tage.cc

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 * Copyright (c) 2014 The University of Wisconsin
 *
 * Copyright (c) 2006 INRIA (Institut National de Recherche en
 * Informatique et en Automatique  / French National Research Institute
 * for Computer Science and Applied Mathematics)
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/* @file
 * Implementation of a TAGE branch predictor
 */
 
#include "cpu/pred/tage.hh"
 
#include "base/intmath.hh"
#include "base/logging.hh"
#include "base/random.hh"
#include "base/trace.hh"
#include "debug/Fetch.hh"
#include "debug/Tage.hh"
 
namespace gem5
{
 
namespace branch_prediction
{
 
TAGE::TAGE(const TAGEParams &params)
    : ConditionalPredictor(params),
      tage(params.tage)
{
}
 
// PREDICTOR UPDATE
void
TAGE::update(ThreadID tid, Addr pc, bool taken, void * &bp_history,
             bool squashed, const StaticInstPtr & inst, Addr target)
{
    assert(bp_history);
 
    TageBranchInfo *bi = static_cast<TageBranchInfo*>(bp_history);
    TAGEBase::BranchInfo *tage_bi = bi->tageBranchInfo;
 
    if (squashed) {
        // This restores the global history, then update it
        // and recomputes the folded histories.
        tage->squash(tid, taken, target, inst, tage_bi);
        return;
    }
 
    int nrand = rng->random<int>() & 3;
    if (bi->tageBranchInfo->condBranch) {
        DPRINTF(Tage, "Updating tables for branch:%lx; taken?:%d\n",
                pc, taken);
        tage->updateStats(taken, bi->tageBranchInfo);
        tage->condBranchUpdate(tid, pc, taken, tage_bi, nrand,
                               target, bi->tageBranchInfo->tagePred);
    }
 
    // optional non speculative update of the histories
    tage->updateHistories(tid, pc, false, taken, target, inst, tage_bi);
    delete bi;
    bp_history = nullptr;
}
 
void
TAGE::squash(ThreadID tid, void * &bp_history)
{
    TageBranchInfo *bi = static_cast<TageBranchInfo*>(bp_history);
    tage->restoreHistState(tid, bi->tageBranchInfo);
    DPRINTF(Tage, "Deleting branch info: %lx\n", bi->tageBranchInfo->branchPC);
    delete bi;
    bp_history = nullptr;
}
 
bool
TAGE::predict(ThreadID tid, Addr pc, bool cond_branch, void* &b)
{
    TageBranchInfo *bi = new TageBranchInfo(*tage, pc, cond_branch);
    b = (void*)(bi);
    return tage->tagePredict(tid, pc, cond_branch, bi->tageBranchInfo);
}
 
bool
TAGE::lookup(ThreadID tid, Addr pc, void* &bp_history)
{
    bool retval = predict(tid, pc, true, bp_history);
 
    DPRINTF(Tage, "Lookup branch: %lx; predict:%d\n", pc, retval);
 
    return retval;
}
 
void
TAGE::updateHistories(ThreadID tid, Addr pc, bool uncond, bool taken,
                      Addr target, const StaticInstPtr &inst,
                      void * &bp_history)
{
    if (bp_history == nullptr) {
 
        // We should only see unconditional branches
        assert(uncond);
 
        predict(tid, pc, false, bp_history);
    }
 
    // Update the global history for all branches
    TageBranchInfo *bi = static_cast<TageBranchInfo*>(bp_history);
    tage->updateHistories(tid, pc, true, taken, target, inst,
                          bi->tageBranchInfo);
}
 
void
TAGE::branchPlaceholder(ThreadID tid, Addr pc, bool uncond, void * &bpHistory)
{
    TageBranchInfo *bi = new TageBranchInfo(*tage, pc, !uncond);
    bpHistory = (void*)(bi);
}
 
} // namespace branch_prediction
} // namespace gem5