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Re: MS/CW-style inline assembly for GCC
On Mon, May 10, 2004 at 12:43:43PM -0400, Ian Lance Taylor wrote:
> Jack Lloyd <lloyd@randombit.net> writes:
>
> > Actually pretty much all MPI math can be implemented efficiently in plain
> > C.
>
> Is this really true? What about the simple case of unlimited-
> precision addition on a processor which has a carry flag? Do we
> expect the compiler to be smart enough to turn a loop like
> c = 0;
> for (i = 0; i < len; i++)
> {
> r[i] = a[i] + b[i] + c;
> c = (int) (((long long) a[i] + (long long) b[i]) >> 32);
> }
> into code which uses the "add with carry" instruction?
Actually, usually one uses compares, GNU MP's add loop looks like this:
do
{
ul = *up++;
vl = *vp++;
sl = ul + vl;
cy1 = sl < ul;
rl = sl + cy;
cy2 = rl < sl;
cy = cy1 | cy2;
*rp++ = rl;
}
while (--n != 0);
The code you posted doesn't exactly work if you're using 64 bit words, which is
kind of a must.
And yes, I expect the compiler to figure it out. :) I actually didn't even
think about CPUs with a cc, most of my asm programming has been on Alpha. My
fault on that.
-Jack