtual machine.

===Other implementationsVariables===

Perl variables can be global, dynamic (''local'' keyword), or lexical (''my'' and ''our'' keywords).

Global variables are accessible via the stash and the corresponding [[typeglob]].

There is no standarization for Perl language and Perl virtual machine. The internal API should be considered as non-stable and changes from version to version. The Perl virtual machine is tied closely to compiler. These things make very hard to reimplement Perl virtual machine.

Local variables are the same as global variables but a special opcode is generated to save its value on the ''savestack'' and restore it later.

Lexical variables are stored in the ''padlist''.

===Data structures===

Perl VM data structures are represented internally by [[typedef]]s.

The internal data structures can be examined with the B Perl module<ref name="B" /> or other specialized tools like the Devel::Peek Perl module.<ref>{{cite web | url=https://metacpan.org/module/Devel::Peek | title=Devel::Peek - A data debugging tool for the XS programmer}}</ref>

====data types====

Perl has three typedefs that handle Perl's three main data types: Scalar Value (''SV''), Array Value (''AV''), Hash Value (''HV''). Perl uses a special typedef for the simple signed integer type (''IV''), unsigned integers (''UV''), floating point numbers (''NV'') and strings (''PV'').

Perl uses a [[Reference counting|reference count]]-driven garbage collection mechanism. SVs, AVs, or HVs start their life with a reference count of 1. If the reference count of a data value drops to 0, then it will be destroyed and its memory is made available for reuse.

Other typedefs are Glob Value (''GV'') which contain named references to various objects, Code Value (''CV'') which contain a reference to a Perl subroutine, I/O Handler (''IO''), a reference to [[regular expression]] (''REGEXP''; ''RV'' in Perl before 5.11), reference to compiled format for output record (''FM'') and simple reference which is a special type of scalar that point to other data types (''RV'').

====stash====

Special Hash Value is ''stash'', a hash that contains all variables that are defined within a package. Each value in this hash table is a Glob Value (''GV'').

====padlist====

Special Array Value is ''padlist'' which is an array of array. Its 0th element to an AV containing all lexical variable names (with prefix symbols) used within that subroutine. The padlist's first element points to a scratchpad AV, whose elements contain the values corresponding to the lexical variables named in the 0th row. Another elements of padlist are created when the subroutine recurses or new thread is created.

===Stacks===

Perl has a number of stacks to store things it is working on.

====Argument stack====

Arguments are passed to opcode and returned from opcode using the argument stack. The typical way to handle arguments is to pop them off the stack, and then push the result back onto the stack.

====Mark stack====

This stack saves bookmarks to locations in the argument stack usable by each function so the functions doesn't necessarily get the whole argument stack to itself.

====Save stack====

This stack is used for saving and restoring values of dynamically [[Scope (programming)|scoped]] local variables.

====Scope stack====

This stack stores information about the actual scope and it is used only for debugging purposes.

===Other implementations===

There is no standarizationstandardization for the Perl language and Perl virtual machine. The internal API should beis considered as non-stable and changes from version to version. The Perl virtual machine is tied closely to the compiler. These things make very hard to reimplement Perl virtual machine.

The most known and most stable implementation is athe B::C Perl module<ref>{{cite web | url=httphttps://search.cpanmetacpan.org/perldoc?module/B::C | title=B::C - Perl compiler's C backend}}</ref> which translates opcodes tree to a representation in the C programming language and adds its own tree walker.

Another implementation is an Acme::Perl::VM Perl module<ref>{{cite web | url=httphttps://search.cpanmetacpan.org/perldoc?module/Acme::Perl::VM | title=Acme::Perl::VM - A Perl5 Virtual Machine in Pure Perl (APVM)}}</ref> which is an implementation coded in Perl language only but it is still tied with original Perl virtual machine via B:: modules.

==See also==

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==External links==

*[httphttps://perldoc.perl.org/perlhack.html#Running The Perl internals: running stage]

*[httphttps://perldoc.perl.org/perlguts.html Introduction to the Perl API]

*[httphttps://search.cpanmetacpan.org/perldoc?module/RURBAN/B-C-1.42/perloptree.pod The "B" op tree.]

{{Perl}}

{{DEFAULTSORT:Perl Virtual Machine}}

[[Category:Perl programming language]]

[[Category:Virtual machinesPerl]]

[[Category:Stack-based virtual machines]]