Merge pull request rust-lang#3 from bluss/memrchr-must-go-faster

Provide a faster fallback for memrchr

Author: BurntSushi

benches/bench.rs

@@ -14,6 +14,7 @@ fn iterator(b: &mut test::Bencher) {
     b.iter(|| {
         assert!(haystack.iter().position(|&b| b == needle).is_none());
     });
+    b.bytes = haystack.len() as u64;
 }
 
 #[bench]
@@ -23,6 +24,7 @@ fn libc_memchr(b: &mut test::Bencher) {
     b.iter(|| {
         assert!(memchr::memchr(needle, &haystack).is_none());
     });
+    b.bytes = haystack.len() as u64;
 }
 
 #[bench]
@@ -32,6 +34,7 @@ fn iterator_reversed(b: &mut test::Bencher) {
     b.iter(|| {
         assert!(haystack.iter().rposition(|&b| b == needle).is_none());
     });
+    b.bytes = haystack.len() as u64;
 }
 
 #[bench]
@@ -41,4 +44,5 @@ fn libc_memrchr(b: &mut test::Bencher) {
     b.iter(|| {
         assert!(memchr::memrchr(needle, &haystack).is_none());
     });
+    b.bytes = haystack.len() as u64;
 }

src/lib.rs

@@ -8,8 +8,8 @@ to the corresponding functions in `libc`.
 extern crate libc;
 
 use libc::funcs::c95::string;
-use libc::types::common::c95::c_void;
-use libc::types::os::arch::c95::{c_int, size_t};
+use libc::c_void;
+use libc::{c_int, size_t};
 
 /// A safe interface to `memchr`.
 ///
@@ -78,18 +78,127 @@ pub fn memrchr(needle: u8, haystack: &[u8]) -> Option<usize> {
         }
     }
 
-    #[cfg(not(target_os = "linux"))]
+    #[cfg(all(not(target_os = "linux"),
+              any(target_pointer_width = "32", target_pointer_width = "64")))]
+    fn memrchr_specific(needle: u8, haystack: &[u8]) -> Option<usize> {
+        fallback::memrchr(needle, haystack)
+    }
+
+    // For the rare case of neither 32 bit nor 64-bit platform.
+    #[cfg(all(not(target_os = "linux"),
+              not(target_pointer_width = "32"),
+              not(target_pointer_width = "64")))]
     fn memrchr_specific(needle: u8, haystack: &[u8]) -> Option<usize> {
         haystack.iter().rposition(|&b| b == needle)
     }
 
     memrchr_specific(needle, haystack)
 }
 
+#[cfg(not(target_os = "linux"))]
+mod fallback {
+    use std::cmp;
+
+    const LO_U64: u64 = 0x0101010101010101;
+    const HI_U64: u64 = 0x8080808080808080;
+
+    // use truncation
+    const LO_USIZE: usize = LO_U64 as usize;
+    const HI_USIZE: usize = HI_U64 as usize;
+
+    #[cfg(target_pointer_width = "32")]
+    const USIZE_BYTES: usize = 4;
+    #[cfg(target_pointer_width = "64")]
+    const USIZE_BYTES: usize = 8;
+
+    /// Return `true` if `x` contains any zero byte.
+    ///
+    /// From *Matters Computational*, J. Arndt
+    ///
+    /// "The idea is to subtract one from each of the bytes and then look for
+    /// bytes where the borrow propagated all the way to the most significant
+    /// bit."
+    #[inline]
+    fn contains_zero_byte(x: usize) -> bool {
+        x.wrapping_sub(LO_USIZE) & !x & HI_USIZE != 0
+    }
+
+    #[cfg(target_pointer_width = "32")]
+    #[inline]
+    fn repeat_byte(b: u8) -> usize {
+        let mut rep = (b as usize) << 8 | b as usize;
+        rep = rep << 16 | rep;
+        rep
+    }
+
+    #[cfg(target_pointer_width = "64")]
+    #[inline]
+    fn repeat_byte(b: u8) -> usize {
+        let mut rep = (b as usize) << 8 | b as usize;
+        rep = rep << 16 | rep;
+        rep = rep << 32 | rep;
+        rep
+    }
+
+    /// Return the last index matching the byte `a` in `text`.
+    pub fn memrchr(x: u8, text: &[u8]) -> Option<usize> {
+        // Scan for a single byte value by reading two `usize` words at a time.
+        //
+        // Split `text` in three parts
+        // - unaligned tail, after the last word aligned address in text
+        // - body, scan by 2 words at a time
+        // - the first remaining bytes, < 2 word size
+        let len = text.len();
+        let ptr = text.as_ptr();
+
+        // search to an aligned boundary
+        let endptr = unsafe { ptr.offset(text.len() as isize) };
+        let align = (endptr as usize) & (USIZE_BYTES - 1);
+        let tail;
+        if align > 0 {
+            tail = cmp::min(USIZE_BYTES - align, len);
+            for (index, &byte) in text[len - tail..].iter().enumerate().rev() {
+                if byte == x {
+                    return Some(len - tail + index);
+                }
+            }
+        } else {
+            tail = 0;
+        }
+
+        // search the body of the text
+        let repeated_x = repeat_byte(x);
+        let mut offset = len - tail;
+
+        while offset >= 2 * USIZE_BYTES {
+            unsafe {
+                let u = *(ptr.offset(offset as isize - 2 * USIZE_BYTES as isize) as *const usize);
+                let v = *(ptr.offset(offset as isize - USIZE_BYTES as isize) as *const usize);
+
+                // break if there is a matching byte
+                let zu = contains_zero_byte(u ^ repeated_x);
+                let zv = contains_zero_byte(v ^ repeated_x);
+                if zu || zv {
+                    break;
+                }
+            }
+            offset -= 2 * USIZE_BYTES;
+        }
+
+        // find a zero after the point the body loop stopped
+        for (index, &byte) in text[..offset].iter().enumerate().rev() {
+            if byte == x {
+                return Some(index);
+            }
+        }
+        None
+    }
+}
+
 #[cfg(target_os = "linux")]
 mod ffi {
-    use libc::types::common::c95::c_void;
-    use libc::types::os::arch::c95::{c_int, size_t};
+    use libc::c_void;
+    use libc::{c_int, size_t};
     extern {
         pub fn memrchr(cx: *const c_void, c: c_int, n: size_t) -> *mut c_void;
     }
@@ -186,4 +295,18 @@ mod tests {
         }
         quickcheck::quickcheck(prop as fn(u8, Vec<u8>) -> bool);
     }
+
+    #[test]
+    fn qc_correct_reversed() {
+        fn prop(a: Vec<u8>) -> bool {
+            for byte in 0..256u32 {
+                let byte = byte as u8;
+                if memrchr(byte, &a) != a.iter().rposition(|elt| *elt == byte) {
+                    return false;
+                }
+            }
+            true
+        }
+        quickcheck::quickcheck(prop as fn(Vec<u8>) -> bool);
+    }
 }