Parallelize in_place_bit_reverse_permute

crypto/math/src/fft/cpu/bit_reversing.rs

@@ -1,7 +1,42 @@
 /// In-place bit-reverse permutation algorithm. Requires input length to be a power of two.
-pub fn in_place_bit_reverse_permute<E>(input: &mut [E]) {
-    for i in 0..input.len() {
-        let bit_reversed_index = reverse_index(i, input.len() as u64);
+pub fn in_place_bit_reverse_permute<E: Send>(input: &mut [E]) {
+    let n = input.len();
+    #[cfg(feature = "parallel")]
+    {
+        // Pair-parallel swap: each pair (i, br(i)) with i < br(i) is independent of all
+        // other pairs (disjoint indices), so threads can swap concurrently provided they
+        // never touch the same memory location. `if br > i` selects exactly one owner
+        // per pair, so no two threads ever write the same slot.
+        const PARALLEL_BITREV_THRESHOLD: usize = 1 << 14;
+        if n >= PARALLEL_BITREV_THRESHOLD {
+            use rayon::prelude::*;
+            struct SendPtr<E>(*mut E);
+            impl<E> Copy for SendPtr<E> {}
+            impl<E> Clone for SendPtr<E> {
+                fn clone(&self) -> Self {
+                    *self
+                }
+            }
+            unsafe impl<E> Send for SendPtr<E> {}
+            unsafe impl<E> Sync for SendPtr<E> {}
+            let ptr = SendPtr(input.as_mut_ptr());
+            (0..n).into_par_iter().for_each(|i| {
+                let br = reverse_index(i, n as u64);
+                if br > i {
+                    // SAFETY: (i, br) uniquely identifies this pair (smaller index is owner),
+                    // so no two threads race on the same `ptr.0.add(k)` slot. Both indices
+                    // are in-bounds since i < n and br < n.
+                    let p = ptr;
+                    unsafe {
+                        core::ptr::swap(p.0.add(i), p.0.add(br));
+                    }
+                }
+            });
+            return;
+        }
+    }
+    for i in 0..n {
+        let bit_reversed_index = reverse_index(i, n as u64);
         if bit_reversed_index > i {
             input.swap(i, bit_reversed_index);
         }