Re: Bug in GCC code generation (powerpc-eabi)

["Andrew Pinski" <pinskia at gmail dot com>]

On 3/21/07, Informatica Tecnologia Dual <informatica@tecnologiadual.com> wrote:
> Hi.
>
> I'm using the GCC compiler for PPC405 as delivered with Xilinx EDK 8.1.
> The exact version is:  powerpc-eabi-gcc (GCC) 3.4.1 ( Xilinx EDK 8.1.1
> Build EDK_I.19.3 190106 )
>
> I found a bug in the code generation.
>
> My code is a serial receiver and uses an uninitialized uchar nData:
>
>   uchar nBits;
>   uchar nData;
>   for (nBits=0; nBits<8; nBits++) {
>     nData >>= 1;                // the bug is related to this statement
>     if (function()) nData |= 0x80;
>   }
>
> Although nData is not initialized, the correct state after the first
> "nData >>= 1" statement would be any value between 0x00 and 0x7f.
> However, on my target board I found it to be >= 0x80.

And if nData is not initialized you are invoking undefined behavior
according to the C/C++ standards so the behavior here is ok.  Yes you
are not expecting this behavor but that is not our fault you are
depening on undefined behavior.

Thanks,
Andrew Pinski