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+
+Strange floating-point behaviour under Windows with some CPU models
+Description of problem:
+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
+
+(I also reported this via the mailing list, but I think it might've swallowed my post)