Bug#345435: gcc-3.3: wrong evaluation of ++ expressions

[John Mysoul <katsankat@wanadoo.fr>]

On Saturday 31 December 2005 14:47, Martin Stumpf wrote:
> Package: gcc-3.3
> Version: 1:3.3.5-13
> Severity: normal
>
> The following program evaluates d to 13. On other architectures (and in
> java) the same code evaluates to 12.
> (2.95, 3.3, 3.4, 4.0, 4.1 do have the same bug)
>
> #include <iostream>
>
> using namespace std;
>
> int main(int argc, char* argv[]) {
>   int b = 2;
>   int d = ++b + ++b + ++b;
>   std::cout << "b: " << b << "d: " << d << endl;
>   return 0;
> }
>
>
> -- System Information:
> Debian Release: 3.1
> Architecture: i386 (i686)
> Kernel: Linux 2.6.8-2-k7
> Locale: LANG=C, LC_CTYPE=de_DE (charmap=ISO-8859-1)
>
> Versions of packages gcc-3.3 depends on:
> ii  binutils                    2.15-6       The GNU assembler, linker and
> bina ii  cpp-3.3                     1:3.3.5-13   The GNU C preprocessor ii
>  gcc-3.3-base                1:3.3.5-13   The GNU Compiler Collection (base
> ii  libc6                       2.3.2.ds1-22 GNU C Library: Shared
> libraries an ii  libgcc1                     1:3.4.3-13   GCC support
> library
>
> -- no debconf information

It's not a bug but the way the compiler evaluates expressions.
With the -Wall argument, GCC reports the result may be "undefined"
whereas g++ doesn't warn at all.

It's somewhat fascinating 12 can be obtained with a
  int d = (b+= ++b, b+= b);
or with a
  int d = (++b + b++ , ++b + b++ , b + b) ;
or a 
  b+=++b;  int d = b+=b;
but not int d = ++b + ++b + ++b;

Thanks for pointing this out.