[Bug c/54558] New: get wrong answer in {static int a; a = 4; a += (a++);}

[soarsky at foxmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54558

             Bug #: 54558
           Summary: get wrong answer in {static int a; a = 4; a += (a++);}
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: soarsky@foxmail.com


1. Code below should output "a=9", however it output "a=5"

#include <stdio.h>
int main()
{
    static int a;
    a = 4;
    a += (a++);
    printf("a=%d\n", a);
    return 0;
}

2. If omit "static" in "static int a", compile and re-run, it output expected
result "a=9"

3. If omit "++" in "a += (a++)", compile and re-run, it output expected result
"a=8"

4. Using gcc -S code in 1, below is a segment of assembly code, hope that helps
    movl     $4, a.1817
    movl        a.1817, %eax
    movl        a.1817, %edx
    addl        %eax, %edx        // %edx equals to 8
    movl        %edx, a.1817
    addl        $1, %eax            // %eax equals to 5
    movl        %eax, a.1817
    movl        a.1817, %eax
    movl        %eax, 4(%esp)
    movl        $.LC0, (%esp)
    call        printf
    movl        $0, %eax
    leave

5. information obtained by "gcc -v -save-temps all-your-options source-file"

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/lto-wrapper
Target: i686-pc-linux-gnu
Configured with: /build/src/gcc-4.7-20120721/configure --prefix=/usr
--libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch
--enable-libstdcxx-time --enable-gnu-unique-object --enable-linker-build-id
--with-ppl --enable-cloog-backend=isl --disable-ppl-version-check
--disable-cloog-version-check --enable-lto --enable-gold --enable-ld=default
--enable-plugin --with-plugin-ld=ld.gold --with-linker-hash-style=gnu
--disable-multilib --disable-libssp --disable-build-with-cxx
--disable-build-poststage1-with-cxx --enable-checking=release
Thread model: posix
gcc version 4.7.1 20120721 (prerelease) (GCC) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=pentiumpro'
 /usr/lib/gcc/i686-pc-linux-gnu/4.7.1/cc1 -E -quiet -v app.c -mtune=generic
-march=pentiumpro -fpch-preprocess -o app.i
ignoring nonexistent directory
"/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/../../../../i686-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/i686-pc-linux-gnu/4.7.1/include
 /usr/local/include
 /usr/lib/gcc/i686-pc-linux-gnu/4.7.1/include-fixed
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=pentiumpro'
 /usr/lib/gcc/i686-pc-linux-gnu/4.7.1/cc1 -fpreprocessed app.i -quiet -dumpbase
app.c -mtune=generic -march=pentiumpro -auxbase app -version -o app.s
GNU C (GCC) version 4.7.1 20120721 (prerelease) (i686-pc-linux-gnu)
    compiled by GNU C version 4.7.1 20120721 (prerelease), GMP version 5.0.5,
MPFR version 3.1.1, MPC version 1.0
warning: MPC header version 1.0 differs from library version 1.0.1.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C (GCC) version 4.7.1 20120721 (prerelease) (i686-pc-linux-gnu)
    compiled by GNU C version 4.7.1 20120721 (prerelease), GMP version 5.0.5,
MPFR version 3.1.1, MPC version 1.0
warning: MPC header version 1.0 differs from library version 1.0.1.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: b205659ec6a57a28c245e2c9c58a01b0
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=pentiumpro'
 as -v --32 -o app.o app.s
GNU assembler version 2.22.0 (i686-pc-linux-gnu) using BFD version (GNU
Binutils) 2.22.0.20120323
COMPILER_PATH=/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/:/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/:/usr/lib/gcc/i686-pc-linux-gnu/:/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/:/usr/lib/gcc/i686-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/:/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-mtune=generic' '-march=pentiumpro'
 /usr/lib/gcc/i686-pc-linux-gnu/4.7.1/collect2 --build-id --eh-frame-hdr
--hash-style=gnu -m elf_i386 -dynamic-linker /lib/ld-linux.so.2
/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/../../../crt1.o
/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/../../../crti.o
/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/crtbegin.o
-L/usr/lib/gcc/i686-pc-linux-gnu/4.7.1
-L/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/../../.. app.o -lgcc --as-needed -lgcc_s
--no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed
/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/crtend.o
/usr/lib/gcc/i686-pc-linux-gnu/4.7.1/../../../crtn.o