[Bug middle-end/102276] -ftrivial-auto-var-init fails to initialize a variable, causes a spurious warning

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102276

--- Comment #15 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Qing Zhao <qinzhao@gcc.gnu.org>:

https://gcc.gnu.org/g:dbaabd06aaf4a1b0f2a20671c39148a0bd6ccf0e

commit r12-7452-gdbaabd06aaf4a1b0f2a20671c39148a0bd6ccf0e
Author: Qing Zhao <qing.zhao@oracle.com>
Date:   Wed Mar 2 16:48:37 2022 +0000

    Don't emit switch-unreachable warnings for -ftrivial-auto-var-init
(PR102276)

    At the same time, adding -Wtrivial-auto-var-init and update documentation.
     -Wtrivial-auto-var-init and update documentation.

    for the following testing case:
    1 int g(int *);
    2 int f1()
    3 {
    4     switch (0) {
    5         int x;
    6         default:
    7         return g(&x);
    8     }
    9 }
    compiling with -O -ftrivial-auto-var-init causes spurious warning:
    warning: statement will never be executed [-Wswitch-unreachable]
    5 |         int x;
      |             ^
    This is due to the compiler-generated initialization at the point of
    the declaration.

    We could avoid the warning  to exclude the following cases:

    when
    flag_auto_var_init > AUTO_INIT_UNINITIALIZED
    And
    1) call to .DEFERRED_INIT
    2) call to __builtin_clear_padding if the 2nd argument is present and
non-zero
    3) a gimple assign store right after the .DEFERRED_INIT call that has the
LHS
       as RHS

    However, we still need to warn users about the incapability of the option
    -ftrivial-auto-var-init by adding a new warning option
-Wtrivial-auto-var-init
    to report cases when it cannot initialize the auto variable. At the same
    time, update documentation for -ftrivial-auto-var-init to connect it with
    the new warning option -Wtrivial-auto-var-init,  and add documentation
    for -Wtrivial-auto-var-init.

    gcc/ChangeLog:

            PR middle-end/102276
            * common.opt (-Wtrivial-auto-var-init): New option.
            * doc/invoke.texi (-Wtrivial-auto-var-init): Document new option.
            (-ftrivial-auto-var-init): Update option;
            * gimplify.cc (emit_warn_switch_unreachable): New function.
            (warn_switch_unreachable_r): Rename to ...
            (warn_switch_unreachable_and_auto_init_r): This.
            (maybe_warn_switch_unreachable): Rename to ...
            (maybe_warn_switch_unreachable_and_auto_init): This.
            (gimplify_switch_expr): Update calls to renamed function.

    gcc/testsuite/ChangeLog:

            PR middle-end/102276
            * gcc.dg/auto-init-pr102276-1.c: New test.
            * gcc.dg/auto-init-pr102276-2.c: New test.
            * gcc.dg/auto-init-pr102276-3.c: New test.
            * gcc.dg/auto-init-pr102276-4.c: New test.