Fix fast masking of unaligned data

Signed-off-by: Alexey Galakhov <agalakhov@snapview.de>

src/protocol/frame/mask.rs

@@ -1,4 +1,6 @@
-use std::mem::transmute;
+use std::cmp::min;
+use std::mem::uninitialized;
+use std::ptr::copy_nonoverlapping;
 use rand;
 
 /// Generate a random frame mask.
@@ -10,13 +12,12 @@ pub fn generate_mask() -> [u8; 4] {
 /// Mask/unmask a frame.
 #[inline]
 pub fn apply_mask(buf: &mut [u8], mask: &[u8; 4]) {
-    // Assume that the memory is 32-bytes aligned.
-    // FIXME: this assumption is not correct.
-    unsafe { apply_mask_aligned32(buf, mask) }
+    apply_mask_fast32(buf, mask)
 }
 
 /// A safe unoptimized mask application.
 #[inline]
+#[allow(dead_code)]
 fn apply_mask_fallback(buf: &mut [u8], mask: &[u8; 4]) {
     for (i, byte) in buf.iter_mut().enumerate() {
         *byte ^= mask[i & 3];
@@ -24,30 +25,72 @@ fn apply_mask_fallback(buf: &mut [u8], mask: &[u8; 4]) {
 }
 
 /// Faster version of `apply_mask()` which operates on 4-byte blocks.
-///
-/// Safety: `buf` must be at least 4-bytes aligned.
 #[inline]
-unsafe fn apply_mask_aligned32(buf: &mut [u8], mask: &[u8; 4]) {
-    debug_assert_eq!(buf.as_ptr() as usize % 4, 0);
+#[allow(dead_code)]
+fn apply_mask_fast32(buf: &mut [u8], mask: &[u8; 4]) {
+    // TODO replace this with read_unaligned() as it stabilizes.
+    let mask_u32 = unsafe {
+        let mut m: u32 = uninitialized();
+        #[allow(trivial_casts)]
+        copy_nonoverlapping(mask.as_ptr(), &mut m as *mut _ as *mut u8, 4);
+        m
+    };
 
-    let mask_u32 = transmute(*mask);
+    let mut ptr = buf.as_mut_ptr();
+    let mut len = buf.len();
 
-    let mut ptr = buf.as_mut_ptr() as *mut u32;
-    for _ in 0..(buf.len() / 4) {
-        *ptr ^= mask_u32;
-        ptr = ptr.offset(1);
+    // Possible first unaligned block.
+    let head = min(len, (4 - (ptr as usize & 3)) & 3);
+    let mask_u32 = if head > 0 {
+        unsafe {
+            xor_mem(ptr, mask_u32, head);
+            ptr = ptr.offset(head as isize);
+        }
+        len -= head;
+        if cfg!(target_endian = "big") {
+            mask_u32.rotate_left(8 * head as u32)
+        } else {
+            mask_u32.rotate_right(8 * head as u32)
+        }
+    } else {
+        mask_u32
+    };
+
+    if len > 0 {
+        debug_assert_eq!(ptr as usize % 4, 0);
+    }
+
+    // Properly aligned middle of the data.
+    while len > 4 {
+        unsafe {
+            *(ptr as *mut u32) ^= mask_u32;
+            ptr = ptr.offset(4);
+            len -= 4;
+        }
+    }
+
+    // Possible last block.
+    if len > 0 {
+        unsafe { xor_mem(ptr, mask_u32, len); }
+    }
 }
 
-    // Possible last block with less than 4 bytes.
-    let last_block_start = buf.len() & !3;
-    let last_block = &mut buf[last_block_start..];
-    apply_mask_fallback(last_block, mask);
+#[inline]
+// TODO: copy_nonoverlapping here compiles to call memcpy. While it is not so inefficient,
+// it could be done better. The compiler does not see that len is limited to 3.
+unsafe fn xor_mem(ptr: *mut u8, mask: u32, len: usize) {
+    let mut b: u32 = uninitialized();
+    #[allow(trivial_casts)]
+    copy_nonoverlapping(ptr, &mut b as *mut _ as *mut u8, len);
+    b ^= mask;
+    #[allow(trivial_casts)]
+    copy_nonoverlapping(&b as *const _ as *const u8, ptr, len);
 }
 
 #[cfg(test)]
 mod tests {
 
-    use super::{apply_mask_fallback, apply_mask_aligned32};
+    use super::{apply_mask_fallback, apply_mask_fast32};
 
     #[test]
     fn test_apply_mask() {
@@ -55,16 +98,31 @@ mod tests {
             0x6d, 0xb6, 0xb2, 0x80,
         ];
         let unmasked = vec![
-            0xf3, 0x00, 0x01, 0x02, 0x03, 0x80, 0x81, 0x82, 0xff, 0xfe, 0x00,
+            0xf3, 0x00, 0x01, 0x02,  0x03, 0x80, 0x81, 0x82,
+            0xff, 0xfe, 0x00, 0x17,  0x74, 0xf9, 0x12, 0x03,
         ];
 
+        // Check masking with proper alignment.
+        {
             let mut masked = unmasked.clone();
             apply_mask_fallback(&mut masked, &mask);
 
-        let mut masked_aligned = unmasked.clone();
-        unsafe { apply_mask_aligned32(&mut masked_aligned, &mask) };
+            let mut masked_fast = unmasked.clone();
+            apply_mask_fast32(&mut masked_fast, &mask);
+
+            assert_eq!(masked, masked_fast);
+        }
+
+        // Check masking without alignment.
+        {
+            let mut masked = unmasked.clone();
+            apply_mask_fallback(&mut masked[1..], &mask);
+
+            let mut masked_fast = unmasked.clone();
+            apply_mask_fast32(&mut masked_fast[1..], &mask);
 
-        assert_eq!(masked, masked_aligned);
+            assert_eq!(masked, masked_fast);
+        }
     }
 
 }