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

[WineHQ Bugzilla]

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: [e-mail]
          Reporter: [e-mail]
                CC: [e-mail]
   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.