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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: OutOfOrder / InOrder
// 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, OutOfOrder/InOrder, at least in the current CPU
// landscape. This could change in the future.
// # Implementation notes and alternatives.
// The current implementation uses a nano-benchmark, see
// That is why it's quite expensive, making caching /
// statefulness necessary (see TuningResolver class comment).
// An interesting alternative, which was explained to us by Marat Dukhan
// (maratek@) after this was implemented, would be to use the
// getcpu(2) system call on Linux. This returns a
// numeric CPU identifier that could be mapped to a OutOfOrder/InOrder
// classification given additional information about the CPU. Such
// additional information could be obtained by the cpuinfo library,
// which obtains this information mainly from parsing /proc/cpuinfo.
// Pros:
// * Would remove the need for the relatively expensive nano-benchmark
// (dozens of microseconds, which have to be reevaluated again several
// times per second).
// * Would conceivably be more reliable.
// Cons:
// * Linux-specific.
// * Modest binary size increase (Marat mentioned the cpuinfo lib is 20k).
// * Won't support exactly 100% of devices (nonstandard /proc/cpuinfo etc).
// We could also have both:
// * Maybe by trying getcpu first if supported, then falling back to a
// nano-benchmark.
// * Maybe using getcpu in conjunction with the nano-benchmark to cache
// per-CPU-id nano-benchmark results.
#ifndef RUY_RUY_TUNE_H_
#define RUY_RUY_TUNE_H_
#include "ruy/opt_set.h"
#include "ruy/platform.h"
#include "ruy/time.h"
// Tuning only implemented on NEON_64 at the moment (see assembly code
// in the nano-benchmark) and not on Apple (some Apple CPUs produce incorrect
// results on in-order-tuned kernels combining ARM and NEON load instructions
// and NEON `ins` instructions).
// When tuning is not implemented, we simply always use Tuning::kOutOfOrder.
namespace ruy {
enum class Tuning {
// kAuto means please use auto-detection. It's the default in the
// user-visible parts (see Context). It's meant to be resolved to an
// actual tuning at some point by means of TuningResolver.
// Target an out-order CPU. Example: ARM Cortex-A75.
// Target an in-order CPU. Example: ARM Cortex-A55.
// 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 {
// Allows the user to specify an explicit Tuning value, bypassing auto
// detection; or to specify Tuning::kAuto, reverting to auto detection.
void SetTuning(Tuning tuning) { unresolved_tuning_ = tuning; }
// Get an actual tuning --- that is the function that this class wanted to be.
Tuning Resolve();
TuningResolver(const TuningResolver&) = delete;
// TuningTool is a demo/tool used to tweak the tuning implementation to
// specific devices. It needs to access some finer granularity information
// than just the Tuning returned by Resolve. Nothing else should need
// access to that.
friend class TuneTool;
// Actually runs a nano-benchmark, producing a real number called 'ratio'
// whose meaning is generally opaque / implementation defined. Typically,
// this would be the ratio between the latencies of two different
// pieces of asm code differing only by the ordering of instructions,
// revealing whether the CPU cares about such ordering details.
// An implementation may just return a dummy value if it is not based on
// such nanobenchmarking / ratio evaluation.
float EvalRatio();
// Empirically determined threshold on ratio values delineating
// out-of-order (ratios closer to 1) from in-order (ratios farther from 1).
// An implementation may just return a dummy value if it is not based on
// such nanobenchmarking / ratio evaluation.
float ThresholdRatio();
// Perform the tuning resolution now. That may typically use EvalRatio and
// ThresholdRatio, but an implementation may use a different approach instead.
Tuning ResolveNow();
// The tuning as specified by the user, before actual resolution happens
// i.e. before querying any specifics of the current CPU.
// The default value kAuto means try to auto-detect. Other values mean
// bypass auto-detect, use explicit value instead. See SetTuning().
Tuning unresolved_tuning_ = Tuning::kAuto;
// Cached last resolved tuning.
Tuning last_resolved_tuning_ = Tuning::kAuto;
// Timepoint of cached last resolved tuning, for invalidation purposes.
TimePoint last_resolved_timepoint_;
// Cached last resolved tunings that are older than this age are invalid.
const Duration expiry_duration_;
} // namespace ruy
#endif // RUY_RUY_TUNE_H_