This is a complete rewrite of the PEG.js code generator. Its goals are:
1. Allow optimizing the generated parser code for code size as well as
for parsing speed.
2. Prepare ground for future optimizations and big features (like
incremental parsing).
2. Replace the old template-based code-generation system with
something more lightweight and flexible.
4. General code cleanup (structure, style, variable names, ...).
New Architecture
----------------
The new code generator consists of two steps:
* Bytecode generator -- produces bytecode for an abstract virtual
machine
* JavaScript generator -- produces JavaScript code based on the
bytecode
The abstract virtual machine is stack-based. Originally I wanted to make
it register-based, but it turned out that all the code related to it
would be more complex and the bytecode itself would be longer (because
of explicit register specifications in instructions). The only downsides
of the stack-based approach seem to be few small inefficiencies (see
e.g. the |NIP| instruction), which seem to be insignificant.
The new generator allows optimizing for parsing speed or code size (you
can choose using the |optimize| option of the |PEG.buildParser| method
or the --optimize/-o option on the command-line).
When optimizing for size, the JavaScript generator emits the bytecode
together with its constant table and a generic bytecode interpreter.
Because the interpreter is small and the bytecode and constant table
grow only slowly with size of the grammar, the resulting parser is also
small.
When optimizing for speed, the JavaScript generator just compiles the
bytecode into JavaScript. The generated code is relatively efficient, so
the resulting parser is fast.
Internal Identifiers
--------------------
As a small bonus, all internal identifiers visible to user code in the
initializer, actions and predicates are prefixed by |peg$|. This lowers
the chance that identifiers in user code will conflict with the ones
from PEG.js. It also makes using any internals in user code ugly, which
is a good thing. This solves GH-92.
Performance
-----------
The new code generator improved parsing speed and parser code size
significantly. The generated parsers are now:
* 39% faster when optimizing for speed
* 69% smaller when optimizing for size (without minification)
* 31% smaller when optimizing for size (with minification)
(Parsing speed was measured using the |benchmark/run| script. Code size
was measured by generating parsers for examples in the |examples|
directory and adding up the file sizes. Minification was done by |uglify
--ascii| in version 1.3.4.)
Final Note
----------
This is just a beginning! The new code generator lays a foundation upon
which many optimizations and improvements can (and will) be made.
Stay tuned :-)
Previously, the report-left-recursion and report-missing-rules passes
used PEG.GrammarError without requiring it, causing a ReferenceError.
Since requiring lib/peg.js would cause circular requirements, this
commit imports lib/grammar-error.js as GrammarError.
The bug was introduced in commit
4cda79951a.
Fixes GH-135.
When called inside an action, the |text| function returns the text
matched by action's expression. It can be also called inside an
initializer or a predicate where it returns an empty string.
The |text| function will be useful mainly in cases where one needs a
structured representation of the input and simultaneously the raw text.
Until now, the only way to get the raw text in these cases was to
painfully build it from the structured representation.
Fixes GH-131.
Implement a new syntax to extract matched strings from expressions. For
example, instead of:
identifier = first:[a-zA-Z_] rest:[a-zA-Z0-9_]* { return first + rest.join(""); }
you can now just write:
identifier = $([a-zA-Z_] [a-zA-Z0-9_]*)
This is useful mostly for "lexical" rules at the bottom of many
grammars.
Note that structured match results are still built for the expressions
prefixed by "$", they are just ignored. I plan to optimize this later
(sometime after the code generator rewrite).
Cache the last reported position info. If the position advances, the
code uses the cache and only computes the differnece. If the position
goes back, the cache is simply dropped.
Getting rid of the |trackLineAndColumn| simplifies the code generator
(by unifying two paths in the code).
The |line| and |column| functions currently always compute all the
position info from scratch, which is horribly ineffective. This will be
improved in later commit(s).
This will allow to compute position data lazily and get rid of the
|trackLineAndColumn| option without affecting performance of generated
parsers that don't use position data.
Before this commit, incorrect regexps were produced for classes starting
with "\^". For example, this grammar:
start = [\^a]
didn't match "a" because the generated regexp inside the parser was
/^[^a]/, not /^[\^a]/ as it should be.
This commit fixes the issue by escaping "^" in |quoteForRegexpClass|.
Fixes GH-125.
Before this commit, |PEG.buildParser| always returned a parser object.
The only way to get its source code was to call the |toSource| method on
it. While this method worked for parsers produced by |PEG.buildParser|
directly, it didn't work for parsers instantiated by executing their
source code. In other words, it was unreliable.
This commit remvoes the |toSource| method on generated parsers and
introduces a new |output| option to |PEG.buildParser|. It allows callers
to specify whether they want to get back the parser object
(|options.output === "parser"|) or its source code (|options.output ===
"source"|). This is much better and more reliable API.
Includes:
* Moving the source code from /src to /lib.
* Adding an explicit file list to package.json
* Updating the Makefile.
* Updating the spec and benchmark suites and their READMEs.
Part of a fix for GH-32.
The source code is now in the src directory. The library needs to be
built using "rake", which creates the lib/peg.js file by combining the
source files.
1. |PEG.Compiler| -> |PEG.compiler|
2. |PEG.grammarParser| -> |PEG.parser|
This brings us closer to the desired structure of the PEG object, which
is:
+-PEG
|- parser
+- compiler
|- checks
|- passes
+- emitter
These are the only things (together with the |PEG.buildParser| function
and exceptions) that I want to be publicly accessible -- as extension
points and also for easy testing of PEG.js's components.
Before this change, the start rule was the one named "start" and there
was an option to override that. This is now impossible.
The goal of this change is to contain all information for the parser
generation in the grammar itself.
In the future, some override directive for the start rule (like Bison's
"%start") may be added to the grammar.
Little change in the source grammar now does not change variables in all
the generated code. This is helpful especially when one has the
generated grammar stored in a VCS (this is true e.g. for our
metagrammar).
We want to have the rule parsing functions inside the |parse| method
because we want them to share a common environment. In the future,
initializers will be executed in this enviromnent and thus functions and
variables defined by them will be accessible to the rule parsing
functions.
Moving various private properties from the parser object into the
|parse| method was not strictly necessary, but it was a natural step
after moving the functions.
Labeled expressions lead to more maintainable code and also will allow
certain optimizations (we can ignore results of expressions not passed
to the actions).
This does not speed up the benchmark suite execution statistically
significantly on V8.
Detailed results (benchmark suite totals):
---------------------------------
Test # Before After
---------------------------------
1 28.43 kB/s 28.46 kB/s
2 28.38 kB/s 28.56 kB/s
3 28.22 kB/s 28.58 kB/s
4 28.76 kB/s 28.55 kB/s
5 28.57 kB/s 28.48 kB/s
---------------------------------
Average 28.47 kB/s 28.53 kB/s
---------------------------------
Mozilla/5.0 (X11; U; Linux i686; en-US) AppleWebKit/533.4 (KHTML, like Gecko) Chrome/5.0.375.55 Safari/533.4
I'll introduce labelled expressions shortly and I want to use ":" as a
label-expression separator. This change avoids conflict between the two
meanings of ":". (What would e.g. "foo: 'bar'" mean? Rule "foo"
matching string "bar", or string "bar" labelled "foo"?)
This shouldn't have measurable effect on the benchmarks as there are no
proxy rules in the grammars the benchamrk uses. However the effect on
generated parsers' speed should be positive generally.
David Majda
Justin Blank