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From: "pozar at ecs dot umass.edu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug fortran/98201] New: CSQRT function gives bad resuts at runtime
Date: Tue, 08 Dec 2020 14:44:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98201

            Bug ID: 98201
           Summary: CSQRT function gives bad resuts at runtime
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pozar at ecs dot umass.edu
  Target Milestone: ---

I am running mingw gfortran 9.2.0 on Windows 7, and have a problem with res=
ults
from the CSQRT function during runtime. Sample code below, with results.
Starting with a complex argument having increasingly large real part, and a
relatively small imaginary part, csqrt initially gives a correct result (as
tested by squaring the result from CSQRT). But the second row shows some
significant error, and later rows show 'Infinity' for the imaginary part.
(using sq =3D z**.5 gives correct results)


F95 CODE;

! test CSQRT
    complex z, sq
    write(6,*) "         Argument                SquareRoot               SR
squared"
    do n=3D1,9
        z=3D-10.**n+(0.,.1)
        sq=3Dcsqrt(z)
        write(*,1) z, sq, sq*sq
    end do
1   format(3("(",e10.4,2x,e10.4,")",3x))
pause

RESULTS:
          Argument                  SquareRoot               SR squared
(-.1000E+02  0.1000E+00)   (0.1581E-01  0.3163E+01)   (-.1001E+02  0.1000E+=
00)
(-.1000E+03  0.1000E+00)   (0.5167E-02  0.9676E+01)   (-.9362E+02  0.1000E+=
00)
(-.1000E+04  0.1000E+00)   (0.0000E+00    Infinity)   ( -Infinity         N=
aN)
(-.1000E+05  0.1000E+00)   (0.0000E+00    Infinity)   ( -Infinity         N=
aN)
(-.1000E+06  0.1000E+00)   (0.0000E+00    Infinity)   ( -Infinity         N=
aN)
(-.1000E+07  0.1000E+00)   (0.0000E+00    Infinity)   ( -Infinity         N=
aN)
(-.1000E+08  0.1000E+00)   (0.0000E+00    Infinity)   ( -Infinity         N=
aN)
(-.1000E+09  0.1000E+00)   (0.0000E+00    Infinity)   ( -Infinity         N=
aN)
(-.1000E+10  0.1000E+00)   (0.0000E+00    Infinity)   ( -Infinity         N=
aN)
PAUSE=