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