It is now the job of the pipeline to monitor the frontend's valid mask (of
instructions) and the BTB's suggested valid mask (based on the prediction it
makes). Some processors may want to ignore or override the BTB's prediction and
thus can supply their own instruction mask.
- BTBUpdate only occurs on mispredicts now.
- RASUpdate broken out from BTBUpdate (allows RASUpdate to be performed in
Decode).
- Added optional 2nd CAM port to BTB for updates (for when updates to the
BTB may occur out-of-order).
- Fixed resp.mask bit logic.
- Global history only contains branches.
- Only update BHT and history on BTB hits.
- Gate off speculative update on stall or icmiss.
- Fixed bug where BHT updates were delayed a cycle.
- No longer update global history in fetch stage.
- Only update global history when instruction is a branch.
- Does allow for the possibility of back-to-back branches to see
slightly different histories on subsequent executions.
- Updated in fetch speculatively.
* Updates gated off by cpu.resp.fire().
* BTB direction factored into history update.
- All branches update the BHT.
- Each instruction carries history; index into BHT is recomputed by
passing in mem_reg_pc.
- The nbdache (among others?) use CoreParameters, which has nothing to do with RetireWidth requirements.
- This conflicts with other cores which uses nbdcache.
- RocketCoreParameters may be unneccessary, and the require() check can be moved deeper into Rocket.
Christopher Celio