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

[Steve Kargl <sgk@troutmask.apl.washington.edu>]

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.


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.

-- 
Steve