Implementing _mm_shuffle_epi8. (rust-lang#18)

Implementing `_mm_shuffle_epi8`.

Author: fulmicoton

TODO.md

@@ -422,7 +422,7 @@ ssse3
 * [ ] `_mm_abs_epi16`
 * [ ] `_mm_abs_pi32`
 * [ ] `_mm_abs_epi32`
-* [ ] `_mm_shuffle_epi8`
+* [x] `_mm_shuffle_epi8`
 * [ ] `_mm_shuffle_pi8`
 * [ ] `_mm_alignr_epi8`
 * [ ] `_mm_alignr_pi8`

src/x86/ssse3.rs

@@ -8,10 +8,46 @@ pub fn _mm_abs_epi8(a: i8x16) -> u8x16 {
     unsafe { pabsb128(a) }
 }
 
+
+
+/// Shuffle bytes from `a` according to the content of `b`.
+///
+/// The last 4 bits of each byte of `b` are used as addresses
+/// into the 16 bytes of `a`.
+///
+/// In addition, if the highest significant bit of a byte of `b`
+/// is set, the respective destination byte is set to 0.
+///
+/// Picturing `a` and `b` as `[u8; 16]`, `_mm_shuffle_epi8` is
+/// logically equivalent to:
+///
+/// ```
+/// fn mm_shuffle_epi8(a: [u8; 16], b: [u8; 16]) -> [u8; 16] {
+///     let mut r = [0u8; 16];
+///     for i in 0..16 {
+///         // if the most significant bit of b is set,
+///         // then the destination byte is set to 0.
+///         if b[i] & 0x80 == 0u8 {
+///             r[i] = a[(b[i] % 16) as usize];
+///         }
+///     }
+///     r
+/// }
+/// ```
+#[inline(always)]
+#[target_feature = "+ssse3"]
+pub fn _mm_shuffle_epi8(a: u8x16, b: u8x16) -> u8x16 {
+    unsafe { pshufb128(a, b) }
+}
+
+
 #[allow(improper_ctypes)]
 extern {
     #[link_name = "llvm.x86.ssse3.pabs.b.128"]
     fn pabsb128(a: i8x16) -> u8x16;
+
+    #[link_name = "llvm.x86.ssse3.pshuf.b.128"]
+    fn pshufb128(a: u8x16, b: u8x16) -> u8x16;
 }
 
 #[cfg(test)]
@@ -25,4 +61,14 @@ mod tests {
         let r = ssse3::_mm_abs_epi8(i8x16::splat(-5));
         assert_eq!(r, u8x16::splat(5));
     }
+
+    #[test]
+    #[target_feature = "+ssse3"]
+    fn _mm_shuffle_epi8() {
+        let a = u8x16::new(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16);
+        let b = u8x16::new(4, 128, 4, 3, 24, 12, 6, 19, 12, 5, 5, 10, 4, 1, 8, 0);
+        let expected = u8x16::new(5, 0, 5, 4, 9, 13, 7, 4, 13, 6, 6, 11, 5, 2, 9, 1);
+        let r = ssse3::_mm_shuffle_epi8(a, b);
+        assert_eq!(r, expected);
+    }
 }