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by scoutt
2378 days ago
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Thank you and the other users for the patience. I guess in the case of your code, the checks are removed/optimized because the compiler knows the size of the array (since it's static, as in non-dynamically-allocated)? Probably the out-of-bounds runtime checks will (or should be) enforced when it's about dynamically allocated arrays. |
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Rust can do a lot, even for dynamically allocated ones. Here's a fun example: https://godbolt.org/z/n4ubZS
You can see that add has to do the bounds checks, because it doesn't know how long the slice is. But, foo gets compiled down to a single lea, because even though we create a dynamic length vector, the compiler can tell that it has two elements, and get rid of all of it: the malloc, the bounds checks, everything.
Or take these two versions of a summation function: https://godbolt.org/z/LzGURB
The first uses a manual loop, but rustc eliminates all of the bounds checks, because it knows that it can't possibly go out of bounds. For completeness, I included the iterator version too; you can see the ASM is (I think) identical. (I only looked at the line count and glanced, I didn't compare all 89 lines exactly.)
If rustc can't analyze something, but you can, you can also help hint it with asserts. I don't have an easy example handy, but for example, if rustc does a bounds check in the body of the loop, you could assert! outside of the loop and it has the effect of hoisting the check manually. It's pretty good at doing this on its own, though.