/*

 * Copyright (c) 2018, Google and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 *

 */

#include "precompiled.hpp"

#include "runtime/handles.inline.hpp"

#include "runtime/orderAccess.hpp"

#include "runtime/sharedRuntime.hpp"

#include "runtime/threadHeapSampler.hpp"

// Cheap random number generator

uint64_t ThreadHeapSampler::_rnd;

// Default is 512kb.

int ThreadHeapSampler::_sampling_rate = 512 * 1024;

int ThreadHeapSampler::_enabled;

// Statics for the fast log

static const int FastLogNumBits = 10;

static const int FastLogMask = (1 << FastLogNumBits) - 1;

static double log_table[1<<FastLogNumBits];  // Constant

static bool log_table_initialized;

// Returns the next prng value.

// pRNG is: aX+b mod c with a = 0x5DEECE66D, b =  0xB, c = 1<<48

// This is the lrand64 generator.

static uint64_t next_random(uint64_t rnd) {

  const uint64_t PrngMult = 0x5DEECE66DLL;

  const uint64_t PrngAdd = 0xB;

  const uint64_t PrngModPower = 48;

  const uint64_t PrngModMask = ((uint64_t)1 << PrngModPower) - 1;

  //assert(IS_SAFE_SIZE_MUL(PrngMult, rnd), "Overflow on multiplication.");

  //assert(IS_SAFE_SIZE_ADD(PrngMult * rnd, PrngAdd), "Overflow on addition.");

  return (PrngMult * rnd + PrngAdd) & PrngModMask;

}

static double fast_log2(const double & d) {

  assert(d>0, "bad value passed to assert");

  uint64_t x = 0;

  assert(sizeof(d) == sizeof(x),

         "double and uint64_t do not have the same size");

  x = *reinterpret_cast<const uint64_t*>(&d);

  const uint32_t x_high = x >> 32;

  assert(FastLogNumBits <= 20, "FastLogNumBits should be less than 20.");

  const uint32_t y = x_high >> (20 - FastLogNumBits) & FastLogMask;

  const int32_t exponent = ((x_high >> 20) & 0x7FF) - 1023;

  return exponent + log_table[y];

}

// Generates a geometric variable with the specified mean (512K by default).

// This is done by generating a random number between 0 and 1 and applying

// the inverse cumulative distribution function for an exponential.

// Specifically: Let m be the inverse of the sample rate, then

// the probability distribution function is m*exp(-mx) so the CDF is

// p = 1 - exp(-mx), so

// q = 1 - p = exp(-mx)

// log_e(q) = -mx

// -log_e(q)/m = x

// log_2(q) * (-log_e(2) * 1/m) = x

// In the code, q is actually in the range 1 to 2**26, hence the -26 below

void ThreadHeapSampler::pick_next_geometric_sample() {

  _rnd = next_random(_rnd);

  // Take the top 26 bits as the random number

  // (This plus a 1<<58 sampling bound gives a max possible step of

  // 5194297183973780480 bytes.  In this case,

  // for sample_parameter = 1<<19, max possible step is

  // 9448372 bytes (24 bits).

  const uint64_t PrngModPower = 48;  // Number of bits in prng

  // The uint32_t cast is to prevent a (hard-to-reproduce) NAN

  // under piii debug for some binaries.

  double q = static_cast<uint32_t>(_rnd >> (PrngModPower - 26)) + 1.0;

  // Put the computed p-value through the CDF of a geometric.

  // For faster performance (save ~1/20th exec time), replace

  // min(0.0, FastLog2(q) - 26)  by  (Fastlog2(q) - 26.000705)

  // The value 26.000705 is used rather than 26 to compensate

  // for inaccuracies in FastLog2 which otherwise result in a

  // negative answer.

  double log_val = (fast_log2(q) - 26);

  double result =

      (0.0 < log_val ? 0.0 : log_val) * (-log(2.0) * (get_sampling_rate())) + 1;

  assert(result > 0 && result < SIZE_MAX, "Result is not in an acceptable range.");

  size_t rate = static_cast<size_t>(result);

  _bytes_until_sample = rate;

}

void ThreadHeapSampler::pick_next_sample(size_t overflowed_bytes) {

  if (get_sampling_rate() == 1) {

    _bytes_until_sample = 1;

    return;

  }

  pick_next_geometric_sample();

  // Try to correct sample size by removing extra space from last allocation.

  if (overflowed_bytes > 0 && _bytes_until_sample > overflowed_bytes) {

    _bytes_until_sample -= overflowed_bytes;

  }

}

void ThreadHeapSampler::check_for_sampling(HeapWord* ptr, size_t allocation_size, size_t bytes_since_allocation) {

  oopDesc* oop = reinterpret_cast<oopDesc*>(ptr);

  size_t total_allocated_bytes = bytes_since_allocation + allocation_size;

  // If not yet time for a sample, skip it.

  if (total_allocated_bytes < _bytes_until_sample) {

    _bytes_until_sample -= total_allocated_bytes;

    return;

  }

  JvmtiExport::sampled_object_alloc_event_collector(oop);

  size_t overflow_bytes = total_allocated_bytes - _bytes_until_sample;

  pick_next_sample(overflow_bytes);

}

void ThreadHeapSampler::init_log_table() {

  MutexLockerEx mu(ThreadHeapSampler_lock, Mutex::_no_safepoint_check_flag);

  if (log_table_initialized) {

    return;

  }

  for (int i = 0; i < (1 << FastLogNumBits); i++) {

    log_table[i] = (log(1.0 + static_cast<double>(i+0.5) / (1 << FastLogNumBits))

                    / log(2.0));

  }

  log_table_initialized = true;

}

void ThreadHeapSampler::enable() {

  // Done here to be done when things have settled. This adds a mutex lock but

  // presumably, users won't be enabling and disabling all the time.

  init_log_table();

  OrderAccess::release_store(&_enabled, 1);

}

int ThreadHeapSampler::enabled() {

  return OrderAccess::load_acquire(&_enabled);

}

void ThreadHeapSampler::disable() {

  OrderAccess::release_store(&_enabled, 0);

}

int ThreadHeapSampler::get_sampling_rate() {

  return OrderAccess::load_acquire(&_sampling_rate);

}

void ThreadHeapSampler::set_sampling_rate(int sampling_rate) {

  OrderAccess::release_store(&_sampling_rate, sampling_rate);

}

// Methods used in assertion mode to check if a collector is present or not at

// the moment of TLAB sampling, ie a slow allocation path.

bool ThreadHeapSampler::sampling_collector_present() const {

  return _collectors_present > 0;

}

bool ThreadHeapSampler::remove_sampling_collector() {

  assert(_collectors_present > 0, "Problem with collector counter.");

  _collectors_present--;

  return true;

}

bool ThreadHeapSampler::add_sampling_collector() {

  _collectors_present++;

  return true;

}