[Bug c/39143] New: Incorrect compilation involving assignment by addition/substraction

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

A friend of mine have noticed an error in GCC when he was developing his own C
compiler. The problem happens when using -O0 (no optimization) and local vars.
See sample code:

#include <stdio.h>

int jj = 0, ii = 0; //global vars

int main()
{
    int j = 0, i = 0; // local vars

    i -= i += 2; // i = 0 is correct
    ii -= ii += 2; // ii = -2 => incorrect, should be 0 too

    printf("i: %d ii: %d\n", i, ii);
    return 0;
}

The output is:
$ gcc main.c -o main -O0 && ./main
i: 0 ii: -2

As you can see, the same code (using code "var -= var += value;") returns a
different value depending if the var is local or global. Also note that, oddly
enough, when we use optimizations then the compilated code turns correct in
both cases:

$ gcc main.c -o main -O1 && ./main
i: 0 ii: 0

Same happens when using C++. This is the gcc version:
$ gcc -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,fortran,objc,obj-c++,treelang --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++/4.2 --program-suffix=-4.2
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr
--enable-targets=all --enable-checking=release --build=i486-linux-gnu
--host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.2.4 (Ubuntu 4.2.4-1ubuntu3)


-- 
           Summary: Incorrect compilation involving assignment by
                    addition/substraction
           Product: gcc
           Version: 4.2.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: edulix at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39143