Re: OoO execution

On Tue, 20 May 2025 0:04:03 +0000, Lawrence D'Oliveiro wrote:

On Mon, 19 May 2025 19:09:12 +0000, Ze wrote:
>

Wasn't one of the earliest forms of branch prediction the simple
heuristic of always taking it in one direction and not taking it in the
other direction , I seem to remember that being the case for some of the
early pipelined microprocessors. I believe it was called static branch
prediction compared to the more modern dynamic branch prediction.

>
The simple heuristic I remember was to assume that backward branches
would
be more likely to be taken than not (on the grounds that they were
probably loops) while forward ones would more likely not be taken (I
guess
as an excuse for not disturbing the pipeline too much).

CDC 7600 used this scheme. Backwards taken, forwards not-taken.
Was about 70% accurate for essentially zero storage and 1 (or few)
gates.
This scheme might have been limited in scope (backwards into the
instruction stack was predicted taken, farther than stack was
predicted not-taken:: I don't remember exactly.