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TagePredictorOpt.cc
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TagePredictorOpt.cc
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#include "predictor.h"
#include <math.h>
/////////////// STORAGE BUDGET JUSTIFICATION ////////////////
// Total storage budget: 32KB + 17 bits
// Total PHT counters: 2^17
// Total PHT size = 2^17 * 2 bits/counter = 2^18 bits = 32KB
// GHR size: 17 bits
// Total Size = PHT size + GHR size
/////////////////////////////////////////////////////////////
#define TAGE_TABLE_NUM 4
#define BASE_TABLE_INDEX_WIDTH 14
#define BASE_CTR_WIDTH 2
#define BASE_CTR_INIT 2
#define BASE_CTR_MAX 3
#define TAG_WIDTH 9
#define U_WIDTH 2
#define CTR_WIDTH 3
#define TAGGED_ENTRY_LEN 12 // each tagged table has 2^10 entry
#define TAGGED_CTR_INIT 0
#define TAGGED_CTR_MAX 7
#define TAGGED_WEAK_CORRECT 4
#define L1 5
#define L2 14
#define L3 37
#define L4 100
#define USE_ALT_MAX 15
#define USE_ALT_INIT 7
/////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////
PREDICTOR::PREDICTOR(void){
historyLength = L4;
ghr = 0;
numBaseTableEntries = 1 << BASE_TABLE_INDEX_WIDTH;
base_table = new uint8_t[numBaseTableEntries];
for(UINT32 ii=0; ii < numBaseTableEntries; ii++){
base_table[ii] = BASE_CTR_INIT;
}
numTageTableEntries = 1 << TAGGED_ENTRY_LEN;
tage_table_len = new UINT32[4];
tage_table_len[0] = L1;
tage_table_len[1] = L2;
tage_table_len[2] = L3;
tage_table_len[3] = L4;
tag_table = new TageEntry*[TAGE_TABLE_NUM];
for (UINT32 j = 0; j < TAGE_TABLE_NUM; j++){
tag_table[j] = new TageEntry[numTageTableEntries];
for(UINT32 ii=0; ii< numTageTableEntries; ii++){
tag_table[j][ii].tag = 0;
tag_table[j][ii].u = 0;
tag_table[j][ii].ctr = TAGGED_CTR_INIT;
}
}
clock = 0;
tag = new UINT32[TAGE_TABLE_NUM];
tag_table_idx = new UINT32[TAGE_TABLE_NUM];
use_alt = USE_ALT_INIT;
pred_is_new_entry = false;
}
/////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////
bool PREDICTOR::GetPrediction(UINT32 PC){
UINT32 base_index = PC % numBaseTableEntries;
uint8_t base_counter = base_table[base_index];
pred = base_counter > BASE_CTR_MAX/2;
provider_component = -1;
altpred_component = -1;
altpred = pred;
for(int i = 0; i < TAGE_TABLE_NUM; i++){
tag[i] = get_tag(PC, i);
tag_table_idx[i] = get_tagged_idx(PC, i);
if(tag_table[i][tag_table_idx[i]].tag == tag[i]){
altpred_component = provider_component;
altpred = pred;
provider_component = i;
pred = tag_table[i][tag_table_idx[i]].ctr > TAGGED_CTR_MAX / 2;
}
}
if(provider_component != -1 && tag_table[provider_component][tag_table_idx[provider_component]].u == 0 &&
(tag_table[provider_component][tag_table_idx[provider_component]].ctr == TAGGED_CTR_MAX / 2 ||
tag_table[provider_component][tag_table_idx[provider_component]].ctr == TAGGED_CTR_MAX / 2 + 1)){
pred_is_new_entry = true;
}
else{
pred_is_new_entry = false;
}
if(pred_is_new_entry && use_alt > USE_ALT_MAX / 2 + 1){
return (int)altpred;
}else{
return (int)pred;
}
}
/////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////
void PREDICTOR::UpdatePredictor(UINT32 PC, bool resolveDir, bool predDir, UINT32 branchTarget){
UINT32 base_index = PC % numBaseTableEntries;
uint8_t base_counter = base_table[base_index];
// update counter of provider component
if(provider_component == -1){
if(resolveDir == TAKEN){
base_table[base_index] = SatIncrement(base_counter, BASE_CTR_MAX);
}else{
base_table[base_index] = SatDecrement(base_counter);
}
}
else{
uint8_t pred_ctr = tag_table[provider_component][tag_table_idx[provider_component]].ctr;
if(resolveDir == TAKEN){
tag_table[provider_component][tag_table_idx[provider_component]].ctr = SatIncrement(pred_ctr, TAGGED_CTR_MAX);
}
else{
tag_table[provider_component][tag_table_idx[provider_component]].ctr = SatDecrement(pred_ctr);
}
}
// if prediction is incorrect, allocate entry
// don't need to allocate entry when altpred is false and pred is right, the u tag will do it(otherwise, we will always get the new entry?)
if(resolveDir != pred && provider_component != TAGE_TABLE_NUM - 1 /*&& !(pred == resolveDir && pred_is_new_entry)*/){
int unalloc_idx[TAGE_TABLE_NUM] = {-1, -1, -1, -1};
int count = 0;
for(int i = provider_component + 1; i < TAGE_TABLE_NUM; i++){
if(tag_table[i][tag_table_idx[i]].u == 0){
unalloc_idx[count] = i;
count ++;
}
}
// if uk > 0 for k in (i, M), then uk = uk-1 for all uk
if(count == 0){
for(int i = provider_component + 1; i < TAGE_TABLE_NUM; i++){
tag_table[i][tag_table_idx[i]].u = SatDecrement(tag_table[i][tag_table_idx[i]].u);
}
}
else{
srand(3407);
// allocate one entry each time
// if more than one T_i need allocate, for i < j, the probility of allocate entry in T_i = 2 * T_j
// example:count = 3, rand = {0} for unalloc[2], rand = {1, 2} for unalloc[1], rand = {3,4,5,6} for unalloc[0]
int total_pro = (1 << count) - 1;
int r = rand() % total_pro;
int choose_idx = -1;
for (int i = 0; i < count; i++){
if( r >= (1<<i) - 1 && r < ( 1 << (i + 1) ) - 1 ){
choose_idx = unalloc_idx[count - 1 - i];
}
}
UINT32 idx_in_tag_table_choose = tag_table_idx[choose_idx];
tag_table[choose_idx][idx_in_tag_table_choose].tag = tag[choose_idx];
tag_table[choose_idx][idx_in_tag_table_choose].u = 0;
if(resolveDir)
tag_table[choose_idx][idx_in_tag_table_choose].ctr = TAGGED_WEAK_CORRECT;
else
tag_table[choose_idx][idx_in_tag_table_choose].ctr = TAGGED_WEAK_CORRECT - 1;
}
}
// update use_alt
if(altpred != pred && provider_component != -1){
if(pred != resolveDir){
use_alt = SatIncrement(use_alt, USE_ALT_MAX);
}
else{
use_alt = SatDecrement(use_alt);
}
}
// update u
if(altpred != pred && provider_component != -1){
uint8_t u = tag_table[provider_component][tag_table_idx[provider_component]].u;
if(pred == resolveDir){
tag_table[provider_component][tag_table_idx[provider_component]].u = SatIncrement(u, 3);
}
else{
tag_table[provider_component][tag_table_idx[provider_component]].u = SatDecrement(u);
}
}
// After 256k branch, reset u
clock ++;
uint8_t mask = 0;
if(clock == 1 << 18){
mask = 1;
}
else if(clock == 1 << 19){
mask = 2;
clock = 0;
}
if(mask){
for(int i = 0; i < TAGE_TABLE_NUM; i++){
for (UINT32 j = 0; j < numTageTableEntries; j++){
tag_table[i][j].u = tag_table[i][j].u & mask;
}
}
}
// update ghr
ghr = ghr << 1;
if(resolveDir){
ghr++;
}
}
UINT32 PREDICTOR::get_tagged_idx(UINT32 PC, int bank_no){
__uint128_t temp_ghr = ghr;
int history_width = tage_table_len[bank_no];
UINT32 temp_pc = PC & ( (1 << TAGGED_ENTRY_LEN) - 1 );
// folder
while(history_width > 0){
int block_width = min(history_width, TAGGED_ENTRY_LEN);
temp_pc ^= temp_ghr & ( (1 << block_width) - 1 );
temp_ghr = temp_ghr >> block_width;
history_width -= block_width;
}
return temp_pc & ( (1 << TAGGED_ENTRY_LEN) - 1 ) ;
}
uint16_t PREDICTOR::get_tag(UINT32 PC, int bank_no){
__uint128_t temp_ghr = ghr & ((1 << tage_table_len[bank_no])-1);
// TODO
return (temp_ghr + PC * 1000000007) & ((1<<TAG_WIDTH) - 1);
}
/////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////
void PREDICTOR::TrackOtherInst(UINT32 PC, OpType opType, UINT32 branchTarget){
// This function is called for instructions which are not
// conditional branches, just in case someone decides to design
// a predictor that uses information from such instructions.
// We expect most contestants to leave this function untouched.
return;
}
/////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////