Domain-Specific Language Compilation in Elfeed

Last night I pushed another performance enhancement for Elfeed, this time reducing the time spent parsing feeds. It’s accomplished by compiling, during macro expansion, a jQuery-like domain-specific language within Elfeed.

Heuristic parsing

Given the nature of the domain — an under-specified standard and a lack of robust adherence — feed parsing is much more heuristic than strict. Sure, everyone’s feed XML is strictly conforming since virtually no feed reader tolerates invalid XML (thank you, XML libraries), but, for the schema, the situation resembles the de facto looseness of HTML. Sometimes important or required information is missing, or is only available in a different namespace. Sometimes, especially in the case of timestamps, it’s in the wrong format, or encoded incorrectly, or ambiguous. It’s real world data.

To get a particular piece of information, Elfeed looks in a number of different places within the feed, starting with the preferred source and stopping when the information is found. For example, to find the date of an Atom entry, Elfeed first searches for elements in this order:

  1. <published>
  2. <updated>
  3. <date>
  4. <modified>
  5. <issued>

Failing to find any of these elements, or if no parsable date is found, it settles on the current time. Only the updated element is required, but published usually has the desired information, so it goes first. The last three are only valid for another namespace, but are useful fallbacks.

Before Elfeed even starts this search, the XML text is parsed into an s-expression using xml-parse-region — a pure Elisp XML parser included in Emacs. The search is made over the resulting s-expression.

For example, here’s a sample from the Atom specification.

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">

  <title>Example Feed</title>
  <link href="http://example.org/"/>
  <updated>2003-12-13T18:30:02Z</updated>
  <author>
    <name>John Doe</name>
  </author>
  <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>

  <entry>
    <title>Atom-Powered Robots Run Amok</title>
    <link rel="alternate" href="http://example.org/2003/12/13/atom03"/>
    <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
    <updated>2003-12-13T18:30:02Z</updated>
    <summary>Some text.</summary>
  </entry>

</feed>

Which is parsed to into this s-expression.

((feed ((xmlns . "http://www.w3.org/2005/Atom"))
       (title () "Example Feed")
       (link ((href . "http://example.org/")))
       (updated () "2003-12-13T18:30:02Z")
       (author () (name () "John Doe"))
       (id () "urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6")
       (entry ()
              (title () "Atom-Powered Robots Run Amok")
              (link ((rel . "alternate")
                     (href . "http://example.org/2003/12/13/atom03")))
              (id () "urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a")
              (updated () "2003-12-13T18:30:02Z")
              (summary () "Some text."))))

Each XML element is converted to a list. The first item is a symbol that is the element’s name. The second item is an alist of attributes — cons pairs of symbols and strings. And the rest are its children, both string nodes and other elements. I’ve trimmed the extraneous string nodes from the sample s-expression.

A subtle detail is that xml-parse-region doesn’t just return the root element. It returns a list of elements, which always happens to be a single element list, which is the root element. I don’t know why this is, but I’ve built everything to assume this structure as input.

For Elfeed, all namespaces stripped from both elements and attributes to make parsing simpler. As I said, it’s heuristic rather than strict, so namespaces are treated as noise.

A domain-specific language

Coding up Elfeed’s s-expression searches in straight Emacs Lisp would be tedious, error-prone, and difficult to understand. It’s a lot of loops, assoc, etc. So instead I invented a jQuery-like, CSS selector-like, domain-specific language (DSL) to express these searches concisely and clearly.

For example, all of the entry links are “selected” using this expression:

(feed entry link [rel "alternate"] :href)

Reading right-to-left, this matches every href attribute under every link element with the rel="alternate" attribute, under every entry element, under the feed root element. Symbols match element names, two-element vectors match elements with a particular attribute pair, and keywords (which must come last) narrow the selection to a specific attribute value.

Imagine hand-writing the code to navigate all these conditions for each piece of information that Elfeed requires. The RSS parser makes up to 16 such queries, and the Atom parser makes as many as 24. That would add up to a lot of tedious code.

The package (included with Elfeed) that executes this query is called “xml-query.” It comes in two flavors: xml-query and xml-query-all. The former returns just the first match, and the latter returns all matches. The naming parallels the querySelector() and querySelectorAll() DOM methods in JavaScript.

(let ((xml (elfeed-xml-parse-region)))
  (xml-query-all '(feed entry link [rel "alternate"] :href) xml))

;; => ("http://example.org/2003/12/13/atom03")

That date search I mentioned before looks roughly like this. The * matches text nodes within the selected element. It must come last just like the keyword matcher.

(or (xml-query '(feed entry published *))
    (xml-query '(feed entry updated *))
    (xml-query '(feed entry date *))
    (xml-query '(feed entry modified *))
    (xml-query '(feed entry issued *))
    (current-time))

Over the past three years, Elfeed has gained more and more of these selectors as it collects more and more information from feeds. Most recently, Elfeed collects author and category information provided by feeds. Each new query slows feed parsing a little bit, and it’s a perfect example of a program slowing down as it gains more features and capabilities.

But I don’t want Elfeed to slow down. I want it to get faster!

Optimizing the domain-specific language

Just like the primary jQuery function ($), both xml-query and xml-query-all are functions. The xml-query engine processes the selector from scratch on each invocation. It examines the first element, dispatches on its type/value to apply it to the input, and then recurses on the rest of selector with the narrowed input, stopping when it hits the end of the list. That’s the way it’s worked from the start.

However, every selector argument in Elfeed is a static, quoted list. Unlike user-supplied filters, I know exactly what I want to execute ahead of time. It would be much better if the engine didn’t have to waste time reparsing the DSL for each query.

This is the classic split between interpreters and compilers. An interpreter reads input and immediately executes it, doing what the input tells it to do. A compiler reads input and, rather than execute it, produces output, usually in a simpler language, that, when evaluated, has the same effect as executing the input.

Rather than interpret the selector, it would be better to compile it into Elisp code, compile that into byte-code, and then have the Emacs byte-code virtual machine (VM) execute the query each time it’s needed. The extra work of parsing the DSL is performed ahead of time, the dispatch is entirely static, and the selector ultimately executes on a much faster engine (byte-code VM). This should be a lot faster!

So I wrote a function that accepts a selector expression and emits Elisp source that implements that selector: a compiler for my DSL. Having a readily-available syntax tree is one of the big advantages of homoiconicity, and this sort of function makes perfect sense in a lisp. For the external interface, this compiler function is called by a new pair of macros, xml-query* and xml-query-all*. These macros consume a static selector and expand into the compiled Elisp form of the selector.

To demonstrate, remember that link query from before? Here’s the macro version of that selection, but only returning the first match. Notice the selector is no longer quoted. This is because it’s consumed by the macro, not evaluated.

(xml-query* (feed entry title [rel "alternate"] :href) xml)

This will expand into the following code.

(catch 'done
  (dolist (v xml)
    (when (and (consp v) (eq (car v) 'feed))
      (dolist (v (cddr v))
        (when (and (consp v) (eq (car v) 'entry))
          (dolist (v (cddr v))
            (when (and (consp v) (eq (car v) 'title))
              (let ((value (cdr (assq 'rel (cadr v)))))
                (when (equal value "alternate")
                  (let ((v (cdr (assq 'href (cadr v)))))
                    (when v
                      (throw 'done v))))))))))))

As soon as it finds a match, it’s thrown to the top level and returned. Without the DSL, the expansion is essentially what would have to be written by hand. This is exactly the sort of leverage you should be getting from a compiler. It compiles to around 130 byte-code instructions.

The xml-query-all* form is nearly the same, but instead of a throw, it pushes the result into the return list. Only the prologue (the outermost part) and the epilogue (the innermost part) are different.

Parsing feeds is a hot spot for Elfeed, so I wanted the compiler’s output to be as efficient as possible. I had three goals for this:

The end result is at least as optimal as hand-written code, but without the chance of human error (typos, fat fingering) and sourced from an easy-to-read DSL.

Performance

In my tests, the xml-query macros are a full order of magnitude faster than the functions. Yes, ten times faster! It’s an even bigger gain than I expected.

In the full picture, xml-query is only one part of parsing a feed. Measuring the time starting from raw XML text (as delivered by cURL) to a list of database entry objects, I’m seeing an overall 25% speedup with the macros. The remaining time is dominated by xml-parse-region, which is mostly out of my control.

With xml-query so computationally cheap, I don’t need to worry about using it more often. Compared to parsing XML text, it’s virtually free.

When it came time to validate my DSL compiler, I was really happy that Elfeed had a test suite. I essentially rewrote a core component from scratch, and passing all of the unit tests was a strong sign that it was correct. Many times that test suite has provided confidence in changes made both by me and by others.

I’ll end by describing another possible application: Apply this technique to regular expressions, such that static strings containing regular expressions are compiled into Elisp/byte-code via macro expansion. I wonder if situationally this would be faster than Emacs’ own regular expression engine.

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Chris Wellons

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