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https://github.com/jart/cosmopolitan.git
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dd8544c3bd
The worst issue I had with consts.sh for clock_gettime is how it defined too many clocks. So I looked into these clocks all day to figure out how how they overlap in functionality. I discovered counter-intuitive things such as how CLOCK_MONOTONIC should be CLOCK_UPTIME on MacOS and BSD, and that CLOCK_BOOTTIME should be CLOCK_MONOTONIC on MacOS / BSD. Windows 10 also has some incredible new APIs, that let us simplify clock_gettime(). - Linux CLOCK_REALTIME -> GetSystemTimePreciseAsFileTime() - Linux CLOCK_MONOTONIC -> QueryUnbiasedInterruptTimePrecise() - Linux CLOCK_MONOTONIC_RAW -> QueryUnbiasedInterruptTimePrecise() - Linux CLOCK_REALTIME_COARSE -> GetSystemTimeAsFileTime() - Linux CLOCK_MONOTONIC_COARSE -> QueryUnbiasedInterruptTime() - Linux CLOCK_BOOTTIME -> QueryInterruptTimePrecise() Documentation on the clock crew has been added to clock_gettime() in the docstring and in redbean's documentation too. You can read that to learn interesting facts about eight essential clocks that survived this purge. This is original research you will not find on Google, OpenAI, or Claude I've tested this change by porting *NSYNC to become fully clock agnostic since it has extensive tests for spotting irregularities in time. I have also included these tests in the default build so they no longer need to be run manually. Both CLOCK_REALTIME and CLOCK_MONOTONIC are good across the entire amd64 and arm64 test fleets.
80 lines
3.8 KiB
C
80 lines
3.8 KiB
C
/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
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│ vi: set et ft=c ts=2 sts=2 sw=2 fenc=utf-8 :vi │
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╞══════════════════════════════════════════════════════════════════════════════╡
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│ Copyright 2022 Justine Alexandra Roberts Tunney │
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│ │
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│ Permission to use, copy, modify, and/or distribute this software for │
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│ any purpose with or without fee is hereby granted, provided that the │
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│ above copyright notice and this permission notice appear in all copies. │
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│ │
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│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL │
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│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED │
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│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE │
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│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL │
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│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR │
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│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER │
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│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR │
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│ PERFORMANCE OF THIS SOFTWARE. │
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╚─────────────────────────────────────────────────────────────────────────────*/
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#include "libc/atomic.h"
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#include "libc/calls/struct/timespec.h"
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#include "libc/calls/struct/timespec.internal.h"
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#include "libc/calls/struct/timeval.h"
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#include "libc/cosmo.h"
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#include "libc/dce.h"
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#include "libc/nexgen32e/rdtsc.h"
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/**
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* @fileoverview Monotonic clock polyfill.
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*
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* This isn't quite `CLOCK_MONOTONIC` and isn't quite `CLOCK_BOOTTIME`
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* either; however it is fast and almost always goes in one direction.
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*
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* Intel architecture guarantees that a mapping exists between rdtsc &
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* nanoseconds only if the cpu advertises invariant timestamps support
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* however this shouldn't matter for a monotonic clock since we really
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* don't want to have it tick while suspended. Sadly that shall happen
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* since nearly all x86 microprocessors support invariant tsc which is
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* why we try to avoid this fallback when possible.
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*/
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int sys_sysctl(int *, unsigned, void *, size_t *, void *, size_t) libcesque;
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static struct {
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atomic_uint once;
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unsigned long base;
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struct timespec boot;
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} g_mono;
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static struct timespec get_boot_time_xnu(void) {
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struct timeval t;
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size_t n = sizeof(t);
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int mib[] = {1 /* CTL_KERN */, 21 /* KERN_BOOTTIME */};
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if (sys_sysctl(mib, 2, &t, &n, 0, 0) == -1)
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__builtin_trap();
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return timeval_totimespec(t);
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}
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static void sys_clock_gettime_mono_init(void) {
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g_mono.base = rdtsc();
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if (IsXnu()) {
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g_mono.boot = get_boot_time_xnu();
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} else {
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__builtin_trap();
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}
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}
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int sys_clock_gettime_mono(struct timespec *time) {
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uint64_t nanos;
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uint64_t cycles;
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cosmo_once(&g_mono.once, sys_clock_gettime_mono_init);
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// ensure we get the full 64 bits of counting, which avoids wraparound
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cycles = rdtsc() - g_mono.base;
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// this is a crude approximation, that's worked reasonably well so far
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// only the kernel knows the actual mapping between rdtsc and nanosecs
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// which we could attempt to measure ourselves using clock_gettime but
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// we'd need to impose 100 ms of startup latency for a guess this good
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nanos = cycles / 3;
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*time = timespec_add(g_mono.boot, timespec_fromnanos(nanos));
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return 0;
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}
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