Rollup merge of #149539 - quaternic:gather-scatter-bits, r=Mark-Simulacrum

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

Feature gate: #![feature(uint_gather_scatter_bits)]
Tracking issue: https://github.com/rust-lang/rust/issues/149069
Accepted ACP: https://github.com/rust-lang/libs-team/issues/695#issuecomment-3549284861

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.

library/coretests/tests/num/uint_macros.rs

@@ -387,6 +387,68 @@ macro_rules! uint_module {
             }
         }
 
+        #[cfg(not(miri))] // Miri is too slow
+        #[test]
+        fn test_lots_of_gather_scatter() {
+            // Generate a handful of bit patterns to use as inputs
+            let xs = {
+                let mut xs = vec![];
+                let mut x: $T = !0;
+                let mut w = $T::BITS;
+                while w > 0 {
+                    w >>= 1;
+                    xs.push(x);
+                    xs.push(!x);
+                    x ^= x << w;
+                }
+                xs
+            };
+            if $T::BITS == 8 {
+                assert_eq!(&xs, &[0xff, 0x00, 0x0f, 0xf0, 0x33, 0xcc, 0x55, 0xaa]);
+            }
+
+            // `256 * (BITS / 5)` masks
+            let sparse_masks = (i8::MIN..=i8::MAX)
+                .map(|i| (i as i128 as $T).rotate_right(4))
+                .flat_map(|x| (0..$T::BITS).step_by(5).map(move |r| x.rotate_right(r)));
+
+            for sparse in sparse_masks {
+                // Collect the set bits to sequential low bits
+                let dense = sparse.gather_bits(sparse);
+                let count = sparse.count_ones();
+                assert_eq!(count, dense.count_ones());
+                assert_eq!(count, dense.trailing_ones());
+
+                // Check that each bit is individually mapped correctly
+                let mut t = sparse;
+                let mut bit = 1 as $T;
+                for _ in 0..count {
+                    let lowest_one = t.isolate_lowest_one();
+                    assert_eq!(lowest_one, bit.scatter_bits(sparse));
+                    assert_eq!(bit, lowest_one.gather_bits(sparse));
+                    t ^= lowest_one;
+                    bit <<= 1;
+                }
+                // Other bits are ignored
+                assert_eq!(0, bit.wrapping_neg().scatter_bits(sparse));
+                assert_eq!(0, (!sparse).gather_bits(sparse));
+
+                for &x in &xs {
+                    // Gather bits from `x & sparse` to `dense`
+                    let dx = x.gather_bits(sparse);
+                    assert_eq!(dx & !dense, 0);
+
+                    // Scatter bits from `x & dense` to `sparse`
+                    let sx = x.scatter_bits(sparse);
+                    assert_eq!(sx & !sparse, 0);
+
+                    // The other recovers the input (within the mask)
+                    assert_eq!(dx.scatter_bits(sparse), x & sparse);
+                    assert_eq!(sx.gather_bits(sparse), x & dense);
+                }
+            }
+        }
+
         test_runtime_and_compiletime! {
             fn test_div_floor() {
                 assert_eq_const_safe!($T: (8 as $T).div_floor(3), 2);