[Bug c/34070] New: Wrong code for (int)x%4

[Bug c/34070] New: Wrong code for (int)x%4

[simonmar at microsoft dot com]

The following code generates the wrong result:

--------------------
#include <stdio.h>

int f(unsigned int x)
{
    printf("%x %d\n", x, (int)x);
    return ((int)x) % 4;
}

int main(int argc, char *argv[])
{
    printf("%d\n", f((unsigned int)(-1)));
    return 0;
}
--------------------

I expect this:

$ gcc-3.4.3 ctest33.c -Wall  && ./a.out
ffffffff -1
-1

and with gcc-4 and greater I get this:

$ gcc-4.2.1 ctest33.c -Wall  && ./a.out
ffffffff -1
3

Why do I think this is a bug?  Well, initially I thought I'd run into undefined
behaviour, but on closer reading of the C spec it seems the behaviour should be
implementation-defined, and gcc is not implementing the documented behaviour. 
Furthermore, gcc's behaviour is not consistent, as implementation-defined
behaviour should be.

The bug appears to be centered around conversion from unsigned to signed
integers.  We convert from unsigned to signed in f(), and the value passed is
0xffffffff.  The result is therefore implementation-defined (C99 6.3.1.3), and
gcc defines it (section 4.5 of the gcc docs) as: "For conversion to a type of
width N, the value is reduced modulo 2^N to be within range of the type".  I
presume this means that the value is truncated to N bits and the result
interpreted as twos-complement, which in this case should mean that (int)x is
-1, and the expression is (-1 % 4), which has value -1.

We can see from the printf output that (int)x has value -1.  Since this is its
implementation-defined value, it should have the same value in the expression
(int)x % 4.

Indeed, several minor variations of this code give the expected output. 
Substituting 0xffffffffU for x in the definition of f(), for example.

Optimisation level has no effect.  Bug also observed on i686-unknown-linux.


-- 
           Summary: Wrong code for (int)x%4
           Product: gcc
           Version: 4.2.1
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: simonmar at microsoft dot com
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34070

[Bug rtl-optimization/34070] [4.1/4.2/4.3 Regression] Wrong code for (int)x%4

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2007-11-12 12:41 -------
The difference is that we now generate

f:
.LFB2:
        andl    $3, %edi
        movl    %edi, %eax
        ret

while previously we'd get the correct

f:
.LFB3:
        leal    3(%rdi), %eax
        cmpl    $-1, %edi
        cmovg   %edi, %eax
        andl    $-4, %eax
        subl    %eax, %edi
        movl    %edi, %eax
        ret

for

int f(unsigned int x)
{
    return ((int)x) % 4;
}

This is a problem in fold, as we get initially:

;; Function f (f)
;; enabled by -tree-original


{
  return (int) x & 3;
}

which is wrong.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
          Component|c                           |rtl-optimization
     Ever Confirmed|0                           |1
           Keywords|                            |wrong-code
   Last reconfirmed|0000-00-00 00:00:00         |2007-11-12 12:41:59
               date|                            |
            Summary|Wrong code for (int)x%4     |[4.1/4.2/4.3 Regression]
                   |                            |Wrong code for (int)x%4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34070

[Bug rtl-optimization/34070] [4.1/4.2/4.3 Regression] Wrong code for (int)x%4

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2007-11-12 12:45 -------
Mine.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |rguenth at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2007-11-12 12:41:59         |2007-11-12 12:45:59
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34070

[Bug middle-end/34070] [4.1/4.2/4.3 Regression] Wrong code for (int)x%4

[rguenth at gcc dot gnu dot org]

-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|rtl-optimization            |middle-end
      Known to fail|                            |4.1.0
      Known to work|                            |4.0.4
   Target Milestone|---                         |4.2.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34070

[Bug middle-end/34070] [4.1/4.2 Regression] Wrong code for (int)x%4

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2007-11-12 14:16 -------
Fixed for 4.3.0.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|4.0.4                       |4.0.4 4.3.0
            Summary|[4.1/4.2/4.3 Regression]    |[4.1/4.2 Regression] Wrong
                   |Wrong code for (int)x%4     |code for (int)x%4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34070

[Bug middle-end/34070] [4.1/4.2/4.3 Regression] Wrong code for (int)x%4

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2007-11-12 14:16 -------
Subject: Bug 34070

Author: rguenth
Date: Mon Nov 12 14:16:05 2007
New Revision: 130098

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=130098
Log:
2007-11-12  Richard Guenther  <rguenther@suse.de>

        PR middle-end/34070
        * fold-const.c (fold_binary): If testing for non-negative
        operands with tree_expr_nonnegative_warnv_p make sure to
        use op0 which has all (sign) conversions retained.

        * gcc.c-torture/execute/pr34070-1.c: New testcase.
        * gcc.c-torture/execute/pr34070-2.c: Likewise.

Added:
    trunk/gcc/testsuite/gcc.c-torture/execute/pr34070-1.c
    trunk/gcc/testsuite/gcc.c-torture/execute/pr34070-2.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/fold-const.c
    trunk/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34070

[Bug middle-end/34070] [4.1/4.2 Regression] Wrong code for (int)x%4

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2008-01-22 14:46 -------
Subject: Bug 34070

Author: rguenth
Date: Tue Jan 22 14:45:56 2008
New Revision: 131723

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=131723
Log:
2008-01-22  Richard Guenther  <rguenther@suse.de>

        PR middle-end/34739
        Backport from mainline
        2008-01-16  Richard Guenther  <rguenther@suse.de>

        PR c/34768
        * c-typeck.c (common_pointer_type): Do not merge inconsistent
        type qualifiers for function types.

        2007-11-12  Richard Guenther  <rguenther@suse.de>

        PR middle-end/34070
        * fold-const.c (fold_binary): If testing for non-negative
        operands with tree_expr_nonnegative_warnv_p make sure to
        use op0 which has all (sign) conversions retained.

        2006-10-24  Richard Guenther  <rguenther@suse.de>

        PR middle-end/28796
        * builtins.c (fold_builtin_classify): Use HONOR_INFINITIES
        and HONOR_NANS instead of MODE_HAS_INFINITIES and MODE_HAS_NANS
        for deciding optimizations in consistency with fold-const.c
        (fold_builtin_unordered_cmp): Likewise.

Added:
    branches/gcc-4_2-branch/gcc/testsuite/gcc.c-torture/execute/pr34070-1.c
      - copied unchanged from r130098,
trunk/gcc/testsuite/gcc.c-torture/execute/pr34070-1.c
    branches/gcc-4_2-branch/gcc/testsuite/gcc.c-torture/execute/pr34070-2.c
      - copied unchanged from r130098,
trunk/gcc/testsuite/gcc.c-torture/execute/pr34070-2.c
    branches/gcc-4_2-branch/gcc/testsuite/gcc.c-torture/execute/pr34768-1.c
      - copied unchanged from r131568,
trunk/gcc/testsuite/gcc.c-torture/execute/pr34768-1.c
    branches/gcc-4_2-branch/gcc/testsuite/gcc.c-torture/execute/pr34768-2.c
      - copied unchanged from r131568,
trunk/gcc/testsuite/gcc.c-torture/execute/pr34768-2.c
    branches/gcc-4_2-branch/gcc/testsuite/gcc.dg/pr28796-1.c
      - copied unchanged from r118001, trunk/gcc/testsuite/gcc.dg/pr28796-1.c
    branches/gcc-4_2-branch/gcc/testsuite/gcc.dg/pr28796-2.c
      - copied unchanged from r118001, trunk/gcc/testsuite/gcc.dg/pr28796-2.c
Modified:
    branches/gcc-4_2-branch/gcc/ChangeLog
    branches/gcc-4_2-branch/gcc/builtins.c
    branches/gcc-4_2-branch/gcc/c-typeck.c
    branches/gcc-4_2-branch/gcc/fold-const.c
    branches/gcc-4_2-branch/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34070

[Bug middle-end/34070] [4.1 Regression] Wrong code for (int)x%4

[rguenth at gcc dot gnu dot org]

------- Comment #6 from rguenth at gcc dot gnu dot org  2008-01-22 14:52 -------
Fixed on the 4.2 branch.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|rguenth at gcc dot gnu dot  |unassigned at gcc dot gnu
                   |org                         |dot org
             Status|ASSIGNED                    |NEW
      Known to work|4.0.4 4.3.0                 |4.0.4 4.2.3 4.3.0
            Summary|[4.1/4.2 Regression] Wrong  |[4.1 Regression] Wrong code
                   |code for (int)x%4           |for (int)x%4
   Target Milestone|4.2.3                       |4.1.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34070

[Bug middle-end/34070] [4.1 Regression] Wrong code for (int)x%4

[jsm28 at gcc dot gnu dot org]

------- Comment #7 from jsm28 at gcc dot gnu dot org  2008-07-04 16:16 -------
Closing 4.1 branch.


-- 

jsm28 at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
      Known to fail|4.1.0                       |4.1.0 4.1.3
         Resolution|                            |FIXED
   Target Milestone|4.1.3                       |4.2.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34070