[Bug middle-end/104504] New: spurious -Wswitch-unreachable warning with -ftrivial-auto-var-init=zero

[Bug middle-end/104504] New: spurious -Wswitch-unreachable warning with -ftrivial-auto-var-init=zero

[nsz at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104504

            Bug ID: 104504
           Summary: spurious -Wswitch-unreachable warning with
                    -ftrivial-auto-var-init=zero
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nsz at gcc dot gnu.org
  Target Milestone: ---

reduced from linux code on which gcc-12 warns now:

int foo(int x) {
    switch(x) {
        int y;
        /* spuriously warns with -ftrivial-auto-var-init=zero */
    default:
        y = x * 2;
        return y;
    }
}

$ gcc -Wall -ftrivial-auto-var-init=zero -c a.c
a.c: In function 'foo':
a.c:3:13: warning: statement will never be executed [-Wswitch-unreachable]
    3 |         int y;
      |             ^

i can see why gcc warns, but it would be better not to.

[Bug middle-end/104504] spurious -Wswitch-unreachable warning with -ftrivial-auto-var-init=zero

[kees at outflux dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104504

Kees Cook <kees at outflux dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kees at outflux dot net

--- Comment #1 from Kees Cook <kees at outflux dot net> ---
Similar issue exists in Clang, though Clang doesn't warn:
https://github.com/llvm/llvm-project/issues/44261

[Bug middle-end/104504] spurious -Wswitch-unreachable warning with -ftrivial-auto-var-init=zero

[kees at outflux dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104504

--- Comment #2 from Kees Cook <kees at outflux dot net> ---
As mentioned in a Linux kernel thread, isn't it possible to transform this:


  switch (x) {
      int y;
  default:
      y = x * 2;
      return y;
  }

into this:

  {
      int y;
      switch (x) {
      default:
          y = x * 2;
          return y;
      }
  }

[Bug middle-end/104504] spurious -Wswitch-unreachable warning with -ftrivial-auto-var-init=zero

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104504

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

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

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
At the C/C++ level certainly not, it could be
int foo(int x, int y) {
  switch (bar (x, y)) {
    int y;
    default:
      y = x * 2;
      return y;
  }
}
and the inner y shouldn't be in scope of the switch control expression.
Now, for this particular case, it surely could be handled like:
  auto tmp_ = bar (x, y);
  {
    int y;
    switch (tmp_) {
    default:
      y = x * 2;
      return y;
    }
  }
but I really don't understand why exactly this case should get extra special
care when
  switch (x) {
  case 4:
    ++y;
  case 5:
    int y;
  default:
    y = x * 2;
    return y;
  }
can't easily.

[Bug middle-end/104504] spurious -Wswitch-unreachable warning with -ftrivial-auto-var-init=zero

[kees at outflux dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104504

--- Comment #4 from Kees Cook <kees at outflux dot net> ---
(Ah, I knew this had been reported before. I found it now...)
Duplicate of: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102276

[Bug middle-end/104504] spurious -Wswitch-unreachable warning with -ftrivial-auto-var-init=zero

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104504

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

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

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 102276.

*** This bug has been marked as a duplicate of bug 102276 ***