benchtests/bench-util.c

   1 /* Benchmark utility functions.
   2    Copyright (C) 2015-2020 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19
  20 static volatile unsigned int dontoptimize = 0;
  21
  22 void
  23 bench_start (void)
  24 {
  25   /* This loop should cause CPU to switch to maximal freqency.
  26      This makes subsequent measurement more accurate.  We need a side effect
  27      to prevent the loop being deleted by compiler.
  28      This should be enough to cause CPU to speed up and it is simpler than
  29      running loop for constant time.  This is used when user does not have root
  30      access to set a constant freqency.  */
  31
  32   for (int k = 0; k < START_ITER; k++)
  33     dontoptimize += 23 * dontoptimize + 2;
  34 }