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cpu: Fix BTB threading oversight

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The extant BTB code doesn't hash on the thread id but does check the
thread id for 'btb hits'.  This results in 1-thread of a multi-threaded
workload taking a BTB entry, and all other threads missing for the same branch
missing.
This commit is contained in:
Mitch Hayenga 2016-04-05 11:44:27 -05:00
parent f902c0218a
commit 7bc52af771
3 changed files with 18 additions and 10 deletions
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3
src/cpu/pred/bpred_unit.cc
View file

@ -59,7 +59,8 @@ BPredUnit::BPredUnit(const Params *params)
predHist(numThreads),
BTB(params->BTBEntries,
params->BTBTagSize,
params->instShiftAmt),
params->instShiftAmt,
params->numThreads),
RAS(numThreads),
instShiftAmt(params->instShiftAmt)
{

18
src/cpu/pred/btb.cc
View file

@ -35,10 +35,12 @@
DefaultBTB::DefaultBTB(unsigned _numEntries,
unsigned _tagBits,
unsigned _instShiftAmt)
unsigned _instShiftAmt,
unsigned _num_threads)
: numEntries(_numEntries),
tagBits(_tagBits),
instShiftAmt(_instShiftAmt)
instShiftAmt(_instShiftAmt),
log2NumThreads(floorLog2(_num_threads))
{
DPRINTF(Fetch, "BTB: Creating BTB object.\n");
@ -69,10 +71,12 @@ DefaultBTB::reset()
inline
unsigned
DefaultBTB::getIndex(Addr instPC)
DefaultBTB::getIndex(Addr instPC, ThreadID tid)
{
// Need to shift PC over by the word offset.
return (instPC >> instShiftAmt) & idxMask;
return ((instPC >> instShiftAmt)
^ (tid << (tagShiftAmt - instShiftAmt - log2NumThreads)))
& idxMask;
}
inline
@ -85,7 +89,7 @@ DefaultBTB::getTag(Addr instPC)
bool
DefaultBTB::valid(Addr instPC, ThreadID tid)
{
unsigned btb_idx = getIndex(instPC);
unsigned btb_idx = getIndex(instPC, tid);
Addr inst_tag = getTag(instPC);
@ -106,7 +110,7 @@ DefaultBTB::valid(Addr instPC, ThreadID tid)
TheISA::PCState
DefaultBTB::lookup(Addr instPC, ThreadID tid)
{
unsigned btb_idx = getIndex(instPC);
unsigned btb_idx = getIndex(instPC, tid);
Addr inst_tag = getTag(instPC);
@ -124,7 +128,7 @@ DefaultBTB::lookup(Addr instPC, ThreadID tid)
void
DefaultBTB::update(Addr instPC, const TheISA::PCState &target, ThreadID tid)
{
unsigned btb_idx = getIndex(instPC);
unsigned btb_idx = getIndex(instPC, tid);
assert(btb_idx < numEntries);

7
src/cpu/pred/btb.hh
View file

@ -66,7 +66,7 @@ class DefaultBTB
* @param instShiftAmt Offset amount for instructions to ignore alignment.
*/
DefaultBTB(unsigned numEntries, unsigned tagBits,
unsigned instShiftAmt);
unsigned instShiftAmt, unsigned numThreads);
void reset();
@ -97,7 +97,7 @@ class DefaultBTB
* @param inst_PC The branch to look up.
* @return Returns the index into the BTB.
*/
inline unsigned getIndex(Addr instPC);
inline unsigned getIndex(Addr instPC, ThreadID tid);
/** Returns the tag bits of a given address.
* @param inst_PC The branch's address.
@ -125,6 +125,9 @@ class DefaultBTB
/** Number of bits to shift PC when calculating tag. */
unsigned tagShiftAmt;
/** Log2 NumThreads used for hashing threadid */
unsigned log2NumThreads;
};
#endif // __CPU_PRED_BTB_HH__