Re: [PATCH][RFC] target/hppa: Avoid nullification for uaddcmt instruction

[Richard Henderson]

On 3/23/24 11:15, Helge Deller wrote:
> The uaddcmt (UNIT ADD COMPLEMENT AND TRAP ON CONDITION) instruction
> triggers a trap if the condition is true, and stores the result of the
> addition in the target register otherwise.
> It does not use the condition to nullify the following instruction, so
> drop the calculated condition and do not install it as null_cond.

That's not what the manual says:

if (trapc == TC && cond_satisfied)
    conditional_trap;
else {
    GR[t] ← res;
    if (cond_satisfied) PSW[N] ← 1;
}

We have implemented this like so:

if (trapc == TC)
    conditional_trap(cond_satisfied);
GR[t] ← res;
if (cond_satisfied) PSW[N] ← 1;

because the conditional trap step does not return if it traps.


> This patch is not tested and as such sent as RFC.
> I just stumbled over the apparently wrong behaviour while debugging the
> uaddcm instruction.

Under separate cover you said the condition might be computed incorrectly.  I think that 
is more likely the root cause of the wrong behaviour.


r~