[Bug c/114763] New: Wduplicated-branches just check last else if-else case?

[Bug c/114763] New: Wduplicated-branches just check last else if-else case?

[hanwei62 at huawei dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114763

            Bug ID: 114763
           Summary: Wduplicated-branches just check last else if-else
                    case?
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hanwei62 at huawei dot com
  Target Milestone: ---

This code:

void foo(int x) {
    if (x) {
        x -= 2;
    } else if (x > 50) {
        x -= 2;
    } else
        x += 2;
}

# gcc -c test.c -Wduplicated-branches

not warning, even if-else if branches is same.

void foo(int x) {
    if (x) {
        x -= 2;
    } else if (x > 50) {
        x -= 2;
    } else if (x < 20) {
        x += 2;
    } else {
        x += 2;
    }
}

warning.

Same like https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85094

What I find that Wduplicated-branches just check the last `else if-else`
branches.

https://godbolt.org/z/WP8v5P4Yf

It's misleading.

[Bug c/114763] Wduplicated-branches just check last else if-else case?

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114763

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note only the second case has if's 2 sides which are the same;
it is basically `a ? b : (c ? d : d)`.

While the first case you have `a ? b : (c ? d : b)` which is not supposed to
warn about at all because the 2 sides are not the same.

The reason is because that is how the grammar of C/C++ works.

[Bug c/114763] Wduplicated-branches just check last else if-else case?

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114763

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement

[Bug c/114763] Wduplicated-branches just check last else if-else case?

[hanwei62 at huawei dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114763

--- Comment #2 from hanwei (K) <hanwei62 at huawei dot com> ---
(In reply to Andrew Pinski from comment #1)
> Note only the second case has if's 2 sides which are the same;
> it is basically `a ? b : (c ? d : d)`.
> 
> While the first case you have `a ? b : (c ? d : b)` which is not supposed to
> warn about at all because the 2 sides are not the same.
> 
> The reason is because that is how the grammar of C/C++ works.

So, Wduplicated-branches just check bottom two branches actually.
Because only the bottom two branches will same if we check followed the grammar
of C/C++.

[Bug c/114763] Wduplicated-branches just check last else if-else case?

[hanwei62 at huawei dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114763

--- Comment #3 from hanwei (K) <hanwei62 at huawei dot com> ---
(In reply to Andrew Pinski from comment #1)
> Note only the second case has if's 2 sides which are the same;
> it is basically `a ? b : (c ? d : d)`.
> 
> While the first case you have `a ? b : (c ? d : b)` which is not supposed to
> warn about at all because the 2 sides are not the same.
> 
> The reason is because that is how the grammar of C/C++ works.

So, Wduplicated-branches just check bottom two branches actually.
Because only the bottom two branches will same if we check followed the grammar
of C/C++.

[Bug c/114763] Wduplicated-branches just check last else if-else case?

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114763

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
-Wduplicate-branches compares the substatements of an if statement.
In C/C++, there is not a statement like if ... elseif ... elseif ... else, just
if statement.
In your first example, there are 2 if statements, one has
{ x -= 2; }
and
if (x > 50) { x -= 2; } else x += 2;
substatements (and those are not the same), and the other has
{ x -= 2; }
and
x += 2;
substatements and those aren't the same either.