Additional test for uN::{gather,scatter}_bits

[quaternic]

Feature gate: #![feature(uint_gather_scatter_bits)]
Tracking issue: #149069
Accepted ACP: rust-lang/libs-team#695 (comment)

Adds an additional runtime test for uN::gather_bits and uN::scatter_bits in coretests. They are each other's inverses in a sense, so a shared test can test both with relative ease.

I plan to follow up with optimized implementations for these functions.

[rustbot]

r? @Mark-Simulacrum

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[quaternic]

Given the below loop, the total count of calls to one of the tested functions comes up to about 512 * BITS * BITS. As the current implementation is itself a 0..BITS-loop (in the worst case when the mask has most bits set), the u128 test would be doing on the order of 10^9 operations, and completes in ~2s locally, which might be excessive for a single test. Any ideas on what to trim?

[okaneco]

In the for k in 0..BITS scatter loop, t will be zeroed out before the loop is complete so some iterations could be shaved.
It won't have the same coverage, but you could combine the for k in 0..BITS and for k in 0..count loops together since their iteration count would both be the number of mask bits.

let mut t = sparse; for k in 0..count { let lowest_one = t.isolate_lowest_one(); let gathered_bits = lowest_one.gather_bits(sparse); let shifted_bit = ((1 as $T) << k); let scattered_bits = shifted_bit.scatter_bits(sparse); assert_eq!(gathered_bits, shifted_bit); assert_eq!(scattered_bits, lowest_one); t ^= lowest_one; } assert_eq!(t, 0);

Then, outside of the sparse_masks loop, we can explicitly check the property.

// If t.trailing_zeros() >= the mask's bit width, the result is 0 let mut t = !(0 as $T); for _ in 0..$T::BITS { assert_eq!(0, t.scatter_bits(!t)); t <<= 1; }