id = 2318
title = "SH4: SUBV instruction not emulated properly"
state = "closed"
created_at = "2024-04-29T17:36:33.900Z"
closed_at = "2024-05-04T02:01:16.498Z"
labels = ["target: sh4", "workflow::Patch available"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/2318"
host-os = "Debian testing"
host-arch = "x86_64"
qemu-version = "qemu-sh4 version 8.2.1 (Debian 1:8.2.1+ds-2)"
guest-os = "Buildroot uClibc (qemu-user)"
guest-arch = "SH4"
description = """SUBV opcode is emulated incorrectly.

The documentation says:

`SUBV Rm, Rn        Rn - Rm -> Rn, underflow -> T`

Qemu seems to perform the subtraction correctly, but will not detect an underflow."""
reproduce = """```c
#include <stdio.h>

int main(void)
{
\tregister unsigned int a asm("r8") = 0x80000001;
\tregister unsigned int b asm("r9") = 0x2;
\tregister unsigned int c asm("r10");

\tasm volatile("subv %2,%0\\n"
\t\t     "movt %1\\n"
\t\t     : "+r"(a), "=r"(c) : "r"(b) :);

\tprintf("Values: a=0x%x b=0x%x c=0x%x\\n", a, b, c);

\treturn 0;
}
```"""
additional = """Tested on real hardware (SEGA Dreamcast, GCC 15.0), the program above prints:
`Values: a=0x7fffffff b=0x2 c=0x1`

Running with Qemu (and GCC 13.0), the same program prints:
`Values: a=0x7fffffff b=0x2 c=0x0`"""