fill_via_chunks: on BE, swap then copy

Author: dhardy

rand_core/src/block.rs

@@ -226,7 +226,7 @@ where
                 self.generate_and_set(0);
             }
             let (consumed_u32, filled_u8) =
-                fill_via_u32_chunks(&self.results.as_ref()[self.index..], &mut dest[read_len..]);
+                fill_via_u32_chunks(&mut self.results.as_mut()[self.index..], &mut dest[read_len..]);
 
             self.index += consumed_u32;
             read_len += filled_u8;
@@ -399,7 +399,7 @@ where
             }
 
             let (consumed_u64, filled_u8) = fill_via_u64_chunks(
-                &self.results.as_ref()[self.index as usize..],
+                &mut self.results.as_mut()[self.index as usize..],
                 &mut dest[read_len..],
             );
 

rand_core/src/impls.rs

@@ -58,28 +58,21 @@ macro_rules! fill_via_chunks {
         let chunk_size_u8 = min($src.len() * SIZE, $dst.len());
         let chunk_size = (chunk_size_u8 + SIZE - 1) / SIZE;
 
-        if cfg!(target_endian = "little") {
-            // On LE we can do a simple copy, which is 25-50% faster:
-            unsafe {
-                core::ptr::copy_nonoverlapping(
-                    $src.as_ptr() as *const u8,
-                    $dst.as_mut_ptr(),
-                    chunk_size_u8);
-            }
-        } else {
-            // This code is valid on all arches, but slower than the above:
-            let mut i = 0;
-            let mut iter = $dst[..chunk_size_u8].chunks_exact_mut(SIZE);
-            while let Some(chunk) = iter.next() {
-                chunk.copy_from_slice(&$src[i].to_le_bytes());
-                i += 1;
-            }
-            let chunk = iter.into_remainder();
-            if !chunk.is_empty() {
-                chunk.copy_from_slice(&$src[i].to_le_bytes()[..chunk.len()]);
+        // Byte-swap for portability of results:
+        if cfg!(target_endian = "big") {
+            for x in &mut $src[..chunk_size] {
+                *x = x.to_le();
             }
         }
 
+        // We do a simple copy, which is 25-50% faster:
+        unsafe {
+            core::ptr::copy_nonoverlapping(
+                $src.as_ptr() as *const u8,
+                $dst.as_mut_ptr(),
+                chunk_size_u8);
+        }
+
         (chunk_size, chunk_size_u8)
     }};
 }
@@ -89,6 +82,9 @@ macro_rules! fill_via_chunks {
 ///
 /// The return values are `(consumed_u32, filled_u8)`.
 ///
+/// On big-endian systems, endianness of `src[..consumed_u32]` values is
+/// swapped. No other adjustments to `src` are made.
+///
 /// `filled_u8` is the number of filled bytes in `dest`, which may be less than
 /// the length of `dest`.
 /// `consumed_u32` is the number of words consumed from `src`, which is the same
@@ -114,21 +110,25 @@ macro_rules! fill_via_chunks {
 ///     }
 /// }
 /// ```
-pub fn fill_via_u32_chunks(src: &[u32], dest: &mut [u8]) -> (usize, usize) {
+pub fn fill_via_u32_chunks(src: &mut [u32], dest: &mut [u8]) -> (usize, usize) {
     fill_via_chunks!(src, dest, u32)
 }
 
 /// Implement `fill_bytes` by reading chunks from the output buffer of a block
 /// based RNG.
 ///
 /// The return values are `(consumed_u64, filled_u8)`.
+///
+/// On big-endian systems, endianness of `src[..consumed_u64]` values is
+/// swapped. No other adjustments to `src` are made.
+///
 /// `filled_u8` is the number of filled bytes in `dest`, which may be less than
 /// the length of `dest`.
 /// `consumed_u64` is the number of words consumed from `src`, which is the same
 /// as `filled_u8 / 8` rounded up.
 ///
 /// See `fill_via_u32_chunks` for an example.
-pub fn fill_via_u64_chunks(src: &[u64], dest: &mut [u8]) -> (usize, usize) {
+pub fn fill_via_u64_chunks(src: &mut [u64], dest: &mut [u8]) -> (usize, usize) {
     fill_via_chunks!(src, dest, u64)
 }
 
@@ -152,33 +152,41 @@ mod test {
 
     #[test]
     fn test_fill_via_u32_chunks() {
-        let src = [1, 2, 3];
+        let src_orig = [1, 2, 3];
+
+        let mut src = src_orig;
         let mut dst = [0u8; 11];
-        assert_eq!(fill_via_u32_chunks(&src, &mut dst), (3, 11));
+        assert_eq!(fill_via_u32_chunks(&mut src, &mut dst), (3, 11));
         assert_eq!(dst, [1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0]);
 
+        let mut src = src_orig;
         let mut dst = [0u8; 13];
-        assert_eq!(fill_via_u32_chunks(&src, &mut dst), (3, 12));
+        assert_eq!(fill_via_u32_chunks(&mut src, &mut dst), (3, 12));
         assert_eq!(dst, [1, 0, 0, 0, 2, 0, 0, 0, 3, 0, 0, 0, 0]);
 
+        let mut src = src_orig;
         let mut dst = [0u8; 5];
-        assert_eq!(fill_via_u32_chunks(&src, &mut dst), (2, 5));
+        assert_eq!(fill_via_u32_chunks(&mut src, &mut dst), (2, 5));
         assert_eq!(dst, [1, 0, 0, 0, 2]);
     }
 
     #[test]
     fn test_fill_via_u64_chunks() {
-        let src = [1, 2];
+        let src_orig = [1, 2];
+
+        let mut src = src_orig;
         let mut dst = [0u8; 11];
-        assert_eq!(fill_via_u64_chunks(&src, &mut dst), (2, 11));
+        assert_eq!(fill_via_u64_chunks(&mut src, &mut dst), (2, 11));
         assert_eq!(dst, [1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0]);
 
+        let mut src = src_orig;
         let mut dst = [0u8; 17];
-        assert_eq!(fill_via_u64_chunks(&src, &mut dst), (2, 16));
+        assert_eq!(fill_via_u64_chunks(&mut src, &mut dst), (2, 16));
         assert_eq!(dst, [1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0]);
 
+        let mut src = src_orig;
         let mut dst = [0u8; 5];
-        assert_eq!(fill_via_u64_chunks(&src, &mut dst), (1, 5));
+        assert_eq!(fill_via_u64_chunks(&mut src, &mut dst), (1, 5));
         assert_eq!(dst, [1, 0, 0, 0, 0]);
     }
 }