Simple Rotation and bitmasking problem

[Patrick]

Hi

I have a basic question concerning rotations and bitmasking.

Assume the following code fragement.

uint32 p_lo[] = { 0x00, 0x00};

for (j = 0; j < 64; j++ )

p_lo[0] <<= (j % 32);

p_lo[1] <<= (j % 32);

p_lo[1] |= (p_hi[0] >> (32 - (j % 32)));
}

I itend to do the following. In each iteration shift each element of
the array by an amount of
n. Additionally, the n-bits that are shifted out of each word should
be the least significant bits of the next higher
word. This I try to do by shifting the lower word by 32 - n positions
to the left and OR then the result
with the next higher word. Unfortunately the output does not make
sense.

Has anyone an idea what I am missing?

Many thanks,
Patrick

 

[Old Wolf]

uint32 p_lo[] = { 0x00, 0x00};

for (j = 0; j < 64; j++ )
p_lo[0] <<= (j % 32);
p_lo[1] <<= (j % 32);
p_lo[1] |= (p_hi[0] >> (32 - (j % 32)));
}

I suppose you wanted a '{' after the 'for' statement.
You haven't shown the definition of p_hi. But if it
is an array of 32-bit entities, then you have undefined
behaviour when j==0 and j==32 because you shift its
first element by the entire width of the element.

I itend to do the following. In each iteration shift
each element of the array by an amount of n.
Additionally, the n-bits that are shifted out of each
word should be the least significant bits of the next
higher word.

Don't quite follow the description because you
haven't shown enough code, but in the above
snippets the bits that you shift out of p_lo[0]
and p_lo[1] are all lost.