Strange floating-point behaviour with some CPU models

Strange floating-point behaviour with some CPU models

[Daniel Collins]

Hello all,

I'm encountering a very weird bug with some floating-point maths code, but only under very specific configurations. First I thought it was a Clang bug, but then further digging eventually showed it to only occur under Windows VMs with specific QEMU CPU options, I'm not certain whether it is a QEMU/KVM bug or a Windows bug, but thought starting here would be easiest.

When compiled under MSVC Clang with modern CPU instructions disabled (e.g. -march=pentium3 or -march=pentium-mmx), the floorf() call in the following program always returns 0.0, while the truncation works correctly:

#include <math.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
     float n = atof(argv[1]);
     printf("n = %f\n", n);

     float f = floorf(n);
     printf("f = %f\n", f);

     float c = (int)(n);
     printf("c = %f\n", c);

     return 0;
}

Example output on an affected VM:

C:\Users\Administrator> floorf-p3.exe 10
n = 10.000000
f = 0.000000
c = 10.000000

C:\Users\Administrator> floorf-p4.exe 10
n = 10.000000
f = 10.000000
c = 10.000000

(floorf-p3.exe was compiled with -march=pentium3 and floorf-p4.exe with -march=pentium4 above)

I've tried a few QEMU CPU models on a variety of Intel/AMD VM hosts and two different Windows versions (10 and Server 2022), and observed the following:

host-passthrough: works (on AMD and Intel hosts)
qemu64: broken
EPYC-Milan: works
Westmere: works
Penryn: broken

Happy to provide executables and/or disassembly to aid in debugging this.

Daniel