[Bug libfortran/108937] New: Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[Bug libfortran/108937] New: Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[saitofuyuki at jamstec dot go.jp]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

            Bug ID: 108937
           Summary: Intrinsic IBITS(I,POS,LEN) fails when LEN equals to
                    BIT_SIZE(I).
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libfortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: saitofuyuki at jamstec dot go.jp
  Target Milestone: ---

Created attachment 54539
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54539&action=edit
A minimum patch to fix ibits().

gfc_conv_intrinsic_ishft(), e.g, in gcc/fortran/trans-intrinsic.cc
catches a special case when shift widths GE BIT_SIZE at LSHIFT_EXPR
operation, which is not performed in gfc_conv_intrinsic_ibits.

I attached a minimum patch, and also a test program.

The output of the program before/after patch are as follow:

------------------------ before patch
IBITS(-1,0,32)
intrinsic 0 00000000000000000000000000000000
expected -1 11111111111111111111111111111111

------------------------ after patch
IBITS(-1,0,32)
intrinsic -1 11111111111111111111111111111111
expected -1 11111111111111111111111111111111

As far as I checked, this is a long-standing bug.  At least gfortran
10.4.0, 11.3.0, 12.2.0, and the latest one
(4341106354c6a463ce3628a4ef9c1a1d37193b59).

[Bug libfortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[saitofuyuki at jamstec dot go.jp]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

--- Comment #1 from saitofuyuki at jamstec dot go.jp ---
Sorry, I missed to attach the test program.


program test_bits
  implicit none
  integer,parameter :: KT = 4
  integer,parameter :: lbits = bit_size(0_KT)
  integer(kind=KT) x, y0, y1
  integer(kind=KT) p, l

  x = -1
  p = 0
  l = lbits

100 format('IBITS(', I0, ',', I0, ',', I0, ')')
101 format(A, 1x, I0, 1x, B32.32)
  y0 = ibits(x, p, l)
  y1 = ibits_1(x, p, l)
  write(*, 100) x, p, l
  write(*, 101) 'intrinsic', y0, y0
  write(*, 101) 'expected',  y1, y1
  stop
contains
  elemental integer(kind=KT) function ibits_1(I, POS, LEN) result(n)
    !! IBITS(I, POS, LEN) = (I >> POS) & ~((~0) << LEN)
    implicit none
    integer(kind=KT),intent(in) :: I
    integer,         intent(in) :: POS, LEN
    n = IAND(ISHFT(I, - POS), NOT(ISHFT(-1_KT, LEN)))
  end function ibits_1
end program test_bits

[Bug libfortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[kargl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

kargl at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kargl at gcc dot gnu.org

--- Comment #2 from kargl at gcc dot gnu.org ---
Not sure it's a bug.

16.3.1
...
The interpretation of a negative integer as a sequence of bits
is processor dependent.

[Bug libfortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[saitofuyuki at jamstec dot go.jp]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

--- Comment #3 from saitofuyuki at jamstec dot go.jp ---
Thanks a lot.

> Not sure it's a bug.

I see.  I do agree it's not a bug and the answer of the particular case is
undefined.

But (for 32-bit integer case), since implemented ISHFT(I,32) returns 0 for any
representation of I, I suppose it may be better to return I as it is by
IBITS(I,0,32) (again, I agree it is not defined by standard).

Anyway, thanks a lot for your comment.

[Bug libfortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[kargl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

--- Comment #4 from kargl at gcc dot gnu.org ---
(In reply to saitofuyuki from comment #3)
> Thanks a lot.
> 
> > Not sure it's a bug.
> 
> I see.  I do agree it's not a bug and the answer of the particular case is
> undefined.
> 
> But (for 32-bit integer case), since implemented ISHFT(I,32) returns 0 for
> any representation of I, I suppose it may be better to return I as it is by
> IBITS(I,0,32) (again, I agree it is not defined by standard).
> 
> Anyway, thanks a lot for your comment.

In the end, gfortran should probably do something that a user
might expect, and that would likely be two-complements wrap-around
unsigned integer arithmetic.

[Bug libfortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[anlauf at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

anlauf at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |anlauf at gcc dot gnu.org

--- Comment #5 from anlauf at gcc dot gnu.org ---
Interestingly, we simplify IBITS(-1,0,32) to -1, and also the code generated
for ibits(x, p, lbits) produces the result expected by the reporter,
and which is also generated by Intel and NAG.

For ISHFT we have

/* ISHFT (I, SHIFT) = (abs (shift) >= BIT_SIZE (i))
                        ? 0
                        : ((shift >= 0) ? i << shift : i >> -shift)
   where all shifts are logical shifts.  */

so we might as well consider doing what the reporter suggests.

[Bug fortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[anlauf at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

anlauf at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|libfortran                  |fortran
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-02-27
     Ever confirmed|0                           |1

--- Comment #6 from anlauf at gcc dot gnu.org ---
Adjusting component as the issue is in the frontend generating the code.

[Bug fortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[anlauf at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

anlauf at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |anlauf at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #7 from anlauf at gcc dot gnu.org ---
Submitted: https://gcc.gnu.org/pipermail/fortran/2023-February/058984.html

[Bug fortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Harald Anlauf <anlauf@gcc.gnu.org>:

https://gcc.gnu.org/g:6cce953ebec274f1468d5d3a0697cf05bb43b8f6

commit r13-6362-g6cce953ebec274f1468d5d3a0697cf05bb43b8f6
Author: Harald Anlauf <anlauf@gmx.de>
Date:   Mon Feb 27 21:37:11 2023 +0100

    Fortran: fix corner case of IBITS intrinsic [PR108937]

    gcc/fortran/ChangeLog:

            PR fortran/108937
            * trans-intrinsic.cc (gfc_conv_intrinsic_ibits): Handle corner case
            LEN argument of IBITS equal to BITSIZE(I).

    gcc/testsuite/ChangeLog:

            PR fortran/108937
            * gfortran.dg/ibits_2.f90: New test.

[Bug fortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[saitofuyuki at jamstec dot go.jp]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

--- Comment #9 from saitofuyuki at jamstec dot go.jp ---
Thanks a lot for your effort.  I am very happy I can contribute.

[Bug fortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Harald Anlauf
<anlauf@gcc.gnu.org>:

https://gcc.gnu.org/g:bb68c20e1f3c673ed00e0e1abd468d8f19cd2072

commit r12-9222-gbb68c20e1f3c673ed00e0e1abd468d8f19cd2072
Author: Harald Anlauf <anlauf@gmx.de>
Date:   Mon Feb 27 21:37:11 2023 +0100

    Fortran: fix corner case of IBITS intrinsic [PR108937]

    gcc/fortran/ChangeLog:

            PR fortran/108937
            * trans-intrinsic.cc (gfc_conv_intrinsic_ibits): Handle corner case
            LEN argument of IBITS equal to BITSIZE(I).

    gcc/testsuite/ChangeLog:

            PR fortran/108937
            * gfortran.dg/ibits_2.f90: New test.

    (cherry picked from commit 6cce953ebec274f1468d5d3a0697cf05bb43b8f6)

[Bug fortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

--- Comment #11 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Harald Anlauf
<anlauf@gcc.gnu.org>:

https://gcc.gnu.org/g:adc4c8eb79a75bd0f38a461c299b37c643a1153c

commit r11-10560-gadc4c8eb79a75bd0f38a461c299b37c643a1153c
Author: Harald Anlauf <anlauf@gmx.de>
Date:   Mon Feb 27 21:37:11 2023 +0100

    Fortran: fix corner case of IBITS intrinsic [PR108937]

    gcc/fortran/ChangeLog:

            PR fortran/108937
            * trans-intrinsic.c (gfc_conv_intrinsic_ibits): Handle corner case
            LEN argument of IBITS equal to BITSIZE(I).

    gcc/testsuite/ChangeLog:

            PR fortran/108937
            * gfortran.dg/ibits_2.f90: New test.

    (cherry picked from commit 6cce953ebec274f1468d5d3a0697cf05bb43b8f6)

[Bug fortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

--- Comment #12 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Harald Anlauf
<anlauf@gcc.gnu.org>:

https://gcc.gnu.org/g:570828e84f751730743093e5f060eeaf98c08cac

commit r10-11241-g570828e84f751730743093e5f060eeaf98c08cac
Author: Harald Anlauf <anlauf@gmx.de>
Date:   Mon Feb 27 21:37:11 2023 +0100

    Fortran: fix corner case of IBITS intrinsic [PR108937]

    gcc/fortran/ChangeLog:

            PR fortran/108937
            * trans-intrinsic.c (gfc_conv_intrinsic_ibits): Handle corner case
            LEN argument of IBITS equal to BITSIZE(I).

    gcc/testsuite/ChangeLog:

            PR fortran/108937
            * gfortran.dg/ibits_2.f90: New test.

    (cherry picked from commit 6cce953ebec274f1468d5d3a0697cf05bb43b8f6)

[Bug fortran/108937] Intrinsic IBITS(I,POS,LEN) fails when LEN equals to BIT_SIZE(I).

[anlauf at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108937

anlauf at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |10.5

--- Comment #13 from anlauf at gcc dot gnu.org ---
Fixed on all open branches.  Closing.

Thanks for the report!