Currently, our next address prediction cannot distinguish between a
taken compressed branch to PC+4 and an uncompressed instruction that
falls through. We can instead make the NAP machinery much more robust by
keying it on the 16b parcels, with uncompressed branches having their
taken prediction on the second 16b parcel. This also removes the need
for the address prediction requests to be chained.
Moreover, if we decode more than 2 instructions in one cycle due to
decompression, we throw away any subsequent instructions and treat it
like a branch miss, redirecting and thus restarting the pipeline from
the first discarded PC. We should therefore instead save them for
issuing on the next cycle and avoid the redirects. To ensure we don't
needlessly reduce our IPC, if we have a partial issue's width of
instructions saved, we should also support issuing instructions from the
next ICache response if valid, which should be the case in hot
correctly-predicted code paths, especially tight loops. As part of this
change, we also keep the pending straddle state in Fetch3 rather than
sending it to Decode only to have it be forwarded back.
Combined, these two approaches ensure the fetch unit can maintain an IPC
of 2 after it has had time to be correctly trained, regardless of the
distribution of compressed instructions.
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