[Bug c/44677] Warn for variables incremented but not used

[Bug c/44677] Warn for variables incremented but not used

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2010-06-28 00:34:58         |2020-5-19

--- Comment #10 from Martin Sebor <msebor at gcc dot gnu.org> ---
See pr95217 for other cases to handle (-Wunused-but-set-parameter).  For the
test case there, Clang's static analyzer diagnoses two out of the four cases:

$ cat pr95217.c && clang -S -Wall -Wextra --analyze pr95217.c
void f0 (int *p)
{
  p = 0;           // -Wunused-but-set-parameter (expected)
}

void f1 (int *p)
{
  p += 1;          // missing warning
}

void f2 (int *p)
{
  p = p + 1;       // missing warning
}

void f3 (int *p)
{
  ++p;             // missing warning
}

pr95217.c:8:3: warning: Value stored to 'p' is never read
  p += 1;          // missing warning
  ^    ~
pr95217.c:13:3: warning: Value stored to 'p' is never read
  p = p + 1;       // missing warning
  ^   ~~~~~
2 warnings generated.

[Bug c/44677] Warn for variables incremented but not used

[jsm28 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677

Joseph S. Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |eyalroz1 at gmx dot com

--- Comment #11 from Joseph S. Myers <jsm28 at gcc dot gnu.org> ---
*** Bug 100184 has been marked as a duplicate of this bug. ***

[Bug c/44677] Warn for variables incremented but not used

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mytbk920423 at gmail dot com

--- Comment #12 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 102909 has been marked as a duplicate of this bug. ***

[Bug c/44677] Warn for variables incremented but not used

[eggert at cs dot ucla.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677

eggert at cs dot ucla.edu changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |eggert at cs dot ucla.edu

--- Comment #13 from eggert at cs dot ucla.edu ---
I ran into this issue today on Emacs master, with both += and |=. Clang
correctly diagnosed unused local variables, but GCC did not.

It would be nice if GCC could do at least as well as Clang does.

Here are references to patches I installed into Emacs to fix these issues that
Clang found but GCC did not:

https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=d9bffa1f3b121085fd8f954eb9446a4a5241c062
https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=68bc1446855c86b96d5bc22f819e63358ab250ac

[Bug c/44677] Warn for variables incremented but not used

[stormbyte at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677

David Manuelda <stormbyte at gmail dot com> changed:

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

--- Comment #14 from David Manuelda <stormbyte at gmail dot com> ---
It is worth to notice that this bug propagates to C++ whenever an object uses
an int that increments but is never used, like the following example:

class Test{
        public:
                Test() { i = 0; i++; }
        private:
                int i;
};

int main(int, char**) {
        Test unused;

        return 0;
}

[Bug c/44677] Warn for variables incremented but not used

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |muecker at gwdg dot de

--- Comment #15 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 108156 has been marked as a duplicate of this bug. ***

[Bug c/44677] Warn for variables incremented but not used (+=, ++)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Warn for variables          |Warn for variables
                   |incremented but not used    |incremented but not used
                   |                            |(+=, ++)

--- Comment #16 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
"set but not used" (just for future searches)

[Bug c/44677] Warn for variables incremented but not used (+=, ++)

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44677

Vincent Lefèvre <vincent-gcc at vinc17 dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vincent-gcc at vinc17 dot net

--- Comment #17 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Martin Sebor from comment #10)
> $ cat pr95217.c && clang -S -Wall -Wextra --analyze pr95217.c
[...]
> pr95217.c:8:3: warning: Value stored to 'p' is never read
>   p += 1;          // missing warning
>   ^    ~
> pr95217.c:13:3: warning: Value stored to 'p' is never read
>   p = p + 1;       // missing warning
>   ^   ~~~~~
> 2 warnings generated.

Clang (15 and above) with -Wunused-but-set-variable now detects the issue on
the "p++" and "p += 1" forms (ditto with other combined assignment operators),
but not on "p = p + 1".

Such forms (p++, etc.) are common, so that detecting an unused variable is very
useful.

Like Paul did for Emacs (comment 13), I've just fixed two issues in GNU MPFR
(one cosmetic about a useless loop variable and one important in the
testsuite), found with Clang 16. The references:
https://gitlab.inria.fr/mpfr/mpfr/-/commit/4c110cf4773b3c07de2a33acbee591cedb083c80
https://gitlab.inria.fr/mpfr/mpfr/-/commit/b34d867fa41934d12d0d4dbaaa0242d6d3eb32c7

For the second MPFR issue, there was an "err++" for each error found by the
function in the testsuite, but err was not tested at the end, so that potential
errors were never reported.