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| author | Christian Krinitsin <mail@krinitsin.com> | 2025-05-30 16:52:07 +0200 |
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| committer | Christian Krinitsin <mail@krinitsin.com> | 2025-05-30 16:52:17 +0200 |
| commit | 9260319e7411ff8281700a532caa436f40120ec4 (patch) | |
| tree | 2f6bfe5f3458dd49d328d3a9eb508595450adec0 /gitlab/issues_text/target_i386/host_missing/accel_missing/2817 | |
| parent | 225caa38269323af1bfc2daadff5ec8bd930747f (diff) | |
| download | qemu-analysis-9260319e7411ff8281700a532caa436f40120ec4.tar.gz qemu-analysis-9260319e7411ff8281700a532caa436f40120ec4.zip | |
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Diffstat (limited to 'gitlab/issues_text/target_i386/host_missing/accel_missing/2817')
| -rw-r--r-- | gitlab/issues_text/target_i386/host_missing/accel_missing/2817 | 51 |
1 files changed, 51 insertions, 0 deletions
diff --git a/gitlab/issues_text/target_i386/host_missing/accel_missing/2817 b/gitlab/issues_text/target_i386/host_missing/accel_missing/2817 new file mode 100644 index 000000000..0f00a05f5 --- /dev/null +++ b/gitlab/issues_text/target_i386/host_missing/accel_missing/2817 @@ -0,0 +1,51 @@ +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) |