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Sunday, February 06, 2011

Branch Prediction Stew

{{Terminology Term}}

The [[stew]] is a form of history used by certain branch predictors.

See, for example, US patent 7143273,
Method and apparatus for dynamic branch prediction utilizing multiple stew algorithms for indexing a global history,
Mile, Slade, and Jourdan,
filed March 31, 2003,
assignee Intel.

A simple [[branch predictor history]] might be  a simple [[TNT]] history,
with 0s corresponding to non-taken and 1s corresponding to taken.
Such a simple TNT history cannot distinguish some convergeing paths,
and indirect branches.

describes one embodiment of a stew as
 stew = ((stew << 1)|new_bit ^  ip)

A stew formed  in this way can distinguish converging paths such as  ("if true" means a condition that evaluates as true, not an unconditional branch):
   L1: if true got L2
   L2: if true goto L99
   L10: if true goto L11
   L11: if true  goto L
   L:  if ?? goto L99

However, it does not distinguish multiple  branch targets and paths out of an indirect branch, such as
   L1: Reg:= IL1; if true goto L2
   L2: if true goto L99
   L10: Reg:=IL2; if true goto L11
   L11: if true  goto L
   L:  if ?? goto [Reg]
   IL1: ...
   IL2: ...
It can be seen that mixing in arc information as well as node information remedies this situation,
and distinguishes different paths so long as the hashes do not collide:

 stew <<= number_of_bits_to_discard
 stew = hash( stew, from_IP, to_IP, taken/not_taken, ...)

Issue: how many bits to use?  Which may vary as a function of the type of branch: e.g. a direct conditional branch
may not need as many to_ip bits to be hashed in
as a completely random indirect branch.
Similarly, indirect calls and returns may be handled separately.


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