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std.time.Timer.lap: only read system time once (#4533)

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Calling Timer.lap queried the system time twice; once to compute the lap
time and once to reset the timer. This can lead to time discrepancies
between actual and computed durations when summing the result of
Timer.lap in a loop. This commit fixes that.

also fix Timer.read to not require a pointer
This commit is contained in:
Heide Onas Auri 2020-02-24 00:25:52 +01:00 committed by GitHub
parent cfe1fbad0f
commit 907c5589ae
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GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 13 additions and 9 deletions
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22
lib/std/time.zig
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@ -153,15 +153,9 @@ pub const Timer = struct {
}
/// Reads the timer value since start or the last reset in nanoseconds
pub fn read(self: *Timer) u64 {
pub fn read(self: Timer) u64 {
var clock = clockNative() - self.start_time;
if (builtin.os == .windows) {
return @divFloor(clock * ns_per_s, self.frequency);
}
if (comptime std.Target.current.isDarwin()) {
return @divFloor(clock * self.frequency.numer, self.frequency.denom);
}
return clock;
return self.nativeDurationToNanos(clock);
}
/// Resets the timer value to 0/now.
@ -172,7 +166,7 @@ pub const Timer = struct {
/// Returns the current value of the timer in nanoseconds, then resets it
pub fn lap(self: *Timer) u64 {
var now = clockNative();
var lap_time = self.read();
var lap_time = self.nativeDurationToNanos(now - self.start_time);
self.start_time = now;
return lap_time;
}
@ -188,6 +182,16 @@ pub const Timer = struct {
os.clock_gettime(monotonic_clock_id, &ts) catch unreachable;
return @intCast(u64, ts.tv_sec) * @as(u64, ns_per_s) + @intCast(u64, ts.tv_nsec);
}
fn nativeDurationToNanos(self: Timer, duration: u64) u64 {
if (builtin.os == .windows) {
return @divFloor(duration * ns_per_s, self.frequency);
}
if (comptime std.Target.current.isDarwin()) {
return @divFloor(duration * self.frequency.numer, self.frequency.denom);
}
return duration;
}
};
test "sleep" {