Optimize clock_gettime.

src/backend/linux_raw/vdso_wrappers.rs

@@ -45,6 +45,10 @@ pub(crate) fn clock_gettime(which_clock: ClockId) -> __kernel_timespec {
             None => init_clock_gettime(),
         };
         let r0 = callee(which_clock as c::c_int, result.as_mut_ptr());
+        // The `ClockId` enum only contains clocks which never fail. It may be
+        // tempting to change this to `debug_assert_eq`, however they can still
+        // fail on uncommon kernel configs, so we leave this in place to ensure
+        // that we don't execute UB if they ever do fail.
         assert_eq!(r0, 0);
         result.assume_init()
     }
@@ -227,6 +231,7 @@ pub(super) type SyscallType = unsafe extern "C" fn();
 
 /// Initialize `CLOCK_GETTIME` and return its value.
 #[cfg(feature = "time")]
+#[cold]
 fn init_clock_gettime() -> ClockGettimeType {
     init();
     // SAFETY: Load the function address from static storage that we
@@ -236,6 +241,7 @@ fn init_clock_gettime() -> ClockGettimeType {
 
 /// Initialize `SYSCALL` and return its value.
 #[cfg(target_arch = "x86")]
+#[cold]
 fn init_syscall() -> SyscallType {
     init();
     // SAFETY: Load the function address from static storage that we

tests/time/clocks.rs

@@ -0,0 +1,85 @@
+//! Test all the `ClockId` clocks, which should never fail.
+
+#![cfg(not(any(apple, target_os = "wasi")))]
+
+#[cfg(not(any(solarish, target_os = "netbsd", target_os = "redox")))]
+use rustix::time::{clock_gettime, ClockId};
+
+/// Attempt to test that the uptime clock is monotonic. Time may or may not
+/// advance, but it shouldn't regress.
+#[cfg(any(freebsdlike, target_os = "openbsd"))]
+#[test]
+fn test_uptime_clock() {
+    let a = clock_gettime(ClockId::Uptime);
+    let b = clock_gettime(ClockId::Uptime);
+    if b.tv_sec == a.tv_sec {
+        assert!(b.tv_nsec >= a.tv_nsec);
+    } else {
+        assert!(b.tv_sec > a.tv_sec);
+    }
+}
+
+/// Attempt to test that the process CPU-time clock is monotonic. Time may or
+/// may not advance, but it shouldn't regress.
+#[cfg(not(any(solarish, target_os = "netbsd", target_os = "redox")))]
+#[test]
+fn test_process_cputime_clock() {
+    let a = clock_gettime(ClockId::ProcessCPUTime);
+    let b = clock_gettime(ClockId::ProcessCPUTime);
+    if b.tv_sec == a.tv_sec {
+        assert!(b.tv_nsec >= a.tv_nsec);
+    } else {
+        assert!(b.tv_sec > a.tv_sec);
+    }
+}
+
+/// Attempt to test that the thread CPU-time clock is monotonic. Time may or
+/// may not advance, but it shouldn't regress.
+#[cfg(not(any(solarish, target_os = "netbsd", target_os = "redox")))]
+#[test]
+fn test_thread_cputime_clock() {
+    let a = clock_gettime(ClockId::ThreadCPUTime);
+    let b = clock_gettime(ClockId::ThreadCPUTime);
+    if b.tv_sec == a.tv_sec {
+        assert!(b.tv_nsec >= a.tv_nsec);
+    } else {
+        assert!(b.tv_sec > a.tv_sec);
+    }
+}
+
+#[cfg(any(linux_kernel, target_os = "freebsd"))]
+#[test]
+fn test_realtime_coarse_clock() {
+    let a = clock_gettime(ClockId::RealtimeCoarse);
+
+    // Test that the timespec is valid; there's not much else we can say.
+    assert!(a.tv_nsec < 1_000_000_000);
+}
+
+/// Attempt to test that the coarse monotonic clock is monotonic. Time may or
+/// may not advance, but it shouldn't regress.
+#[cfg(any(linux_kernel, target_os = "freebsd"))]
+#[test]
+fn test_monotonic_coarse_clock() {
+    let a = clock_gettime(ClockId::MonotonicCoarse);
+    let b = clock_gettime(ClockId::MonotonicCoarse);
+    if b.tv_sec == a.tv_sec {
+        assert!(b.tv_nsec >= a.tv_nsec);
+    } else {
+        assert!(b.tv_sec > a.tv_sec);
+    }
+}
+
+/// Attempt to test that the raw monotonic clock is monotonic. Time may or
+/// may not advance, but it shouldn't regress.
+#[cfg(linux_kernel)]
+#[test]
+fn test_monotonic_raw_clock() {
+    let a = clock_gettime(ClockId::MonotonicRaw);
+    let b = clock_gettime(ClockId::MonotonicRaw);
+    if b.tv_sec == a.tv_sec {
+        assert!(b.tv_nsec >= a.tv_nsec);
+    } else {
+        assert!(b.tv_sec > a.tv_sec);
+    }
+}

tests/time/main.rs

@@ -3,6 +3,7 @@
 #![cfg(feature = "time")]
 #![cfg(not(any(windows, target_os = "espidf")))]
 
+mod clocks;
 #[cfg(not(any(target_os = "redox", target_os = "wasi")))]
 mod dynamic_clocks;
 #[cfg(not(any(target_os = "redox", target_os = "wasi")))]