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
