reduced branching in get_bits, improving performance of various data structures

Author: Cydhra

src/bit_vec/fast_rs_vec/mod.rs

@@ -373,10 +373,8 @@ impl RsVec {
     pub fn get_bits_unchecked(&self, pos: usize, len: usize) -> u64 {
         debug_assert!(len <= WORD_SIZE);
         let partial_word = self.data[pos / WORD_SIZE] >> (pos % WORD_SIZE);
-        if pos % WORD_SIZE + len == WORD_SIZE {
-            partial_word
-        } else if pos % WORD_SIZE + len < WORD_SIZE {
-            partial_word & ((1 << (len % WORD_SIZE)) - 1)
+        if pos % WORD_SIZE + len <= WORD_SIZE {
+            partial_word & 1u64.checked_shl(len as u32).unwrap_or(0).wrapping_sub(1)
         } else {
             (partial_word | (self.data[pos / WORD_SIZE + 1] << (WORD_SIZE - pos % WORD_SIZE)))
                 & 1u64.checked_shl(len as u32).unwrap_or(0).wrapping_sub(1)