111 lines
2.7 KiB
C++
111 lines
2.7 KiB
C++
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#include "base/trace.hh"
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#include "cpu/beta_cpu/2bit_local_pred.hh"
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DefaultBP::SatCounter::SatCounter(unsigned bits)
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: maxVal((1 << bits) - 1), counter(0)
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{
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}
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DefaultBP::SatCounter::SatCounter(unsigned bits, unsigned initial_val)
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: maxVal((1 << bits) - 1), counter(initial_val)
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{
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// Check to make sure initial value doesn't exceed the max counter value.
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if (initial_val > maxVal) {
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panic("BP: Initial counter value exceeds max size.");
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}
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}
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void
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DefaultBP::SatCounter::increment()
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{
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if(counter < maxVal) {
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++counter;
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}
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}
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void
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DefaultBP::SatCounter::decrement()
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{
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if(counter > 0) {
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--counter;
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}
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}
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DefaultBP::DefaultBP(unsigned _localPredictorSize,
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unsigned _localCtrBits,
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unsigned _instShiftAmt)
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: localPredictorSize(_localPredictorSize),
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localCtrBits(_localCtrBits),
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instShiftAmt(_instShiftAmt)
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{
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// Should do checks here to make sure sizes are correct (powers of 2).
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// Setup the index mask.
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indexMask = localPredictorSize - 1;
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DPRINTF(Fetch, "Branch predictor: index mask: %#x\n", indexMask);
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// Setup the array of counters for the local predictor.
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localCtrs = new SatCounter[localPredictorSize](localCtrBits);
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DPRINTF(Fetch, "Branch predictor: local predictor size: %i\n",
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localPredictorSize);
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DPRINTF(Fetch, "Branch predictor: local counter bits: %i\n", localCtrBits);
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DPRINTF(Fetch, "Branch predictor: instruction shift amount: %i\n",
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instShiftAmt);
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}
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inline
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bool
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DefaultBP::getPrediction(uint8_t &count)
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{
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// Get the MSB of the count
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return (count >> (localCtrBits - 1));
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}
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inline
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unsigned
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DefaultBP::getLocalIndex(Addr &branch_addr)
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{
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return (branch_addr >> instShiftAmt) & indexMask;
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}
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bool
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DefaultBP::lookup(Addr &branch_addr)
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{
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uint8_t local_prediction;
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unsigned local_predictor_idx = getLocalIndex(branch_addr);
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DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
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local_predictor_idx);
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local_prediction = localCtrs[local_predictor_idx].read();
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DPRINTF(Fetch, "Branch predictor: prediction is %i.\n",
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(int)local_prediction);
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return getPrediction(local_prediction);
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}
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void
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DefaultBP::update(Addr &branch_addr, bool taken)
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{
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unsigned local_predictor_idx;
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// Update the local predictor.
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local_predictor_idx = getLocalIndex(branch_addr);
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DPRINTF(Fetch, "Branch predictor: Looking up index %#x\n",
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local_predictor_idx);
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if (taken) {
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DPRINTF(Fetch, "Branch predictor: Branch updated as taken.\n");
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localCtrs[local_predictor_idx].increment();
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} else {
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DPRINTF(Fetch, "Branch predictor: Branch updated as not taken.\n");
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localCtrs[local_predictor_idx].decrement();
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}
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}
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