gcc doesn't warn about uninitialized variable use in switch/case with -O (was: Re: [PATCH] Staging: dgnc: Using temporary value for repeated dereferences)

[Joe Perches <joe@perches.com>]

On Mon, 2015-04-20 at 11:33 +0300, Dan Carpenter wrote:
> You would hope that GCC would warn about the uninitialized variable but
> it doesn't.

That's odd.  So I filed this new gcc bugzilla:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65812)

gcc 4.9.1 doesn't warn about uninitialized variable use
declared in a switch/case statement when compiled with -O

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 ‘foo_1’:
t.c:9:14: warning: ‘m’ is used uninitialized in this function
[-Wuninitialized]
  struct foo *a = array[m];
              ^
t.c: In function ‘foo_2’:
t.c:21:15: warning: ‘m’ 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 ‘foo_1’:
t.c:9:14: warning: ‘m’ 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)