Re: Decrement And Branch

Lawrence D'Oliveiro <ldo@nz.invalid> writes:

On Wed, 14 Aug 2024 09:10:01 GMT, Anton Ertl wrote:

Like I said, I wondered why this sort of thing wasn't more common ...

 
For the early RISCs, the pipeline was designed for early branch
execution.

>
Note that I was referring to the decrement-down-to-minus-1 form, as
opposed to the decrement-down-to-zero form.

I guess what you want to point out is that

x = x-1
if (x!=-1) goto ...

is equivalent to

flag = x!=0; x = x-1; if (flag) goto ...

but in the latter the branch does not need to wait for the decrement
to complete.  As for x!=0 vs. x!=1, the CPU may already have special
circuits for x!=0.

Ok, so this is not the reason for not having this instruction.  Which
leaves: It is not useful that often.

- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>