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More code = more better (lemmy.world)
submitted 11 months ago by to c/programmerhumor@lemmy.ml
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[–] 18 points 11 months ago* (last edited 11 months ago) (11 children)

The compiler should be able to optimize all of them to the same machine code.

  1. This is already good.
  2. Easily optimized by constant folding.
  3. This one depends on the semantics of signed underflow, so it may not do what you want.
  4. The loop can only exit if x==10, so as long as the nextInt() method doesn't have side effects, the loop should be eliminated. But, again, language semantics can affect this.

Edit: Very wrong for 3 & 4, see replies.

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  • [–] 17 points 11 months ago (6 children)

    I'd be shocked if 4 got optimised out

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  • [–] 2 points 11 months ago (3 children)

    I feel like an idiot. Also, in the "Good" example, no underflow occurs. i goes from 0 to -10, and x is assigned to -i every loop.

    It might still be possible to optimize away the random number example, if the random function were made a magic language item, but it would not be even remotely close to being worth the effort.

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  • [–] 2 points 11 months ago (2 children)

    The question the optimizer can't really answer is: will Random.nextInt() ever return 10? If that's a 64 bit integer it could be a LOOOOOONG time before 10 ever shows up.

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  • [–] 1 point 11 months ago

    If it were a magic lang item, you could treat the resulting value in a special way. Then, you could create an optimization pass for this situation: if a variable is assigned random in a loop and the loop can only be exited with a certain value, the compiler can coerce the magic rand value to it.

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  • [–] 6 points 11 months ago (1 child)

    For #4 if the Random instance weren't "new", then calling the nextInt() function would definitely have side effects, since the next integer would pull one away from the random stack.

    However unlike the first three which will run within a consistent amount of time, #4 will take an unknown amount of time to run, so you can't just collapse it and eliminate the loop.

    For example a very simple race game where a participant moves a random number of steps each turn, we may want to time how long that race takes. We can't just say that they will reach the end immediately. In fact technically we don't know that they will ever finish the race... But that's the halting problem and a whole other issue.

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