utf8 validation: Compute block end upfront

Simplify the conditional used for ensuring that the whole word loop is
only used if there are at least two whole words left to read.

This makes the function slightly smaller and simpler, a 0-5% reduction
in runtime for various test cases.

Author: bluss

src/libcore/str/mod.rs

@@ -1217,6 +1217,11 @@ fn contains_nonascii(x: usize) -> bool {
 fn run_utf8_validation(v: &[u8]) -> Result<(), Utf8Error> {
     let mut offset = 0;
     let len = v.len();
+
+    let usize_bytes = mem::size_of::<usize>();
+    let ascii_block_size = 2 * usize_bytes;
+    let blocks_end = if len >= ascii_block_size { len - ascii_block_size + 1 } else { 0 };
+
     while offset < len {
         let old_offset = offset;
         macro_rules! err { () => {{
@@ -1282,26 +1287,22 @@ fn run_utf8_validation(v: &[u8]) -> Result<(), Utf8Error> {
             // Ascii case, try to skip forward quickly.
             // When the pointer is aligned, read 2 words of data per iteration
             // until we find a word containing a non-ascii byte.
-            let usize_bytes = mem::size_of::<usize>();
-            let bytes_per_iteration = 2 * usize_bytes;
             let ptr = v.as_ptr();
             let align = (ptr as usize + offset) & (usize_bytes - 1);
             if align == 0 {
-                if len >= bytes_per_iteration {
-                    while offset <= len - bytes_per_iteration {
-                        unsafe {
-                            let u = *(ptr.offset(offset as isize) as *const usize);
-                            let v = *(ptr.offset((offset + usize_bytes) as isize) as *const usize);
-
-                            // break if there is a nonascii byte
-                            let zu = contains_nonascii(u);
-                            let zv = contains_nonascii(v);
-                            if zu || zv {
-                                break;
-                            }
+                while offset < blocks_end {
+                    unsafe {
+                        let u = *(ptr.offset(offset as isize) as *const usize);
+                        let v = *(ptr.offset((offset + usize_bytes) as isize) as *const usize);
+
+                        // break if there is a nonascii byte
+                        let zu = contains_nonascii(u);
+                        let zv = contains_nonascii(v);
+                        if zu || zv {
+                            break;
                         }
-                        offset += bytes_per_iteration;
                     }
+                    offset += ascii_block_size;
                 }
                 // step from the point where the wordwise loop stopped
                 while offset < len && v[offset] < 128 {