> On nov. 23, 2016, 3:17 après-midi, Jason Lowe-Power wrote: > > src/cpu/pred/BranchPredictor.py, line 100 > > <http://reviews.gem5.org/r/3743/diff/2/?file=63759#file63759line100> > > > > Comment is now wrong. > > > > Also, does it make more sense to call this histBufferEntries (or > > histBufferBits) and make the default 2**21? (or would it be 2**21*8? <-- > > this is the confusion.) > > > > I think it would be more clear to use bits or entries, personally. But > > if the branch predictor papers usually talk about the history buffer in > > terms of bytes instead of bits, I guess the current version makes more > > sense. > > Nathanael Premillieu wrote: > This big buffer is just a software structure and it does not represent a > hardware one. > It is just done like this to simplify the management of the branch > history and avoid copying those bits too often. > The modeled hardware history is implemented as a sliding window on this > big buffer.
In effect, the predictor will create an array of 2**21 entries, each entry containing an uint8_t, so the buffer is indeed 2MB, even though it contains 2Mbits of history information. As Nathanael pointed out, this is a trick to avoid using a linked-list/deque which is more costly. The buffer is much bigger than the actual history size to remove bound checks and wrap-around adjustment that would be necessary with a circular buffer in *most* places where it is accessed, at the cost of copying the whole history from the beginning to the end of the buffer when the buffer is filled (so it has to be big enough to amortize the cost of the copy). If you really think it should be changed to Param.Unsigned, I'll change it. I'll wait for some input before posting the version with the correct comment (my bad). - Arthur ----------------------------------------------------------- This is an automatically generated e-mail. To reply, visit: http://reviews.gem5.org/r/3743/#review9160 ----------------------------------------------------------- On nov. 23, 2016, 2:52 après-midi, Arthur Perais wrote: > > ----------------------------------------------------------- > This is an automatically generated e-mail. To reply, visit: > http://reviews.gem5.org/r/3743/ > ----------------------------------------------------------- > > (Updated nov. 23, 2016, 2:52 après-midi) > > > Review request for Default. > > > Repository: gem5 > > > Description > ------- > > Changeset 11707:1d085f66c4ca > --------------------------- > > cpu: implement an L-TAGE branch predictor > > This patch implements an L-TAGE predictor, based on André Seznec's code > available from > CBP-2 > (http://hpca23.cse.tamu.edu/taco/camino/cbp2/cbp-src/realistic-seznec.h). The > patch also changes the default branch predictor of o3 from the tournament > predictor > to L-TAGE. > > This patch requires patch #3727 (http://reviews.gem5.org/r/3727/) to compile. > > > Diffs > ----- > > src/cpu/pred/BranchPredictor.py 1d085f66c4ca > src/cpu/pred/SConscript 1d085f66c4ca > src/cpu/pred/ltage.hh PRE-CREATION > src/cpu/pred/ltage.cc PRE-CREATION > > Diff: http://reviews.gem5.org/r/3743/diff/ > > > Testing > ------- > > > Thanks, > > Arthur Perais > > _______________________________________________ gem5-dev mailing list gem5-dev@gem5.org http://m5sim.org/mailman/listinfo/gem5-dev
