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commit 04efdcfac405723c23b25d124817bcfc1697e2d8
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Author: Noah Goldstein <goldstein.w.n@gmail.com>
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Date: Wed Apr 27 15:13:02 2022 -0500
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sysdeps: Add 'get_fast_jitter' interace in fast-jitter.h
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'get_fast_jitter' is meant to be used purely for performance
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purposes. In all cases it's used it should be acceptable to get no
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randomness (see default case). An example use case is in setting
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jitter for retries between threads at a lock. There is a
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performance benefit to having jitter, but only if the jitter can
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be generated very quickly and ultimately there is no serious issue
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if no jitter is generated.
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The implementation generally uses 'HP_TIMING_NOW' iff it is
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inlined (avoid any potential syscall paths).
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Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
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(cherry picked from commit 911c63a51c690dd1a97dfc587097277029baf00f)
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diff --git a/sysdeps/generic/fast-jitter.h b/sysdeps/generic/fast-jitter.h
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new file mode 100644
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index 0000000000000000..4dd53e3475c3dfe6
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--- /dev/null
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+++ b/sysdeps/generic/fast-jitter.h
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@@ -0,0 +1,42 @@
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+/* Fallback for fast jitter just return 0.
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+ Copyright (C) 2019-2022 Free Software Foundation, Inc.
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+ This file is part of the GNU C Library.
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+
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+ The GNU C Library is free software; you can redistribute it and/or
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+ modify it under the terms of the GNU Lesser General Public
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+ License as published by the Free Software Foundation; either
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+ version 2.1 of the License, or (at your option) any later version.
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+
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+ The GNU C Library is distributed in the hope that it will be useful,
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+ but WITHOUT ANY WARRANTY; without even the implied warranty of
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+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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+ Lesser General Public License for more details.
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+
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+ You should have received a copy of the GNU Lesser General Public
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+ License along with the GNU C Library; if not, see
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+ <https://www.gnu.org/licenses/>. */
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+
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+#ifndef _FAST_JITTER_H
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+# define _FAST_JITTER_H
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+
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+# include <stdint.h>
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+# include <hp-timing.h>
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+
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+/* Baseline just return 0. We could create jitter using a clock or
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+ 'random_bits' but that may imply a syscall and the goal of
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+ 'get_fast_jitter' is minimal overhead "randomness" when such
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+ randomness helps performance. Adding high overhead the function
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+ defeats the purpose. */
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+static inline uint32_t
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+get_fast_jitter (void)
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+{
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+# if HP_TIMING_INLINE
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+ hp_timing_t jitter;
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+ HP_TIMING_NOW (jitter);
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+ return (uint32_t) jitter;
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+# else
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+ return 0;
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+# endif
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+}
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+
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+#endif
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