[Bug c/49971] New: Missing "uninitialized" warning; may involve "return" statements

["cernekee at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49971

           Summary: Missing "uninitialized" warning; may involve "return"
                    statements
           Product: gcc
           Version: 4.5.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: cernekee@gmail.com


This code does not produce any warnings ("uninitialized" or otherwise) on gcc
4.4.3, 4.4.5, or 4.5.3:

int testcase(unsigned int in)
{
    int a = 0, b;

    a++;
    if (in == 1)
        return -1;
    b++;
    return a + b;
}

Command line: gcc -O2 -c test.c -o test.o -Wall

Disassembly:

0000000000000000 <testcase>:
   0:   b8 ff ff ff ff          mov    $0xffffffff,%eax
   5:   c3                      retq   

(IOW, it silently changed the entire function into "return -1")

Compiler:

$ gcc -v
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.4.3-4ubuntu5'
--with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--enable-multiarch --enable-linker-build-id --with-system-zlib
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls
--enable-clocale=gnu --enable-libstdcxx-debug --enable-plugin --enable-objc-gc
--disable-werror --with-arch-32=i486 --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 4.4.3 (Ubuntu 4.4.3-4ubuntu5) 


gcc 4.2.0 and 4.3.5 do produce a warning, using the same command lines.

-O0 on gcc 4.4.3 / 4.4.5 / 4.5.3 does produce a warning.  -O[123s] do not.

If I leave "a" uninitialized, gcc will produce a warning.  "a" does not seem to
be strictly necessary to reproduce the problem involving "b".

If I assign the value of "b" to a global variable before returning, gcc will
produce a warning.