New subject: [Bug 56372] musl based exp2() gives very inaccurate results on i686

26 Feb 2024


      https://bugs.winehq.org/show_bug.cgi?id=56372
Bug ID: 56372
           Summary: musl based exp2() gives very inaccurate results on
                    i686
           Product: Wine
           Version: 8.6
          Hardware: x86-64
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: msvcrt
          Assignee: wine-bugs@winehq.org
          Reporter: martin@martin.st
                CC: piotr@codeweavers.com
   Regression SHA1: ca0643875961ef62c76e0b9cbd0433002a948d9b
      Distribution: ---
Since switching the msvcrt implementation of exp2() from internal to musl
based, in ca0643875961ef62c76e0b9cbd0433002a948d9b, the accuracy on i686 has
become quite bad, to the point of breaking a handful of tests in ffmpeg.
The issue can be reproduced with the following test snippet:
---
#include <stdio.h>
#include <math.h>
double input = 5.979471;
int main(int argc, char* argv[]) {
    printf("exp2(%f) = %f\n", input, exp2(input));
    return 0;
}
---
Prior to this commit, and with later versions on e.g. x86_64, the output is:
exp2(5.979471) = 63.095753
However, on i686, the output now is:
exp2(5.979471) = 62.968753
This is an error of around 0.2%, which is a couple orders of magnitudes larger
than expected/tolerated. It's curious how this issue only appears on i686
though, i.e. it doesn't look like an issue with the musl exp2() implementation
algorithm, but some component of it misbehaving on i686.
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[Bug 56372] New: musl based exp2() gives very inaccurate results on i686