[Bug c/55985] New: Misleading message about which variable 'may be used uninitialized in this function'

["jonathan.leffler at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55985

             Bug #: 55985
           Summary: Misleading message about which variable 'may be used
                    uninitialized in this function'
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jonathan.leffler@gmail.com


The problem has been reduced to the following 30 lines of code with no
preprocessing required.

typedef struct vtype { int type; } vtype_t;
typedef struct field { int length; } field_t;
typedef struct clist { char *tuple; field_t *fields; } clist_t;

extern int  cvint(const char *p);
extern void updtypes(clist_t *clist, int ocnt, vtype_t *obind);

void
updtypes(clist_t *clist, int ocnt, vtype_t *obind)
{
    char    *tupstart = clist->tuple;
    char    *savtupstart;
    int      colsize = 0;
    field_t *field = clist->fields;

    for (int cnt = ocnt; cnt--; field++, obind++)
    {
        unsigned col_flags = 0;

        tupstart += colsize;

        colsize = cvint(tupstart+1);
        if (*tupstart & 1)
            col_flags |= 0x04;
        tupstart += 5;

        if (col_flags & 0x04)
            tupstart = savtupstart;
    }
}

The problem is that compilation like this refers to 'tupstart' as the
uninitialized variable, not 'savtupstart'.  The source file was called
'gccbug.c' for this reproduction.

$ gcc-4.7.1 -O3 -Werror -Wall -g -std=c99 -c gccbug.c
gccbug.c: In function âupdtypesâ:
gccbug.c:20:18: error: âtupstartâ may be used uninitialized in this function
[-Werror=maybe-uninitialized]
cc1: all warnings being treated as errors
$ gcc-4.7.1 -Werror -Wall -g -std=c99 -c gccbug.c
gccbug.c: In function âupdtypesâ:
gccbug.c:29:22: error: âsavtupstartâ may be used uninitialized in this function
[-Werror=uninitialized]
cc1: all warnings being treated as errors
$

The real problem is that savtupstart is not initialized.

Compilation with '-v' too gives:

$ gcc-4.7.1 -v -O3 -Werror -Wall -g -std=c99 -c gccbug.c
Using built-in specs.
COLLECT_GCC=/usr/gcc/v4.7.1/bin/gcc
Target: x86_64-unknown-linux-gnu
Configured with: /work4/jleffler/open.source/GCC/gcc-4.7.1/configure
--prefix=/usr/gcc/v4.7.1 --with-gmp=/usr/gnu64 --with-mpfr=/usr/gnu64
-with-mpc=/usr/gnu64
Thread model: posix
gcc version 4.7.1 (GCC) 
COLLECT_GCC_OPTIONS='-v' '-O3' '-Werror' '-Wall' '-g' '-std=c99' '-c'
'-mtune=generic' '-march=x86-64'
 /work5/gcc/v4.7.1/bin/../libexec/gcc/x86_64-unknown-linux-gnu/4.7.1/cc1 -quiet
-v -iprefix /work5/gcc/v4.7.1/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.7.1/
gccbug.c -quiet -dumpbase gccbug.c -mtune=generic -march=x86-64 -auxbase gccbug
-g -O3 -Werror -Wall -std=c99 -version -o /tmp/ccYrtwtL.s
GNU C (GCC) version 4.7.1 (x86_64-unknown-linux-gnu)
    compiled by GNU C version 4.7.1, GMP version 5.0.2, MPFR version 3.1.0, MPC
version 0.9
GGC heuristics: --param ggc-min-expand=89 --param ggc-min-heapsize=112207
ignoring nonexistent directory
"/work5/gcc/v4.7.1/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.7.1/../../../../x86_64-unknown-linux-gnu/include"
ignoring duplicate directory
"/work5/gcc/v4.7.1/bin/../lib/gcc/../../lib/gcc/x86_64-unknown-linux-gnu/4.7.1/include"
ignoring duplicate directory
"/work5/gcc/v4.7.1/bin/../lib/gcc/../../lib/gcc/x86_64-unknown-linux-gnu/4.7.1/include-fixed"
ignoring nonexistent directory
"/work5/gcc/v4.7.1/bin/../lib/gcc/../../lib/gcc/x86_64-unknown-linux-gnu/4.7.1/../../../../x86_64-unknown-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /work5/gcc/v4.7.1/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.7.1/include
 /work5/gcc/v4.7.1/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.7.1/include-fixed
 /usr/local/include
 /work5/gcc/v4.7.1/bin/../lib/gcc/../../include
 /usr/include
End of search list.
GNU C (GCC) version 4.7.1 (x86_64-unknown-linux-gnu)
    compiled by GNU C version 4.7.1, GMP version 5.0.2, MPFR version 3.1.0, MPC
version 0.9
GGC heuristics: --param ggc-min-expand=89 --param ggc-min-heapsize=112207
Compiler executable checksum: 2d58955a5e5707524f76b1886c2957fc
gccbug.c: In function âupdtypesâ:
gccbug.c:20:18: error: âtupstartâ may be used uninitialized in this function
[-Werror=maybe-uninitialized]
cc1: all warnings being treated as errors
$

The machine where it is running, and was built, is RHEL5:

$ uname -a
Linux toru 2.6.18-128.el5 #1 SMP Wed Dec 17 11:41:38 EST 2008 x86_64 x86_64
x86_64 GNU/Linux
$

(Yes, archaic...sorry about that.)

The variable names and function names have been renamed from the original code;
it is unlikely that anyone could spot where it comes from. The structure types
have been renamed and their content minimized. The reduced code does not do
anything useful. Most changes to the code seem to give the correct
'savtupstart' message; I'm sure there are changes I've not tried that don't
change the erroneous output, but not all that many.  Removing the call to
'cvint()' leads to successful compilation (no warning at all), while removing
the test before `tupstart = savtupstart;` leads to the warning mentioning
'savtupstart'.

The problem was originally spotted in a 2850 line source file which includes
many files and generates over 18,000 lines of preprocessor output.  The
original function is just about 1030 lines in the body, plus the declaration
which has more parameters and is spread over 8 lines.