Re: Tonights Tradeoff

On Fri, 13 Sep 2024 15:08:42 +0000, EricP wrote:

MitchAlsup1 wrote:

On Thu, 12 Sep 2024 19:28:19 +0000, Robert Finch wrote:
>

Would not want to queue to the reorder buffer from the alternate path,
as there is a bit of a bottleneck at queue. Not wondering what to do
about multiple branches. Multiple pipelines and more switches? Front-end
would look like a pipeline tree to handle multiple outstanding branches.
>
Was wondering what to do with the extra fetch bandwidth. Fetching two
cache-lines at once means there may have been up to 21 instructions
fetched. But its only a four-wide machine.

>
For my 6-wide machine I am fetching 1/2 a cache line twice for the
sequential path and 1/2 a cache line for the alternate path from
an 8 banked ICache.

>
Why 8 banks if you are fetching just three 32-byte buffers at once?
I suppose 8 minimizes the chance of colliding on a bank access.
Still, it seems like 4 banks would be sufficient.

3 banks for the predicted fetch stuff, 1-2 banks for the mispredicted
fetches.
You not only fetch instructions on the predicted directions, you fetch
instructions on the predicted non-taken directions so they are ready
for insertion should that branch need backup.