Re: [avr-gcc-list] Re: C aliasing rules

[Lars Noschinski]

Hi!

* Bernard Fouché <address@hidden> [10-05-19 17:06]:
[reordered quote]
> >>>// 
> >>>---------------------------------------------------------------------------
> >>>void xteaDecrypt(uint32_t v[2], uint32_t const k[4]) {
> >>>    uint32_t v0=v[0], v1=v[1], delta=0x9E3779B9, sum=delta*XTEA_ROUNDS;
> >>>    for (uint8_t i=0; i<  XTEA_ROUNDS; i++) {
> >>>        v1 -= (((v0<<  4) ^ (v0>>  5)) + v0) ^ (sum + k[(sum>>11)&  3]);
> >>>        sum -= delta;
> >>>        v0 -= (((v1<<  4) ^ (v1>>  5)) + v1) ^ (sum + k[sum&  3]);
> >>>    }
> >>>    v[0]=v0; v[1]=v1;
> >>>}
> >>>
> >>>void xteaDecryptCbc(uint8_t v[8], uint8_t cb[8], uint8_t const k[16]) {
> >>>    static uint8_t tmpbuf[8];
> >>>    memcpy(tmpbuf, v, 8);
> >>>    xteaDecrypt((uint32_t*)v, (uint32_t*)k);
> >>>    for (uint8_t i=0; i<  8; i++)
> >>>        v[i] ^= cb[i];
> >>>    memcpy(cb, tmpbuf, 8);
> >>>}
> >>>
> >>>int main(void) {
> >>>    uint8_t b1[8], b2[8], b3[16];
> >>>    xteaDecryptCbc(b1, b2 b3);
> >>>}
> >>>// 
> >>>---------------------------------------------------------------------------
>
> I don't understand what 'tmpbuf' is declared 'static'.

tmpbuf is declared static as the compiler produces better code in this
case (arrays on the stack are costly).

> And what do you mean by 'broken'? In your example, main() calls
> functions by providing  pointers to uninitialized data located in the
> stack.

This was just an incomplete example, in my application those buffers are
filled with real data. I provided the declarations in main() because for
alias analysis the declared type of a variable matters.

 -- Lars.