[Bug c/65812] New: gcc 4.9.1 doesn't warn about uninitialized variable use declared in a switch/case statement when compiled with -O

["joe at perches dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65812

            Bug ID: 65812
           Summary: gcc 4.9.1 doesn't warn about uninitialized variable
                    use declared in a switch/case statement when compiled
                    with -O
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: joe at perches dot com

Created attachment 35365
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35365&action=edit
sample code

Here is a small example:

$ cat t.c
struct foo {
       int bar[100];
};

void foo_1(void)
{
    unsigned int m;
    extern struct foo *array[100];
    struct foo *a = array[m];

    a = a;
}

void foo_2(void)
{
    int i = 1;

    switch (i) {
    case 1: {
        unsigned int m;
        extern struct foo *array[100];
        struct foo *a = array[m];

        a = a;
        break;
    }
    }
}

$

gcc warns properly about both foo_1 and foo_2 without -O

$ gcc -c -Wall t.c
t.c: In function âfoo_1â:
t.c:9:14: warning: âmâ is used uninitialized in this function [-Wuninitialized]
  struct foo *a = array[m];
              ^
t.c: In function âfoo_2â:
t.c:21:15: warning: âmâ may be used uninitialized in this function
[-Wmaybe-uninitialized]
   struct foo *a = array[m];
               ^
$

but gcc warns about foo_1 but not foo_2 with -O

$ gcc -c -Wall -O t.c
t.c: In function âfoo_1â:
t.c:9:14: warning: âmâ is used uninitialized in this function [-Wuninitialized]
  struct foo *a = array[m];
              ^

$ gcc -v
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.9/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.9.1-16ubuntu6'
--with-bugurl=file:///usr/share/doc/gcc-4.9/README.Bugs
--enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.9 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.9 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-vtable-verify
--enable-plugin --with-system-zlib --disable-browser-plugin
--enable-java-awt=gtk --enable-gtk-cairo
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64/jre --enable-java-home
--with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.9-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.9-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib
--with-tune=generic --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.9.1 (Ubuntu 4.9.1-16ubuntu6)