Optimize BitStreamReader, speed up EV Whitelist loading

components/packed_ct_ev_whitelist/bit_stream_reader.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "components/packed_ct_ev_whitelist/bit_stream_reader.h"
 
 #include "base/big_endian.h"
 #include "base/logging.h"
 #include "base/numerics/safe_conversions.h"
 
@@ skipping 16 matching lines @@
   return true;
 }
 
 bool BitStreamReader::ReadBits(uint8_t num_bits, uint64_t* out) {
   DCHECK_LE(num_bits, 64);
 
   if (BitsLeft() < num_bits)
     return false;
 
   *out = 0;
-  for (uint8_t i = 0; i < num_bits; ++i)
-    (*out) |= (static_cast<uint64_t>(ReadBit()) << (num_bits - (i + 1)));
+
+  for (; num_bits && (current_bit_ != 7); --num_bits)
+    (*out) |= (static_cast<uint64_t>(ReadBit()) << (num_bits - 1));
+  for (; num_bits / 8; num_bits -= 8)
+    (*out) |= (static_cast<uint64_t>(ReadByte()) << (num_bits - 8));
+  for (; num_bits; --num_bits)
+    (*out) |= (static_cast<uint64_t>(ReadBit()) << (num_bits - 1));
 
   return true;
 }
 
 uint64_t BitStreamReader::BitsLeft() const {
   if (current_byte_ == source_.length())
     return 0;
   DCHECK_GT(source_.length(), current_byte_);
   return (source_.length() - (current_byte_ + 1)) * 8 + current_bit_ + 1;
 }
 
 uint8_t BitStreamReader::ReadBit() {
   DCHECK_GT(BitsLeft(), 0u);
   DCHECK(current_bit_ < 8 && current_bit_ >= 0);
   uint8_t res =
       (source_.data()[current_byte_] & (1 << current_bit_)) >> current_bit_;
   current_bit_--;
   if (current_bit_ < 0) {
     current_byte_++;
     current_bit_ = 7;
   }
 
   return res;
 }
 
+uint8_t BitStreamReader::ReadByte() {
+  DCHECK_GT(BitsLeft(), 7u);
+  DCHECK_EQ(current_bit_, 7);
+
+  return source_.data()[current_byte_++];
+
+}
+
 }  // namespace internal
 }  // namespace packed_ct_ev_whitelist