Friday, 20 January 2012

C ternary operator hack

Here is a simple bit of C that sets either x or y to value v depending on the value of c..

if (c)   
    x = v;  
else  
    y = v;

..but why not "improve" this by using the C ternary operator ? : as follows:

 *(c ? &x : &y) = v;

Now, how does this shape up when compiled on an x86 with gcc -O2 ?  Well, the first example compiles down to a test and a branch where as the second example uses a conditional move instruction (cmove) and avoids the test and branch and is faster code.  Result!

OK, so this isn't rocket science, but does show that a little bit of abuse of the ternary operator can save me a few cycles if the compiler is clued up to use cmove.
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17 comments:

  1. Miek Gieben20 January 2012 at 17:02

    This is just awful. Make the compiler smarter and leave this code alone.

    ReplyDelete
  2. Colin Ian King20 January 2012 at 18:02

    I did say it was a hack ;-)

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  3. Anonymous20 January 2012 at 18:04

    This is nice, but I'm often hesitant to design around GCC's heuristics.

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  4. Colin Ian King20 January 2012 at 18:24

    I'm not going to advocate the use of it. It just was a small C mind doodle really.

    ReplyDelete
  5. Anonymous20 January 2012 at 18:30

    In C++ one can do this directly:

    ( c ? x : y) = v;

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    Replies
    1. Colin Ian King20 January 2012 at 18:51

      And the resulting assembler is just like the if statement using g++ -O2

      Delete
  6. Kamal Mostafa20 January 2012 at 19:40

    But doesn't the hack version force the compiler to use actual memory for x and y, where the normal version would allow it to use registers for those?

    Is the hack version *really* faster in a real-world-ish test case (where x and y aren't declared as volatile or otherwise forced to land in memory locations?)

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    Replies
    1. Colin Ian King20 January 2012 at 21:44

      Good observation Kamal. Not sure. I suspect I need to really see how it works in real world examples. We can learn from this is never make rash statements that one saves cycles until one has used the code in real examples rather than simple tests.

      Delete
  7. Anonymous21 January 2012 at 03:54

    Nice. I might have to add this to my list of evil C interview questions. ;-)

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    Replies
    1. Colin Ian King21 January 2012 at 11:12

      In which case, perhaps you should also consider: http://smackerelofopinion.blogspot.com/2009/07/abusing-c-for-fun.html

      Delete
  8. Anonymous21 January 2012 at 07:48

    How about just:
    c ? x = v : y = v;

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    Replies
    1. Colin Ian King21 January 2012 at 11:10

      c ? x = v : y = v; produces "error: lvalue required as left operand of assignment", so I suggest using instead the following:

      c ? (x = v) : (y = v);

      or even the more uglier:

      c ? 1, x = y : 1, y = v;

      These compile down to a jump and no cmove.

      Delete
  9. Tim Penhey25 January 2012 at 00:53

    I'm sure there are many other things to worry about before you care about optimizing this use-case.

    Personally I'm all for making the code easily readable. But then I prefer C++ to C.

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    Replies
    1. Colin Ian King25 January 2012 at 09:43

      I agree, I don't want to promote bad code style to save a cycle or two. I thought it was an interesting abuse of C really.

      Delete
  10. Anonymous29 January 2012 at 14:16

    This works fine in other architectures and compiler?

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  11. Admin Eye17 October 2012 at 10:18

    Linus Torvalds posted an interesting discussion on the CMOVE instruction here. He appears to be deprecating its use:

    http://yarchive.net/comp/linux/cmov.html

    ReplyDelete
    Replies
    1. Colin Ian King17 October 2012 at 11:02

      I wonder how relevant that is to newer Intel CPUs? I need to investigate that.

      Delete