[LLVMdev] Signed NaNs in APFloat arithmetic

[Keno Fischer]

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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>