[LLVMdev] Signed NaNs in APFloat arithmetic

[Keno Fischer]

Ok. That you for clarifying the point for me. I was primed for a
regression because this behavior changed over llvm versions and was
causing my tests to fail ;). I'm now doing bitcasting to int, xoring
with the signbit and bitcasting back.

On Thu, Aug 7, 2014 at 2:59 AM, Owen Anderson <resistor at mac.com> wrote:
> Subtraction is also not a correct implementation of negation, for exactly the same reason.  LLVM is simply wrong in this context.
> Generally speaking, correct implementations of fabs, fneg, and copysign are built out of logical operations rather than arithmetic ones.
>
> I don’t know offhand why the behavior of multiplication of multiplication in APFloat was changed.
>
> —Owen
>
> On Aug 6, 2014, at 10:45 PM, Keno Fischer <kfischer at college.harvard.edu> wrote:
>
>> Ok, I had forgotten about sNaNs. Doesn't the same caveat apply to
>> 0-sNaN then though or does that not signal? Does that mean we need a
>> separate way to handle negate in the IR? Funnily enough, historically
>> I believe we were using the multiplication by -1.0 because it was a
>> more reliable negation that 0-x (from 3.0 until 3.3 at least). Is
>> there a good reason why multiplication by NaN should kill the sign
>> bit?
>>
>> On Thu, Aug 7, 2014 at 1:22 AM, Owen Anderson <resistor at mac.com> wrote:
>>> Hi Keno,
>>>
>>> From IEEE 754-2008, §5.5.1:
>>>        Implementations shall provide the following homogeneous quiet-computational sign bit operations for all
>>>        supported arithmetic formats; they only affect the sign bit. The operations treat floating-point numbers and
>>>        NaNs alike, and signal no exception. These operations may propagate non-canonical encodings.
>>>
>>>        sourceFormat copy(source)
>>>        sourceFormat negate(source)
>>>        sourceFormat abs(source)
>>>
>>> Multiplying by -1.0 has the potential to raise a floating point exception on sNaN inputs, and hence is not a valid implementation of negation per IEEE 754.
>>>
>>> —Owen
>>>
>>> On Aug 6, 2014, at 9:51 PM, Keno Fischer <kfischer at college.harvard.edu> wrote:
>>>
>>>> In r187314, APFloat multiplication by with NaNs was made to always
>>>> yield a positive NaN. I am wondering whether that was the correct
>>>> decision. It is of course true that the result of a multiplication is
>>>> undefined in IEEE, however, we were using multiplication by -1.0 to
>>>> implement IEEE negate, which is defined to preserve the sign bit.
>>>> r210428 made 0-NaN have IEEE negate behavior, which is good because it
>>>> seems to me from r187314 to r210428 there was no compliant way to
>>>> implement it in LLVM. Does somebody remember what the arguments for
>>>> the behavior change in r187314 were? It seems more sane to me to
>>>> preserve the sign bit than to unconditionally overwrite it, especially
>>>> considering that the hardware doesn't do it this way.
>>>>
>>>> Thanks,
>>>> Keno
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>>>
>