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+// Copyright (c) 2008, Google Inc.
+// All rights reserved.
+// 
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+// 
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+// 
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// ---
+// All Rights Reserved.
+//
+// Author: Daniel Ford
+
+#ifndef TCMALLOC_SAMPLER_H_
+#define TCMALLOC_SAMPLER_H_
+
+#include "config.h"
+#include "common.h"
+#include "static_vars.h"
+
+namespace tcmalloc {
+
+//-------------------------------------------------------------------
+// Sampler to decide when to create a sample trace for an allocation
+// Not thread safe: Each thread should have it's own sampler object.
+// Caller must use external synchronization if used
+// from multiple threads.
+//
+// With 512K average sample step (the default):
+//  the probability of sampling a 4K allocation is about 0.00778
+//  the probability of sampling a 1MB allocation is about 0.865
+//  the probability of sampling a 1GB allocation is about 1.00000
+// In general, the probablity of sampling is an allocation of size X
+// given a flag value of Y (default 1M) is:
+//  1 - e^(-X/Y)
+//
+// With 128K average sample step:
+//  the probability of sampling a 1MB allocation is about 0.99966
+//  the probability of sampling a 1GB allocation is about 1.0
+//  (about 1 - 2**(-26))
+// With 1M average sample step:
+//  the probability of sampling a 4K allocation is about 0.00390
+//  the probability of sampling a 1MB allocation is about 0.632
+//  the probability of sampling a 1GB allocation is about 1.0
+//
+// The sampler works by representing memory as a long stream from
+// which allocations are taken. Some of the bytes in this stream are
+// marked and if an allocation includes a marked byte then it is
+// sampled. Bytes are marked according to a Poisson point process
+// with each byte being marked independently with probability
+// p = 1/tcmalloc_sample_parameter.  This makes the probability
+// of sampling an allocation of X bytes equal to the CDF of
+// a geometric with mean tcmalloc_sample_parameter. (ie. the
+// probability that at least one byte in the range is marked). This
+// is accurately given by the CDF of the corresponding exponential
+// distribution : 1 - e^(X/tcmalloc_sample_parameter_)
+// Independence of the byte marking ensures independence of
+// the sampling of each allocation.
+//
+// This scheme is implemented by noting that, starting from any
+// fixed place, the number of bytes until the next marked byte
+// is geometrically distributed. This number is recorded as
+// bytes_until_sample_.  Every allocation subtracts from this
+// number until it is less than 0. When this happens the current
+// allocation is sampled.
+//
+// When an allocation occurs, bytes_until_sample_ is reset to
+// a new independtly sampled geometric number of bytes. The
+// memoryless property of the point process means that this may
+// be taken as the number of bytes after the end of the current
+// allocation until the next marked byte. This ensures that
+// very large allocations which would intersect many marked bytes
+// only result in a single call to PickNextSamplingPoint.
+//-------------------------------------------------------------------
+
+class PERFTOOLS_DLL_DECL Sampler {
+ public:
+  // Initialize this sampler.
+  // Passing a seed of 0 gives a non-deterministic
+  // seed value given by casting the object ("this")
+  void Init(uint32_t seed);
+  void Cleanup();
+
+  // Record allocation of "k" bytes.  Return true iff allocation
+  // should be sampled
+  bool SampleAllocation(size_t k);
+
+  // Generate a geometric with mean 512K (or FLAG_tcmalloc_sample_parameter)
+  size_t PickNextSamplingPoint();
+
+  // Initialize the statics for the Sampler class
+  static void InitStatics();
+
+  // Returns the current sample period
+  int GetSamplePeriod();
+
+  // The following are public for the purposes of testing
+  static uint64_t NextRandom(uint64_t rnd_);  // Returns the next prng value
+  static double FastLog2(const double & d);  // Computes Log2(x) quickly
+  static void PopulateFastLog2Table();  // Populate the lookup table
+
+ private:
+  size_t        bytes_until_sample_;    // Bytes until we sample next
+  uint64_t      rnd_;                   // Cheap random number generator
+
+  // Statics for the fast log
+  // Note that this code may not depend on anything in //util
+  // hence the duplication of functionality here
+  static const int kFastlogNumBits = 10;
+  static const int kFastlogMask = (1 << kFastlogNumBits) - 1;
+  static double log_table_[1<<kFastlogNumBits];  // Constant
+};
+
+inline bool Sampler::SampleAllocation(size_t k) {
+  if (bytes_until_sample_ < k) {
+    bytes_until_sample_ = PickNextSamplingPoint();
+    return true;
+  } else {
+    bytes_until_sample_ -= k;
+    return false;
+  }
+}
+
+// Inline functions which are public for testing purposes
+
+// Returns the next prng value.
+// pRNG is: aX+b mod c with a = 0x5DEECE66D, b =  0xB, c = 1<<48
+// This is the lrand64 generator.
+inline uint64_t Sampler::NextRandom(uint64_t rnd) {
+  const uint64_t prng_mult = 0x5DEECE66DLL;
+  const uint64_t prng_add = 0xB;
+  const uint64_t prng_mod_power = 48;
+  const uint64_t prng_mod_mask =
+                ~((~static_cast<uint64_t>(0)) << prng_mod_power);
+  return (prng_mult * rnd + prng_add) & prng_mod_mask;
+}
+
+// Adapted from //util/math/fastmath.[h|cc] by Noam Shazeer
+// This mimics the VeryFastLog2 code in those files
+inline double Sampler::FastLog2(const double & d) {
+  ASSERT(d>0);
+  COMPILE_ASSERT(sizeof(d) == sizeof(uint64_t), DoubleMustBe64Bits);
+  uint64_t x;
+  memcpy(&x, &d, sizeof(x));   // we depend on the compiler inlining this
+  const uint32_t x_high = x >> 32;
+  const uint32_t y = x_high >> (20 - kFastlogNumBits) & kFastlogMask;
+  const int32_t exponent = ((x_high >> 20) & 0x7FF) - 1023;
+  return exponent + log_table_[y];
+}
+
+}  // namespace tcmalloc
+
+#endif  // TCMALLOC_SAMPLER_H_