On Sat, Jul 29, 2023 at 10:08:51PM +0100, Stafford Horne wrote:
> The architecture specification calls for the EPCR to be set to "Address
> of next not executed instruction" when there is a floating point
> exception (FPE). This was not being done, so fix it by using the same
> method as syscall. Note, this may need a lot more work if we start
> seeing floating point operations in delay slots which exceptions
> enabled.
It should be "with exceptions enabled."
>
> Without this patch FPU exceptions will loop, as the exception hanlding
"handling"
> will always return back to the failed floating point instruction.
>
> This was not noticed in earlier testing because:
>
> 1. The compiler usually generates code which clobbers the input operand
> such as:
>
> lf.div.s r19,r17,r19
>
> 2. The target will store the operation output before to the register
> before handling the exception. So an operation such as:
>
> float a = 100.0f;
> float b = 0.0f;
> float c = a / b; /* lf.div.s r19,r17,r19 */
>
> Will first execute:
>
> 100 / 0 -> Store inf to c (r19)
> -> triggering divide by zero exception
> -> handle and return
>
> Then it will exectute:
>
> 100 / inf -> Store 0 to c (no exception)
>
> To confirm the looping behavoid and the fix I used the following:
"behavior"
> float fpu_div(float a, float b) {
> float c;
> asm volatile("lf.div.s %0, %1, %2"
> : "+r" (c)
> : "r" (a), "r" (b));
> return c;
> }
>
> Signed-off-by: Stafford Horne <sho...@gmail.com>
-Stafford
> ---
> target/openrisc/interrupt.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/target/openrisc/interrupt.c b/target/openrisc/interrupt.c
> index 3887812810..9b14b8a2c6 100644
> --- a/target/openrisc/interrupt.c
> +++ b/target/openrisc/interrupt.c
> @@ -34,7 +34,7 @@ void openrisc_cpu_do_interrupt(CPUState *cs)
> int exception = cs->exception_index;
>
> env->epcr = env->pc;
> - if (exception == EXCP_SYSCALL) {
> + if (exception == EXCP_SYSCALL || exception == EXCP_FPE) {
> env->epcr += 4;
> }
> /* When we have an illegal instruction the error effective address
> --
> 2.39.1
>
