This simplifies the code a bit and makes the AST more regular (each node
type has a fixed set of properties). The latter may get useful later
when generalizing visitors.
This has two main benefits:
1. The knowledge about scoping params in at one designated place,
making all future adjustments in this area easier.
2. Action-related code does not handle sequences specially anymore.
Such knowledge/behavior doesn't belong there.
Before this change, knowledge about variable names was spread between
the |computeStackDepths| pass and the code emitter code. For example,
the fact that the |&...| expression needs one variable to store a
position was represented in both places.
This changes consolidates that knowledge and introduces a new
|computeVarNames| pass. This pass replaces old |computeStackDepths|
pass, does all computations realted to variable names and stores the
results in the AST. Note that some knowledge about variables
(inevitably) remained in emitter code templates.
Beside DRYing things up, this change simplifies the emitter
significantly. By storing variable names in the AST it also allows
introduction of a pass that will identify parameters passed to actions
using proper symbol tables. Right now, this is done in a hackish way
directly in the emitter, which won't work well with changes planned in
GH-69.
Before this commit, variables for saving match results and parse
positions in generated parsers were not used efficiently. Each rule
basically used its own variable(s) for storing the data, with names
generated sequentially during code emitting. There was no reuse of
variables and a lot of unnecessary assignments between them.
It is easy to see that both match results and parse positions can
actually be stored on a stack that grows as the parser walks deeper in
the grammar tree and shrinks as it returns. Moreover, if one creates a
new stack for each rule the parser enters, its maximum depth can be
computed statically from the grammar. This allows us to implement the
stack not as an array, but as a set of numbered variables in each
function that handles parsing of a grammar rule, avoiding potentially
slow array accesses.
This commit implements the idea from the previous paragraph, using
separate stack for match results and for parse positions. As a result,
defined variables are reused and unnecessary copying avoided.
Speed implications
------------------
This change speeds up the benchmark suite execution by 2.14%.
Detailed results (benchmark suite totals as reported by "jake benchmark"
on Node.js 0.4.8):
-----------------------------------
Test # Before After
-----------------------------------
1 129.01 kB/s 131.98 kB/s
2 129.39 kB/s 130.13 kB/s
3 128.63 kB/s 132.57 kB/s
4 127.53 kB/s 129.82 kB/s
5 127.98 kB/s 131.80 kB/s
-----------------------------------
Average 128.51 kB/s 131.26 kB/s
-----------------------------------
Size implications
-----------------
This change makes a sample of generated parsers 8.60% smaller:
Before:
$ wc -c src/parser.js examples/*.js
110867 src/parser.js
13886 examples/arithmetics.js
450125 examples/css.js
632390 examples/javascript.js
61365 examples/json.js
1268633 total
After:
$ wc -c src/parser.js examples/*.js
99597 src/parser.js
13077 examples/arithmetics.js
399893 examples/css.js
592044 examples/javascript.js
54797 examples/json.js
1159408 total
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.
David Majda