[Bug c++/95942] New: [11 regression] offsetof on an array: error: 'e' is not a constant expression

[Bug c++/95942] New: [11 regression] offsetof on an array: error: 'e' is not a constant expression

[slyfox at inbox dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942

            Bug ID: 95942
           Summary: [11 regression] offsetof on an array: error: 'e' is
                    not a constant expression
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: slyfox at inbox dot ru
  Target Milestone: ---

The original error is seen on https://www.dolphin-emu.org/ project as a build
error. Minimized example looks the following:

"""
  // $ cat.bug.cc
  struct a {
    int b;
    char c[100];
  };
  unsigned long d(long e) { return __builtin_offsetof(a, c[e]); }
"""

OK:
  $ LANG=C g++-10.1.0 -c bug.cc -o bug.o

OK:
  $ LANG=C clang++-10 -c bug.cc -o bug.o

FAIL:
  $ LANG=C g++-11.0.0 -c bug.cc -o bug.o
  bug.cc: In function 'void d(long int)':
  bug.cc:8:42: error: 'e' is not a constant expression
      8 | void d(long e) { __builtin_offsetof(a, c[e]); }
        |

Who is wrong here? gcc-10 or gcc-11?

[Bug c++/95942] [11 regression] offsetof on an array: error: 'e' is not a constant expression

[slyfox at inbox dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942

--- Comment #1 from Sergei Trofimovich <slyfox at inbox dot ru> ---
Original code looks is at
https://github.com/dolphin-emu/dolphin/blob/2e8d1dd1dbcca7095e9d842f1df037cbe76868e4/Source/Core/Core/DSP/Jit/x64/DSPEmitter.cpp#L476:

"""
...
Gen::OpArg DSPEmitter::M_SDSP_r_st(size_t index)
{
  return MDisp(R15, static_cast<int>(offsetof(SDSP, r.st[index])));
}
...
"""

[Bug c++/95942] [11 regression] offsetof on an array: error: 'e' is not a constant expression

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942

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

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

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Changed in r11-557-gbeb019d346b903c16b9fd349937de444b6a8b6c0

[Bug c++/95942] [11 regression] offsetof on an array: error: 'e' is not a constant expression

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
   Target Milestone|---                         |11.0

[Bug c++/95942] [11 regression] offsetof on an array: error: 'e' is not a constant expression

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-07-15
     Ever confirmed|0                           |1
                 CC|                            |ppalka at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW

[Bug c++/95942] [11 regression] offsetof on an array: error: 'e' is not a constant expression

[slyfox at inbox dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942

--- Comment #3 from Sergei Trofimovich <slyfox at inbox dot ru> ---
Another example is edb-debugger project
https://github.com/eteran/edb-debugger/blob/070d0196227a58ce2ba15c695944ba16ce66c080/plugins/DebuggerCore/unix/linux/arch/x86-generic/PlatformThread.cpp#L331:

  long PlatformThread::setDebugRegister(std::size_t n, unsigned long value) {
        return ptrace(PTRACE_POKEUSER, tid_, offsetof(struct user,
u_debugreg[n]), value);
  }

Fails build on gcc-11 as:

 
edb-debugger/plugins/DebuggerCore/unix/linux/arch/x86-generic/PlatformThread.cpp:331:72:
error: 'n' is not a constant expression
    331 |  return ptrace(PTRACE_POKEUSER, tid_, offsetof(struct user,
u_debugreg[n]), value);
        |                                       ^

[Bug c++/95942] [11 regression] offsetof on an array: error: 'e' is not a constant expression

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
struct S { int a; int b[15]; };

int
foo (int n)
{
  int m = n;
  return __builtin_offsetof (S, b[m]);
}

has been rejected for years, while
struct S { int a; int b[15]; };

int
foo (int n)
{
  return __builtin_offsetof (S, b[n]);
}
has been accepted.
The comment in cp/parser.c says:
  /* ??? For offsetof, there is a question of what to allow here.  If
     offsetof is not being used in an integral constant expression context,
     then we *could* get the right answer by computing the value at runtime.
     If we are in an integral constant expression context, then we might
     could accept any constant expression; hard to say without analysis.
     Rather than open the barn door too wide right away, allow only integer
     constant expressions here.  */

[Bug c++/95942] [11 regression] offsetof on an array: error: 'e' is not a constant expression

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Version|10.1.0                      |11.0
           Priority|P3                          |P1

[Bug c++/95942] [11 regression] offsetof on an array: error: 'e' is not a constant expression

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|NEW                         |RESOLVED

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> ---
C11 says,

....and
  offsetof(type, member-designator)
which expands to an integer constant expression that has type size_t, the value
of which is the offset in bytes, to the structure member (designated by
member-designator), from the beginning of its structure (designated by type).
The type and member designator shall be such that given
  static type t;
then the expression &(t.member-designator) evaluates to an address constant.
(If the specified member is a bit-field, the behavior is undefined.)

And __builtin_offsetof is only documented to be usable as an implementation of
the offsetof macro.

Since e is not constant, the expression above is not an address constant, so
the offsetof expression is ill-formed.  The change mentioned in comment #2
fixed this and various related issues with wrongly treating parameters as
sufficiently constant.

[Bug c++/95942] [11 regression] offsetof on an array: error: 'e' is not a constant expression

[slyfox at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942

Sergei Trofimovich <slyfox at gcc dot gnu.org> changed:

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

--- Comment #6 from Sergei Trofimovich <slyfox at gcc dot gnu.org> ---
Thank you! I'll report the bugs to affected upstreams.

[Bug c++/95942] [11 regression] offsetof on an array: error: 'e' is not a constant expression

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |davem at devkitpro dot org

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
*** Bug 100296 has been marked as a duplicate of this bug. ***

[Bug c++/95942] [11 regression] offsetof on an array: error: 'e' is not a constant expression

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95942

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jlame646 at gmail dot com

--- Comment #8 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 113804 has been marked as a duplicate of this bug. ***