[Libre-soc-dev] gigabit router design decisions

Cesar Strauss cestrauss at gmail.com
Tue Nov 2 12:03:30 GMT 2021

Em 01/11/2021 22:21, Jacob Lifshay escreveu:
> On Mon, Nov 1, 2021, 18:09 Jacob Lifshay <programmerjake at gmail.com> wrote:
>> Please don't reject cpu design conversation, remember this project is a
>> community effort, not just you dictating top-down design decisions.
> To be clear, this isn't merely about having a branch predictor or not, but
> about us cooperating as a community to come up with the best design. I will
> be fine if we decide we don't want a branch predictor, but that should be a
> community decision.

My proposal is to plan ahead, and:

1) Design the pipeline *without* branch prediction, but in a way which 
is compatible with easily adding one.

This involves using a "predict always not taken" approach, with the 
ability to flush the Fetch/Decode stages if, after executing a branch, 
Issue's idea of the next PC disagrees with the one at Decode.

This involves just a correct PC x actual PC comparator at Issue, a flush 
input signal on Fetch/Decode, and a "PC override" port on Fetch. No 
complex speculative logic, and no need to panic and restart whenever 
Decode sees a Branch.

2) As we (hopefully) reach the baseline ahead of schedule, we can 
prioritize additional features. By then, simply inserting a branch 
predictor to the "PC override" Fetch port can be seen as "low hanging 
fruit" and "nice to have", not being canceled due to throwing away 
months of existing code.



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