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From: "lavr at ncbi dot nlm.nih.gov" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/107465] New: Bogus warning: promoted bitwise complement of an
 unsigned value is always nonzero
Date: Sun, 30 Oct 2022 17:59:24 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 11.3.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: lavr at ncbi dot nlm.nih.gov
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107465

            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 proble=
m:

$ 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 =E2=80=98fun=E2=80=99:
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 t=
he
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 f=
lip
all the bits in an int, but only the lower 16 (so there's no promotion of a=
ny
sort, IMO).  Also, it's weird that the warning is only issued with a typedef
for the type of "x".=