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 :-)
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).
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, |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.
Before this commit, generated parser were able to start parsing from any
rule. This was nice, but it made rule code inlining impossible.
Since this commit, the list of allowed start rules has to be specified
explicitly using the |allowedStartRules| option of the |PEG.buildParser|
method (or the --allowed-start-rule option on the command-line). These
rules will be excluded from inlining when it's implemented.
This commit replaces the |startRule| parameter of the |parse| method in
generated parsers with more generic |options| -- an options object. This
options object can be used to pass custom options to the parser because
it is visible as the |options| variable inside parser code.
The start rule can now be specified as the |startRule| option. This
means you have to replace all calls like:
parser.parse("input", "myStartRule");
with
parser.parse("input", { startRule: "myStartRule" });
Closes GH-37.
Or, swapped Ruby dependency for a Node dependency.
The build script was also modified to always regenerate the parser (in
case of the "parser" task) or rebuild the library (in case of the
"build" task) even if the source files were not modified. Not doing this
led to problems when the generating code changed but the files didn't
(which happened often during development).
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.
This and also speeds up the benchmark suite execution by 7.83 % on V8.
Detailed results (benchmark suite totals):
---------------------------------
Test # Before After
---------------------------------
1 26.17 kB/s 28.16 kB/s
2 26.05 kB/s 28.16 kB/s
3 25.99 kB/s 28.10 kB/s
4 26.13 kB/s 28.11 kB/s
5 26.14 kB/s 28.07 kB/s
---------------------------------
Average 26.10 kB/s 28.14 kB/s
---------------------------------
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David Majda