[Bug c/44547] New: -Wuninitialized reports false warning in nested switch statements

["anthony dot penniston at hotmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

GCC with -Wuninitialized enabled and -On where n>= 1 will report falsely that a
variable is uninitialized despite having sufficient information to determine
that the variable must necessarily be initialized.

The erroneous warning is:
In function ‘main’:
warning: ‘n’ may be used uninitialized in this function

The output from running "gcc -v -save-temps -Wuninitialized -O1 source-file"
is: 

Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.3.2-1.1'
--with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.3
--program-suffix=-4.3 --enable-clocale=gnu --enable-libstdcxx-debug
--enable-objc-gc --enable-mpfr --enable-cld --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.3.2 (Debian 4.3.2-1.1) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wuninitialized' '-O1' '-mtune=generic'
 /usr/lib/gcc/x86_64-linux-gnu/4.3.2/cc1 -E -quiet -v test.c -mtune=generic
-Wuninitialized -O1 -fpch-preprocess -o test.i
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../../x86_64-linux-gnu/include"
ignoring nonexistent directory "/usr/include/x86_64-linux-gnu"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/4.3.2/include
 /usr/lib/gcc/x86_64-linux-gnu/4.3.2/include-fixed
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wuninitialized' '-O1' '-mtune=generic'
 /usr/lib/gcc/x86_64-linux-gnu/4.3.2/cc1 -fpreprocessed test.i -quiet -dumpbase
test.c -mtune=generic -auxbase test -O1 -Wuninitialized -version -o test.s
GNU C (Debian 4.3.2-1.1) version 4.3.2 (x86_64-linux-gnu)
        compiled by GNU C version 4.3.2, GMP version 4.2.2, MPFR version 2.3.2.
warning: MPFR header version 2.3.2 differs from library version 2.3.1.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 3741a6052d20b6389f93b1cc4a619780
test.c: In function &#8216;main&#8217;:
test.c:14: warning: &#8216;n&#8217; may be used uninitialized in this function
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wuninitialized' '-O1' '-mtune=generic'
 as -V -Qy -o test.o test.s
GNU assembler version 2.18.0 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Debian) 2.18.0.20080103
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.3.2/:/usr/lib/gcc/x86_64-linux-gnu/4.3.2/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.3.2/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.3.2/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.3.2/:/usr/lib/gcc/x86_64-linux-gnu/4.3.2/:/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wuninitialized' '-O1' '-mtune=generic'
 /usr/lib/gcc/x86_64-linux-gnu/4.3.2/collect2 --eh-frame-hdr -m elf_x86_64
--hash-style=both -dynamic-linker /lib64/ld-linux-x86-64.so.2
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../../lib/crt1.o
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../../lib/crti.o
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/crtbegin.o
-L/usr/lib/gcc/x86_64-linux-gnu/4.3.2 -L/usr/lib/gcc/x86_64-linux-gnu/4.3.2
-L/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../../lib -L/lib/../lib
-L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../.. test.o -lgcc
--as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/crtend.o
/usr/lib/gcc/x86_64-linux-gnu/4.3.2/../../../../lib/crtn.o


*note: the output above lists version 4.3.2; however I have tested with the
same effect on gcc version 4.5.0 and have no reason to believe it doesn't exist
in trunk.

The minimal source code example:
// ---------------------------
/* GCC bug: -Wuninitialized produces false warning
 * compile with gcc -Wuninitialized -On where n >= 1
 */

int main( int argc, char *argv[] )
{
    switch( argc )
    {
        // note: bug no longer occurs if case values are consective (i.e.
1,2,3)
        case 1:
        case 2:
        case 4:
        {
            int n;
            switch( argc )
            {
                case 1:
                case 2:
                case 4:
                    n = argc;
                    break;
            }

            return n;

            break;
        }
    }

    return 0;
}

// ---------------------------

>From the example above, the first switch limits the value of argc to cases 1,
2, or 4, so that in the second switch statement the compiler should be able to
determine that its value must necessarily be 1,2, or 4 and thus n must
necessarily become initialized. This is the case when compiling with no
optimizations or when using consecutive values for cases (i.e. 1,2,3 instead of
1,2,4); however if neither is the case, the compiler issues an erroneous
warning that n is uninitialized.


-- 
           Summary: -Wuninitialized reports false warning in nested switch
                    statements
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: anthony dot penniston at hotmail dot com
 GCC build triplet: x86_64-linux-gnu
  GCC host triplet: x86_64-linux-gnu
GCC target triplet: x86_64-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44547