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starlark: profiler: stop using runtime.nanotime via linkname hack #552

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May 20, 2024
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starlark: profiler: stop using runtime.nanotime via linkname hack #552

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41 changes: 25 additions & 16 deletions starlark/profile.go
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Original file line number Diff line number Diff line change
Expand Up @@ -357,28 +357,37 @@ func profile(w io.Writer) {
profiler.done <- err
}

// nanotime returns the time in nanoseconds since epoch.
// It is implemented by runtime.nanotime using the linkname hack;
// runtime.nanotime is defined for all OSs/ARCHS and uses the
// monotonic system clock, which there is no portable way to access.
// Should that function ever go away, these alternatives exist:
// nanotime returns the time in nanoseconds since process start.
//
// // POSIX only. REALTIME not MONOTONIC. 17ns.
// var tv syscall.Timeval
// syscall.Gettimeofday(&tv) // can't fail
// return tv.Nano()
// This approach, described at
// https://github.com/golang/go/issues/61765#issuecomment-1672090302,
// is fast, monotonic, and portable, and avoids the previous
// dependence on runtime.nanotime using the (unsafe) linkname hack.
// In particular, time.Since does less work than time.Now.
//
// // Portable. REALTIME not MONOTONIC. 46ns.
// return time.Now().Nanoseconds()
// Rejected approaches:
//
// // POSIX only. Adds a dependency.
// Using the linkname hack to unsafely access runtime.nanotime.
// See #546 and golang/go#67401.
//
// // POSIX only. REALTIME not MONOTONIC. 17ns.
// var tv syscall.Timeval
// syscall.Gettimeofday(&tv) // can't fail
// return tv.Nano()
//
// // Portable. REALTIME not MONOTONIC. 46ns.
// return time.Now().Nanoseconds()
//
// // POSIX only. Adds a dependency.
// import "golang.org/x/sys/unix"
// var ts unix.Timespec
// unix.ClockGettime(CLOCK_MONOTONIC, &ts) // can't fail
// unix.ClockGettime(CLOCK_MONOTONIC, &ts) // can't fail
// return unix.TimespecToNsec(ts)
//
//go:linkname nanotime runtime.nanotime
func nanotime() int64
func nanotime() int64 {
return time.Since(processStart).Nanoseconds()
}

var processStart = time.Now()

// profFuncAddr returns the canonical "address"
// of a Callable for use by the profiler.
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