[Bug c++/49136] New: [C++0x][constexpr] Incorrect constexpr c'tor evaluation with bitfields

[Bug c++/49136] New: [C++0x][constexpr] Incorrect constexpr c'tor evaluation with bitfields

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49136

           Summary: [C++0x][constexpr] Incorrect constexpr c'tor
                    evaluation with bitfields
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: daniel.kruegler@googlemail.com
                CC: jason@redhat.com


gcc 4.7.0 20110521 (experimental) in C++0x mode fires a static assertion at the
line marked with #:

//----
struct day {
  unsigned d : 5;
  unsigned n : 3;
  constexpr explicit day(int dd) : d(dd), n(7) {}
};

struct date {
  int d;
  constexpr date(day dd) : d((dd.n != 7) ? 7 : dd.d) {}
};

constexpr day d(0);
constexpr date dt(d);
static_assert(dt.d == 0, "Error"); // #
//----

Further testing shows, that dt.d has the value 7 instead of 0. The error only
occurs, if the day object d is defined as constexpr variable. E.g. given the
above shown types day and date the following program

//---
extern "C" int printf(const char*, ...);

int main()
{
  constexpr day d(0);
  date dt(d);
  printf("%d\n", d.d);  // Prints 0
  printf("%d", dt.d);   // Prints 7
}
//---

still produces the calculation error. If we remove the constexpr specifier of
the local object d (or if we replace it by a const specifier), the calculation
is correct, the output is 

0
0

as expected. Interestingly, the internal value of the object d itself is always
correct. The defect seems to happen if a constexpr day object is used in
another constexpr constructor. It seems that the type date itself is required
as well, I could not replace the constexpr date constructor call by a constexpr
function like this

constexpr int date_func(day dd) { return (dd.n != 7) ? 7 : dd.d; }

to produce the same error. While trying to simplify the test case I found that
the struct day must contain at least two such bitfields.

Let me add that this problem occurred while trying to "constexpressify" an
existing date library of Howard Hinnant, so this is not just a theoretical
case.

[Bug c++/49136] [C++0x][constexpr] Incorrect constexpr c'tor evaluation with bitfields

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49136

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.05.24 10:03:34
                 CC|                            |jakub at gcc dot gnu.org
     Ever Confirmed|0                           |1

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-24 10:03:34 UTC ---
The problem is that fold optimizes dd.n != 7 comparison into
(bit_field_ref <dd, 8, 0> & 224) != 224 - see optimize_bit_field_compare -
and cxx_eval_bit_field_ref doesn't expect a bit_field_ref which covers more
than one bit field.  One possible fix would be to check also the size in
cxx_eval_bit_field_ref and or in at the right position all the bit fields in
the range, another would be make sure such folding doesn't happen during
constexpr processing.

[Bug c++/49136] [C++0x][constexpr] Incorrect constexpr c'tor evaluation with bitfields

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49136

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-24 10:49:09 UTC ---
Another testcase, this one ICEs:

// PR c++/49136
// { dg-do compile }
// { dg-options "-std=c++0x" }

struct S
{
  unsigned : 1; unsigned s : 27; unsigned : 4;
  constexpr S (unsigned int x) : s(x) {}
};

struct T
{
  unsigned int t;
  constexpr T (S s) : t(s.s != 77 ? 0 : s.s) {}
};

constexpr S s (77);
constexpr T t (s);
static_assert (t.t == 77, "Error");

[Bug c++/49136] [C++0x][constexpr] Incorrect constexpr c'tor evaluation with bitfields

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49136

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-24 12:50:10 UTC ---
Created attachment 24341
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24341
gcc47-pr49136.patch

WIP patch, works on little-endian, but doesn't on big endian, so some
correction somewhere is needed.

[Bug c++/49136] [C++0x][constexpr] Incorrect constexpr c'tor evaluation with bitfields

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49136

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
         AssignedTo|unassigned at gcc dot       |jakub at gcc dot gnu.org
                   |gnu.org                     |

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-24 17:08:22 UTC ---
Created attachment 24345
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24345
gcc47-pr49136.patch

Adjusted fix that passes also on powerpc on the testcases.

[Bug c++/49136] [C++0x][constexpr] Incorrect constexpr c'tor evaluation with bitfields

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49136

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

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

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> 2011-05-24 20:24:47 UTC ---
(In reply to comment #4)
> Created attachment 24345 [details]
> gcc47-pr49136.patch
> 
> Adjusted fix that passes also on powerpc on the testcases.

OK.

[Bug c++/49136] [C++0x][constexpr] Incorrect constexpr c'tor evaluation with bitfields

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49136

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-25 07:00:06 UTC ---
Author: jakub
Date: Wed May 25 07:00:01 2011
New Revision: 174168

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=174168
Log:
    PR c++/49136
    * semantics.c (cxx_eval_bit_field_ref): Handle the
    case when BIT_FIELD_REF doesn't cover only a single field.

    * g++.dg/cpp0x/constexpr-bitfield2.C: New test.
    * g++.dg/cpp0x/constexpr-bitfield3.C: New test.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-bitfield2.C
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-bitfield3.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/semantics.c
    trunk/gcc/testsuite/ChangeLog

[Bug c++/49136] [C++0x][constexpr] Incorrect constexpr c'tor evaluation with bitfields

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49136

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-25 07:16:03 UTC ---
Fixed.

[Bug c++/49136] [C++0x][constexpr] Incorrect constexpr c'tor evaluation with bitfields

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49136

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-05-25 07:03:28 UTC ---
Author: jakub
Date: Wed May 25 07:03:25 2011
New Revision: 174169

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=174169
Log:
    PR c++/49136
    * semantics.c (cxx_eval_bit_field_ref): Handle the
    case when BIT_FIELD_REF doesn't cover only a single field.

    * g++.dg/cpp0x/constexpr-bitfield2.C: New test.
    * g++.dg/cpp0x/constexpr-bitfield3.C: New test.

Added:
    branches/gcc-4_6-branch/gcc/testsuite/g++.dg/cpp0x/constexpr-bitfield2.C
    branches/gcc-4_6-branch/gcc/testsuite/g++.dg/cpp0x/constexpr-bitfield3.C
Modified:
    branches/gcc-4_6-branch/gcc/cp/ChangeLog
    branches/gcc-4_6-branch/gcc/cp/semantics.c
    branches/gcc-4_6-branch/gcc/testsuite/ChangeLog