[avr-gcc-list] one-byte stack bug in tiny26

[Svein E. Seldal]

Hi,

I'm using tiny26 with gcc (3.3.2). As we all know, the tiny26 has only
one byte stack. My problem arises because of this code:

        void func(void)
        {
                unsigned char array[4];

                array[0] = 1;
                array[1] = 2;
        }

When the function is initialized, gcc copies SPL and SPH (which is
reserved) into YREG in the function prolog.

In the tiny26 the SPH register is reserved; It contains bogus data, 
making YREG into an invalid pointer. When the array is allocated on top 
of the frame, using the YREG pointer, accesses to or from the array data 
is invalid becuase of the corrupted YREG framepointer.

The -mtiny-stack option, does not help either. SPL and SPH is still 
loaded/unloaded in the function's prologues and epiloges.

I found this:

http://www.avr1.org/pipermail/avr-gcc-list/2001-October/000895.html

where it is stated that the loading/unloading of the two stack bytes in 
the function prologue is intentional, despite the usage of the 
-mtiny-stack option.

According to the datasheet, the register of SPH is reserved. This 
implies that we cannot predict the contents of that register, thus gcc 
should never read SPH on this target. (Even if its done intentional.)


Svein