Hi,
I was checking out the branch prediction implementation in InorderCPU of M5.
I am modeling ALPHA in the SE mode.
I am seeing the following sequence of operations, which seems odd to me.
Like, the sequence num 33 was predcited taken initially, but later it was
predicted as not taken( as the PC was not present in the BTB). This was a
misprediction implies, the initial taken prediction was correct. But the
squash updates the predictor with a not taken direction(see actually taken
below). Or am I missing something here?
648500: BranchPredictor_unit: [tid:0] [sn:33] beq r1,0x120002ac4 ...
PC (0x120002ab4=>0x120002ab8) doing branch prediction
648500: BranchPredictor_unit: [tid:0]: *Branch predictor predicted 1 for PC
* (0x120002ab4=>0x120002ab8)
648500: BranchPredictor_unit: [tid:0]: *BTB doesn't have a valid entry.*
648500: BranchPredictor_unit: [tid:0] [sn:33] pushed onto front of predHist
...predHist.size(): 1
648500: system.cpu.Branch-Predictor: [tid:0]: [sn:33]: *Branch predicted
false.*
648500: system.cpu.Branch-Predictor: [tid:0]: [sn:33]: Predicted PC is
(0x120002ab8=>0x120002abc).
666500: system.cpu.Execution-Unit: [tid:0] Executing [sn:33]
[PC:(0x120002ab4=>0x120002ab8)] beq.
666500: system.cpu.Execution-Unit: [tid:0]: Misprediction detected at
[sn:33] PC (0x120002ab4=>0x120002ac4),
squashing after delay slot instruction [sn:33].
666500: system.cpu.Execution-Unit: [tid:0] Redirecting fetch to
(0x120002ac4=>0x120002ac8).
666500: system.cpu.Execution-Unit: [tid:0] Squashing will start from stage
2.
666500: system.cpu.Execution-Unit: [tid:0] [sn:33] beq
r1,0x120002ac4 ...PC (0x120002ab4=>0x120002ac4) ... Mispredicts! (Not Taken)
666500: system.cpu.Execution-Unit: [tid:0] Executing [sn:34]
[PC:(0x120002ab8=>0x120002abc)] lda.
666500: system.cpu.Execution-Unit: [tid:0]: [sn:34]: The result of
execution is 0x0.
666500: system.cpu.Branch-Predictor: [tid:0][sn:33] Squashing...
666500: BranchPredictor_unit: [tid:0]: Squashing from sequence number 33,
setting target to (0x120002ac4=>0x120002ac8).
666500: BranchPredictor_unit: BranchPred: [tid:0]: Squashing branch
sequence number 33 , PC = 0x120002ab8, *actually_taken = 0*
666500: BranchPredictor_unit: [tid:0]: Removing history for [sn:33] PC
(0x120002ab8=>0x120002abc).
666500: BranchPredictor_unit: BranchPred: [tid:0]: Removing history for
[sn:33] PC (0x120002ab8=>0x120002abc), Actually Taken = 0
666500: BranchPredictor_unit: [tid:0]: predHist.size(): 0
Actually_taken value is passed on from branch_predictor.cc, in the squash
function.
bool taken = inst->predTaken();
branchPred->squash(squash_seq_num, inst->readPredTarg(), taken,
tid);
Where should this inst->predTaken updated now?
Thanks,
Reena
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