I installed the new Feb 2001 distribution and I saw that NUM_FIXUPS with value 1000 is only possible for large version of hugs interpreter. It would be great if I could get this version because the standard interpreter only has a NUM_FIXUPS of 400 and so I get the error message "Compiled code too complex". Thank you very much. Christian

-----Urspr> üngliche Nachricht----- Von: Mark P Jones [SMTP:mpj@cse.ogi.edu] Gesendet am: Montag, 12. Februar 2001 20:07 An: kort@wins.uva.nl; Laaser Christian Cc: hugs-bugs@haskell.org Betreff: RE: AW: What does "Compiled code too complex" error message of Hugs mean?

I know this is more than a week old, but I've been away ... and now that I'm back, I'd like to clear up a possible misunderstanding that began with an exchange along the following lines:

| > > When loading some Haskell files with Hugs, I get the error | > > message "Compiled code too complex". However, the compilation | > > with GHC 4.08.1 succeeds. | > > What does this message mean? What can I do about it? | ... | > You can grep for that sentence in "hugs98/src", it will point to the | > file "machine.c". There you will see it says "if nextLab>=NUM_FIXUPS) ...". | > So grep for "NUM_FIXUPS" it will point to the file "prelude.h". I | > think the default value is 400, you should increase it to 1000 or so. | > I have it at 10000, but that's probably not necesary in your case | > and if you increase constants too much starting up Hugs will become | > slower.

First of all, an explanation. Inside Hugs, the code for Haskell functions is translated into a low-level, abstract machine language. (In today's environment, the way that Java programs are translated into JVM bytecode is probably a good analogy.) As the machine language code is generated, the compiler sometimes needs to insert "jump" instructions to addresses that are not yet known. In this situation, it instead inserts a dummy address, but adds an entry to a simple table of "fixups" that will later be filled in with the correct address. Once the complete section of code has been generated, Hugs scans over it once again and replaces each unknown address with the correct value from the fixups table. This process is also quite commonly described as "back-patching".

The fixups table can contain at most NUM_FIXUPS entries, which, in the current distribution, is set to 400. Programs that require more entries than this in a single block of code will lead to the "Compiled code too complex" error message that you have seen. There is not particular reason for the choice of 400; this is just a number that seemed to work ok for most practical purposes. If you get a compiled code too complex message, it is perhaps a sign that you would benefit from looking at ways to break your code down into smaller, simpler, and more understandable pieces. More likely, however, you will see this error with code that was generated automatically, in response to a "deriving" request on a datatype, or by a tool like a parser generator. In this case, changing the Haskell code that is generated is not an option.

Increasing the standard setting for NUM_FIXUPS is certainly an option here. You would have to increase it a great deal for there to be any impact on the speed with which Hugs starts. In comparison to the standard heap sizes that people use, the fixups table is a drop in the ocean. I think Johan has already increased the size for the Feb 2001 distribution that will be out in a couple of days.

The fixups table could be allocated dynamically, and expand on demand. To understand why Hugs doesn't do it that way already, you need to go back more than a decade to Gofer, the system from which Hugs was derived, which was designed to work on an 8086 with segmented memory> and a maximum of 640K. Back in those days, when loading the prelude took 30 seconds (and it was much smaller then too!), a statically allocated table made sense because it was slightly faster and because there wasn't any spare memory for it to expand into anyway, even if you went to the trouble of dynamically allocating it.

Historical note: If you'd like to see the heart of the machine that ran those original versions of Gofer, come visit me; it's sitting here on the desk lamp in my office :-)

All the best, Mark