Update from https://crrev.com/316786

net/quic/congestion_control/cube_root.cc

-// Copyright (c) 2013 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 "net/quic/congestion_control/cube_root.h"
-
-#include "base/logging.h"
-
-namespace {
-
-// Find last bit in a 64-bit word.
-int FindMostSignificantBit(uint64 x) {
-  if (!x) {
-    return 0;
-  }
-  int r = 0;
-  if (x & 0xffffffff00000000ull) {
-    x >>= 32;
-    r += 32;
-  }
-  if (x & 0xffff0000u) {
-    x >>= 16;
-    r += 16;
-  }
-  if (x & 0xff00u) {
-    x >>= 8;
-    r += 8;
-  }
-  if (x & 0xf0u) {
-    x >>= 4;
-    r += 4;
-  }
-  if (x & 0xcu) {
-    x >>= 2;
-    r += 2;
-  }
-  if (x & 0x02u) {
-    x >>= 1;
-    r++;
-  }
-  if (x & 0x01u) {
-    r++;
-  }
-  return r;
-}
-
-// 6 bits table [0..63]
-const uint32 cube_root_table[] = {
-    0,  54,  54,  54, 118, 118, 118, 118, 123, 129, 134, 138, 143, 147, 151,
-  156, 157, 161, 164, 168, 170, 173, 176, 179, 181, 185, 187, 190, 192, 194,
-  197, 199, 200, 202, 204, 206, 209, 211, 213, 215, 217, 219, 221, 222, 224,
-  225, 227, 229, 231, 232, 234, 236, 237, 239, 240, 242, 244, 245, 246, 248,
-  250, 251, 252, 254
-};
-}  // namespace
-
-namespace net {
-
-// Calculate the cube root using a table lookup followed by one Newton-Raphson
-// iteration.
-uint32 CubeRoot::Root(uint64 a) {
-  uint32 msb = FindMostSignificantBit(a);
-  DCHECK_LE(msb, 64u);
-
-  if (msb < 7) {
-    // MSB in our table.
-    return ((cube_root_table[a]) + 31) >> 6;
-  }
-  // MSB          7,  8,  9, 10, 11, 12, 13, 14, 15, 16, ...
-  // cubic_shift  1,  1,  1,  2,  2,  2,  3,  3,  3,  4, ...
-  uint32 cubic_shift = (msb - 4);
-  cubic_shift = ((cubic_shift * 342) >> 10);  // Div by 3, biased high.
-
-  // 4 to 6 bits accuracy depending on MSB.
-  uint64 root =
-      ((cube_root_table[a >> (cubic_shift * 3)] + 10) << cubic_shift) >> 6;
-
-  // Make one Newton-Raphson iteration.
-  // Since x has an error (inaccuracy due to the use of fix point) we get a
-  // more accurate result by doing x * (x - 1) instead of x * x.
-  root = 2 * root + (a / (root * (root - 1)));
-  root = ((root * 341) >> 10);  // Div by 3, biased low.
-  return static_cast<uint32>(root);
-}
-
-}  // namespace net