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[BOLT] Use offset deduplication for cold fragments
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Apply deduplication for uniformity and BAT section size reduction.

Changes BAT section size to:
- large binary: 39541552 bytes (1.02x original),
- medium binary: 3828996 bytes (0.64x),
- small binary: 928 bytes (0.65x).

Test Plan: Updated bolt-address-translation.test

Reviewers: rafaelauler, dcci, ayermolo, JDevlieghere, maksfb

Reviewed By: maksfb

Pull Request: llvm#87853
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@aaupov
aaupov authored Apr 15, 2024
1 parent 0794298 commit b79b6f9
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Showing 4 changed files with 53 additions and 50 deletions.
9 changes: 5 additions & 4 deletions bolt/docs/BAT.md
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Original file line number Diff line number Diff line change
Expand Up @@ -81,9 +81,10 @@ Hot indices are delta encoded, implicitly starting at zero.
| `FuncHash` | 8b | Function hash for input function | Hot |
| `NumBlocks` | ULEB128 | Number of basic blocks in the original function | Hot |
| `NumSecEntryPoints` | ULEB128 | Number of secondary entry points in the original function | Hot |
| `ColdInputSkew` | ULEB128 | Skew to apply to all input offsets | Cold |
| `NumEntries` | ULEB128 | Number of address translation entries for a function | Both |
| `EqualElems` | ULEB128 | Number of equal offsets in the beginning of a function | Hot |
| `BranchEntries` | Bitmask, `alignTo(EqualElems, 8)` bits | If `EqualElems` is non-zero, bitmask denoting entries with `BRANCHENTRY` bit | Hot |
| `EqualElems` | ULEB128 | Number of equal offsets in the beginning of a function | Both |
| `BranchEntries` | Bitmask, `alignTo(EqualElems, 8)` bits | If `EqualElems` is non-zero, bitmask denoting entries with `BRANCHENTRY` bit | Both |

Function header is followed by *Address Translation Table* with `NumEntries`
total entries, and *Secondary Entry Points* table with `NumSecEntryPoints`
Expand All @@ -99,8 +100,8 @@ entry is encoded. Input offsets implicitly start at zero.
| `BBHash` | Optional, 8b | Basic block hash in input binary | BB |
| `BBIdx` | Optional, Delta, ULEB128 | Basic block index in input binary | BB |

For hot fragments, the table omits the first `EqualElems` input offsets
where the input offset equals output offset.
The table omits the first `EqualElems` input offsets where the input offset
equals output offset.

`BRANCHENTRY` bit denotes whether a given offset pair is a control flow source
(branch or call instruction). If not set, it signifies a control flow target
Expand Down
6 changes: 3 additions & 3 deletions bolt/include/bolt/Profile/BoltAddressTranslation.h
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Original file line number Diff line number Diff line change
Expand Up @@ -149,9 +149,9 @@ class BoltAddressTranslation {
/// entries in function address translation map.
APInt calculateBranchEntriesBitMask(MapTy &Map, size_t EqualElems);

/// Calculate the number of equal offsets (output = input) in the beginning
/// of the function.
size_t getNumEqualOffsets(const MapTy &Map) const;
/// Calculate the number of equal offsets (output = input - skew) in the
/// beginning of the function.
size_t getNumEqualOffsets(const MapTy &Map, uint32_t Skew) const;

std::map<uint64_t, MapTy> Maps;

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86 changes: 44 additions & 42 deletions bolt/lib/Profile/BoltAddressTranslation.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -153,12 +153,13 @@ APInt BoltAddressTranslation::calculateBranchEntriesBitMask(MapTy &Map,
return BitMask;
}

size_t BoltAddressTranslation::getNumEqualOffsets(const MapTy &Map) const {
size_t BoltAddressTranslation::getNumEqualOffsets(const MapTy &Map,
uint32_t Skew) const {
size_t EqualOffsets = 0;
for (const std::pair<const uint32_t, uint32_t> &KeyVal : Map) {
const uint32_t OutputOffset = KeyVal.first;
const uint32_t InputOffset = KeyVal.second >> 1;
if (OutputOffset == InputOffset)
if (OutputOffset == InputOffset - Skew)
++EqualOffsets;
else
break;
Expand Down Expand Up @@ -196,12 +197,17 @@ void BoltAddressTranslation::writeMaps(std::map<uint64_t, MapTy> &Maps,
SecondaryEntryPointsMap.count(Address)
? SecondaryEntryPointsMap[Address].size()
: 0;
uint32_t Skew = 0;
if (Cold) {
auto HotEntryIt = Maps.find(ColdPartSource[Address]);
assert(HotEntryIt != Maps.end());
size_t HotIndex = std::distance(Maps.begin(), HotEntryIt);
encodeULEB128(HotIndex - PrevIndex, OS);
PrevIndex = HotIndex;
// Skew of all input offsets for cold fragments is simply the first input
// offset.
Skew = Map.begin()->second >> 1;
encodeULEB128(Skew, OS);
} else {
// Function hash
size_t BFHash = getBFHash(HotInputAddress);
Expand All @@ -217,24 +223,21 @@ void BoltAddressTranslation::writeMaps(std::map<uint64_t, MapTy> &Maps,
<< '\n');
}
encodeULEB128(NumEntries, OS);
// For hot fragments only: encode the number of equal offsets
// (output = input) in the beginning of the function. Only encode one offset
// in these cases.
const size_t EqualElems = Cold ? 0 : getNumEqualOffsets(Map);
if (!Cold) {
encodeULEB128(EqualElems, OS);
if (EqualElems) {
const size_t BranchEntriesBytes = alignTo(EqualElems, 8) / 8;
APInt BranchEntries = calculateBranchEntriesBitMask(Map, EqualElems);
OS.write(reinterpret_cast<const char *>(BranchEntries.getRawData()),
BranchEntriesBytes);
LLVM_DEBUG({
dbgs() << "BranchEntries: ";
SmallString<8> BitMaskStr;
BranchEntries.toString(BitMaskStr, 2, false);
dbgs() << BitMaskStr << '\n';
});
}
// Encode the number of equal offsets (output = input - skew) in the
// beginning of the function. Only encode one offset in these cases.
const size_t EqualElems = getNumEqualOffsets(Map, Skew);
encodeULEB128(EqualElems, OS);
if (EqualElems) {
const size_t BranchEntriesBytes = alignTo(EqualElems, 8) / 8;
APInt BranchEntries = calculateBranchEntriesBitMask(Map, EqualElems);
OS.write(reinterpret_cast<const char *>(BranchEntries.getRawData()),
BranchEntriesBytes);
LLVM_DEBUG({
dbgs() << "BranchEntries: ";
SmallString<8> BitMaskStr;
BranchEntries.toString(BitMaskStr, 2, false);
dbgs() << BitMaskStr << '\n';
});
}
const BBHashMapTy &BBHashMap = getBBHashMap(HotInputAddress);
size_t Index = 0;
Expand Down Expand Up @@ -315,10 +318,12 @@ void BoltAddressTranslation::parseMaps(std::vector<uint64_t> &HotFuncs,
uint64_t HotAddress = Cold ? 0 : Address;
PrevAddress = Address;
uint32_t SecondaryEntryPoints = 0;
uint64_t ColdInputSkew = 0;
if (Cold) {
HotIndex += DE.getULEB128(&Offset, &Err);
HotAddress = HotFuncs[HotIndex];
ColdPartSource.emplace(Address, HotAddress);
ColdInputSkew = DE.getULEB128(&Offset, &Err);
} else {
HotFuncs.push_back(Address);
// Function hash
Expand All @@ -339,28 +344,25 @@ void BoltAddressTranslation::parseMaps(std::vector<uint64_t> &HotFuncs,
getULEB128Size(SecondaryEntryPoints)));
}
const uint32_t NumEntries = DE.getULEB128(&Offset, &Err);
// Equal offsets, hot fragments only.
size_t EqualElems = 0;
// Equal offsets.
const size_t EqualElems = DE.getULEB128(&Offset, &Err);
APInt BEBitMask;
if (!Cold) {
EqualElems = DE.getULEB128(&Offset, &Err);
LLVM_DEBUG(dbgs() << formatv("Equal offsets: {0}, {1} bytes\n",
EqualElems, getULEB128Size(EqualElems)));
if (EqualElems) {
const size_t BranchEntriesBytes = alignTo(EqualElems, 8) / 8;
BEBitMask = APInt(alignTo(EqualElems, 8), 0);
LoadIntFromMemory(
BEBitMask,
reinterpret_cast<const uint8_t *>(
DE.getBytes(&Offset, BranchEntriesBytes, &Err).data()),
BranchEntriesBytes);
LLVM_DEBUG({
dbgs() << "BEBitMask: ";
SmallString<8> BitMaskStr;
BEBitMask.toString(BitMaskStr, 2, false);
dbgs() << BitMaskStr << ", " << BranchEntriesBytes << " bytes\n";
});
}
LLVM_DEBUG(dbgs() << formatv("Equal offsets: {0}, {1} bytes\n", EqualElems,
getULEB128Size(EqualElems)));
if (EqualElems) {
const size_t BranchEntriesBytes = alignTo(EqualElems, 8) / 8;
BEBitMask = APInt(alignTo(EqualElems, 8), 0);
LoadIntFromMemory(
BEBitMask,
reinterpret_cast<const uint8_t *>(
DE.getBytes(&Offset, BranchEntriesBytes, &Err).data()),
BranchEntriesBytes);
LLVM_DEBUG({
dbgs() << "BEBitMask: ";
SmallString<8> BitMaskStr;
BEBitMask.toString(BitMaskStr, 2, false);
dbgs() << BitMaskStr << ", " << BranchEntriesBytes << " bytes\n";
});
}
MapTy Map;

Expand All @@ -375,7 +377,7 @@ void BoltAddressTranslation::parseMaps(std::vector<uint64_t> &HotFuncs,
PrevAddress = OutputAddress;
int64_t InputDelta = 0;
if (J < EqualElems) {
InputOffset = (OutputOffset << 1) | BEBitMask[J];
InputOffset = (OutputOffset + ColdInputSkew << 1) | BEBitMask[J];
} else {
InputDelta = DE.getSLEB128(&Offset, &Err);
InputOffset += InputDelta;
Expand Down
2 changes: 1 addition & 1 deletion bolt/test/X86/bolt-address-translation.test
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Original file line number Diff line number Diff line change
Expand Up @@ -37,7 +37,7 @@
# CHECK: BOLT: 3 out of 7 functions were overwritten.
# CHECK: BOLT-INFO: Wrote 6 BAT maps
# CHECK: BOLT-INFO: Wrote 3 function and 58 basic block hashes
# CHECK: BOLT-INFO: BAT section size (bytes): 924
# CHECK: BOLT-INFO: BAT section size (bytes): 928
#
# usqrt mappings (hot part). We match against any key (left side containing
# the bolted binary offsets) because BOLT may change where it puts instructions
Expand Down

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