Re: [PATCH] Split mantissa calculation loop and add branchprediction to mp multiplication

[Siddhesh Poyarekar <siddhesh at redhat dot com>]

On Wed, Jan 02, 2013 at 02:20:13PM -0600, Steven Munroe wrote:
> I do not understand what you are doing here. If the intent is to replace
> the X[], Y[], Z[] doubles with int's you will get overflows in Z[] if
> you are changing X[], y[]. Z[] with uint64_t then you avoid the
> overflows but (Z[k] + CUTTER)-CUTTER has no effect and you have not
> saved any space. Also u is still a double, so you are adding some
> expensive int->float->int converts to the inter loop. 

I don't convert mantissa to int and leave everything as is.  I had
posted the patch to do that earlier, which has not been commented upon
yet and that's the one you should be looking at; this patch has a
different purpose:

http://sourceware.org/ml/libc-alpha/2012-12/msg00354.html

None of the problems you're claiming will exist because:

(1) The product is computed and stored in 64-bit

(2) u does not exist since it is replaced by a much simpler operation,
    which results in that snippet looking like this:

    int64_t tmp = Z[k];
    for (i=i1,j=i2-1; i<i2; i++,j--)
      tmp += (int64_t) X[i]*Y[j];
 
    Z[k]  = (int) (tmp % (1 << 24));
    Z[--k] = (int) (tmp / (1 << 24));


Siddhesh