Re: [Qemu-devel] [PATCH] target-mips: remove wrong checks for recip.fmt and rsqrt.fmt

[Leon Alrae]

On 25/08/2015 23:40, Petar Jovanovic wrote:
>> @@ -9839,7 +9837,6 @@ static void gen_farith (DisasContext *ctx, enum
> fopcode op1,
>>          opn = "movn.d";
>>          break;
>>      case OPC_RECIP_D:
>> -        check_cp1_64bitmode(ctx);
> 
>> I think this needs check_cp1_registers() now, i.e. check for odd fpu
> register access when Status.FR = 0.
> 
> This would raise a "reserved instruction" exception. I am not aware that any
> MIPS CPU implementation would throw an exception for e.g. "recip.d
> $f21,$f11" (let me know if that is not the case), and I do not think MIPS
> documentation obliges us to throw an exception either.

MIPS documentation says that this operation is "UNPREDICTABLE" -- software can
never depend on a result and in QEMU we usually raise RI in such cases in
other *.D instructions which is quite handy (it usually indicates the "forgot
to set Status.FR bit" bug in the guest).

Leon