From e8c2efb0fe23e8fa8d8e33ec85b01c04ec7cb505 Mon Sep 17 00:00:00 2001 From: Zihad Date: Sun, 23 Nov 2025 16:15:20 +0600 Subject: [PATCH] std.Io.Threaded: fix QueryPerformanceCounter usage Co-authored-by: Andrew Kelley --- lib/std/Io/Threaded.zig | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) diff --git a/lib/std/Io/Threaded.zig b/lib/std/Io/Threaded.zig index c4d9f1fe4618..0e5c12504b15 100644 --- a/lib/std/Io/Threaded.zig +++ b/lib/std/Io/Threaded.zig @@ -2956,7 +2956,22 @@ fn nowWindows(userdata: ?*anyopaque, clock: Io.Clock) Io.Clock.Error!Io.Timestam }, .awake, .boot => { // QPC on windows doesn't fail on >= XP/2000 and includes time suspended. - return .{ .nanoseconds = windows.QueryPerformanceCounter() }; + const qpc = windows.QueryPerformanceCounter(); + // We don't need to cache QPF as it's internally just a memory read to KUSER_SHARED_DATA + // (a read-only page of info updated and mapped by the kernel to all processes): + // https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntddk/ns-ntddk-kuser_shared_data + // https://www.geoffchappell.com/studies/windows/km/ntoskrnl/inc/api/ntexapi_x/kuser_shared_data/index.htm + const qpf = windows.QueryPerformanceFrequency(); + + // 10Mhz (1 qpc tick every 100ns) is a common enough QPF value that we can optimize on it. + // https://github.com/microsoft/STL/blob/785143a0c73f030238ef618890fd4d6ae2b3a3a0/stl/inc/chrono#L694-L701 + const common_qpf = 10_000_000; + if (qpf == common_qpf) return .{ .nanoseconds = qpc * (std.time.ns_per_s / common_qpf) }; + + // Convert to ns using fixed point. + const scale = @as(u64, std.time.ns_per_s << 32) / @as(u32, @intCast(qpf)); + const result = (@as(u96, qpc) * scale) >> 32; + return .{ .nanoseconds = @intCast(result) }; }, .cpu_process, .cpu_thread,