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[DebugInfo] Add fast path for parsing DW_TAG_compile_unit abbrevs #108757

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[DebugInfo] Add fast path for parsing DW_TAG_compile_unit abbrevs #108757

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d3c7bae
[DebugInfo] Add fast path for parsing DW_TAG_compile_unit abbrevs
BertalanD Sep 15, 2024
ca8106a
Don't crash on empty optional deref
BertalanD Sep 15, 2024
ba0d7f4
Propagate error from tryExtractCUAbbrevFast()
BertalanD Sep 17, 2024
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4 changes: 4 additions & 0 deletions llvm/include/llvm/DebugInfo/DWARF/DWARFDebugAbbrev.h
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Expand Up @@ -62,13 +62,17 @@ class DWARFDebugAbbrev {
mutable DWARFAbbreviationDeclarationSetMap AbbrDeclSets;
mutable DWARFAbbreviationDeclarationSetMap::const_iterator PrevAbbrOffsetPos;
mutable std::optional<DataExtractor> Data;
mutable std::map<uint64_t, DWARFAbbreviationDeclaration> CUAbbrevs;

public:
DWARFDebugAbbrev(DataExtractor Data);

Expected<const DWARFAbbreviationDeclarationSet *>
getAbbreviationDeclarationSet(uint64_t CUAbbrOffset) const;

Expected<const DWARFAbbreviationDeclaration *>
tryExtractCUAbbrevFast(uint64_t CUAbbrOffset) const;

void dump(raw_ostream &OS) const;
Error parse() const;

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4 changes: 4 additions & 0 deletions llvm/include/llvm/DebugInfo/DWARF/DWARFUnit.h
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Expand Up @@ -419,6 +419,10 @@ class DWARFUnit {

uint64_t getAbbreviationsOffset() const { return Header.getAbbrOffset(); }

/// Extracts only the abbreviation declaration with code 1, which is
/// typically the compile unit DIE (DW_TAG_compile_unit).
const DWARFAbbreviationDeclaration *tryExtractCUAbbrevFast() const;

const DWARFAbbreviationDeclarationSet *getAbbreviations() const;

static bool isMatchingUnitTypeAndTag(uint8_t UnitType, dwarf::Tag Tag) {
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22 changes: 22 additions & 0 deletions llvm/lib/DebugInfo/DWARF/DWARFDebugAbbrev.cpp
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Expand Up @@ -168,3 +168,25 @@ DWARFDebugAbbrev::getAbbreviationDeclarationSet(uint64_t CUAbbrOffset) const {
.first;
return &PrevAbbrOffsetPos->second;
}

Expected<const DWARFAbbreviationDeclaration *>
DWARFDebugAbbrev::tryExtractCUAbbrevFast(uint64_t CUAbbrOffset) const {
if (auto AbbrevDecl = CUAbbrevs.find(CUAbbrOffset);
AbbrevDecl != CUAbbrevs.end())
return &AbbrevDecl->second;

if (!Data || CUAbbrOffset >= Data->getData().size())
return make_error<llvm::object::GenericBinaryError>(
"the abbreviation offset into the .debug_abbrev section is not valid");

DWARFAbbreviationDeclaration Decl;
uint64_t Offset = CUAbbrOffset;
Expected<DWARFAbbreviationDeclaration::ExtractState> ES =
Decl.extract(*Data, &Offset);
if (!ES)
return ES.takeError();
if (Decl.getCode() != 1)
return nullptr;

return &(CUAbbrevs[CUAbbrOffset] = std::move(Decl));
}
57 changes: 35 additions & 22 deletions llvm/lib/DebugInfo/DWARF/DWARFDebugInfoEntry.cpp
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Expand Up @@ -34,36 +34,49 @@ bool DWARFDebugInfoEntry::extractFast(const DWARFUnit &U, uint64_t *OffsetPtr,
return false;
}
assert(DebugInfoData.isValidOffset(UEndOffset - 1));
AbbrevDecl = nullptr;

uint64_t AbbrCode = DebugInfoData.getULEB128(OffsetPtr);
if (0 == AbbrCode) {
// NULL debug tag entry.
AbbrevDecl = nullptr;
return true;
}
const auto *AbbrevSet = U.getAbbreviations();
if (!AbbrevSet) {
U.getContext().getWarningHandler()(
createStringError(errc::invalid_argument,
"DWARF unit at offset 0x%8.8" PRIx64 " "
"contains invalid abbreviation set offset 0x%" PRIx64,
U.getOffset(), U.getAbbreviationsOffset()));
// Restore the original offset.
*OffsetPtr = Offset;
return false;

// Fast path: parsing the entire abbreviation table is wasteful if we only
// need the unit DIE (typically AbbrCode == 1).
if (1 == AbbrCode) {
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  1. minor, we don't usually do these "constant on the LHS" style comparison in LLVM, I think?
  2. The abbreviation numbers in the abbrev table are arbitrary/not ordered - so we probably shouldn't be needing/trying to check for the value is 1.

Could this either generalize this to check the first abbrev in the table, regardless of numbering - or change abbrev parsing to be lazy in general? (like parse as much of the table as is needed to find the number - oh, I guess we do have some optimizations that only fire if the abbrev numbering is strictly increasing (doesn't have to start at 1, doesn't have to increase by 1 each time - but so long as it's always increasing we can still binary search))

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I agree with David here, this is seems like an overfit. Both suggestions make sense to me, either check the first entry in the table (which seems to be what you're doing anyway) or make abbreviation parsing lazier.

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@BertalanD BertalanD Sep 17, 2024 •
edited
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or change abbrev parsing to be lazy in general?

I have considered this, but this would likely pessimize the general case.

Profiling shows that the majority of ld64.lld's DWARF parsing time is split evenly between ULEB128 parsing and appending to the DWARFAbbreviationDeclaration::attribute_specs vector.

As far as I can tell, everything is ULEB128-encoded in the abbrev table, so if we do it lazily, we'll end up constantly re-parsing the abbrev declarations that come before the one we're looking for. So we'd parse these numbers O(n^2) times instead of O(n). I guess we could do a first pass where we only note down the begin offsets of the abbreviations, but that would bring us back to the situation with the large number of vector appends (although the count is |abbrevs|, not |attributes|, that might be better)

And if we do need everything, the total cost of building e.g. an std::map would end up being larger than the vector appends' cost. For the use cases where everything is needed (LLDB, dsymutil, etc.), constructing the single sorted vector upfront (i.e. what DWARFAbbreviationDeclarationSet does) is faster than the above approach.

Could this either generalize this to check the first abbrev in the table

Do you mean always trying to parse the first abbreviation (so no AbbrCode == 1 check), and checking afterwards if the code happens to be what we need? That sounds okay to me, since after the first call, the value is cached, so its cost would be a single std::map lookup.

Just out of curiosity, how common is the first abbreviation code not being 1 or code 1 not being DW_TAG_compile_unit? All C++/Swift/Rust programs I tested hit my fast path.

Flame graph of DWARF parsing image

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(re 1: Agreed, it's not common in LLVM, but if (0 == AbbrCode) { is just 7 lines above in this file. So it does match local style.)

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or change abbrev parsing to be lazy in general?

I have considered this, but this would likely pessimize the general case.

Ah, sorry, not /that/ lazy. Like we could store the existing vector of abbrevs, and an offset to past the end of the last abbrev we parsed (or some sentinel if we reached the end of the abbrev list, with its 0 marker). Then if an abbrev number is requested that isn't in the list already, we parse abbrevs (& put them in the vector, etc) until we find the one we're looking for.

But I think the only place I'm thinking of that'd benefit from lazy abbrev parsing is just for the CU abbrev too anyway (building an address lookup table when the DWARF doesn't include .debug_aranges - currently that involves parsing all the abbrevs, and should only need the CU's abbrev too)

& also there's some code in llvm-dwp that tries to parse just the first CU to access the dwo_id for DWARFv4. Though I'm not sure that code's using libDebugInfoDWARF at all, since it mostly doesn't need to parse DWARF.

I guess it'd help to be more generalized laziness would handle cases where the CU DIE abbrev isn't the first one (like with type units, but those aren't used on MacOS, so aren't relevant to ld64 perf).

Why is ld64 reading the CUs anyway?

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@BertalanD BertalanD Sep 17, 2024 •
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(will reflect on the other bits later):

Why is ld64 reading the CUs anyway?

For emitting N_OSO stabs; which indicate the source file names of symbols. We need to read DW_AT_source_dir for this, see ObjFile::sourceFile in LLD.

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One generalization would be to try symbolizing (llvm-symbolizer) where the abbrev parsing probably isn't /as/ large a part of the profile, but probably it shows up and lazy CU abbrev parsing probably helps there too?

AbbrevDecl = U.tryExtractCUAbbrevFast();
assert(!AbbrevDecl || AbbrevDecl->getCode() == AbbrCode);
}
AbbrevDecl = AbbrevSet->getAbbreviationDeclaration(AbbrCode);

if (!AbbrevDecl) {
U.getContext().getWarningHandler()(
createStringError(errc::invalid_argument,
"DWARF unit at offset 0x%8.8" PRIx64 " "
"contains invalid abbreviation %" PRIu64 " at "
"offset 0x%8.8" PRIx64 ", valid abbreviations are %s",
U.getOffset(), AbbrCode, *OffsetPtr,
AbbrevSet->getCodeRange().c_str()));
// Restore the original offset.
*OffsetPtr = Offset;
return false;
const auto *AbbrevSet = U.getAbbreviations();
if (!AbbrevSet) {
U.getContext().getWarningHandler()(createStringError(
errc::invalid_argument,
"DWARF unit at offset 0x%8.8" PRIx64 " "
"contains invalid abbreviation set offset 0x%" PRIx64,
U.getOffset(), U.getAbbreviationsOffset()));
// Restore the original offset.
*OffsetPtr = Offset;
return false;
}
AbbrevDecl = AbbrevSet->getAbbreviationDeclaration(AbbrCode);

if (!AbbrevDecl) {
U.getContext().getWarningHandler()(createStringError(
errc::invalid_argument,
"DWARF unit at offset 0x%8.8" PRIx64 " "
"contains invalid abbreviation %" PRIu64 " at "
"offset 0x%8.8" PRIx64 ", valid abbreviations are %s",
U.getOffset(), AbbrCode, *OffsetPtr,
AbbrevSet->getCodeRange().c_str()));
// Restore the original offset.
*OffsetPtr = Offset;
return false;
}
}

// See if all attributes in this DIE have fixed byte sizes. If so, we can
// just add this size to the offset to skip to the next DIE.
if (std::optional<size_t> FixedSize =
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11 changes: 11 additions & 0 deletions llvm/lib/DebugInfo/DWARF/DWARFUnit.cpp
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Expand Up @@ -1051,6 +1051,17 @@ DWARFUnit::getLastChildEntry(const DWARFDebugInfoEntry *Die) const {
return nullptr;
}

const DWARFAbbreviationDeclaration *DWARFUnit::tryExtractCUAbbrevFast() const {
Expected<const DWARFAbbreviationDeclaration *> AbbrevOrError =
Abbrev->tryExtractCUAbbrevFast(getAbbreviationsOffset());
if (!AbbrevOrError) {
// FIXME: We should propagate this error upwards.
consumeError(AbbrevOrError.takeError());
return nullptr;
}
return *AbbrevOrError;
}

const DWARFAbbreviationDeclarationSet *DWARFUnit::getAbbreviations() const {
if (!Abbrevs) {
Expected<const DWARFAbbreviationDeclarationSet *> AbbrevsOrError =
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