Math problems and marriage

January 12, 2013 § 1 Comment

So, at tea this afternoon:

Me: Hey, Vivienne, here’s a cool math problem:

Vivienne: [skeptical expression]

Me: Suppose I give you an envelope with $20 in it, and I have another
envelope with either half that much or twice that much in it. I offer
to trade the envelope you’ve got for the one I have. Do you take the

Vivienne: No. $20 is enough.

After twelve years of marriage, sometimes I can’t tell whether she
loves me, or whether she hates me but we get along well.


Happy Birthday, Ben!

January 5, 2013 § Leave a comment

My dear friend Ben Collins-Sussman has just turned 40! Please join me in congratulating him, and in associating his name with that web page. Ahem.

Support for debugging SpiderMonkey with GDB now landed

January 2, 2013 § 1 Comment

The main source tree for Firefox, Mozilla Central, now includes some code that should make debugging SpiderMonkey with GDB on Linux much more pleasant and productive.

GDB understands C++ types and values, but naturally it has no idea what the debuggee intends them to represent. As a result, GDB’s attempts to display those values can sometimes be worse than printing nothing at all. For example, here’s how a stock GDB displays a stack frame for a call to js::baseops::SetPropertyHelper:

(gdb) frame
#0 js::baseops::SetPropertyHelper (cx=0xc648b0, obj={<js::HandleBase> = {}, ptr = 0x7fffffffc960}, receiver={<js::HandleBase> = {}, ptr = 0x7fffffffc960}, id={<js::HandleBase> = {}, ptr = 0x7fffffffc1e0}, defineHow=4, vp={<js::MutableHandleBase> = {<js::MutableValueOperations<JS::MutableHandle >> = {<js::ValueOperations<JS::MutableHandle >> = {}, }, }, ptr = 0x7fffffffc1f0}, strict=0) at /home/jimb/moz/dbg/js/src/jsobj.cpp:3593

There exist people who can pick through that, but for me it’s just a pile of hexadecimal noise. And yet, if you persevere, what those arguments represent is quite simple: in this case, obj and receiver are both the JavaScript global object; id is the identifier "x"; and vp refers to the JavaScript string value "foo". This SetPropertyHelper call is simply storing "foo" as the value of the global variable x. But it sure is hard to tell—and that’s an annoyance for SpiderMonkey developers.

As of early December, Mozilla Central includes Python scripts for GDB that define custom printers for SpiderMonkey types, so that when GDB comes across a SpiderMonkey type like MutableValueHandle, it can print it in a meaningful way. With these changes, GDB displays the stack frame shown above like this:

(gdb) frame
#0 js::baseops::SetPropertyHelper (cx=0xc648b0, obj=(JSObject * const) 0x7ffff151f060 [object global] delegate, receiver=(JSObject * const) 0x7ffff151f060 [object global] delegate, id=$jsid("x"), defineHow=4, vp=$jsval("foo"), strict=0) at /home/jimb/moz/dbg/js/src/jsobj.cpp:3593

Here it’s much easier to see what’s going on. Objects print with their class, like “global” or “Object”; strings print as strings; jsval values print as appropriate for their tags; and so on. (The line breaks could still be improved, but that’s GDB for you.)

Naturally, the pretty-printers work with any command in GDB that displays values: print, backtrace, display, and so on. Each type requires custom Python code to decode it; at present we have pretty-printers for JSObject, JSString, jsval, the various Rooted and Handle types, and things derived from those. The list will grow.

GDB picks up the SpiderMonkey support scripts automatically when you’re debugging the JavaScript shell, as directed by the file that the build system places in the same directory as the js executable. We haven’t yet made the support scripts load automatically when debugging Firefox, as that’s a much larger audience of developers, and we’d like a chance to shake out bugs before foisting it on everyone.

Some versions of GDB are patched to trust only auto-load files found in directories you’ve whitelisted; if this is the case for you, GDB will complain, and you’ll need to add a command like the following to your ~/.gdbinit file:

# Tell GDB to trust auto-load files found under ~/moz.
add-auto-load-safe-path ~/moz

If you need to see a value in its plain C++ form, with no pretty-printing applied, you can add the /r format modifier to the print command:

(gdb) print vp
$1 = $jsval("foo")
(gdb) print/r vp
$2 = {
  <js::MutableHandleBase> = {
    <js::MutableValueOperations<JS::MutableHandle >> = {
      <js::ValueOperations<JS::MutableHandle >> = {}, }, },
  members of JS::MutableHandle:
  ptr = 0x7fffffffc1f0

If you run into troubles with a pretty-printer, please file a bug in Bugzilla, under the “Core” product, for the “JavaScript Engine” component. (The pretty-printers have their own unit and regression tests; we do want them to be reliable.) In the mean time, you can disable the pretty-printers with the disable pretty-printer command:

(gdb) disable pretty-printer .* SpiderMonkey
12 printers disabled
128 of 140 printers enabled

For more details, see the GDB support directory’s README file.

Linear algebra textbooks

December 31, 2011 § 3 Comments

On Twitter, I asked:

Sincere question: do any of you have a linear algebra introductory text that you really liked?

(Man, this blog is just going to rocket to the top of the charts, I can tell.)

Here’s what people mentioned:

Prices are from Powell’s Books, for comparison. Powell’s prices aren’t the lowest, but I consider the physical bookstore (and especially the technical bookstore, oh my goodness) a substantial side benefit, so I try to buy from them anyway. But, wow, tutelage on the fundamental truths of the universe is not cheap. However much I love bookstores, I think I might like open-sourced textbooks even more…

Anime series we watch with our daughters

December 18, 2011 § Leave a comment

On Twitter, I wrote:

Between ARIA, Saiunkoku, and Akage no Anne, we may’ve watched enough anime with our girls about women who work hard and achieve their goals.

Some folks were interested, so here are some details on those series.
First, some things you should know about our tastes in anime:

  • They run strongly towards the gentle and slow-paced, because that’s what our girls (9 and 6) like. If your kids think Jurassic Park is great fun, this isn’t the list for you. If we ever watched something even as severe as, say, Star Wars, they would be crying so loudly I wouldn’t be able to hear Luke tell Obi-wan, “I want to come with you to Alderaan.” (Which is a problem… how?)
  • Every single one of these series extolls the virtues of hard work and an honest, forthright character. The truth is, I’m not sure I could explain to the next generation why being an anarchist punk isn’t the better choice. I sure hope good parenting doesn’t entail making the family a miniature of the world outside.
  • It’s surprisingly easy to find anime that passes the Bechdel Test, so why settle for less? (I hear a lot about how constraining Japanese society is for women, and I believe it, but I also come across a lot that doesn’t fit neatly into that picture.)

So, the anime:

  • Aria
    A young woman leaves Earth to be an apprentice gondolier in a rebuilt Venice on a terraformed Mars. But it’s barely sci-fi at all; it’s entirely about the characters, about the nature of kindness, friendship, study, and community. The sense of place in Aria is so strong, I wanted to move to Neo-Venezia myself. The plots are idiosyncratic: nobody else has ever had a character undertake a punishment campaign against her own left hand.

    If you have a limited tolerance for sweetness, Aria will probably give you diabetes. “Cool” is big in American culture, and Aria has nothing to do with detachment and independence. But from 2004 to 2009, when I was avidly (compulsively) following the cruel and banal mendacity that is American politics, watching Aria on Fridays was like a balm for my soul. There’s something there. Aria makes you wonder why arguments and conflict are such essential ingredients in almost all drama when you can actually have great stories without them.

    There’s a bit of Fan service in one early episode, which usually bugs me (it’s condescending to the viewers, and also strikes me as disrespectful to the characters—whatever that means), but please forgive it. It’s nothing my girls even blinked at.

  • Saiunkoku
    Shuurei, a young woman in a poor but noble family, is roped into being the king’s consort in an attempt to awaken his interest in ruling. He falls in love with her, but she aspires to become a government official, even though the civil service exam is not open to women. Predictably, the king gets the rules changed, and Shuurei aces the exam.

    A good bit of the series deals with Shuurei’s troubles gaining the respect of her colleagues despite her manifest competence, and the reactions of the male-dominated court culture to the introduction of a woman. Her supporters hesitate to intervene on her behalf, knowing that her detractors are eager to attribute her success to favoritism. But she perseveres and triumphs.

    Visually, Saiunkoku is weak; this is classic cheap Japanimation. The fun is all in the plot, which is pretty complicated. It definitely goes over our 6-year-old’s head, and our 9-year-old misses a lot too, but there’s enough for the kids to latch on to that they seem to enjoy it as well.

  • Akage no Anne
    This is an anime adaptation of Anne of Green Gables, a novel set in 1878 Canada about a young girl who is adopted by an elderly couple. She goes to school; makes friends; climbs a roof on a dare and falls off; accidentally gets her best friend drunk on currant wine; and so on. She is an accomplished student, gains admission to Queen’s Academy, and wins a scholarship to college.

    The story often sympathetically articulates Anne’s adoptive parents’ feelings; I was glad to have that angle set out to our girls.

    Isao Takahata and Hayao Miyazaki worked together on this anime.

A favorite quote

January 6, 2011 § Leave a comment

The future masters of technology will have to be light-hearted and intelligent. The machine easily masters the grim and the dumb.

—Marshall McLuhan, 1969

I love this quote. It roughly fits what I’ve seen. But while it seems clear that being ‘dumb’ probably dooms you to being mastered by the first anything (person, cause, addiction, …) you encounter, why would being light-hearted have anything to do with it?

Emacs hacks for Mozilla hackers

January 5, 2011 § 3 Comments

If you use Emacs to work on Mozilla code, you might find something you like in my mozilla-elisp repository. It currently includes:

  • mozilla-c-style.el, a package which defines a C/C++ indentation style named “Mozilla”, which you can select with C-c . (c-set-style). This properly handles case labels, long return expressions, and things like that. The comments at the top explain how to turn it on by default in Mozilla code. It may have some SpiderMonkey-isms in it; if you send me bug reports, I will split it into “Mozilla” and “SpiderMonkey” styles.
  • mercurial-queues.el, a major mode for editing Mercurial Queues series files, pushing, popping, and refreshing patches, and so on. In particular, the push and pop commands take care of refreshing any buffers visiting modified files. (This is currently slow if you have many buffers open, due to a shortcoming in vc‘s Mercurial support that makes refreshing a buffer take a significant fraction of a second. I have an unpolished workaround; if anyone besides me begins to use this and complains to me, then I’ll certainly finish it up and get it into the code.)
  • diff-find-mq-file.el, a quick hack to help ‘diff-goto-source‘ and ‘diff-apply-hunk‘, commands that jump from a hunk in a patch file to the place where that hunk would apply, find the file to patch when they’re invoked on a patch in a .hg/patches directory.

Happy Hacking!


December 20, 2010 § Leave a comment

I ran a half-marathon last Sunday, the first race I’ve ever run.

It had been raining and miserable all week, but on Sunday morning it was clear, warm, and beautiful—we couldn’t have asked for better weather. The bus wasn’t convenient, and it was too early in the morning to ask our wives for a ride, so Kevin and I decided to bike there: 5 miles for me, 9 miles for Kevin. We left my house at 7:15 and got there on time. It was a big crowd, 1200 runners, people of all ages and shapes (but not all sizes). The starting gun was at 8:15; with everyone lined up, it took us 30 seconds just to get to the starting line. They had the streets blocked off, with traffic cones, police directing traffic, mile markers, and so on.

The route started at Adidas’ North American headquarters, went up to Pier Park at the northern end of Portland, and then turned around and came back. My map omits about two miles of squiggles they added at the beginning to get the requisite length, but you can see how it goes along the cliffs to the east of the Willamette, past the University of Portland campus, under the St. Johns bridge, and past Pier Park. From the cliffs, we could see all the way to downtown. I was surprised at how far away it was; we’d started the day on the far side of downtown, and come the distance on our own power.

About two thirds of the way to the turnaround, the lead runner passed us coming back—so he was going about twice as fast as we were! Kevin and I ran at our own pace, and even walked a bit when we felt like it. Many more people passed us, including plenty of little old ladies, than vice versa. But it’s a race against yourself, not some random person next to you.

Every so often they had a station set up where you could grab a cup of water or a packet of sugary energy goop. Water is obviously a good thing; I ate some goop about half-way through, and it definitely helped.

At roughly the half-way mark, we passed one woman standing off to the side throwing up. Later on, she passed us! Don’t you think that, if you’re throwing up, perhaps you should slow down?

We finished in 2 hours and 10 minutes, roughly a ten-minute mile on average. I’d been expecting to take between two to two and a half hours, so I was happy to be towards the faster end. I bet we could have done even better if we hadn’t biked. After we stopped, I felt fantastic—my heart and lungs were saying, “Piece of cake!”—but I could tell that if I didn’t keep walking around, my legs were ready to cramp up in a way far worse than anything I’d ever experienced before. I did keep walking, and things were fine. (Well, “within expectations” would be more honest.) At the finishing site there was more water, free soup, cornbread muffins, and… free beer! Kevin had a bowl of soup and a (pretty big) cup of beer. I had a bowl of soup. I started a beer, but my stomach said, “EVIDENTLY THERE HAS BEEN SOME SORT OF MISCOMMUNICATION”, so I dumped the beer under a bush.

Biking back, it rained on us a bit, but not with much sincerity. I was grateful for the essentially perfect weather during the race. Vivienne had french toast coming off the griddle as we walked in the door; Kevin stayed for a quick bite, and then biked the rest of the way home.

I had a great time, and I definitely want to do it again. I’d like to beat 2 hours. I have no interest in doing a full marathon.

Why did you run? Were you chased?

I was both chased and led, by the usual things.

I can’t honestly claim that fear of heart disease played any factor in the decision, although it should have. I haven’t lost weight. It hasn’t helped my sleep as much as switching pillows. And I don’t think it has helped my concentration. Some people say exercise brings a general sense of well-being, but, aside from the occasional bout of black despair, I’m generally pretty cheerful on my own. And I do go down stairs clutching the railing like an old man, after a longer run.

But I turn out to be immune to equipment fetishism. And I seem to be able to avoid the sorts of competitive feelings I find ugly.

I think the real reasons are these: It had aspects of a dare, mostly against my self-image as a pale geek who’d gotten used to being the last pick at school by third grade. And once I’d gotten started, I found that it felt really good, in a way that nothing else I do does; there’s surprising satisfaction in knowing that you can handle it. And I enjoy Kevin’s company.

SpiderMonkey’s conservative collector still needs some rooting

December 20, 2010 § 2 Comments

We’ve just landed the definition in <jsapi.h> of a new template type, js::Anchor, in TraceMonkey. Developers writing code against the JSAPI will need to understand it to avoid garbage collection hazards. The need for js::Anchor became clear in bug 614138, but I’ll let the comments from <jsapi.h> tell the story:

Protecting non-jsval, non-JSObject *, non-JSString * values from collection

Most of the time, the garbage collector’s conservative stack scanner works behind the scenes, finding all live values and protecting them from being collected. However, when JSAPI client code obtains a pointer to data the scanner does not know about, owned by an object the scanner does know about, Care Must Be Taken.

The scanner recognizes only a select set of types: pointers to JSObjects and similar things (JSFunctions, and so on), pointers to JSStrings, and jsvals. So while the scanner finds all live JSString pointers, it does not notice jschar pointers.

So suppose we have:

  void f(JSString *str) {
    const jschar *ch = JS_GetStringCharsZ(str);
    ... do stuff with ch, but no uses of str ...

After the call to JS_GetStringCharsZ, there are no further uses of str, which means that the compiler is within its rights to not store it anywhere. But because the stack scanner will not notice ch, there is no longer any live value in this frame that would keep the string alive. If str is the last reference to that JSString, and the collector runs while we are using ch, the string’s array of jschars may be freed out from under us.

Note that there is only an issue when 1) we extract a thing X the scanner doesn’t recognize from 2) a thing Y the scanner does recognize, and 3) if Y gets garbage-collected, then X gets freed. If we have code like this:

  void g(JSObject *obj) {
    jsval x;
    JS_GetProperty(obj, "x", &x);
    ... do stuff with x ...

there’s no problem, because the value we’ve extracted, x, is a jsval, a type that the conservative scanner recognizes.

Conservative GC frees us from the obligation to explicitly root the types it knows about, but when we work with derived values like ch, we must root their owners, as the derived value alone won’t keep them alive.

A js::Anchor is a kind of GC root that allows us to keep the owners of derived values like ch alive throughout the Anchor’s lifetime. We could fix the above code as follows:

  void f(JSString *str) {
    js::Anchor<JSString *> a_str(str);
    const jschar *ch = JS_GetStringCharsZ(str);
    ... do stuff with ch, but no uses of str ...

This simply ensures that str will be live until a_str goes out of scope. As long as we don’t retain a pointer to the string’s characters for longer than that, we have avoided all garbage collection hazards.

Update: Of course, this belongs in the JSAPI docs, not a blog post or a header file. I’ll get it up there shortly; third try’s a charm.

Debugging SpiderMonkey with Archer

December 20, 2010 § 4 Comments

If you’re hacking on SpiderMonkey, the JavaScript engine used in Mozilla Firefox, the debugger can be less than helpful when viewing SpiderMonkey’s data structures:

(gdb) print str
$1 = (JSString *) 0x7ffff680a440
(gdb) print *str
$2 = {mLengthAndFlags = 148, {mChars = 0x7ffff680a450, mLeft = 0x7ffff680a450}, {mInlineStorage = {118, 97, 114, 32}, e = {{mCapacity = 9007688887369846, mParent = 0x20007200610076, mBufferWithInfo = 0x20007200610076}, {mBase = 0x20003d00200078, mRight = 0x20003d00200078}}, externalStringType = 6357110}}
(gdb) print/c str->mChars[0]@(str->mLengthAndFlags / 16)
$4 = {118 'v', 97 'a', 114 'r', 32 ' ', 120 'x', 32 ' ', 61 '=', 32 ' ', 53 '5'}

If you know enough magic, you can make it work, but there’s a lot of clutter, and it’s a distraction from the task at hand.

The Archer Project is a development branch of GDB that you can customize in Python. In particular, you can have Archer automatically call your own Python functions to format particular types of values. I’ve written a set of pretty-printers for SpiderMonkey’s datatypes, so for me the above interaction would go:

(gdb) print str
$1 = 0x7ffff680a440 "var x=5" ATOMIZED
(gdb) print *str
$2 = {mLengthAndFlags = (7 << 4)|FLAT|ATOMIZED, mChars = 0x7ffff680a450, externalStringType = 6357110}

When printing a pointer to a JSString, Archer shows the string’s contents, and any flags (like ATOMIZED). When printing a JSString struct value, Archer omits dead union branches — in this case, most of the members.

Archer applies the pretty-printers whenever it needs to print a value, so, for example, stack traces are legible, too:

(gdb) where 1
#0  js_SetPropertyHelper (cx=0xa8e550, obj=0x7ffff6803048 [Object global], id=$jsid("x"), defineHow=1, vp=0x7fffffffc7f0, strict=0) at ../jsobj.cpp:5367

Here the JSObject pointer and the jsid have been printed in a more helpful way.

At present, I have pretty-printers for:

  • jsval and js::Value
  • JSString, JSString *, JSAtom *, and jsid
  • JSObject *, JSFunction *, and JSFunction
  • js::PropertyCacheEntry and js::PCVal
  • JSParseNode *, JSDefinition *, JSParseNode, JSDefinition, and JSFunctionBox *
  • js::Shape

You can check out a copy of the current sources using Mercurial:

$ hg clone
destination directory: archer-mozilla
requesting all changes
22 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd archer-mozilla
$ more README

This creates a directory called archer-mozilla; the README file there explains how to make Archer use the pretty-printers.

This is very much a work in progress; there are certainly bugs, and while there are regression tests, they don’t cover all the pretty-printers the package defines. Please let me know if you find a bug, or have suggestions for improvements.

I use Archer on Linux; I don’t know how well it works on Macintosh OS X or Windows. I hear that these pretty-printers can be used with modern stock GDB releases.