Re: [Qemu-devel] Checking underflow condition for FRCPE instr [AARCH64]

[Peter Maydell]

On 8 August 2014 11:02, Gaurav Sharma <address@hidden> wrote:
> I am trying to test the floating point instruction for arm in qemu.
> For floating point reciprocal estimate instruction, for a64 i see a check
> for underflow condition in float64 HELPER(recpe_f64) method :
>
> " else if (f64_exp >= 1023 && fpst->flush_to_zero) "
>
> 1. how do we calculate the value to determine underflow ?

By looking at the pseudocode in the ARM ARM which
defines the condition under which we should signal
underflow.

> 2. Is the number 1023 correct for a64 ?

The underflow condition for 64 bit in the ARM ARM is
"Abs(value) >= 2.0^1022", which is a condition in the
pseudocode on the true real-number value of the input.
For QEMU we don't have a real-number value so we must
figure out the correct checks to be made on the raw
exponent/mantissa/sign fields of the IEEE format double.
(Specifically, f64_exp is the raw exponent field.)
Since we've already dealt with the NaN and infinity and
denormal cases by this point in the code, we know that
   value == (-1)^S * 2^(exp-1023) * 1.frac
If you plug in the value == 2.0^1022 and work it out
you'll find that this corresponds to f64_exp == 1023.

If you think we have a bug here (or elsewhere), it would be
helpful if you either:
 * described a specific case where you put input values
    in and get a wrong value out (as compared with h/w)
 * described your reasoning for why you think we're wrong,
   with reference to the specifications

If you just vaguely say "hey, is this right?" with no further
detail then you're asking us to do all the hard work of thinking
about the corner cases for you. If you think something is odd
and you've thought about it and you still can't figure out
why it's right, that's OK, but you need to tell us what you've
already done and where exactly you're getting confused.

thanks
-- PMM