tune.cc

/* Copyright 2019 Google LLC. 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

    http://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 "tune.h"

#include <algorithm>
#include <cstdint>

namespace ruy {

#ifdef RUY_IMPLEMENT_TUNING

namespace {

void PoorlyOrderedKernel(int iters) {
  asm volatile(
      "mov w0, %w[iters]\n"
      "1:\n"
      "subs w0, w0, #1\n"
      "mul v0.4s, v0.4s, v0.4s\n"
      "mul v0.4s, v0.4s, v0.4s\n"
      "mul v0.4s, v0.4s, v0.4s\n"
      "mul v0.4s, v0.4s, v0.4s\n"
      "mul v1.4s, v1.4s, v1.4s\n"
      "mul v1.4s, v1.4s, v1.4s\n"
      "mul v1.4s, v1.4s, v1.4s\n"
      "mul v1.4s, v1.4s, v1.4s\n"
      "mul v2.4s, v2.4s, v2.4s\n"
      "mul v2.4s, v2.4s, v2.4s\n"
      "mul v2.4s, v2.4s, v2.4s\n"
      "mul v2.4s, v2.4s, v2.4s\n"
      "mul v3.4s, v3.4s, v3.4s\n"
      "mul v3.4s, v3.4s, v3.4s\n"
      "mul v3.4s, v3.4s, v3.4s\n"
      "mul v3.4s, v3.4s, v3.4s\n"
      "bne 1b\n" ::[iters] "r"(iters)
      : "cc", "x0", "v0", "v1", "v2", "v3");
}

void NicelyOrderedKernel(int iters) {
  asm volatile(
      "mov w0, %w[iters]\n"
      "1:\n"
      "subs w0, w0, #1\n"
      "mul v0.4s, v0.4s, v0.4s\n"
      "mul v1.4s, v1.4s, v1.4s\n"
      "mul v2.4s, v2.4s, v2.4s\n"
      "mul v3.4s, v3.4s, v3.4s\n"
      "mul v0.4s, v0.4s, v0.4s\n"
      "mul v1.4s, v1.4s, v1.4s\n"
      "mul v2.4s, v2.4s, v2.4s\n"
      "mul v3.4s, v3.4s, v3.4s\n"
      "mul v0.4s, v0.4s, v0.4s\n"
      "mul v1.4s, v1.4s, v1.4s\n"
      "mul v2.4s, v2.4s, v2.4s\n"
      "mul v3.4s, v3.4s, v3.4s\n"
      "mul v0.4s, v0.4s, v0.4s\n"
      "mul v1.4s, v1.4s, v1.4s\n"
      "mul v2.4s, v2.4s, v2.4s\n"
      "mul v3.4s, v3.4s, v3.4s\n"
      "bne 1b\n" ::[iters] "r"(iters)
      : "cc", "x0", "v0", "v1", "v2", "v3");
}

}  // namespace

float TuningResolver::EvalRatio() {
  // With the current settings, 400 iterations and 4 repeats, this test has
  // a latency of roughly 80 microseconds on a Cortex-A53 at 1.4 GHz.
  static constexpr int kLoopIters = 400;
  static constexpr int kRepeats = 4;

  Duration timing_poorly_ordered = Duration::max();
  Duration timing_nicely_ordered = Duration::max();

  for (int r = 0; r < kRepeats; r++) {
    TimePoint t0 = Now();
    PoorlyOrderedKernel(kLoopIters);
    TimePoint t1 = Now();
    NicelyOrderedKernel(kLoopIters);
    TimePoint t2 = Now();
    timing_poorly_ordered = std::min(timing_poorly_ordered, t1 - t0);
    timing_nicely_ordered = std::min(timing_nicely_ordered, t2 - t1);
  }

  return ToFloatSeconds(timing_nicely_ordered) /
         ToFloatSeconds(timing_poorly_ordered);
}

float TuningResolver::ThresholdRatio() {
  // Empirically (see :tune_tool) determined threshold to distinguish in-order
  // Cortex-A53/A55 cores from out-of-order Cortex-A57/A73/A75/A76 cores. Based
  // on these experimental results, which were obtained with much lower
  // (kLoopIters=1000, kRepeats=1) so as to make them resilient to noise, we
  // have:
  //
  // CPU core type | in/out of order | observed ratio
  // --------------+-----------------+-----------------------------------------
  // Cortex-A53    | in-order        | 0.32 -- 0.329
  // Cortex-A55    | in-order        | 0.319 -- 0.325
  // Cortex-A55r1  | in-order        | 0.319 -- 0.325
  // Cortex-A57    | out-of-order    | 0.99 -- 1.01
  // Cortex-A73    | out-of-order    | 0.922 -- 0.927
  // Cortex-A75    | out-of-order    | 0.921 -- 0.93
  // Cortex-A76    | out-of-order    | 1
  // Kryo (pixel1) | out-of-order    | 0.73 -- 0.76
  //
  // Thus the allowable range for the threshold is [0.35 .. 0.70].
  // We pick a value closer to the upper bound because really any out-of-order
  // CPU should by definition produce a ratio close to 1.
  return 0.65f;
}

Tuning TuningResolver::ResolveNow() {
  const bool is_probably_inorder = EvalRatio() < ThresholdRatio();
  return is_probably_inorder ? Tuning::kInOrder : Tuning::kOutOfOrder;
}

#else  // not defined RUY_IMPLEMENT_TUNING

float TuningResolver::EvalRatio() { return 0; }
float TuningResolver::ThresholdRatio() { return 0; }

Tuning TuningResolver::ResolveNow() { return Tuning::kOutOfOrder; }

#endif

TuningResolver::TuningResolver()
    : expiry_duration_(DurationFromMilliseconds(250)) {}

Tuning TuningResolver::Resolve() {
#ifdef RUY_IMPLEMENT_TUNING
  if (unresolved_tuning_ != Tuning::kAuto) {
    return unresolved_tuning_;
  }
  TimePoint new_timepoint = CoarseNow();
  if (last_resolved_tuning_ != Tuning::kAuto &&
      (new_timepoint - last_resolved_timepoint_) < expiry_duration_) {
    return last_resolved_tuning_;
  }
  last_resolved_timepoint_ = new_timepoint;
  last_resolved_tuning_ = ResolveNow();
  return last_resolved_tuning_;
#else
  return Tuning::kOutOfOrder;
#endif
}

}  // namespace ruy