Older blog entries for crhodes (starting at number 58)

Thesis minus 0 days.

Good thing it's ready, eh? Actually, it went to the binders on Wednesday, so it was with a certain amount of relief that I closed down that emacs instance on Thursday morning and was not told that I had unsaved changes in any buffers, and would I like to save the changes?

Now, all I need to do is find something to do with the rest of my life. Oh yes, and contend with Saruman the viva voce examination.

* most-positive-fixnum


Well, that one was satisfyingly straightforward. Known suboptimalities:

* (1+ most-positive-fixnum)

-100000000000000000*,,,*')*1207959552 * (compile nil '(lambda () 1))

INTERNAL ERROR * (gc) fatal error encountered in SBCL pid 30918: GC invariant lost, file "gc-common.c", line 277

I think it's safe to say that it's not quite working yet.

The lisp system I work on, SBCL, sometimes shows its age. Although SBCL per se has only been around for five years or so, it is an evolution from CMUCL, itself an offshoot from Spice Lisp, which was started in the early 1980s, borrowing from the heritage of 1970s lisps.

Given this history, it's not entirely surprising that the codebase (200kloc of lisp, plus about 10kloc of C and assembler) isn't 64-bit aware. However, today's applications are definitely nudging the space requirements; Lisp is apparently "big in bioinformatics", and fairly obviously there they'll want to be dealing with large databases.

So, a couple of months ago, Dan and I had a quick hack at porting our existing alpha backend (which was originally crafted as a 32-bit application) to be fully 64-bit. We got far enough to be executing plenty of lisp code, but not quite up to the point that the REPL was functional.

Time passed. Various bugs in the alpha backend were fixed, and so (why not?) I tried again yesterday, merging the previous branch with CVS HEAD. After conflict resolution, the addition of the necessary specialized array types, and three or more bugfixes (plus commenting out of things that break weirdly):

/about to set up restarts in TOPLEVEL-REPL
/entering REPL
* (* 2 3)

6 * most-positive-fixnum


Waitaminute! That doesn't look nicely big enough for a 64-bit lisp. Well, not quite everything is working, despite getting to the REPL; here, the printing routines are slightly broken. The most positive fixnum is meant to be (1- (ash 1 60)) (leaving four bits for tagging objects); that number is 1152921504606846975. So, compare and contrast


Other things that appear not to work include the compiler itself; clearly, the utility of a compiler-only implementation of 64-bit lisp without a functioning compiler might be viewed as "limited". Since I'm technically unemployed at this point, I'm taking offers of funding...

Thesis minus 8 days.

I tell a lie: it wasn't that bad after all.

Thesis minus 23 days.

Ow ow ow.

There are days when it might be better not to get out of bed, at least in a metaphorical sense.

On the plus side, we are now thesis minus 28 days. Is that a plus? that doesn't sound like a plus. can't wait 'till it's over, though, and I have some free time, or a job, or something of that sort.


Bugs bugs bugs... some of which were nastily difficult to track down. Particular embarrassments:

  • (ash (1- (ash 1 32)) -40) no longer returns 1;
  • (round 1.3) no longer trashes the stack;
  • (truncate 291351647815394962053040658028983955 10000000000000000000000000) now returns 29135164781, not 29135164782.

How does one find how to fix these bugs? Sometimes by observing strangenesses, and applying binary search to the system until the problem's origin is sufficiently well localized. Sometimes, though, just by thinking very hard...

It is at times like this, though, that the benefits of a test suite are very obvious — if only because some fixes can be deferred on some architectures, while remaining secure in the knowledge that the issue won't be lost.

Inevitably, as one gets nearer to the end of one's funding for any given project (in my case: doctoral studies) one finds less time for peripheral activities. In my case, the first out of the window was keeping this diary up to date: I can offer no apology to the masses of addicted readers other than "sorry". Somewhat more painfully, I've let my music lapse (apart from paying gigs or favours); I haven't even found time to bash through piano duets with another physicist.

I haven't yet managed to kick the Lisp compiler addiction, though. Highlights of the last two months: MacOS X port merged; implementing an ANSIly-correct (stupid) array type structure without any impact on user code; improvement in compiler diagnostic handlability; and a data structure improvement leading to a 25% performance increase in the compiler.

Microoptimizations are the most fun, though; right now, I'm working on implementing automatic detection of when modular arithmetic (as is the default in e.g. C, where overflow wraps round) is wanted, and also optimizing constant multiplies (which, on the x86 processor, is a tricky problem). Obviously, what I should be doing is writing up my thesis...

Rencontres Mondiales du Logiciel Libre

Otherwise known as the Libre Software Meeting. This year, after Bordeaux declined to host it on grounds of organizational difficulties, it was held in Metz; since they started organizing it late, it's not surprising that there were one or two hiccups in the smooth functioning, but there was fortunately nothing which interfered with sitting en terrasse and sampling beers.

Most of the action that I was expressly there for was the dubiously-titled "Very High Level Languages for Writing Applications" track, which in practice was a bit of an excuse for several Lisp bigots to get together and plot the next stages of world domination. Mostly this involved talking about real-time garbage collection, but the demonstrations of McCLIM and especially Marc Battyani's FractalConcept Web Application Framework were very impressive.

Also useful was meeting several researchers who, while not following our track, were nevertheless using Lisp to do neat stuff; Axiom and Maxima developers were on hand to inform us that we needed to implement reliable callbacks from C to Lisp in the next six months.

Apart from that, I managed to avoid most of the politics (not all!) by taking a day to go to visit Strasbourg; highly recommended (though avoid cafés on the Place Kleber; they're overpriced and the beer isn't as good). Then 8 hours of train back to good old England.

So while I'm waiting for the fuss^Wdiscussion about (VECTOR NIL)to die down on comp.lang.lisp, here's a new version of CLX.

"What is CLX?", I hear you cry. CLX is, roughly speaking, Xlib for Lisp. It is an implementation of the client-side of the X protocol. So, why "roughly speaking", then? Well, mostly because by all accounts programming for raw Xlib is extremely painful, whereas the macrology of Common Lisp allows syntactical abstractions to be built, allowing for ease of development.

For instance, the WITH-BUFFER-REQUEST macro allows for rapid implementation of extensions to the X protocol. In fairly short order, we have acquired implementations of the SHAPE, RENDER and XVidModeExtension (sic) extensions, so we can make Lisp applications just as pretty as their counterparts.

And, oh look, something nasty did happen to the guys wearing red.

* Krystof sighs at the irony in CLHS
<pfdietz> And its interaction with the rules for upgraded array element types?
* pfdietz realizes that those bizarre NIL arrays are strings!

Most Common Lisp implementations purporting to conform to the standard have only one internal representation for strings: it simplifies the logic; it allows for fast string access (no need to do a type test to see how big each string element); it makes type derivation easy. Several implementations can talk different representations, but internally, they're canonicalized to ASCII, UCS-16, UCS-32 or whatever.

Sadly, since we've previously deduced that arrays specialized to hold no objects must exist, and since strings are defined to be vectors specialized to hold a subtype of character, vectors specialized to hold objects of the NIL type are indeed strings.

Aargh. I have a working prototype, but there are going to be plenty of issues involved in sorting this out. At least they're mostly the same issues as would be involved in dealing with the Unicodization of SBCL, so it's not wasted work. It might even encourage me to do some more refactoring to share more code among the six backends.

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