sysdeps: Add 'get_fast_jitter' interace in fast-jitter.h

'get_fast_jitter' is meant to be used purely for performance
purposes. In all cases it's used it should be acceptable to get no
randomness (see default case). An example use case is in setting
jitter for retries between threads at a lock. There is a
performance benefit to having jitter, but only if the jitter can
be generated very quickly and ultimately there is no serious issue
if no jitter is generated.

The implementation generally uses 'HP_TIMING_NOW' iff it is
inlined (avoid any potential syscall paths).
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

sysdeps/generic/fast-jitter.h

@@ -0,0 +1,42 @@
+/* Fallback for fast jitter just return 0.
+   Copyright (C) 2019-2022 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _FAST_JITTER_H
+# define _FAST_JITTER_H
+
+# include <stdint.h>
+# include <hp-timing.h>
+
+/* Baseline just return 0.  We could create jitter using a clock or
+   'random_bits' but that may imply a syscall and the goal of
+   'get_fast_jitter' is minimal overhead "randomness" when such
+   randomness helps performance.  Adding high overhead the function
+   defeats the purpose.  */
+static inline uint32_t
+get_fast_jitter (void)
+{
+# if HP_TIMING_INLINE
+  hp_timing_t jitter;
+  HP_TIMING_NOW (jitter);
+  return (uint32_t) jitter;
+# else
+  return 0;
+# endif
+}
+
+#endif