Optimize BitStreamReader, speed up EV Whitelist loading

The EV Certificate Whitelist loads ~1MB of data using BitStreamReader.
Currently this is read in one bit at a time. Changing BitStreamReader to
support byte reads sped up loading by about 65% on my computer.

BUG=457949

TEST=bit_stream_reader_unittest.cc

Review URL: https://codereview.chromium.org/1070903003

Cr-Commit-Position: refs/heads/master@{#325286}

2 files changed, 20 insertions, 2 deletions

diff --git a/components/packed_ct_ev_whitelist/bit_stream_reader.cc b/components/packed_ct_ev_whitelist/bit_stream_reader.cc
index e4f6cc7..94ef009 100644
--- a/components/packed_ct_ev_whitelist/bit_stream_reader.cc
+++ b/components/packed_ct_ev_whitelist/bit_stream_reader.cc
@@ -34,8 +34,13 @@ bool BitStreamReader::ReadBits(uint8_t num_bits, uint64_t* out) {
     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;
 }
@@ -61,5 +66,13 @@ uint8_t BitStreamReader::ReadBit() {
   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
diff --git a/components/packed_ct_ev_whitelist/bit_stream_reader.h b/components/packed_ct_ev_whitelist/bit_stream_reader.h
index 45fa5f5..5c95243 100644
--- a/components/packed_ct_ev_whitelist/bit_stream_reader.h
+++ b/components/packed_ct_ev_whitelist/bit_stream_reader.h
@@ -41,6 +41,11 @@ class BitStreamReader {
   // LSB.
   uint8_t ReadBit();
 
+  // Reads a single byte.
+  // Precondition: The stream must be byte-aligned (current_bit_ == 7) before
+  // calling this function.
+  uint8_t ReadByte();
+
   const base::StringPiece source_;
 
   // Index of the byte currently being read from.