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authorAndreas Sandberg <andreas@sandberg.pp.se>2014-02-09 20:49:28 +0100
committerAndreas Sandberg <andreas@sandberg.pp.se>2014-02-09 20:49:28 +0100
commitc52190a695dc1928713e5bbda85cd17867c7e465 (patch)
tree8e9e03fdd3b1cf367f4fa9e28eaf8458947cfcf7 /src/cpu
parenteb73a14fe29ff4940a206a9961e30c2376412951 (diff)
downloadgem5-c52190a695dc1928713e5bbda85cd17867c7e465.tar.xz
cpu: simple: Add support for using branch predictors
This changesets adds branch predictor support to the BaseSimpleCPU. The simple CPUs normally don't need a branch predictor, however, there are at least two cases where it can be desirable: 1) A simple CPU can be used to warm the branch predictor of an O3 CPU before switching to the slower O3 model. 2) The simple CPU can be used as a quick way of evaluating/debugging new branch predictors since it exposes branch predictor statistics. Limitations: * Since the simple CPU doesn't speculate, only one instruction will be active in the branch predictor at a time (i.e., the branch predictor will never see speculative branches). * The outcome of a branch prediction does not affect the performance of the simple CPU.
Diffstat (limited to 'src/cpu')
-rw-r--r--src/cpu/simple/BaseSimpleCPU.py3
-rw-r--r--src/cpu/simple/base.cc57
-rw-r--r--src/cpu/simple/base.hh16
3 files changed, 73 insertions, 3 deletions
diff --git a/src/cpu/simple/BaseSimpleCPU.py b/src/cpu/simple/BaseSimpleCPU.py
index a4ae7e34b..243a09269 100644
--- a/src/cpu/simple/BaseSimpleCPU.py
+++ b/src/cpu/simple/BaseSimpleCPU.py
@@ -30,6 +30,7 @@ from m5.defines import buildEnv
from m5.params import *
from BaseCPU import BaseCPU
from DummyChecker import DummyChecker
+from BranchPredictor import BranchPredictor
class BaseSimpleCPU(BaseCPU):
type = 'BaseSimpleCPU'
@@ -46,3 +47,5 @@ class BaseSimpleCPU(BaseCPU):
else:
print "ERROR: Checker only supported under ARM ISA!"
exit(1)
+
+ branchPred = Param.BranchPredictor(NULL, "Branch Predictor")
diff --git a/src/cpu/simple/base.cc b/src/cpu/simple/base.cc
index 078f490e8..3adf6d27f 100644
--- a/src/cpu/simple/base.cc
+++ b/src/cpu/simple/base.cc
@@ -60,6 +60,7 @@
#include "cpu/checker/cpu.hh"
#include "cpu/checker/thread_context.hh"
#include "cpu/exetrace.hh"
+#include "cpu/pred/bpred_unit.hh"
#include "cpu/profile.hh"
#include "cpu/simple_thread.hh"
#include "cpu/smt.hh"
@@ -85,7 +86,9 @@ using namespace std;
using namespace TheISA;
BaseSimpleCPU::BaseSimpleCPU(BaseSimpleCPUParams *p)
- : BaseCPU(p), traceData(NULL), thread(NULL)
+ : BaseCPU(p),
+ branchPred(p->branchPred),
+ traceData(NULL), thread(NULL)
{
if (FullSystem)
thread = new SimpleThread(this, 0, p->system, p->itb, p->dtb,
@@ -286,6 +289,21 @@ BaseSimpleCPU::regStats()
idleFraction = constant(1.0) - notIdleFraction;
numIdleCycles = idleFraction * numCycles;
numBusyCycles = (notIdleFraction)*numCycles;
+
+ numBranches
+ .name(name() + ".Branches")
+ .desc("Number of branches fetched")
+ .prereq(numBranches);
+
+ numPredictedBranches
+ .name(name() + ".predictedBranches")
+ .desc("Number of branches predicted as taken")
+ .prereq(numPredictedBranches);
+
+ numBranchMispred
+ .name(name() + ".BranchMispred")
+ .desc("Number of branch mispredictions")
+ .prereq(numBranchMispred);
}
void
@@ -434,6 +452,19 @@ BaseSimpleCPU::preExecute()
curStaticInst->getName(), curStaticInst->machInst);
#endif // TRACING_ON
}
+
+ if (branchPred && curStaticInst && curStaticInst->isControl()) {
+ // Use a fake sequence number since we only have one
+ // instruction in flight at the same time.
+ const InstSeqNum cur_sn(0);
+ const ThreadID tid(0);
+ pred_pc = thread->pcState();
+ const bool predict_taken(
+ branchPred->predict(curStaticInst, cur_sn, pred_pc, tid));
+
+ if (predict_taken)
+ ++numPredictedBranches;
+ }
}
void
@@ -464,6 +495,10 @@ BaseSimpleCPU::postExecute()
CPA::cpa()->swAutoBegin(tc, pc.nextInstAddr());
}
+ if (curStaticInst->isControl()) {
+ ++numBranches;
+ }
+
/* Power model statistics */
//integer alu accesses
if (curStaticInst->isInteger()){
@@ -507,10 +542,11 @@ BaseSimpleCPU::postExecute()
}
}
-
void
BaseSimpleCPU::advancePC(Fault fault)
{
+ const bool branching(thread->pcState().branching());
+
//Since we're moving to a new pc, zero out the offset
fetchOffset = 0;
if (fault != NoFault) {
@@ -526,6 +562,23 @@ BaseSimpleCPU::advancePC(Fault fault)
thread->pcState(pcState);
}
}
+
+ if (branchPred && curStaticInst && curStaticInst->isControl()) {
+ // Use a fake sequence number since we only have one
+ // instruction in flight at the same time.
+ const InstSeqNum cur_sn(0);
+ const ThreadID tid(0);
+
+ if (pred_pc == thread->pcState()) {
+ // Correctly predicted branch
+ branchPred->update(cur_sn, tid);
+ } else {
+ // Mis-predicted branch
+ branchPred->squash(cur_sn, pcState(),
+ branching, tid);
+ ++numBranchMispred;
+ }
+ }
}
void
diff --git a/src/cpu/simple/base.hh b/src/cpu/simple/base.hh
index 8134465af..ad672da6c 100644
--- a/src/cpu/simple/base.hh
+++ b/src/cpu/simple/base.hh
@@ -77,7 +77,7 @@ namespace Trace {
}
struct BaseSimpleCPUParams;
-
+class BPredUnit;
class BaseSimpleCPU : public BaseCPU
{
@@ -87,6 +87,8 @@ class BaseSimpleCPU : public BaseCPU
typedef TheISA::FloatRegBits FloatRegBits;
typedef TheISA::CCReg CCReg;
+ BPredUnit *branchPred;
+
protected:
Trace::InstRecord *traceData;
@@ -272,6 +274,15 @@ class BaseSimpleCPU : public BaseCPU
Stats::Scalar dcacheRetryCycles;
Counter lastDcacheRetry;
+ /// @{
+ /// Total number of branches fetched
+ Stats::Scalar numBranches;
+ /// Number of branches predicted as taken
+ Stats::Scalar numPredictedBranches;
+ /// Number of misprediced branches
+ Stats::Scalar numBranchMispred;
+ /// @}
+
void serializeThread(std::ostream &os, ThreadID tid);
void unserializeThread(Checkpoint *cp, const std::string &section,
ThreadID tid);
@@ -446,6 +457,9 @@ class BaseSimpleCPU : public BaseCPU
bool misspeculating() { return thread->misspeculating(); }
ThreadContext *tcBase() { return tc; }
+
+ private:
+ TheISA::PCState pred_pc;
};
#endif // __CPU_SIMPLE_BASE_HH__