[PATCH][Fortran] Use MIN/MAX_EXPR for intrinsics or __builtin_fmin/max when appropriate

[Janne Blomqvist]

On Tue, Jul 17, 2018 at 11:06 PM, Janne Blomqvist <blomqvist.janne@gmail.com
> wrote:

> On Tue, Jul 17, 2018 at 6:36 PM, Thomas Koenig <tkoenig@netcologne.de>
> wrote:
>
>> Hi Kyrill,
>>
>> The current implementation expands to:
>>>      mvar = a1;
>>>      if (a2 .op. mvar || isnan (mvar))
>>>        mvar = a2;
>>>      if (a3 .op. mvar || isnan (mvar))
>>>        mvar = a3;
>>>      ...
>>>      return mvar;
>>>
>>> That is, if one of the operands is a NaN it will return the other
>>> argument.
>>> If both (all) are NaNs, it will return NaN. This is the same as the
>>> semantics of fmin/max
>>> as far as I can tell.
>>>
>>
>> I've looked at the F2008 standard, and, interestingly enough, the
>> requirement on MIN and MAX do not mention NaNs at all. 13.7.106
>> has, for MAX,
>>
>> Result Value. The value of the result is that of the largest argument.
>>
>> plus some stuff about character variables (not relevant here).  Similar
>> for MIN.
>>
>
> FWIW, this has not changed in the latest(?) draft for F2018 (N2146), see
> 16.9.125.
>
> Also, the section on IEEE_ARITHMETIC (14.9) does not mention
>> comparisons; also, "Complete conformance with IEC 60559:1989 is not
>> required", what is required is the correct support for +,-, and *,
>> plus support for / if IEEE_SUPPORT_DIVIDE is covered.
>>
>
> Interestingly, here the F2018 draft has new intrinsics in the
> IEEE_ARITHMETIC module, IEEE_MAX_NUM, IEEE_MAX_NUM_MAG, IEEE_MIN_NUM,
> IEEE_MIN_NUM_MAG. These correspond to the {max,min}num{,_mag} operations in
> IEEE 754-2008, which AFAICT has the same NaN semantics as __builtin_fmax
> etc.
>
>
>> So, the Fortran standard does not impose many requirements.
>
>
> If so, why don't we just use {MAX,MIN}_EXPR unconditionally? Those who
> worry about the behavior wrt. NaNs, infinities etc. can use the intrinsics
> from IEEE_ARITHMETIC?
>
>
> This thread also has some interesting discussion on the topic:
> https://github.com/JuliaLang/julia/issues/7866
>

Oh, and on http://754r.ucbtest.org/ there is information about the next
update after IEEE 754-2008. In particular,
http://754r.ucbtest.org/changes.html notes that the above mentioned
{max,min}num{,_mag}  have been deleted, and "new
{min,max}imum{,Number,Magnitude,MagnitudeNumber} operations are
recommended; NaN and signed zero handling are changed from 754-2008 5.3.1.
".


-- 
Janne Blomqvist