[OpenSmalltalk/opensmalltalk-vm] primitives 54 and 554 for timesTwoPower: answer incorrect result on 64bits macOS VM (#383) - Vm-dev

26 Mar 2019


      This is due to a macOS bug in 64bits libm! (Hard to believe, isn't it?)
I've opened a ticket https://bugreport.apple.com/web/?problemID=48021471
...
Area:
Something not on this list
Summary: ldexp incorrectly rounds gradual underflow for some specific values.
Steps to Reproduce:
#include <math.h>
#include <stdio.h>
int main() {
int exp=-54; double y=ldexp(11.0,exp); /* y is binary 1.011*2^-51 */
double u=ldexp(1.0,-1074); /* u is the minimal denormalized IEEE754 double */
double v=ldexp(y,-1023); /* v is binary 1.011*2^-1074 and should round to u */
printf("u=%g v=%g\n",u,v);
return 0;
}
Expected Results:
v should be rounded to 1.0*2^-1074 (round to nearest, tie to even default rule)
Thus we should have u == v.
Actual Results:
v is rounded upward to binary 10.0*2^-1074 = 1.0*2^-1073
output is u=4.94066e-324 v=9.88131e-324
Note 1: this fails for 4 different values of exp -54,-53,+968,+969
(replace exponent -1023 by -1074-3-exp, that is  -1023,-1024,-2045,-2046)
Note 2: this did not happen previously with 32bits libm version.
Note 3: this sounds a bit like this bug https://stackoverflow.com/questions/32150888/should-ldexp-round-correctly
Version/Build:
macOS HighSierra 10.13.16
Configuration:
compiled with
clang --version
Apple LLVM version 10.0.0 (clang-1000.11.45.5)
Target: x86_64-apple-darwin17.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
Xcode Version 10.1 (10B61)
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https://github.com/OpenSmalltalk/opensmalltalk-vm/issues/383