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+id = 2817
+title = "Strange floating-point behaviour under Windows with some CPU models"
+state = "opened"
+created_at = "2025-02-18T12:02:17.591Z"
+closed_at = "n/a"
+labels = ["Softfloat", "target: i386"]
+url = "https://gitlab.com/qemu-project/qemu/-/issues/2817"
+host-os = "Debian 12"
+host-arch = "x86_64"
+qemu-version = "QEMU emulator version 7.2.13 (Debian 1:7.2+dfsg-7+deb12u7)"
+guest-os = "Windows 10, Windows Server 2022"
+guest-arch = "x86_64"
+description = """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)
+{
+\tfloat n = atof(argv[1]);
+\tprintf("n = %f\\n", n);
+\t
+\tfloat f = floorf(n);
+\tprintf("f = %f\\n", f);
+\t
+\tfloat c = (int)(n);
+\tprintf("c = %f\\n", c);
+\t
+\treturn 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
+
+(I also reported this via the mailing list, but I think it might've swallowed my post)"""
+reproduce = "n/a"
+additional = "n/a"