[Bug middle-end/65472] -Wunreachable-code failure

[Bug middle-end/65472] -Wunreachable-code failure

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65472

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

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

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
-Wunreachable-code does nothing and is ignored.

[Bug middle-end/65472] New: -Wunreachable-code failure

[skvadrik at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65472

            Bug ID: 65472
           Summary: -Wunreachable-code failure
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: skvadrik at gmail dot com

Created attachment 35065
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35065&action=edit
1.c

Given the following code:

extern void f ();
void g ()
{
    for (;;)
    {
        f ();
        continue;
        f ();
    }
}

$ gcc -Wunreachable-code -c 1.c
$ clang -Wunreachable-code -c 1.c
1.c:9:3: warning: code will never be executed [-Wunreachable-code]
                f ();
                ^
1 warning generated.

[Bug middle-end/65472] -Wunreachable-code failure

[skvadrik at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65472

--- Comment #2 from Ulya <skvadrik at gmail dot com> ---
$ gcc -W -Wall -Wextra -c 1.c

gives the same result: no warning

[Bug middle-end/65472] -Wunreachable-code failure

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65472

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Yes, the warning has been removed altogether.  -Wunreachable-code is retained
only for backwards compatibility.

[Bug middle-end/65472] -Wunreachable-code failure

[skvadrik at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65472

Ulya <skvadrik at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|INVALID                     |FIXED

--- Comment #4 from Ulya <skvadrik at gmail dot com> ---
So GCC's intended behavior is not to warn about unreachable code?

[Bug middle-end/65472] -Wunreachable-code failure

[manu at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65472

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu.org
         Resolution|FIXED                       |DUPLICATE

--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
(In reply to Ulya from comment #4)
> So GCC's intended behavior is not to warn about unreachable code?

No, but the previous implementation of Wunreachable-code was very broken and
nobody knew how to fix it, thus it was eventually removed. Since then (years
ago!), nobody has offered a new implementation, which perhaps shows that there
is actually little interest in this warning in practice. In the past, we got
more reports about Wunreachable-code bogus warnings than about missed ones.

Unfortunately, there are very few GCC developers working on the C/C++ FEs right
now and they are very busy with other more urgent things, like structural work
and updating to new standards. In addition, implementing this properly in the
FE would need some kind of control-flow graph, which GCC only has in the
middle-end.

BTW, I don't think it is useful to close these reports as INVALID, since they
are in essence requests for a new implementation of -Wunreachable-code. On the
other hand, it is not useful to have a PR for each possible testcase, since as
far as we know, there is no one even planning to work on this, unfortunately.

*** This bug has been marked as a duplicate of bug 46476 ***
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46476

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

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----------------------------------------------------------------------------
                 CC|                            |skvadrik at gmail dot com

--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> ---
*** Bug 65472 has been marked as a duplicate of this bug. ***
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Subject: [Bug middle-end/65472] -Wunreachable-code failure
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65472

Ulya <skvadrik at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|DUPLICATE                   |FIXED

--- Comment #6 from Ulya <skvadrik at gmail dot com> ---
(In reply to Manuel López-Ibáñez from comment #5)

I see. Thank you for a detailed answer. :)
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ide465

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Reduced testcase:

struct A {};
struct B { virtual A foo () const; };
struct C { A foo () const; };
struct D : virtual B { A foo () const {} };
struct F : D { virtual int bar () const; };
int F::bar () const { return 0; }
A C::foo () const { return A (); }