[Bug c/105323] New: Unused assignment does not get reported

[Bug c/105323] New: Unused assignment does not get reported

[floridsleeves at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105323

            Bug ID: 105323
           Summary: Unused assignment does not get reported
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: floridsleeves at gmail dot com
  Target Milestone: ---

Hi, I'm trying to compile C program using gcc 9.4.0. The C program is:

#include<stdio.h>
int foo(int a)
{
    if(a==0)
        return 0;
    else
        return 1;//error
}
int main (int argc, char *arg[])
{
    int ret = 0;
    if(ret!=0) return 1;
    ret = foo(6);
    return 0;
}

The command I use to compile: gcc -Wunused test.c

The assignment 'ret = foo(6);' is unused. However, it's not reported by gcc.
Imagine if foo is a function that return error code to variable ret if it has
issues to execute, the error code will be unchecked. And gcc currently does not
even report such an unused assignment. Not sure it's a bug or implementation
choice. 


Output from `gcc --version`:
    gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0

[Bug c/105323] assignment that is unused afterwards is not reported

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105323

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
            Summary|Unused assignment does not  |assignment that is unused
                   |get reported                |afterwards is not reported
           Keywords|                            |diagnostic

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Currently set but unused only tracks if the variable is ever unused and not if
it is unused afterwards.

This requires data flow really to do such a warning.

[Bug c/105323] assignment that is unused afterwards is not reported

[floridsleeves at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105323

--- Comment #2 from Li Zhong <floridsleeves at gmail dot com> ---
Any further comment on this?
To do data flow analysis in local variable relatively costs less time and can
definitely help developers find more bugs like forgetting return value, etc.