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Proposal for an ideal extensible instruction set
Author: Agner Date: 2016-04-11 08:04
HarryDev wrote:
I think the proposed ISA needs specific instruction for memory copying, initialization and comparison.
My experience with x86 processors is that these microcoded instructions have very long setup times and that software loops with vector registers are faster in most cases. I want to avoid complex microcode instructions. Instead, I have designed instructions for very fast software loops using variable-length vector registers.

A-11 wrote:

BOOST: Berkeley's Out-of-Order Stack Thingy
Thanks for the interesting reference. As I understand the article, it is more difficult to do out-of-order processing on a stack machine than on a conventional machine.

zboson wrote:

double-double multiplication can be done with

high = a * b;
low = fma(a, b, -high);

However double-double addition cannot be done so simply. If there was a three operand single rounding mode (a+b+c) instruction then double-double addition would be much faster (actually (a+b-c) is perhaps the most interesting).

Interesting idea. Actually, I once used that fma trick to get extra precision in a multiplication. So as I understand your idea, you want to do:
high = a + b
low = a + b - high // (with extra precision in the intermediate)
I think this would be quite costly to implement in hardware, considering the rare use. Instead, you can do this:
high = a + b;
if (abs(a) > abs(b)) {
   low = b - (high - a);
} else {
   low = a - (high - b);
}
This does not require any extra hardware.
 
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