[tor-commits] [tor/master] Use a slightly more accurate formula for OSX 32-bit msec conversion

[nickm at torproject.org]

commit 4eabc6db47fe64b6757f7a5f0e651a41f02efca3
Author: Nick Mathewson <nickm at torproject.org>
Date:   Fri Sep 14 08:30:24 2018 -0400

    Use a slightly more accurate formula for OSX 32-bit msec conversion
    
    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.
---
 src/common/compat_time.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/common/compat_time.c b/src/common/compat_time.c
index 40847a844..f92dc09c4 100644
--- a/src/common/compat_time.c
+++ b/src/common/compat_time.c
@@ -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;
   }
 }