Re: [PATCH] Fortran: fix compile-time simplification of SET_EXPONENT [PR109511]

[Harald Anlauf <anlauf@gmx.de>]

Hi Steve,

On 4/14/23 21:33, Steve Kargl via Gcc-patches wrote:
> On Fri, Apr 14, 2023 at 08:59:24PM +0200, Harald Anlauf via Fortran wrote:
>>
>> the compile-time simplification of intrinsic SET_EXPONENT was
>> broken since the early days of gfortran for argument X < 1
>> (including negative X) and for I < 0.  I identified and fixed
>> several issues in the implementation.  The testcase explores
>> argument space comparing compile-time and runtime results and
>> is checked against Intel.
>>
>> Regtested on x86_64-pc-linux-gnu.  OK for mainline?
>>
>
> Yes, it is certainly better than the current situation.
>
>> This is not a regression, but can lead to wrong code.
>> Would it be OK to backport to open branches?
>
> Sure.  Looks simply and fairly specific.

OK, thanks, will proceed.


> I was wondering about the difference between set_exponent()
> and scale(), and found that set_exponent() talks about IEEE
> values while scale() doesn't.  I'm wondering if we should
> add the IEEE special cases to the testsuite.  Of particular
> note, I doubt that this is true:
>
>     If X is an IEEE NaN, the result is the same NaN.
>
> program foo
>     real x, y
>     x = 1
>     y = x - x
>     x = (x - x) / y
>     print '(F4.0,1X,Z8.8)', x, x
>     y = set_exponent(x,1)
>     print '(F4.0,1X,Z8.8)', y, y
> end program foo
>
>   gfcx -o z a.f90 && ./z
>   NaN FFC00000
>   NaN 7FC00000
>
> Those are not the same NaN.  The second is a qNaN.
> The first looks like a qNaN with the sign bit set.

Until now there was no testing at all of SET_EXPONENT in the testsuite.
It would be really good to have better coverage of compile-time and
runtime behavior of the intrinsics and checking consistency ... ;-)

I think you have much more experience in that area.  (Hint!)

Cheers,
Harald