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Change-Id: I44b6175022a3a593ed385407bd95ec0c40c74642
Change-Id: I16ce79a7d8488d9a138d0a26f5500576ae54132e
Change-Id: I8be037a1cdacd7151f4fe8e743a32cca90a85036
Change-Id: I6b9c92c18c574a12c19532ae0b894e64c1187342
Change-Id: I953965b8e6feb1c15a238ac832d65bc16b32f496
Change-Id: I39cd471d452aa343c3dd741a80fdfa7d126e3a9f
Change-Id: I3567ae93652aac218c5b4646003abadddaf7cf32
Change-Id: If7c9d3aa68a23c36dde74d8cf3a286c9c48f3e3c
Change-Id: I83277ae5c801e9d22b594286580459d12cdec69b
Change-Id: I394246af184d3f07e02b85f06e4e5ceed368ec22
Change-Id: I762c11f8d15262fb9f1c9d443f77895fa76bbc79
Change-Id: Ia9bcdc028235447e254889d95e5ea98e7f067664
Change-Id: I56614e8ebc2dd33320d353562087ab456fc452da
Change-Id: Icdc7ac7f047a36dba6733a0e1d11e5e37aa4cdf1
Change-Id: I9cb7e7ef04efefbcbdbf705563563f50f3c83324
… update statistics
📝 WalkthroughWalkthroughThis PR adds a new MicroTAGE branch predictor implementation, exposes it as a TimedBaseBTBPredictor-derived SimObject, wires it into the build/BpType enum, and updates DecoupledBPUWithBTB and example configs to reference the new MicroTAGE type and parameters. Changes
Sequence Diagram(s)sequenceDiagram
participant BTB as BTB Entry
participant MTAGE as MicroTAGE Predictor
participant Tables as TAGE Tables
participant History as History Folding
participant Update as Update Engine
rect rgba(200, 150, 100, 0.5)
Note over BTB,MTAGE: Prediction Path
BTB->>MTAGE: putPCHistory(startPC, history)
MTAGE->>History: Fold histories per table
MTAGE->>Tables: Walk tables high→low (index/tag)
Tables-->>MTAGE: Matching entries / counters
MTAGE->>MTAGE: Select main prediction, form meta
MTAGE-->>BTB: Return FullBTBPrediction (meta)
end
rect rgba(100, 150, 200, 0.5)
Note over BTB,Update: Update Path
BTB->>MTAGE: update(FetchStream with actual_taken)
MTAGE->>MTAGE: Prepare update entries & recompute preds
MTAGE->>Tables: Update counters/useful bits
MTAGE->>Update: Check & perform allocation if needed
Update->>Tables: Allocate/adjust replacement
MTAGE->>History: Update folded histories
end
rect rgba(150, 200, 100, 0.5)
Note over BTB,MTAGE: Recovery Path
BTB->>MTAGE: recoverPHist(history, FetchStream)
MTAGE->>History: Restore folded histories from checkpoint
MTAGE-->>BTB: Histories restored
end
Estimated code review effort🎯 4 (Complex) | ⏱️ ~60 minutes Possibly related PRs
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Change-Id: I09779b2dbec44aaae1cae03a6e5366340a0cc9da
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Actionable comments posted: 7
🤖 Fix all issues with AI agents
In `@src/cpu/pred/btb/decoupled_bpred.cc`:
- Line 136: Replace the incomplete comment "//here need" with either a clear,
actionable comment explaining what's required at that spot (e.g., why a
particular operation is necessary, what invariant must hold, or what TODO
remains with expected behavior and a JIRA/issue ID) or simply remove it if no
note is needed; locate the comment near the decoupled branch predictor logic in
decoupled_bpred.cc (around the relevant function handling branch prediction) and
ensure the new comment references the specific reason or next step so future
readers understand intent.
In `@src/cpu/pred/btb/microtage.cc`:
- Around line 1063-1069: The code dereferences meta (auto meta =
std::static_pointer_cast<TageMeta>(stream.predMetas[getComponentIdx()])) without
checking for null, which can crash on commit; add a null-guard after obtaining
meta (from stream.predMetas[getComponentIdx()]) and skip or set safe defaults
for uses of meta (e.g., when accessing meta->preds, meta->preds.find(pc), and
reading it->second.taken) if meta is null; ensure functions/methods using meta
such as the block that computes pc via inst->pcState().instAddr() and sets
pred_taken/pred_hit handle the null case consistently (e.g., leave
pred_taken=false and pred_hit=false or bail out) to avoid dereferencing a null
TageMeta.
- Around line 106-128: The loop assumes tableSizes[i] is a positive power-of-two
and tableTagBits[i] >= 1; add explicit validation before using them: check
tableSizes.size() and tableTagBits.size() as already asserted, then validate
that tableSizes[i] > 0 and is a power-of-two (or replace it with nextPowerOfTwo
if you prefer) before computing tableIndexBits and resizing tageTable[i], and
ensure tableTagBits[i] >= 1 before using tableTagBits[i]-1 for altTagFoldedHist;
if validation fails, assert or throw with a clear message. Also ensure
tableIndexBits[i] is computed from the validated size and use it to size
tableIndexMasks[i] and tableTagMasks[i] safely.
- Around line 192-256: The code currently calls main_info.taken()
unconditionally even when provided is false; change generation of main_taken to
only occur when provided is true by moving the call to main_info.taken() inside
the provided branch (use main_taken only when provided), e.g., compute
main_taken after the loop only if provided is true and otherwise avoid calling
TageTableInfo::taken(); also adjust the DPRINTF lines that reference main_taken
to use conditional values (or print only when provided) so no
unused/uninitialized variable is accessed; update the return to use the computed
values accordingly in generateSinglePrediction.
- Around line 738-769: The mask creation in getTageTag and getTageIndex is
unsafe when tableTagBits[t] or tableIndexBits[t] equals the address width
because `(1ULL << n)` is undefined for n==width; change the mask construction to
handle the full-width case defensively (e.g., if bits >= (sizeof(Addr)*8) use
all-ones mask, otherwise compute `(1ULL << bits) - 1`). Update constructions
that use `tableTagBits[t]` and `tableIndexBits[t]` so they never perform a
shift-by-width and ensure the mask type matches Addr.
- Around line 629-645: predMeta->preds[btb_entry.pc] can insert a default
TagePrediction and corrupt update logic when updateOnRead is false; change the
access in the loop over entries_to_update to first check
predMeta->preds.find(btb_entry.pc) and if not found call
generateSinglePrediction(btb_entry, startAddr, predMeta) (or otherwise fall back
to recomputation) so you only use stored predictions when the key exists; keep
using recomputed when updateOnRead is true or when the lookup fails, and
preserve existing variables (TagePrediction recomputed, utage_hit) and control
flow.
In `@src/cpu/pred/btb/microtage.hh`:
- Around line 4-9: The header is missing direct includes for types it uses: add
`#include` <memory> (for std::shared_ptr used throughout this file, e.g., in
declarations around symbol(s) using shared_ptr) and `#include` <unordered_map>
(for std::unordered_map used near the btb/microtage implementation) to the
top-of-file include list; this ensures the references to std::shared_ptr and
std::unordered_map are correct and not reliant on transitive headers.
🧹 Nitpick comments (1)
src/cpu/pred/btb/decoupled_bpred.hh (1)
18-28: Remove the duplicate btb_mgsc.hh include.Line 20 adds
cpu/pred/btb/btb_mgsc.hh, but it’s already present at Line 23. Keeping one avoids redundancy.♻️ Suggested cleanup
-#include "cpu/pred/btb/btb_mgsc.hh" `#include` "cpu/pred/btb/btb_tage.hh" `#include` "cpu/pred/btb/btb_ubtb.hh" `#include` "cpu/pred/btb/btb_mgsc.hh"
| tage->dryRunCycle(s0PC); | ||
| DPRINTF(Override, "Squashing, BPU state updated.\n"); | ||
| squashing = false; | ||
| //here need |
There was a problem hiding this comment.
Clarify or remove the incomplete comment.
Line 136’s //here need reads like a half‑finished TODO; please replace it with an actionable note or remove it.
🧹 Suggested cleanup
- //here need📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| //here need |
🤖 Prompt for AI Agents
In `@src/cpu/pred/btb/decoupled_bpred.cc` at line 136, Replace the incomplete
comment "//here need" with either a clear, actionable comment explaining what's
required at that spot (e.g., why a particular operation is necessary, what
invariant must hold, or what TODO remains with expected behavior and a
JIRA/issue ID) or simply remove it if no note is needed; locate the comment near
the decoupled branch predictor logic in decoupled_bpred.cc (around the relevant
function handling branch prediction) and ensure the new comment references the
specific reason or next step so future readers understand intent.
| // Initialize base table for fallback predictions | ||
| for (unsigned int i = 0; i < numPredictors; ++i) { | ||
| //initialize ittage predictor | ||
| assert(tableSizes.size() >= numPredictors); | ||
| tageTable[i].resize(tableSizes[i]); | ||
| for (unsigned int j = 0; j < tableSizes[i]; ++j) { | ||
| tageTable[i][j].resize(numWays); | ||
| } | ||
|
|
||
| tableIndexBits[i] = ceilLog2(tableSizes[i]); | ||
| tableIndexMasks[i].resize(tableIndexBits[i], true); | ||
|
|
||
| assert(histLengths.size() >= numPredictors); | ||
|
|
||
| assert(tableTagBits.size() >= numPredictors); | ||
| tableTagMasks[i].resize(tableTagBits[i], true); | ||
|
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||
| assert(tablePcShifts.size() >= numPredictors); | ||
|
|
||
| tagFoldedHist.push_back(PathFoldedHist((int)histLengths[i], (int)tableTagBits[i], 16)); | ||
| altTagFoldedHist.push_back(PathFoldedHist((int)histLengths[i], (int)tableTagBits[i]-1, 16)); | ||
| indexFoldedHist.push_back(PathFoldedHist((int)histLengths[i], (int)tableIndexBits[i], 16)); | ||
| } |
There was a problem hiding this comment.
Validate table sizes/tag bits before computing masks.
Index masking assumes power-of-two table sizes; non-power-of-two sizes can yield indices beyond tageTable[i]. Also tableTagBits[i]-1 requires tableTagBits[i] >= 1.
🛡️ Proposed guardrails
for (unsigned int i = 0; i < numPredictors; ++i) {
//initialize ittage predictor
assert(tableSizes.size() >= numPredictors);
+ assert(tableSizes[i] > 0);
+ assert((tableSizes[i] & (tableSizes[i] - 1)) == 0); // power-of-two
tageTable[i].resize(tableSizes[i]);
for (unsigned int j = 0; j < tableSizes[i]; ++j) {
tageTable[i][j].resize(numWays);
}
@@
assert(tableTagBits.size() >= numPredictors);
+ assert(tableTagBits[i] > 0);
tableTagMasks[i].resize(tableTagBits[i], true);🧰 Tools
🪛 Cppcheck (2.19.0)
[error] 119-119: Reference to temporary returned.
(returnTempReference)
[error] 125-125: Shifting 64-bit value by 64 bits is undefined behaviour
(shiftTooManyBits)
🤖 Prompt for AI Agents
In `@src/cpu/pred/btb/microtage.cc` around lines 106 - 128, The loop assumes
tableSizes[i] is a positive power-of-two and tableTagBits[i] >= 1; add explicit
validation before using them: check tableSizes.size() and tableTagBits.size() as
already asserted, then validate that tableSizes[i] > 0 and is a power-of-two (or
replace it with nextPowerOfTwo if you prefer) before computing tableIndexBits
and resizing tageTable[i], and ensure tableTagBits[i] >= 1 before using
tableTagBits[i]-1 for altTagFoldedHist; if validation fails, assert or throw
with a clear message. Also ensure tableIndexBits[i] is computed from the
validated size and use it to size tableIndexMasks[i] and tableTagMasks[i]
safely.
| MicroTAGE::TagePrediction | ||
| MicroTAGE::generateSinglePrediction(const BTBEntry &btb_entry, | ||
| const Addr &startPC, | ||
| std::shared_ptr<TageMeta> predMeta) { | ||
| DPRINTF(TAGE, "generateSinglePrediction for btbEntry: %#lx\n", btb_entry.pc); | ||
|
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||
| bool provided = false; | ||
| TageTableInfo main_info; | ||
|
|
||
| // Search from highest to lowest table for matches | ||
| // Calculate branch position within the block (like RTL's cfiPosition) | ||
| unsigned position = getBranchIndexInBlock(btb_entry.pc, startPC); | ||
|
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||
| for (int i = numPredictors - 1; i >= 0; --i) { | ||
| // Calculate index and tag: use snapshot if provided, otherwise use current folded history | ||
| // Tag includes position XOR (like RTL: tag = tempTag ^ cfiPosition) | ||
| Addr index = predMeta ? getTageIndex(startPC, i, predMeta->indexFoldedHist[i].get()) | ||
| : getTageIndex(startPC, i); | ||
| Addr tag = predMeta ? getTageTag(startPC, i, | ||
| predMeta->tagFoldedHist[i].get(),predMeta->altTagFoldedHist[i].get(), position) | ||
| : getTageTag(startPC, i, tagFoldedHist[i].get(),altTagFoldedHist[i].get(), position); | ||
|
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| bool match = false; // for each table, only one way can be matched | ||
| TageEntry matching_entry; | ||
| unsigned matching_way = 0; | ||
|
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||
| // Search all ways for a matching entry | ||
| for (unsigned way = 0; way < numWays; way++) { | ||
| auto &entry = tageTable[i][index][way]; | ||
| // entry valid, tag match (position already encoded in tag, no need to check pc) | ||
| if (entry.valid && tag == entry.tag) { | ||
| matching_entry = entry; | ||
| matching_way = way; | ||
| match = true; | ||
|
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| // Do not use LRU; keep logic simple and align with CBP-style replacement | ||
|
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| DPRINTF(TAGE, "hit table %d[%lu][%u]: valid %d, tag %lu, ctr %d, useful %d, btb_pc %#lx, pos %u\n", | ||
| i, index, way, entry.valid, entry.tag, entry.counter, entry.useful, btb_entry.pc, position); | ||
| break; // only one way can be matched, aviod multi hit, TODO: RTL how to do this? | ||
| } | ||
| } | ||
|
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||
| if (match) { | ||
| if (!provided) { | ||
| // First match becomes main prediction | ||
| main_info = TageTableInfo(true, matching_entry, i, index, tag, matching_way); | ||
| provided = true; | ||
| } | ||
| } else { | ||
| DPRINTF(TAGE, "miss table %d[%lu] for tag %lu (with pos %u), btb_pc %#lx\n", | ||
| i, index, tag, position, btb_entry.pc); | ||
| } | ||
| } | ||
|
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||
| // Generate final prediction | ||
| bool main_taken = main_info.taken(); | ||
| bool base_pred = btb_entry.ctr >= 0; | ||
|
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| bool taken = provided ? main_taken : base_pred; | ||
|
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| DPRINTF(TAGE, "tage predict %#lx taken %d\n", btb_entry.pc, taken); | ||
| DPRINTF(TAGE, "tage main prvided %d ? main_taken %d : base_taken %d\n", provided, main_taken, base_pred); | ||
|
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||
| return TagePrediction(btb_entry.pc, main_info, provided, taken, base_pred); |
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Avoid unnecessary taken() call when no provider is available.
main_info.taken() is called unconditionally, but its result is only used when provided is true. Move the call into the conditional to avoid the unnecessary function invocation.
Fix
- bool main_taken = main_info.taken();
+ bool main_taken = provided ? main_info.taken() : false;🤖 Prompt for AI Agents
In `@src/cpu/pred/btb/microtage.cc` around lines 192 - 256, The code currently
calls main_info.taken() unconditionally even when provided is false; change
generation of main_taken to only occur when provided is true by moving the call
to main_info.taken() inside the provided branch (use main_taken only when
provided), e.g., compute main_taken after the loop only if provided is true and
otherwise avoid calling TageTableInfo::taken(); also adjust the DPRINTF lines
that reference main_taken to use conditional values (or print only when
provided) so no unused/uninitialized variable is accessed; update the return to
use the computed values accordingly in generateSinglePrediction.
| auto predMeta = std::static_pointer_cast<TageMeta>(stream.predMetas[getComponentIdx()]); | ||
| if (!predMeta) { | ||
| DPRINTF(TAGE, "update: no prediction meta, skip\n"); | ||
| return; | ||
| } | ||
|
|
||
| bool utage_hit = false; | ||
| // Process each BTB entry | ||
| for (auto &btb_entry : entries_to_update) { | ||
| bool actual_taken = stream.exeTaken && stream.exeBranchInfo == btb_entry; | ||
| TagePrediction recomputed; | ||
| if (updateOnRead) { // if update on read is enabled, re-read providers using snapshot | ||
| // Re-read providers using snapshot (do not rely on prediction-time main/alt) | ||
| recomputed = generateSinglePrediction(btb_entry, startAddr, predMeta); | ||
| } else { // otherwise, use the prediction from the prediction-time main/alt | ||
| recomputed = predMeta->preds[btb_entry.pc]; | ||
| } |
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Use .find() to safely check for prediction metadata instead of inserting defaults.
When updateOnRead is disabled, predMeta->preds[btb_entry.pc] inserts a default TagePrediction if the key doesn't exist (e.g., for new BTB entries). The resulting object has mainInfo.found=false and mainprovided=false, which corrupts the update logic. Use safe lookup with .find() and fallback to recomputation for missing entries, consistent with the approach used in btb_tage.cc and the documented design:
Safer lookup
} else { // otherwise, use the prediction from the prediction-time main/alt
- recomputed = predMeta->preds[btb_entry.pc];
+ auto it = predMeta->preds.find(btb_entry.pc);
+ if (it != predMeta->preds.end()) {
+ recomputed = it->second;
+ } else {
+ // BTB miss/new entry: recompute from snapshot to avoid default insertion
+ recomputed = generateSinglePrediction(btb_entry, startAddr, predMeta);
+ }
}📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| auto predMeta = std::static_pointer_cast<TageMeta>(stream.predMetas[getComponentIdx()]); | |
| if (!predMeta) { | |
| DPRINTF(TAGE, "update: no prediction meta, skip\n"); | |
| return; | |
| } | |
| bool utage_hit = false; | |
| // Process each BTB entry | |
| for (auto &btb_entry : entries_to_update) { | |
| bool actual_taken = stream.exeTaken && stream.exeBranchInfo == btb_entry; | |
| TagePrediction recomputed; | |
| if (updateOnRead) { // if update on read is enabled, re-read providers using snapshot | |
| // Re-read providers using snapshot (do not rely on prediction-time main/alt) | |
| recomputed = generateSinglePrediction(btb_entry, startAddr, predMeta); | |
| } else { // otherwise, use the prediction from the prediction-time main/alt | |
| recomputed = predMeta->preds[btb_entry.pc]; | |
| } | |
| auto predMeta = std::static_pointer_cast<TageMeta>(stream.predMetas[getComponentIdx()]); | |
| if (!predMeta) { | |
| DPRINTF(TAGE, "update: no prediction meta, skip\n"); | |
| return; | |
| } | |
| bool utage_hit = false; | |
| // Process each BTB entry | |
| for (auto &btb_entry : entries_to_update) { | |
| bool actual_taken = stream.exeTaken && stream.exeBranchInfo == btb_entry; | |
| TagePrediction recomputed; | |
| if (updateOnRead) { // if update on read is enabled, re-read providers using snapshot | |
| // Re-read providers using snapshot (do not rely on prediction-time main/alt) | |
| recomputed = generateSinglePrediction(btb_entry, startAddr, predMeta); | |
| } else { // otherwise, use the prediction from the prediction-time main/alt | |
| auto it = predMeta->preds.find(btb_entry.pc); | |
| if (it != predMeta->preds.end()) { | |
| recomputed = it->second; | |
| } else { | |
| // BTB miss/new entry: recompute from snapshot to avoid default insertion | |
| recomputed = generateSinglePrediction(btb_entry, startAddr, predMeta); | |
| } | |
| } |
🤖 Prompt for AI Agents
In `@src/cpu/pred/btb/microtage.cc` around lines 629 - 645,
predMeta->preds[btb_entry.pc] can insert a default TagePrediction and corrupt
update logic when updateOnRead is false; change the access in the loop over
entries_to_update to first check predMeta->preds.find(btb_entry.pc) and if not
found call generateSinglePrediction(btb_entry, startAddr, predMeta) (or
otherwise fall back to recomputation) so you only use stored predictions when
the key exists; keep using recomputed when updateOnRead is true or when the
lookup fails, and preserve existing variables (TagePrediction recomputed,
utage_hit) and control flow.
| // Calculate TAGE tag with folded history - optimized version using bitwise operations | ||
| Addr | ||
| MicroTAGE::getTageTag(Addr pc, int t, uint64_t foldedHist, uint64_t altFoldedHist, Addr position) | ||
| { | ||
| // Create mask for tableTagBits[t] to limit result size | ||
| Addr mask = (1ULL << tableTagBits[t]) - 1; | ||
|
|
||
| // Extract lower bits of PC directly (remove instruction alignment bits) | ||
| Addr pcBits = (pc >> bankBaseShift) & mask; | ||
|
|
||
| // Extract and prepare folded history bits | ||
| Addr foldedBits = foldedHist & mask; | ||
|
|
||
| // Extract alt tag bits and shift left by 1 | ||
| Addr altTagBits = (altFoldedHist << 1) & mask; | ||
|
|
||
| // XOR all components together, including position (like RTL) | ||
| return pcBits ^ foldedBits ^ position ^ altTagBits; | ||
| } | ||
|
|
||
| Addr | ||
| MicroTAGE::getTageIndex(Addr pc, int t, uint64_t foldedHist) | ||
| { | ||
| // Create mask for tableIndexBits[t] to limit result size | ||
| Addr mask = (1ULL << tableIndexBits[t]) - 1; | ||
|
|
||
| const unsigned pcShift = enableBankConflict ? indexShift : bankBaseShift; | ||
| Addr pcBits = (pc >> pcShift) & mask; | ||
| Addr foldedBits = foldedHist & mask; | ||
|
|
||
| return pcBits ^ foldedBits; | ||
| } |
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Guard against shift-by-width when building masks.
1ULL << tableTagBits[t] and 1ULL << tableIndexBits[t] are undefined if the shift count reaches the address width (64 bits for Addr). While current parameter configurations keep these values safely below 64 bits, the code is fragile to future configuration changes. Build masks defensively to prevent latent undefined behavior.
🧯 Defensive mask construction
Addr
MicroTAGE::getTageTag(Addr pc, int t, uint64_t foldedHist, uint64_t altFoldedHist, Addr position)
{
- // Create mask for tableTagBits[t] to limit result size
- Addr mask = (1ULL << tableTagBits[t]) - 1;
+ const unsigned tagBits = tableTagBits[t];
+ const unsigned addrBits = sizeof(Addr) * 8;
+ Addr mask = (tagBits >= addrBits) ? ~Addr(0) : ((Addr(1) << tagBits) - 1);
@@
Addr
MicroTAGE::getTageIndex(Addr pc, int t, uint64_t foldedHist)
{
- // Create mask for tableIndexBits[t] to limit result size
- Addr mask = (1ULL << tableIndexBits[t]) - 1;
+ const unsigned idxBits = tableIndexBits[t];
+ const unsigned addrBits = sizeof(Addr) * 8;
+ Addr mask = (idxBits >= addrBits) ? ~Addr(0) : ((Addr(1) << idxBits) - 1);📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| // Calculate TAGE tag with folded history - optimized version using bitwise operations | |
| Addr | |
| MicroTAGE::getTageTag(Addr pc, int t, uint64_t foldedHist, uint64_t altFoldedHist, Addr position) | |
| { | |
| // Create mask for tableTagBits[t] to limit result size | |
| Addr mask = (1ULL << tableTagBits[t]) - 1; | |
| // Extract lower bits of PC directly (remove instruction alignment bits) | |
| Addr pcBits = (pc >> bankBaseShift) & mask; | |
| // Extract and prepare folded history bits | |
| Addr foldedBits = foldedHist & mask; | |
| // Extract alt tag bits and shift left by 1 | |
| Addr altTagBits = (altFoldedHist << 1) & mask; | |
| // XOR all components together, including position (like RTL) | |
| return pcBits ^ foldedBits ^ position ^ altTagBits; | |
| } | |
| Addr | |
| MicroTAGE::getTageIndex(Addr pc, int t, uint64_t foldedHist) | |
| { | |
| // Create mask for tableIndexBits[t] to limit result size | |
| Addr mask = (1ULL << tableIndexBits[t]) - 1; | |
| const unsigned pcShift = enableBankConflict ? indexShift : bankBaseShift; | |
| Addr pcBits = (pc >> pcShift) & mask; | |
| Addr foldedBits = foldedHist & mask; | |
| return pcBits ^ foldedBits; | |
| } | |
| // Calculate TAGE tag with folded history - optimized version using bitwise operations | |
| Addr | |
| MicroTAGE::getTageTag(Addr pc, int t, uint64_t foldedHist, uint64_t altFoldedHist, Addr position) | |
| { | |
| // Create mask for tableTagBits[t] to limit result size | |
| const unsigned tagBits = tableTagBits[t]; | |
| const unsigned addrBits = sizeof(Addr) * 8; | |
| Addr mask = (tagBits >= addrBits) ? ~Addr(0) : ((Addr(1) << tagBits) - 1); | |
| // Extract lower bits of PC directly (remove instruction alignment bits) | |
| Addr pcBits = (pc >> bankBaseShift) & mask; | |
| // Extract and prepare folded history bits | |
| Addr foldedBits = foldedHist & mask; | |
| // Extract alt tag bits and shift left by 1 | |
| Addr altTagBits = (altFoldedHist << 1) & mask; | |
| // XOR all components together, including position (like RTL) | |
| return pcBits ^ foldedBits ^ position ^ altTagBits; | |
| } | |
| Addr | |
| MicroTAGE::getTageIndex(Addr pc, int t, uint64_t foldedHist) | |
| { | |
| // Create mask for tableIndexBits[t] to limit result size | |
| const unsigned idxBits = tableIndexBits[t]; | |
| const unsigned addrBits = sizeof(Addr) * 8; | |
| Addr mask = (idxBits >= addrBits) ? ~Addr(0) : ((Addr(1) << idxBits) - 1); | |
| const unsigned pcShift = enableBankConflict ? indexShift : bankBaseShift; | |
| Addr pcBits = (pc >> pcShift) & mask; | |
| Addr foldedBits = foldedHist & mask; | |
| return pcBits ^ foldedBits; | |
| } |
🤖 Prompt for AI Agents
In `@src/cpu/pred/btb/microtage.cc` around lines 738 - 769, The mask creation in
getTageTag and getTageIndex is unsafe when tableTagBits[t] or tableIndexBits[t]
equals the address width because `(1ULL << n)` is undefined for n==width; change
the mask construction to handle the full-width case defensively (e.g., if bits
>= (sizeof(Addr)*8) use all-ones mask, otherwise compute `(1ULL << bits) - 1`).
Update constructions that use `tableTagBits[t]` and `tableIndexBits[t]` so they
never perform a shift-by-width and ensure the mask type matches Addr.
| auto meta = std::static_pointer_cast<TageMeta>(stream.predMetas[getComponentIdx()]); | ||
| auto pc = inst->pcState().instAddr(); | ||
| auto it = meta->preds.find(pc); | ||
| bool pred_taken = false; | ||
| bool pred_hit = false; | ||
| if (it != meta->preds.end()) { | ||
| pred_taken = it->second.taken; |
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Guard against null prediction metadata before use.
meta can be null; dereferencing it will crash on commit in that case.
🧷 Null-guard
auto meta = std::static_pointer_cast<TageMeta>(stream.predMetas[getComponentIdx()]);
+ if (!meta) {
+ return;
+ }🤖 Prompt for AI Agents
In `@src/cpu/pred/btb/microtage.cc` around lines 1063 - 1069, The code
dereferences meta (auto meta =
std::static_pointer_cast<TageMeta>(stream.predMetas[getComponentIdx()])) without
checking for null, which can crash on commit; add a null-guard after obtaining
meta (from stream.predMetas[getComponentIdx()]) and skip or set safe defaults
for uses of meta (e.g., when accessing meta->preds, meta->preds.find(pc), and
reading it->second.taken) if meta is null; ensure functions/methods using meta
such as the block that computes pc via inst->pcState().instAddr() and sets
pred_taken/pred_hit handle the null case consistently (e.g., leave
pred_taken=false and pred_hit=false or bail out) to avoid dereferencing a null
TageMeta.
| #include <cstdint> | ||
| #include <deque> | ||
| #include <map> | ||
| #include <utility> | ||
| #include <vector> | ||
|
|
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#!/bin/bash
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head -20 src/cpu/pred/btb/microtage.hh
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Add missing standard headers for shared_ptr and unordered_map.
The file uses std::shared_ptr (line 44 and others) and std::unordered_map (line 384), but lacks the required <memory> and <unordered_map> headers. Relying on transitive includes is brittle and leads to fragile code.
Suggested include fix
`#include` <cstdint>
`#include` <deque>
`#include` <map>
+#include <memory>
+#include <unordered_map>
`#include` <utility>
`#include` <vector>📝 Committable suggestion
‼️ IMPORTANT
Carefully review the code before committing. Ensure that it accurately replaces the highlighted code, contains no missing lines, and has no issues with indentation. Thoroughly test & benchmark the code to ensure it meets the requirements.
| #include <cstdint> | |
| #include <deque> | |
| #include <map> | |
| #include <utility> | |
| #include <vector> | |
| `#include` <cstdint> | |
| `#include` <deque> | |
| `#include` <map> | |
| `#include` <memory> | |
| `#include` <unordered_map> | |
| `#include` <utility> | |
| `#include` <vector> |
🤖 Prompt for AI Agents
In `@src/cpu/pred/btb/microtage.hh` around lines 4 - 9, The header is missing
direct includes for types it uses: add `#include` <memory> (for std::shared_ptr
used throughout this file, e.g., in declarations around symbol(s) using
shared_ptr) and `#include` <unordered_map> (for std::unordered_map used near the
btb/microtage implementation) to the top-of-file include list; this ensures the
references to std::shared_ptr and std::unordered_map are correct and not reliant
on transitive headers.
Change-Id: If50663e7dd3ca7f48f53d9070a65cfb7d53b4513
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Actionable comments posted: 1
🤖 Fix all issues with AI agents
In `@src/cpu/pred/BranchPredictor.py`:
- Around line 1063-1089: The MicroTAGE Python config declares parameters not
used by its C++ constructor; remove the unused Param entries baseTableSize,
enableSC, useAltOnNaSize, and useAltOnNaWidth from the MicroTAGE class so the
Python and C++ interfaces match. Edit the MicroTAGE class (symbols: MicroTAGE,
needMoreHistories, updateOnRead, numPredictors, tableSizes, TTagBitSizes,
TTagPcShifts, histLengths, maxHistLen, numWays, maxBranchPositions,
numTablesToAlloc, numBanks, blockSize, enableBankConflict, numDelay) and delete
the four Param.* lines for baseTableSize, enableSC, useAltOnNaSize, and
useAltOnNaWidth leaving only the actual constructor-backed parameters listed in
the review.
| class MicroTAGE(TimedBaseBTBPredictor): | ||
| """Micro-sized BTB TAGE predictor used alongside uBTB""" | ||
| type = 'MicroTAGE' | ||
| cxx_class = 'gem5::branch_prediction::btb_pred::MicroTAGE' | ||
| cxx_header = "cpu/pred/btb/microtage.hh" | ||
|
|
||
| needMoreHistories = Param.Bool(True, "MicroTAGE needs more histories") | ||
| enableSC = Param.Bool(False, "Enable SC or not") | ||
| updateOnRead = Param.Bool(True,"Enable update on read, no need to save tage meta in FTQ") | ||
| # Keep vector parameters consistent with numPredictors to avoid constructor asserts. | ||
| numPredictors = Param.Unsigned(1, "Number of TAGE predictors") | ||
| tableSizes = VectorParam.Unsigned([512],"the TAGE T0~Tn length") | ||
| TTagBitSizes = VectorParam.Unsigned([16] ,"the T0~Tn entry's tag bit size") | ||
| TTagPcShifts = VectorParam.Unsigned([1] ,"when the T0~Tn entry's tag generating, PC right shift") | ||
| blockSize = Param.Unsigned(32,"tage index function uses 32B aligned block address") | ||
|
|
||
| histLengths = VectorParam.Unsigned([16],"the BTB TAGE T0~Tn history length") | ||
| maxHistLen = Param.Unsigned(970,"The length of history passed from DBP") | ||
| numTablesToAlloc = Param.Unsigned(1,"The number of table to allocated each time") | ||
| numWays = Param.Unsigned(2, "Number of ways per set") | ||
| baseTableSize = Param.Unsigned(256,"Base table size") | ||
| maxBranchPositions = Param.Unsigned(32,"Maximum branch positions per 64-byte block") | ||
| useAltOnNaSize = Param.Unsigned(128,"Size of the useAltOnNa table") | ||
| useAltOnNaWidth = Param.Unsigned(7,"Width of the useAltOnNa table") | ||
| numBanks = Param.Unsigned(4,"Number of banks for bank conflict simulation") | ||
| enableBankConflict = Param.Bool(False,"Enable bank conflict simulation") | ||
| numDelay = Param.Unsigned(0,"Prediction latency in cycles") |
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echo ""
echo "=== Looking for microtage files ==="
fd -e 'cc' -e 'hh' | grep -i microtage
echo ""
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Remove unused parameters from MicroTAGE configuration.
The C++ constructor uses only a subset of the declared parameters. The following parameters are not accessed in the MicroTAGE implementation and should be removed from the Python configuration:
baseTableSize(line 1083)enableSC(line 1070)useAltOnNaSize(line 1086)useAltOnNaWidth(line 1087)
These appear to be copy-pasted from other predictor classes (likely BTBTAGE) but are not integrated into MicroTAGE's C++ implementation. The actual constructor parameters used are: numPredictors, tableSizes, TTagBitSizes, TTagPcShifts, histLengths, maxHistLen, numWays, maxBranchPositions, numTablesToAlloc, updateOnRead, numBanks, blockSize, enableBankConflict, and needMoreHistories.
🧰 Tools
🪛 Ruff (0.14.14)
[error] 1069-1069: Param may be undefined, or defined from star imports
(F405)
[error] 1070-1070: Param may be undefined, or defined from star imports
(F405)
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(F405)
[error] 1073-1073: Param may be undefined, or defined from star imports
(F405)
[error] 1074-1074: VectorParam may be undefined, or defined from star imports
(F405)
[error] 1075-1075: VectorParam may be undefined, or defined from star imports
(F405)
[error] 1076-1076: VectorParam may be undefined, or defined from star imports
(F405)
[error] 1077-1077: Param may be undefined, or defined from star imports
(F405)
[error] 1079-1079: VectorParam may be undefined, or defined from star imports
(F405)
[error] 1080-1080: Param may be undefined, or defined from star imports
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[error] 1081-1081: Param may be undefined, or defined from star imports
(F405)
[error] 1082-1082: Param may be undefined, or defined from star imports
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[error] 1083-1083: Param may be undefined, or defined from star imports
(F405)
[error] 1084-1084: Param may be undefined, or defined from star imports
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[error] 1085-1085: Param may be undefined, or defined from star imports
(F405)
[error] 1086-1086: Param may be undefined, or defined from star imports
(F405)
[error] 1087-1087: Param may be undefined, or defined from star imports
(F405)
[error] 1088-1088: Param may be undefined, or defined from star imports
(F405)
[error] 1089-1089: Param may be undefined, or defined from star imports
(F405)
🤖 Prompt for AI Agents
In `@src/cpu/pred/BranchPredictor.py` around lines 1063 - 1089, The MicroTAGE
Python config declares parameters not used by its C++ constructor; remove the
unused Param entries baseTableSize, enableSC, useAltOnNaSize, and
useAltOnNaWidth from the MicroTAGE class so the Python and C++ interfaces match.
Edit the MicroTAGE class (symbols: MicroTAGE, needMoreHistories, updateOnRead,
numPredictors, tableSizes, TTagBitSizes, TTagPcShifts, histLengths, maxHistLen,
numWays, maxBranchPositions, numTablesToAlloc, numBanks, blockSize,
enableBankConflict, numDelay) and delete the four Param.* lines for
baseTableSize, enableSC, useAltOnNaSize, and useAltOnNaWidth leaving only the
actual constructor-backed parameters listed in the review.
1 participant
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