Use std::find to locate newline.

Benchmark on lofn to count lines in
xzcat /fs/vali0/www/data.statmt.org/ngrams/deduped_en/en.00.xz |head
-n 10000000

With std::find
real    0m0.709s
user    0m0.648s
sys     0m0.060s

Baseline
real    0m0.899s
user    0m0.820s
sys     0m0.076s

C++
int main() {
  std::ios::sync_with_stdio(false);
  std::string line;
  while (getline(std::cin, line)) {}
}
real    0m0.803s
user    0m0.536s
sys     0m0.264s

util/file_piece.cc

@@ -73,17 +73,16 @@ FilePiece::FilePiece(std::istream &stream, const char *name, std::size_t min_buf
 StringPiece FilePiece::ReadLine(char delim, bool strip_cr) {
   std::size_t skip = 0;
   while (true) {
-    for (const char *i = position_ + skip; i < position_end_; ++i) {
-      if (*i == delim) {
-        // End of line.
-        // Take 1 byte off the end if it's an unwanted carriage return.
-        const std::size_t subtract_cr = (
-            (strip_cr && i > position_ && *(i - 1) == '\r') ?
-            1 : 0);
-        StringPiece ret(position_, i - position_ - subtract_cr);
-        position_ = i + 1;
-        return ret;
-      }
+    const char *i = std::find(position_ + skip, position_end_, delim);
+    if (UTIL_LIKELY(i != position_end_)) {
+      // End of line.
+      // Take 1 byte off the end if it's an unwanted carriage return.
+      const std::size_t subtract_cr = (
+          (strip_cr && i > position_ && *(i - 1) == '\r') ?
+          1 : 0);
+      StringPiece ret(position_, i - position_ - subtract_cr);
+      position_ = i + 1;
+      return ret;
     }
     if (at_end_) {
       if (position_ == position_end_) {