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Use a slightly more accurate formula for OSX 32-bit msec conversion

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We use an optimized but less accurate formula for converting coarse
time differences to milliseconds on 32-bit OSX platforms, so that we
can avoid 64-bit division.

The old numbers were off by 0.4%.  The new numbers are off by .006%.

This should make the unit tests a bit cleaner, and our tolerances a
bit closer.
This commit is contained in:
Nick Mathewson 2018-09-14 08:30:24 -04:00
parent 3ddfd5ff25
commit 4eabc6db47
1 changed files with 4 additions and 4 deletions
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8
src/common/compat_time.c
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@ -299,11 +299,11 @@ monotime_init_internal(void)
}
{
// For converting ticks to milliseconds in a 32-bit-friendly way, we
// will first right-shift by 20, and then multiply by 20/19, since
// (1<<20) * 19/20 is about 1e6. We precompute a new numerate and
// will first right-shift by 20, and then multiply by 2048/1953, since
// (1<<20) * 1953/2048 is about 1e6. We precompute a new numerator and
// denominator here to avoid multiple multiplies.
mach_time_info_msec_cvt.numer = mach_time_info.numer * 20;
mach_time_info_msec_cvt.denom = mach_time_info.denom * 19;
mach_time_info_msec_cvt.numer = mach_time_info.numer * 2048;
mach_time_info_msec_cvt.denom = mach_time_info.denom * 1953;
}
}