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* [Bug c/107465] New: Bogus warning: promoted bitwise complement of an unsigned value is always nonzero
@ 2022-10-30 17:59 lavr at ncbi dot nlm.nih.gov
  2022-10-31  1:51 ` [Bug c/107465] " pinskia at gcc dot gnu.org
                   ` (22 more replies)
  0 siblings, 23 replies; 24+ messages in thread
From: lavr at ncbi dot nlm.nih.gov @ 2022-10-30 17:59 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107465

            Bug ID: 107465
           Summary: Bogus warning: promoted bitwise complement of an
                    unsigned value is always nonzero
           Product: gcc
           Version: 11.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lavr at ncbi dot nlm.nih.gov
  Target Milestone: ---

Tried on both Ubuntu and Cygwin, the same gcc-11.3.0 -- and the same problem:

$ gcc --version
gcc (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc --version
gcc (GCC) 11.3.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

The code:
$ cat test.c
extern void fun2(void);

#ifdef BOGUS_WARNING
typedef unsigned short uint2;
#else
#define uint2 unsigned short
#endif

static void fun(uint2 x)
{
    if (!(x ^ 0xFFFF))
        fun2();
}


int main(void)
{
    fun(0);
}


The compilation:
$ gcc -Wall -Wextra -O6 -c test.c
(clean)

BUT:
$ gcc -Wall -Wextra -O6 -c -DBOGUS_WARNING test.c
test.c: In function ‘fun’:
test.c:11:9: warning: promoted bitwise complement of an unsigned value is
always nonzero [-Wsign-compare]
   11 |     if (!(x ^ 0xFFFF))
      |         ^

The variable "x" is an unsigned short, so it gets promoted to int without the
sign extension; the second argument to XOR is a 32-bit int with only 16 bits
set to 1.  The result of XOR is not necessarily non-0 because it does not flip
all the bits in an int, but only the lower 16 (so there's no promotion of any
sort, IMO).  Also, it's weird that the warning is only issued with a typedef
for the type of "x".

^ permalink raw reply	[flat|nested] 24+ messages in thread
end of thread, other threads:[~2023-05-20  4:33 UTC | newest]

Thread overview: 24+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-10-30 17:59 [Bug c/107465] New: Bogus warning: promoted bitwise complement of an unsigned value is always nonzero lavr at ncbi dot nlm.nih.gov
2022-10-31  1:51 ` [Bug c/107465] " pinskia at gcc dot gnu.org
2022-10-31  1:59 ` [Bug c/107465] [10/11/12/13 Regression] " pinskia at gcc dot gnu.org
2022-10-31  2:03 ` pinskia at gcc dot gnu.org
2022-10-31  2:03 ` pinskia at gcc dot gnu.org
2022-11-05 10:25 ` rguenth at gcc dot gnu.org
2022-11-21 14:32 ` jakub at gcc dot gnu.org
2022-11-21 14:51 ` jakub at gcc dot gnu.org
2022-11-21 15:28 ` jakub at gcc dot gnu.org
2022-11-21 19:22 ` jakub at gcc dot gnu.org
2023-03-04  9:19 ` cvs-commit at gcc dot gnu.org
2023-03-04  9:22 ` cvs-commit at gcc dot gnu.org
2023-03-04  9:25 ` [Bug c/107465] [10/11/12 " jakub at gcc dot gnu.org
2023-03-19  5:30 ` cvs-commit at gcc dot gnu.org
2023-03-19  5:30 ` cvs-commit at gcc dot gnu.org
2023-03-20 10:28 ` [Bug c/107465] [10/11 " jakub at gcc dot gnu.org
2023-05-02 20:15 ` cvs-commit at gcc dot gnu.org
2023-05-02 20:15 ` cvs-commit at gcc dot gnu.org
2023-05-03  9:30 ` [Bug c/107465] [10 " jakub at gcc dot gnu.org
2023-05-03 15:22 ` cvs-commit at gcc dot gnu.org
2023-05-03 15:22 ` cvs-commit at gcc dot gnu.org
2023-05-04  7:21 ` jakub at gcc dot gnu.org
2023-05-20  4:26 ` pinskia at gcc dot gnu.org
2023-05-20  4:33 ` pinskia at gcc dot gnu.org

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