Slides from Felix von Leitner's talk on C and compiler optimizations

http://www.linux-kongress.org/2009/slides/compiler_survey_felix_von_leitner.pdf

by nrr • 17 years ago

33 6 17 years ago

linux-kongress.org

aw3c2• 17 years ago

Are video/audio recordings available too?

nrr• 17 years ago

... not that I've been able to tell. At least, they aren't posted yet.

yan• 17 years ago

Hm, regarding the tail-optimization slides, the example used doesn't look like a tail-recursive call. I just tried compiling the function he provided using gcc's -O1, 2 and 3, and neither produced a tail optimized call. I rewrote it to match what my idea of a tail-recursive call is, and sure enough, under -O2 and -O3, gcc tail-optimized it. It didn't however with no optimization settings or -O1. I rewrote it as follows:

  long
  fact(long x, long acc)
  {
     if (x <= 0) return acc;
     return fact(x-1, acc*x);
  }

(edit: the whole slide deck is summarized well on the last slide: "If you do an optimization, test it on real world data. If it’s not drastically faster but makes the code less readable: undo it.")

pauldino• 17 years ago

I think it depends on the version of gcc; I tried compiling his function with 3.4.5 and it didn't look tail-call-optimized, but with -Os on 4.2.3 it did.

keosak• 17 years ago

You are right that it does not look like what is usually called "tail call", ie. call to 'fact' is not the last evaluated expression. However, this doesn't mean that the compiler can't optimize it, provided that the last expression is simple enough. GCC can probably optimize arithmetic expressions. I have read somewhere (can't find it though) that Erlang bytecode compiler also optimizes prepending to a list and some other primitive operations. This means that in these cases you don't have to explicitly use accumulator variable because the compiler will create it for you.

logic• 17 years ago

It appears that he's been updating this presentation over the years:

http://www.fefe.de/know-your-compiler.pdf http://dl.fefe.de/optimizer-isec.pdf

Regardless, it's a good read!

Slides from Felix von Leitner's talk on C and compiler optimizations

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