[Bug c++/95009] New: decltype of increment or decrement bitfield expressions are wrong and causes assembler errors.

[Bug c++/95009] New: decltype of increment or decrement bitfield expressions are wrong and causes assembler errors.

[okannen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95009

            Bug ID: 95009
           Summary: decltype of increment or decrement bitfield
                    expressions are wrong and causes assembler errors.
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: okannen at gmail dot com
  Target Milestone: ---

This bug exists since gcc version 6

The code bellow should not compile:

#include <type_traits>

  struct A
    {
    int i:31;
    };

  A a;

  //I am not sure the standard require that (a.i) and ++a.i have the same type.
  static_assert(!std::is_same_v <decltype ((a.i)), decltype(++a.i)>);
                          //      int&               int:31&

  //but I do believe that decltype(a.i+=1) and decltype(++a.i) shall have the
same type
  static_assert(!std::is_same_v <decltype ((a.i+=1)), decltype(++a.i)>);
                         //                int&            int:31&

[expr.pre.incr]/1 "The expression ++x is equivalent to x+=1."

  Actualy those weird "int:N" types cause a lot of trouble. This code cause  
assembler error:

  //test.cpp
  struct A{
        int i:31;
        };

  template <class T>
  void f(){ 
        }

  int main(){
        A a;
        f<decltype(a.i+=1)>();
        f<decltype(++a.i)>();
        return 0;
        }

`c++ test.cpp` outputs:
/tmp/ccTayNZZ.s: Assembler messages:
/tmp/ccTayNZZ.s:43: Error: symbol `_Z1fIRiEvv' is already defined

[Bug c++/95009] [9/10/11/12 Regression] decltype of increment or decrement bitfield expressions are wrong and causes assembler errors.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95009

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |assemble-failure,
                   |                            |ice-checking,
                   |                            |ice-on-valid-code
   Target Milestone|---                         |9.5
      Known to fail|                            |12.0, 4.8.1
      Known to work|                            |4.7.1
   Last reconfirmed|                            |2021-07-31
            Summary|decltype of increment or    |[9/10/11/12 Regression]
                   |decrement bitfield          |decltype of increment or
                   |expressions are wrong and   |decrement bitfield
                   |causes assembler errors.    |expressions are wrong and
                   |                            |causes assembler errors.
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug c++/95009] [9/10/11/12 Regression] decltype of increment or decrement bitfield expressions are wrong and causes assembler errors.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95009

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There is most likely a missing unlowered_expr_type somewhere in the decltype
handling.

[Bug c++/95009] [9/10/11/12 Regression] decltype of increment or decrement bitfield expressions are wrong and causes assembler errors.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95009

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=70733,
                   |                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=84516

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There is most likely a missing call to unlowered_expr_type somewhere, most
likely in finish_unary_op_expr.

[Bug c++/95009] [9/10/11/12 Regression] decltype of increment or decrement bitfield expressions are wrong and causes assembler errors.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95009

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |84516, 70733

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
PR 84516 and PR 70733 are about other unary operations too (+ and -).


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70733
[Bug 70733] Wrong code with nested structs and bit field.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84516
[Bug 84516] bitfield temporaries > 32-bits have wrong type

[Bug c++/95009] [9/10/11/12 Regression] decltype of increment or decrement bitfield expressions are wrong and causes assembler errors.

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95009

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

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

[Bug c++/95009] [9/10/11/12 Regression] decltype of increment or decrement bitfield expressions are wrong and causes assembler errors.

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95009

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:3a2257e6b3fa288d6c50831987949b9ff7dfb865

commit r12-5823-g3a2257e6b3fa288d6c50831987949b9ff7dfb865
Author: Marek Polacek <polacek@redhat.com>
Date:   Sat Dec 4 12:07:41 2021 -0500

    c++: Fix for decltype and bit-fields [PR95009]

    Here, decltype deduces the wrong type for certain expressions involving
    bit-fields.  Unlike in C, in C++ bit-field width is explicitly not part
    of the type, so I think decltype should never deduce to 'int:N'.  The
    problem isn't that we're not calling unlowered_expr_type--we are--it's
    that is_bitfield_expr_with_lowered_type only handles certain codes, but
    not others.  For example, += works fine but ++ does not.

    This also fixes decltype-bitfield2.C where we were crashing (!), but
    unfortunately it does not fix 84516 or 70733 where the problem is likely
    a missing call to unlowered_expr_type.  It occurs to me now that typeof
    likely has had the same issue, but this patch should fix that too.

            PR c++/95009

    gcc/cp/ChangeLog:

            * typeck.c (is_bitfield_expr_with_lowered_type) <case MODIFY_EXPR>:
            Handle UNARY_PLUS_EXPR, NEGATE_EXPR, NON_LVALUE_EXPR, BIT_NOT_EXPR,
            P*CREMENT_EXPR too.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/decltype-bitfield1.C: New test.
            * g++.dg/cpp0x/decltype-bitfield2.C: New test.

[Bug c++/95009] [9/10/11 Regression] decltype of increment or decrement bitfield expressions are wrong and causes assembler errors.

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95009

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[9/10/11/12 Regression]     |[9/10/11 Regression]
                   |decltype of increment or    |decltype of increment or
                   |decrement bitfield          |decrement bitfield
                   |expressions are wrong and   |expressions are wrong and
                   |causes assembler errors.    |causes assembler errors.

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed on trunk so far, will backport to gcc 11.

[Bug c++/95009] [9/10/11 Regression] decltype of increment or decrement bitfield expressions are wrong and causes assembler errors.

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95009

--- Comment #7 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:e3fab19a79e3dc3cdf3e54ca20107e1dc15a3927

commit r11-9363-ge3fab19a79e3dc3cdf3e54ca20107e1dc15a3927
Author: Marek Polacek <polacek@redhat.com>
Date:   Sat Dec 4 12:07:41 2021 -0500

    c++: Fix for decltype and bit-fields [PR95009]

    Here, decltype deduces the wrong type for certain expressions involving
    bit-fields.  Unlike in C, in C++ bit-field width is explicitly not part
    of the type, so I think decltype should never deduce to 'int:N'.  The
    problem isn't that we're not calling unlowered_expr_type--we are--it's
    that is_bitfield_expr_with_lowered_type only handles certain codes, but
    not others.  For example, += works fine but ++ does not.

    This also fixes decltype-bitfield2.C where we were crashing (!), but
    unfortunately it does not fix 84516 or 70733 where the problem is likely
    a missing call to unlowered_expr_type.  It occurs to me now that typeof
    likely has had the same issue, but this patch should fix that too.

            PR c++/95009

    gcc/cp/ChangeLog:

            * typeck.c (is_bitfield_expr_with_lowered_type) <case MODIFY_EXPR>:
            Handle UNARY_PLUS_EXPR, NEGATE_EXPR, NON_LVALUE_EXPR, BIT_NOT_EXPR,
            P*CREMENT_EXPR too.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/decltype-bitfield1.C: New test.
            * g++.dg/cpp0x/decltype-bitfield2.C: New test.

    (cherry picked from commit 3a2257e6b3fa288d6c50831987949b9ff7dfb865)

[Bug c++/95009] [9/10 Regression] decltype of increment or decrement bitfield expressions are wrong and causes assembler errors.

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95009

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[9/10/11 Regression]        |[9/10 Regression] decltype
                   |decltype of increment or    |of increment or decrement
                   |decrement bitfield          |bitfield expressions are
                   |expressions are wrong and   |wrong and causes assembler
                   |causes assembler errors.    |errors.
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed for GCC 11+.