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nanobenchmark.cc (27528B)

---

// Copyright 2017 Google Inc. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "absl/random/internal/nanobenchmark.h"

#include <sys/types.h>

#include <algorithm>  // sort
#include <atomic>
#include <cstddef>
#include <cstdint>
#include <cstdlib>
#include <cstring>  // memcpy
#include <limits>
#include <string>
#include <utility>
#include <vector>

#include "absl/base/attributes.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/random/internal/platform.h"
#include "absl/random/internal/randen_engine.h"

// OS
#if defined(_WIN32) || defined(_WIN64)
#define ABSL_OS_WIN
#include <windows.h>  // NOLINT

#elif defined(__ANDROID__)
#define ABSL_OS_ANDROID

#elif defined(__linux__)
#define ABSL_OS_LINUX
#include <sched.h>        // NOLINT
#include <sys/syscall.h>  // NOLINT
#endif

#if defined(ABSL_ARCH_X86_64) && !defined(ABSL_OS_WIN)
#include <cpuid.h>  // NOLINT
#endif

// __ppc_get_timebase_freq
#if defined(ABSL_ARCH_PPC)
#include <sys/platform/ppc.h>  // NOLINT
#endif

// clock_gettime
#if defined(ABSL_ARCH_ARM) || defined(ABSL_ARCH_AARCH64)
#include <time.h>  // NOLINT
#endif

// ABSL_RANDOM_INTERNAL_ATTRIBUTE_NEVER_INLINE prevents inlining of the method.
#if ABSL_HAVE_ATTRIBUTE(noinline) || (defined(__GNUC__) && !defined(__clang__))
#define ABSL_RANDOM_INTERNAL_ATTRIBUTE_NEVER_INLINE __attribute__((noinline))
#elif defined(_MSC_VER)
#define ABSL_RANDOM_INTERNAL_ATTRIBUTE_NEVER_INLINE __declspec(noinline)
#else
#define ABSL_RANDOM_INTERNAL_ATTRIBUTE_NEVER_INLINE
#endif

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace random_internal_nanobenchmark {
namespace {

// For code folding.
namespace platform {
#if defined(ABSL_ARCH_X86_64)

// TODO(janwas): Merge with the one in randen_hwaes.cc?
void Cpuid(const uint32_t level, const uint32_t count,
          uint32_t* ABSL_RANDOM_INTERNAL_RESTRICT abcd) {
#if defined(ABSL_OS_WIN)
 int regs[4];
 __cpuidex(regs, level, count);
 for (int i = 0; i < 4; ++i) {
   abcd[i] = regs[i];
 }
#else
 uint32_t a, b, c, d;
 __cpuid_count(level, count, a, b, c, d);
 abcd[0] = a;
 abcd[1] = b;
 abcd[2] = c;
 abcd[3] = d;
#endif
}

std::string BrandString() {
 char brand_string[49];
 uint32_t abcd[4];

 // Check if brand string is supported (it is on all reasonable Intel/AMD)
 Cpuid(0x80000000U, 0, abcd);
 if (abcd[0] < 0x80000004U) {
   return std::string();
 }

 for (int i = 0; i < 3; ++i) {
   Cpuid(0x80000002U + i, 0, abcd);
   memcpy(brand_string + i * 16, &abcd, sizeof(abcd));
 }
 brand_string[48] = 0;
 return brand_string;
}

// Returns the frequency quoted inside the brand string. This does not
// account for throttling nor Turbo Boost.
double NominalClockRate() {
 const std::string& brand_string = BrandString();
 // Brand strings include the maximum configured frequency. These prefixes are
 // defined by Intel CPUID documentation.
 const char* prefixes[3] = {"MHz", "GHz", "THz"};
 const double multipliers[3] = {1E6, 1E9, 1E12};
 for (size_t i = 0; i < 3; ++i) {
   const size_t pos_prefix = brand_string.find(prefixes[i]);
   if (pos_prefix != std::string::npos) {
     const size_t pos_space = brand_string.rfind(' ', pos_prefix - 1);
     if (pos_space != std::string::npos) {
       const std::string digits =
           brand_string.substr(pos_space + 1, pos_prefix - pos_space - 1);
       return std::stod(digits) * multipliers[i];
     }
   }
 }

 return 0.0;
}

#endif  // ABSL_ARCH_X86_64
}  // namespace platform

// Prevents the compiler from eliding the computations that led to "output".
template <class T>
inline void PreventElision(T&& output) {
#ifndef ABSL_OS_WIN
 // Works by indicating to the compiler that "output" is being read and
 // modified. The +r constraint avoids unnecessary writes to memory, but only
 // works for built-in types (typically FuncOutput).
 asm volatile("" : "+r"(output) : : "memory");
#else
 // MSVC does not support inline assembly anymore (and never supported GCC's
 // RTL constraints). Self-assignment with #pragma optimize("off") might be
 // expected to prevent elision, but it does not with MSVC 2015. Type-punning
 // with volatile pointers generates inefficient code on MSVC 2017.
 static std::atomic<T> dummy(T{});
 dummy.store(output, std::memory_order_relaxed);
#endif
}

namespace timer {

// Start/Stop return absolute timestamps and must be placed immediately before
// and after the region to measure. We provide separate Start/Stop functions
// because they use different fences.
//
// Background: RDTSC is not 'serializing'; earlier instructions may complete
// after it, and/or later instructions may complete before it. 'Fences' ensure
// regions' elapsed times are independent of such reordering. The only
// documented unprivileged serializing instruction is CPUID, which acts as a
// full fence (no reordering across it in either direction). Unfortunately
// the latency of CPUID varies wildly (perhaps made worse by not initializing
// its EAX input). Because it cannot reliably be deducted from the region's
// elapsed time, it must not be included in the region to measure (i.e.
// between the two RDTSC).
//
// The newer RDTSCP is sometimes described as serializing, but it actually
// only serves as a half-fence with release semantics. Although all
// instructions in the region will complete before the final timestamp is
// captured, subsequent instructions may leak into the region and increase the
// elapsed time. Inserting another fence after the final RDTSCP would prevent
// such reordering without affecting the measured region.
//
// Fortunately, such a fence exists. The LFENCE instruction is only documented
// to delay later loads until earlier loads are visible. However, Intel's
// reference manual says it acts as a full fence (waiting until all earlier
// instructions have completed, and delaying later instructions until it
// completes). AMD assigns the same behavior to MFENCE.
//
// We need a fence before the initial RDTSC to prevent earlier instructions
// from leaking into the region, and arguably another after RDTSC to avoid
// region instructions from completing before the timestamp is recorded.
// When surrounded by fences, the additional RDTSCP half-fence provides no
// benefit, so the initial timestamp can be recorded via RDTSC, which has
// lower overhead than RDTSCP because it does not read TSC_AUX. In summary,
// we define Start = LFENCE/RDTSC/LFENCE; Stop = RDTSCP/LFENCE.
//
// Using Start+Start leads to higher variance and overhead than Stop+Stop.
// However, Stop+Stop includes an LFENCE in the region measurements, which
// adds a delay dependent on earlier loads. The combination of Start+Stop
// is faster than Start+Start and more consistent than Stop+Stop because
// the first LFENCE already delayed subsequent loads before the measured
// region. This combination seems not to have been considered in prior work:
// http://akaros.cs.berkeley.edu/lxr/akaros/kern/arch/x86/rdtsc_test.c
//
// Note: performance counters can measure 'exact' instructions-retired or
// (unhalted) cycle counts. The RDPMC instruction is not serializing and also
// requires fences. Unfortunately, it is not accessible on all OSes and we
// prefer to avoid kernel-mode drivers. Performance counters are also affected
// by several under/over-count errata, so we use the TSC instead.

// Returns a 64-bit timestamp in unit of 'ticks'; to convert to seconds,
// divide by InvariantTicksPerSecond.
inline uint64_t Start64() {
 uint64_t t;
#if defined(ABSL_ARCH_PPC)
 asm volatile("mfspr %0, %1" : "=r"(t) : "i"(268));
#elif defined(ABSL_ARCH_X86_64)
#if defined(ABSL_OS_WIN)
 _ReadWriteBarrier();
 _mm_lfence();
 _ReadWriteBarrier();
 t = __rdtsc();
 _ReadWriteBarrier();
 _mm_lfence();
 _ReadWriteBarrier();
#else
 asm volatile(
     "lfence\n\t"
     "rdtsc\n\t"
     "shl $32, %%rdx\n\t"
     "or %%rdx, %0\n\t"
     "lfence"
     : "=a"(t)
     :
     // "memory" avoids reordering. rdx = TSC >> 32.
     // "cc" = flags modified by SHL.
     : "rdx", "memory", "cc");
#endif
#else
 // Fall back to OS - unsure how to reliably query cntvct_el0 frequency.
 timespec ts;
 clock_gettime(CLOCK_REALTIME, &ts);
 t = ts.tv_sec * 1000000000LL + ts.tv_nsec;
#endif
 return t;
}

inline uint64_t Stop64() {
 uint64_t t;
#if defined(ABSL_ARCH_X86_64)
#if defined(ABSL_OS_WIN)
 _ReadWriteBarrier();
 unsigned aux;
 t = __rdtscp(&aux);
 _ReadWriteBarrier();
 _mm_lfence();
 _ReadWriteBarrier();
#else
 // Use inline asm because __rdtscp generates code to store TSC_AUX (ecx).
 asm volatile(
     "rdtscp\n\t"
     "shl $32, %%rdx\n\t"
     "or %%rdx, %0\n\t"
     "lfence"
     : "=a"(t)
     :
     // "memory" avoids reordering. rcx = TSC_AUX. rdx = TSC >> 32.
     // "cc" = flags modified by SHL.
     : "rcx", "rdx", "memory", "cc");
#endif
#else
 t = Start64();
#endif
 return t;
}

// Returns a 32-bit timestamp with about 4 cycles less overhead than
// Start64. Only suitable for measuring very short regions because the
// timestamp overflows about once a second.
inline uint32_t Start32() {
 uint32_t t;
#if defined(ABSL_ARCH_X86_64)
#if defined(ABSL_OS_WIN)
 _ReadWriteBarrier();
 _mm_lfence();
 _ReadWriteBarrier();
 t = static_cast<uint32_t>(__rdtsc());
 _ReadWriteBarrier();
 _mm_lfence();
 _ReadWriteBarrier();
#else
 asm volatile(
     "lfence\n\t"
     "rdtsc\n\t"
     "lfence"
     : "=a"(t)
     :
     // "memory" avoids reordering. rdx = TSC >> 32.
     : "rdx", "memory");
#endif
#else
 t = static_cast<uint32_t>(Start64());
#endif
 return t;
}

inline uint32_t Stop32() {
 uint32_t t;
#if defined(ABSL_ARCH_X86_64)
#if defined(ABSL_OS_WIN)
 _ReadWriteBarrier();
 unsigned aux;
 t = static_cast<uint32_t>(__rdtscp(&aux));
 _ReadWriteBarrier();
 _mm_lfence();
 _ReadWriteBarrier();
#else
 // Use inline asm because __rdtscp generates code to store TSC_AUX (ecx).
 asm volatile(
     "rdtscp\n\t"
     "lfence"
     : "=a"(t)
     :
     // "memory" avoids reordering. rcx = TSC_AUX. rdx = TSC >> 32.
     : "rcx", "rdx", "memory");
#endif
#else
 t = static_cast<uint32_t>(Stop64());
#endif
 return t;
}

}  // namespace timer

namespace robust_statistics {

// Sorts integral values in ascending order (e.g. for Mode). About 3x faster
// than std::sort for input distributions with very few unique values.
template <class T>
void CountingSort(T* values, size_t num_values) {
 // Unique values and their frequency (similar to flat_map).
 using Unique = std::pair<T, int>;
 std::vector<Unique> unique;
 for (size_t i = 0; i < num_values; ++i) {
   const T value = values[i];
   const auto pos =
       std::find_if(unique.begin(), unique.end(),
                    [value](const Unique u) { return u.first == value; });
   if (pos == unique.end()) {
     unique.push_back(std::make_pair(value, 1));
   } else {
     ++pos->second;
   }
 }

 // Sort in ascending order of value (pair.first).
 std::sort(unique.begin(), unique.end());

 // Write that many copies of each unique value to the array.
 T* ABSL_RANDOM_INTERNAL_RESTRICT p = values;
 for (const auto& value_count : unique) {
   std::fill_n(p, value_count.second, value_count.first);
   p += value_count.second;
 }
 ABSL_RAW_CHECK(p == values + num_values, "Did not produce enough output");
}

// @return i in [idx_begin, idx_begin + half_count) that minimizes
// sorted[i + half_count] - sorted[i].
template <typename T>
size_t MinRange(const T* const ABSL_RANDOM_INTERNAL_RESTRICT sorted,
               const size_t idx_begin, const size_t half_count) {
 T min_range = (std::numeric_limits<T>::max)();
 size_t min_idx = 0;

 for (size_t idx = idx_begin; idx < idx_begin + half_count; ++idx) {
   ABSL_RAW_CHECK(sorted[idx] <= sorted[idx + half_count], "Not sorted");
   const T range = sorted[idx + half_count] - sorted[idx];
   if (range < min_range) {
     min_range = range;
     min_idx = idx;
   }
 }

 return min_idx;
}

// Returns an estimate of the mode by calling MinRange on successively
// halved intervals. "sorted" must be in ascending order. This is the
// Half Sample Mode estimator proposed by Bickel in "On a fast, robust
// estimator of the mode", with complexity O(N log N). The mode is less
// affected by outliers in highly-skewed distributions than the median.
// The averaging operation below assumes "T" is an unsigned integer type.
template <typename T>
T ModeOfSorted(const T* const ABSL_RANDOM_INTERNAL_RESTRICT sorted,
              const size_t num_values) {
 size_t idx_begin = 0;
 size_t half_count = num_values / 2;
 while (half_count > 1) {
   idx_begin = MinRange(sorted, idx_begin, half_count);
   half_count >>= 1;
 }

 const T x = sorted[idx_begin + 0];
 if (half_count == 0) {
   return x;
 }
 ABSL_RAW_CHECK(half_count == 1, "Should stop at half_count=1");
 const T average = (x + sorted[idx_begin + 1] + 1) / 2;
 return average;
}

// Returns the mode. Side effect: sorts "values".
template <typename T>
T Mode(T* values, const size_t num_values) {
 CountingSort(values, num_values);
 return ModeOfSorted(values, num_values);
}

template <typename T, size_t N>
T Mode(T (&values)[N]) {
 return Mode(&values[0], N);
}

// Returns the median value. Side effect: sorts "values".
template <typename T>
T Median(T* values, const size_t num_values) {
 ABSL_RAW_CHECK(num_values != 0, "Empty input");
 std::sort(values, values + num_values);
 const size_t half = num_values / 2;
 // Odd count: return middle
 if (num_values % 2) {
   return values[half];
 }
 // Even count: return average of middle two.
 return (values[half] + values[half - 1] + 1) / 2;
}

// Returns a robust measure of variability.
template <typename T>
T MedianAbsoluteDeviation(const T* values, const size_t num_values,
                         const T median) {
 ABSL_RAW_CHECK(num_values != 0, "Empty input");
 std::vector<T> abs_deviations;
 abs_deviations.reserve(num_values);
 for (size_t i = 0; i < num_values; ++i) {
   const int64_t abs = std::abs(int64_t(values[i]) - int64_t(median));
   abs_deviations.push_back(static_cast<T>(abs));
 }
 return Median(abs_deviations.data(), num_values);
}

}  // namespace robust_statistics

// Ticks := platform-specific timer values (CPU cycles on x86). Must be
// unsigned to guarantee wraparound on overflow. 32 bit timers are faster to
// read than 64 bit.
using Ticks = uint32_t;

// Returns timer overhead / minimum measurable difference.
Ticks TimerResolution() {
 // Nested loop avoids exceeding stack/L1 capacity.
 Ticks repetitions[Params::kTimerSamples];
 for (size_t rep = 0; rep < Params::kTimerSamples; ++rep) {
   Ticks samples[Params::kTimerSamples];
   for (size_t i = 0; i < Params::kTimerSamples; ++i) {
     const Ticks t0 = timer::Start32();
     const Ticks t1 = timer::Stop32();
     samples[i] = t1 - t0;
   }
   repetitions[rep] = robust_statistics::Mode(samples);
 }
 return robust_statistics::Mode(repetitions);
}

static const Ticks timer_resolution = TimerResolution();

// Estimates the expected value of "lambda" values with a variable number of
// samples until the variability "rel_mad" is less than "max_rel_mad".
template <class Lambda>
Ticks SampleUntilStable(const double max_rel_mad, double* rel_mad,
                       const Params& p, const Lambda& lambda) {
 auto measure_duration = [&lambda]() -> Ticks {
   const Ticks t0 = timer::Start32();
   lambda();
   const Ticks t1 = timer::Stop32();
   return t1 - t0;
 };

 // Choose initial samples_per_eval based on a single estimated duration.
 Ticks est = measure_duration();
 static const double ticks_per_second = InvariantTicksPerSecond();
 const size_t ticks_per_eval = ticks_per_second * p.seconds_per_eval;
 size_t samples_per_eval = ticks_per_eval / est;
 samples_per_eval = (std::max)(samples_per_eval, p.min_samples_per_eval);

 std::vector<Ticks> samples;
 samples.reserve(1 + samples_per_eval);
 samples.push_back(est);

 // Percentage is too strict for tiny differences, so also allow a small
 // absolute "median absolute deviation".
 const Ticks max_abs_mad = (timer_resolution + 99) / 100;
 *rel_mad = 0.0;  // ensure initialized

 for (size_t eval = 0; eval < p.max_evals; ++eval, samples_per_eval *= 2) {
   samples.reserve(samples.size() + samples_per_eval);
   for (size_t i = 0; i < samples_per_eval; ++i) {
     const Ticks r = measure_duration();
     samples.push_back(r);
   }

   if (samples.size() >= p.min_mode_samples) {
     est = robust_statistics::Mode(samples.data(), samples.size());
   } else {
     // For "few" (depends also on the variance) samples, Median is safer.
     est = robust_statistics::Median(samples.data(), samples.size());
   }
   ABSL_RAW_CHECK(est != 0, "Estimator returned zero duration");

   // Median absolute deviation (mad) is a robust measure of 'variability'.
   const Ticks abs_mad = robust_statistics::MedianAbsoluteDeviation(
       samples.data(), samples.size(), est);
   *rel_mad = static_cast<double>(static_cast<int>(abs_mad)) / est;

   if (*rel_mad <= max_rel_mad || abs_mad <= max_abs_mad) {
     if (p.verbose) {
       ABSL_RAW_LOG(INFO,
                    "%6zu samples => %5u (abs_mad=%4u, rel_mad=%4.2f%%)\n",
                    samples.size(), est, abs_mad, *rel_mad * 100.0);
     }
     return est;
   }
 }

 if (p.verbose) {
   ABSL_RAW_LOG(WARNING,
                "rel_mad=%4.2f%% still exceeds %4.2f%% after %6zu samples.\n",
                *rel_mad * 100.0, max_rel_mad * 100.0, samples.size());
 }
 return est;
}

using InputVec = std::vector<FuncInput>;

// Returns vector of unique input values.
InputVec UniqueInputs(const FuncInput* inputs, const size_t num_inputs) {
 InputVec unique(inputs, inputs + num_inputs);
 std::sort(unique.begin(), unique.end());
 unique.erase(std::unique(unique.begin(), unique.end()), unique.end());
 return unique;
}

// Returns how often we need to call func for sufficient precision, or zero
// on failure (e.g. the elapsed time is too long for a 32-bit tick count).
size_t NumSkip(const Func func, const void* arg, const InputVec& unique,
              const Params& p) {
 // Min elapsed ticks for any input.
 Ticks min_duration = ~0u;

 for (const FuncInput input : unique) {
   // Make sure a 32-bit timer is sufficient.
   const uint64_t t0 = timer::Start64();
   PreventElision(func(arg, input));
   const uint64_t t1 = timer::Stop64();
   const uint64_t elapsed = t1 - t0;
   if (elapsed >= (1ULL << 30)) {
     ABSL_RAW_LOG(WARNING,
                  "Measurement failed: need 64-bit timer for input=%zu\n",
                  static_cast<size_t>(input));
     return 0;
   }

   double rel_mad;
   const Ticks total = SampleUntilStable(
       p.target_rel_mad, &rel_mad, p,
       [func, arg, input]() { PreventElision(func(arg, input)); });
   min_duration = (std::min)(min_duration, total - timer_resolution);
 }

 // Number of repetitions required to reach the target resolution.
 const size_t max_skip = p.precision_divisor;
 // Number of repetitions given the estimated duration.
 const size_t num_skip =
     min_duration == 0 ? 0 : (max_skip + min_duration - 1) / min_duration;
 if (p.verbose) {
   ABSL_RAW_LOG(INFO, "res=%u max_skip=%zu min_dur=%u num_skip=%zu\n",
                timer_resolution, max_skip, min_duration, num_skip);
 }
 return num_skip;
}

// Replicates inputs until we can omit "num_skip" occurrences of an input.
InputVec ReplicateInputs(const FuncInput* inputs, const size_t num_inputs,
                        const size_t num_unique, const size_t num_skip,
                        const Params& p) {
 InputVec full;
 if (num_unique == 1) {
   full.assign(p.subset_ratio * num_skip, inputs[0]);
   return full;
 }

 full.reserve(p.subset_ratio * num_skip * num_inputs);
 for (size_t i = 0; i < p.subset_ratio * num_skip; ++i) {
   full.insert(full.end(), inputs, inputs + num_inputs);
 }
 absl::random_internal::randen_engine<uint32_t> rng;
 std::shuffle(full.begin(), full.end(), rng);
 return full;
}

// Copies the "full" to "subset" in the same order, but with "num_skip"
// randomly selected occurrences of "input_to_skip" removed.
void FillSubset(const InputVec& full, const FuncInput input_to_skip,
               const size_t num_skip, InputVec* subset) {
 const size_t count = std::count(full.begin(), full.end(), input_to_skip);
 // Generate num_skip random indices: which occurrence to skip.
 std::vector<uint32_t> omit;
 // Replacement for std::iota, not yet available in MSVC builds.
 omit.reserve(count);
 for (size_t i = 0; i < count; ++i) {
   omit.push_back(i);
 }
 // omit[] is the same on every call, but that's OK because they identify the
 // Nth instance of input_to_skip, so the position within full[] differs.
 absl::random_internal::randen_engine<uint32_t> rng;
 std::shuffle(omit.begin(), omit.end(), rng);
 omit.resize(num_skip);
 std::sort(omit.begin(), omit.end());

 uint32_t occurrence = ~0u;  // 0 after preincrement
 size_t idx_omit = 0;        // cursor within omit[]
 size_t idx_subset = 0;      // cursor within *subset
 for (const FuncInput next : full) {
   if (next == input_to_skip) {
     ++occurrence;
     // Haven't removed enough already
     if (idx_omit < num_skip) {
       // This one is up for removal
       if (occurrence == omit[idx_omit]) {
         ++idx_omit;
         continue;
       }
     }
   }
   if (idx_subset < subset->size()) {
     (*subset)[idx_subset++] = next;
   }
 }
 ABSL_RAW_CHECK(idx_subset == subset->size(), "idx_subset not at end");
 ABSL_RAW_CHECK(idx_omit == omit.size(), "idx_omit not at end");
 ABSL_RAW_CHECK(occurrence == count - 1, "occurrence not at end");
}

// Returns total ticks elapsed for all inputs.
Ticks TotalDuration(const Func func, const void* arg, const InputVec* inputs,
                   const Params& p, double* max_rel_mad) {
 double rel_mad;
 const Ticks duration =
     SampleUntilStable(p.target_rel_mad, &rel_mad, p, [func, arg, inputs]() {
       for (const FuncInput input : *inputs) {
         PreventElision(func(arg, input));
       }
     });
 *max_rel_mad = (std::max)(*max_rel_mad, rel_mad);
 return duration;
}

// (Nearly) empty Func for measuring timer overhead/resolution.
ABSL_RANDOM_INTERNAL_ATTRIBUTE_NEVER_INLINE FuncOutput
EmptyFunc(const void* arg, const FuncInput input) {
 return input;
}

// Returns overhead of accessing inputs[] and calling a function; this will
// be deducted from future TotalDuration return values.
Ticks Overhead(const void* arg, const InputVec* inputs, const Params& p) {
 double rel_mad;
 // Zero tolerance because repeatability is crucial and EmptyFunc is fast.
 return SampleUntilStable(0.0, &rel_mad, p, [arg, inputs]() {
   for (const FuncInput input : *inputs) {
     PreventElision(EmptyFunc(arg, input));
   }
 });
}

}  // namespace

void PinThreadToCPU(int cpu) {
 // We might migrate to another CPU before pinning below, but at least cpu
 // will be one of the CPUs on which this thread ran.
#if defined(ABSL_OS_WIN)
 if (cpu < 0) {
   cpu = static_cast<int>(GetCurrentProcessorNumber());
   ABSL_RAW_CHECK(cpu >= 0, "PinThreadToCPU detect failed");
   if (cpu >= 64) {
     // NOTE: On wine, at least, GetCurrentProcessorNumber() sometimes returns
     // a value > 64, which is out of range. When this happens, log a message
     // and don't set a cpu affinity.
     ABSL_RAW_LOG(ERROR, "Invalid CPU number: %d", cpu);
     return;
   }
 } else if (cpu >= 64) {
   // User specified an explicit CPU affinity > the valid range.
   ABSL_RAW_LOG(FATAL, "Invalid CPU number: %d", cpu);
 }
 const DWORD_PTR prev = SetThreadAffinityMask(GetCurrentThread(), 1ULL << cpu);
 ABSL_RAW_CHECK(prev != 0, "SetAffinity failed");
#elif defined(ABSL_OS_LINUX) && !defined(ABSL_OS_ANDROID)
 if (cpu < 0) {
   cpu = sched_getcpu();
   ABSL_RAW_CHECK(cpu >= 0, "PinThreadToCPU detect failed");
 }
 const pid_t pid = 0;  // current thread
 cpu_set_t set;
 CPU_ZERO(&set);
 CPU_SET(cpu, &set);
 const int err = sched_setaffinity(pid, sizeof(set), &set);
 ABSL_RAW_CHECK(err == 0, "SetAffinity failed");
#endif
}

// Returns tick rate. Invariant means the tick counter frequency is independent
// of CPU throttling or sleep. May be expensive, caller should cache the result.
double InvariantTicksPerSecond() {
#if defined(ABSL_ARCH_PPC)
 return __ppc_get_timebase_freq();
#elif defined(ABSL_ARCH_X86_64)
 // We assume the TSC is invariant; it is on all recent Intel/AMD CPUs.
 return platform::NominalClockRate();
#else
 // Fall back to clock_gettime nanoseconds.
 return 1E9;
#endif
}

size_t MeasureImpl(const Func func, const void* arg, const size_t num_skip,
                  const InputVec& unique, const InputVec& full,
                  const Params& p, Result* results) {
 const float mul = 1.0f / static_cast<int>(num_skip);

 InputVec subset(full.size() - num_skip);
 const Ticks overhead = Overhead(arg, &full, p);
 const Ticks overhead_skip = Overhead(arg, &subset, p);
 if (overhead < overhead_skip) {
   ABSL_RAW_LOG(WARNING, "Measurement failed: overhead %u < %u\n", overhead,
                overhead_skip);
   return 0;
 }

 if (p.verbose) {
   ABSL_RAW_LOG(INFO, "#inputs=%5zu,%5zu overhead=%5u,%5u\n", full.size(),
                subset.size(), overhead, overhead_skip);
 }

 double max_rel_mad = 0.0;
 const Ticks total = TotalDuration(func, arg, &full, p, &max_rel_mad);

 for (size_t i = 0; i < unique.size(); ++i) {
   FillSubset(full, unique[i], num_skip, &subset);
   const Ticks total_skip = TotalDuration(func, arg, &subset, p, &max_rel_mad);

   if (total < total_skip) {
     ABSL_RAW_LOG(WARNING, "Measurement failed: total %u < %u\n", total,
                  total_skip);
     return 0;
   }

   const Ticks duration = (total - overhead) - (total_skip - overhead_skip);
   results[i].input = unique[i];
   results[i].ticks = duration * mul;
   results[i].variability = max_rel_mad;
 }

 return unique.size();
}

size_t Measure(const Func func, const void* arg, const FuncInput* inputs,
              const size_t num_inputs, Result* results, const Params& p) {
 ABSL_RAW_CHECK(num_inputs != 0, "No inputs");

 const InputVec unique = UniqueInputs(inputs, num_inputs);
 const size_t num_skip = NumSkip(func, arg, unique, p);  // never 0
 if (num_skip == 0) return 0;  // NumSkip already printed error message

 const InputVec full =
     ReplicateInputs(inputs, num_inputs, unique.size(), num_skip, p);

 // MeasureImpl may fail up to p.max_measure_retries times.
 for (size_t i = 0; i < p.max_measure_retries; i++) {
   auto result = MeasureImpl(func, arg, num_skip, unique, full, p, results);
   if (result != 0) {
     return result;
   }
 }
 // All retries failed. (Unusual)
 return 0;
}

}  // namespace random_internal_nanobenchmark
ABSL_NAMESPACE_END
}  // namespace absl