gcc generates incorrect code

Thanks dabuk for your correction. You are absolutely correct.

BTW, while consuming BW, there was in fact a funny behaviour remaining in this code after it is corrected as debuk suggests (which in fact is the one I was originally pursuing). Do you see it? It resulted from a more subtle programming error (unfortunately still my fault STS). The following is a very bad idea in portable C code.

uint32_t mask;

if ( l < 32u ) {

static const uint32_t one = 1u;

const unsigned idx = 32u - l;

uint32_t mask = one;

mask <<= idx;

mask -= one;

}

This one is correct:

uint32_t mask;

if ( l > 0 && l < 32u ) {

static const uint32_t one = 1u;

const unsigned idx = 32u - l;

uint32_t mask = one;

mask <<= idx;

mask -= one;

}

The problem in the first version is that the C standard says that the result of a left shift is undefined if we specify a shift of more bits than are in the word, and 32 is in fact too many. The nios2 RTL for the SLL instruction is as follows.

rC &#8592; rA << (rB4..0)

Notice that it uses only 5 bits from the register, shoot self in foot.

Ouch