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bitv: Avoid undefined behavior in mask_for_bits #2

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Dec 10, 2014
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bitv: Avoid undefined behavior in mask_for_bits #2

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2 changes: 1 addition & 1 deletion src/libcollections/bit.rs
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Original file line number Diff line number Diff line change
@@ -177,7 +177,7 @@ fn blocks_for_bits(bits: uint) -> uint {
/// Computes the bitmask for the final word of the vector
fn mask_for_bits(bits: uint) -> u32 {
// Note especially that a perfect multiple of u32::BITS should mask all 1s.
!0u32 >> (u32::BITS - bits % u32::BITS)
!0u32 >> (u32::BITS - bits % u32::BITS) % u32::BITS
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Actually, is it not the case that the internal % can be removed completely because math?

That is (x - (y mod j)) mod j == (x - y) mod (some multiple of j, because uints wrap) mod j

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Yeah, as long as uint wrapping is defined behavior, it could even be (-bits) % u32::BITS. I expect the optimizer will already figure that out, but I can compare.

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Ah yes, that works too! Nice.

All of our primitive integer types overflow in a defined way.

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Bah, -bits triggers #[warn(unsigned_negation)]. But (0 - bits) % u32::BITS is fine. As I expected, the optimized codegen is identical - do you still care to have this updated?

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If codegen is fine, I'm happy with leaving it like this for semantic value. multiple modulo stuff hurts my brain, for the most part.

}

impl Bitv {