tsc.go

package hrtime

import (
	"sync"
	"time"
)

// Count represents represents Time Stamp Counter value, when available.
//
// Count doesn't depend on power throttling making it useful for benchmarking.
// However it is not reliably convertible to a reasonable time-value.
type Count int64

var calibrateOnce sync.Once

// ApproxDuration returns approximate conversion into a Duration.
//
// First call to this function will do calibration and can take several milliseconds.
func (count Count) ApproxDuration() time.Duration {
	calibrateOnce.Do(calculateTSCConversion)
	return time.Duration(count) * ratioNano / time.Duration(ratioCount)
}

// TSC reads the current Time Stamp Counter value.
//
// Reminder: Time Stamp Count are processor specific and need to be converted to
// time.Duration with Count.ApproxDuration.
func TSC() Count { return Count(RDTSC()) }

// TSCSince returns count since start.
//
// Reminder: Count is processor specific and need to be converted to
// time.Duration with Count.ApproxDuration.
func TSCSince(start Count) Count { return TSC() - start }

// TSCSupported returns whether processor supports giving invariant time stamp counter values
func TSCSupported() bool { return rdtscpInvariant }

// TSCOverhead returns overhead of Count call
func TSCOverhead() Count { return readTSCOverhead }

var (
	rdtscpInvariant = false
	readTSCOverhead Count

	cpuid func(op1, op2 uint32) (eax, ebx, ecx, edx uint32)

	ratioNano  time.Duration
	ratioCount Count
)

func calculateTSCOverhead() {
	if !rdtscpInvariant {
		return
	}

	start := TSC()
	for i := 0; i < calibrationCalls; i++ {
		TSC()
	}
	stop := TSC()

	readTSCOverhead = (stop - start) / (calibrationCalls + 1)
}

func calculateTSCConversion() {
	// warmup
	for i := 0; i < 64*calibrationCalls; i++ {
		empty()
	}

	nanostart := Now()
	countstart := TSC()
	for i := 0; i < 64*calibrationCalls; i++ {
		empty()
	}
	nanoend := Now()
	countstop := TSC()

	// TODO: figure out a better way to calculate this
	ratioNano = nanoend - nanostart - Overhead()
	ratioCount = countstop - countstart - TSCOverhead()
}

//go:noinline
func empty() {}