chromium/third_party/ruy/src/ruy/tune.h

/* Copyright 2019 Google LLC. All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License");
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==============================================================================*/

// Library doing minimal CPU detection to decide what to tune asm code for.
//
// # Tuning vs Path
//
// Tunings are merely local variations of optimized code paths, that are
// drop-in replacements for each other --- the input and output data layouts
// are identical.  By contrast, what ruy calls a Path dictates its own
// data layouts. For example, Path::kNeonDotprod will use different
// layouts compared to Path::kNeon; but within each, different tunings
// will share that same layout.
//
// # Tuning is for now only based on 1 bit: Generic / A55ish
//
// In practice, each of our asm code paths only needs one bit information to
// decide on tuning: whether the CPU is out-of-order or in-order.
// That is because out-of-order CPUs are by definition relatively insensitive
// to small-scale asm details (which is what "tuning" is about); and for each
// asm code path, there tends to be one main in-order CPU architecture that
// we focus our tuning effort on. Examples:
//  * For Path::kNeon, the main in-order CPU is Cortex-A53/A55 (pre-dotprod)
//  * For Path::kNeonDotprod, the main in-order CPU is Cortex-A55r1 (dotprod)
//
// Because having tuned code paths is a compromise of efficiency gains
// versus implementation effort and code size, we are happy to stop at just this
// single bit of information, Generic / A55ish, at least in the current CPU
// landscape. This could change in the future.
#ifndef RUY_RUY_TUNE_H_
#define RUY_RUY_TUNE_H_

#include "ruy/cpuinfo.h"
#include "ruy/opt_set.h"
#include "ruy/platform.h"
#include "ruy/time.h"

namespace ruy {

enum class Tuning { … };

// Why a TuningResolver class?
//
// Ideally, this Library would offer a single function,
//   Tuning GetCurrentCPUTuning();
//
// However, determining information about the current CPU is not necessarily
// cheap, so we currently cache that and only invalidate/reevaluate after
// a fixed amount of time. This need to store state is why this library
// has to expose a class, TuningResolver, not just a function.
class TuningResolver { … };

}  // namespace ruy

#endif  // RUY_RUY_TUNE_H_