ltage.cc

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/* @file
 * Implementation of a L-TAGE branch predictor
 */
 
#include "cpu/pred/ltage.hh"
 
#include "base/intmath.hh"
#include "base/logging.hh"
#include "base/trace.hh"
#include "debug/Fetch.hh"
#include "debug/LTage.hh"
 
namespace gem5
{
 
namespace branch_prediction
{
 
LTAGE::LTAGE(const LTAGEParams &params)
  : TAGE(params), loopPredictor(params.loop_predictor)
{
}
 
void
LTAGE::init()
{
    TAGE::init();
}
 
void
LTAGE::branchPlaceholder(ThreadID tid, Addr pc,
                         bool uncond, void * &bpHistory)
{
    LTageBranchInfo *bi = new LTageBranchInfo(*tage, *loopPredictor,
                                              pc, !uncond);
    bpHistory = (void*)(bi);
}
 
//prediction
bool
LTAGE::predict(ThreadID tid, Addr branch_pc, bool cond_branch, void* &b)
{
    LTageBranchInfo *bi = new LTageBranchInfo(*tage, *loopPredictor,
                                              branch_pc, cond_branch);
    b = (void*)(bi);
 
    bool pred_taken = tage->tagePredict(tid, branch_pc, cond_branch,
                                        bi->tageBranchInfo);
 
    pred_taken = loopPredictor->loopPredict(tid, branch_pc, cond_branch,
                                            bi->lpBranchInfo, pred_taken,
                                            instShiftAmt);
    if (cond_branch) {
        if (bi->lpBranchInfo->loopPredUsed) {
            bi->tageBranchInfo->provider = LOOP;
        }
        DPRINTF(LTage, "Predict for %lx: taken?:%d, loopTaken?:%d, "
                "loopValid?:%d, loopUseCounter:%d, tagePred:%d, altPred:%d\n",
                branch_pc, pred_taken, bi->lpBranchInfo->loopPred,
                bi->lpBranchInfo->loopPredValid,
                loopPredictor->getLoopUseCounter(),
                bi->tageBranchInfo->tagePred, bi->tageBranchInfo->altTaken);
    }
 
    // record final prediction
    bi->lpBranchInfo->predTaken = pred_taken;
 
    return pred_taken;
}
 
// PREDICTOR UPDATE
void
LTAGE::update(ThreadID tid, Addr pc, bool taken, void * &bp_history,
              bool squashed, const StaticInstPtr & inst, Addr target)
{
    assert(bp_history);
 
    LTageBranchInfo* bi = static_cast<LTageBranchInfo*>(bp_history);
 
    if (squashed) {
        if (tage->isSpeculativeUpdateEnabled()) {
            // This restores the global history, then update it
            // and recomputes the folded histories.
            tage->squash(tid, taken, target, inst, bi->tageBranchInfo);
 
            if (bi->tageBranchInfo->condBranch) {
                loopPredictor->squashLoop(bi->lpBranchInfo);
            }
        }
        return;
    }
 
    int nrand = rng->random<int>() & 3;
    if (bi->tageBranchInfo->condBranch) {
        DPRINTF(LTage, "Updating tables for branch:%lx; taken?:%d\n",
                pc, taken);
        tage->updateStats(taken, bi->tageBranchInfo);
 
        loopPredictor->updateStats(taken, bi->lpBranchInfo);
 
        loopPredictor->condBranchUpdate(tid, pc, taken,
            bi->tageBranchInfo->tagePred, bi->lpBranchInfo, instShiftAmt);
 
        tage->condBranchUpdate(tid, pc, taken, bi->tageBranchInfo,
            nrand, target, bi->lpBranchInfo->predTaken);
    }
 
    tage->updateHistories(tid, pc, false, taken, target,
                           inst, bi->tageBranchInfo);
 
    delete bi;
    bp_history = nullptr;
}
 
void
LTAGE::squash(ThreadID tid, void * &bp_history)
{
    LTageBranchInfo* bi = (LTageBranchInfo*)(bp_history);
 
    if (bi->tageBranchInfo->condBranch) {
        loopPredictor->squash(tid, bi->lpBranchInfo);
    }
 
    TAGE::squash(tid, bp_history);
}
 
} // namespace branch_prediction
} // namespace gem5