How to Define New Intrinsics in SBCL

How to Define New Intrinsics in SBCL(pvk.ca)

92 points by chiachun 11 years ago | 20 comments

agentultra 11 years ago |

TIL: a solid CL implementation can provide safe maps from high-level intrinsic types to low-level implementation types at run-time. Also, SBCL has at least two IRs with each able to provide optimization hints to the code generator and compiler. So cool.

I love reading Paul's posts. His bit on Tobasco sort[0] was amazing.

[0] http://www.pvk.ca/Blog/2012/08/27/tabasco-sort-super-optimal...

update: link

_delirium 11 years ago | |

That is indeed a good post; thanks for linking it. Beyond the sort itself, there's a nice illustration at the end of dynamically scoped ("special") variables being useful on occasion for something other than the usual example of temporarily shadowing a configuration variable (e.g. the destination of stdout, or logging level).

eudox 11 years ago |

In certain ways, the difference between (Common) Lisp and other languages is the difference between Linux and Windows. When you have a fundamental problem in Windows, your only solution is to wait for a fix then reformat/reinstall. Similarly, when there's a problem in, say, a C compiler, all you can do is code around it, open an issue and wait for a fix in the next release.

In Common Lisp? Well, the compiler's right there under these floorboards. Have a long-running web application that's affected by this bug, and don't want to restart it? Just run this in the REPL.

jlas 11 years ago | |

Not sure exactly what you're trying to say here but there are plenty of open source C compilers [1], including GCC, which allow anyone to fix bugs as they see fit.

To your second point about not restarting a long running web applications — running code in a REPL won't fix your web app since it's a separate process. You might need a remote debugging mechanism to connect a console to the app, but at that point it may be much easier to restart the app.

[1] http://en.wikipedia.org/wiki/List_of_compilers#C_compilers

eudox 11 years ago | | |

>running code in a REPL won't fix your web app since it's a separate process.

In the server you have a script that fires up a Lisp process and starts a Swank[0] server on port whatever. Then the developers, from their own machines, fire up Emacs and SLIME[1] into that port and run something along the lines of

  > (myapp.server:start :port 8000)
  <#server process>

  > (... some work ...)

[0] http://common-lisp.net/project/slime/doc/html/Lisp_002dside....

[1] http://common-lisp.net/project/slime/

zachbeane 11 years ago | | |

> running code in a REPL won't fix your web app since it's a separate process

Not in a Common Lisp web app.

kazinator 11 years ago | | |

Although you can fix a bug in GCC and submit it to the project (been there done that) you cannot deploy a gcc change as part of a C program.

Then again, you can't really deploy a Linux bug fix as part of a Linux program either, so the whole linux-windows-lisp-others analogy is tenuous.

The analogy holds in the sense that in Lisp we have less of a barrier between the programmer and the language, and in free software we have less of a barrier between users and the software. But the barriers are different.

And, ironically, the barrier which is out of your way in Lisp is still out of your way if the Lisp is a closed-source, proprietary Lisp. That is to say, doing many things in the Lispy way does not require you to rebuild the implementation from source, or to have the source.

lelf 11 years ago | |

> when there's a problem in, say, a C compiler, all you can do is code around it, open an issue and wait for a fix in the next release.

Fix at as you'd (presumably) do in lisp. I don't see your point. (Yes, you have to recompile it, C is compiled language.)

> long-running web application that's affected by this bug, and don't want to restart it? Just run this in the REPL.

Yeah, if you it don't have long-running code, it isn't heavy threaded, that change doesn't alter any data structures, you don't care about possible downtime etc

talder 11 years ago |

Does the SBCL compiler use a separate third-party assembler? Otherwise, how does it know the POPCNT opcode/encoding? I didn't see the author specify it in the post.

sedachv 11 years ago | |

SBCL comes with its own assembler: https://github.com/sbcl/sbcl/blob/master/src/compiler/assem....

_delirium 11 years ago | | |

And the instructions themselves are defined here (e.g. for x86-64): https://github.com/sbcl/sbcl/blob/master/src/compiler/x86-64...

I think the question might've been how you can just write things like (instr popcnt...) and expect SBCL to know how to emit the right machine code for popcnt. It's mentioned briefly at the top, but the answer is that this post isn't adding the instruction (it would indeed not work if it were a new instruction SBCL doesn't yet know about). The example is just walking through modifying how it's generated by the VOPs, the code-generation level that sits a level above the assembler. The popcnt instruction itself had already been added to SBCL some time ago, with the define-instruction seen in the link above.