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[AccelTable] Pre-compute entry pool offsets for DW_IDX_parent. NFC (#188915)

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After PR #150846 changed MCFragment to use fixed 16KB blocks, large
sections like .debug_names produce thousands of fragments. The
DW_IDX_parent emission called emitLabelDifference(ParentSymbol,
EntryPool, 4) per entry, which fell through to
attemptToFoldSymbolOffsetDifference's O(n) fragment walk when symbols
were in different fragments. This caused O(entries * fragments)
quadratic behavior, regressing dsymutil on large binaries like clang.

Fix by pre-computing byte offsets from EntryPool for each entry in a
first pass, then emitting emitInt32(offset) directly for DW_IDX_parent
instead of symbol differences. This also eliminates per-entry MCSymbol
allocation that was only needed for parent references.

Fix #188376

(cherry picked from commit 0f63c0a3a5)
This commit is contained in:
Fangrui Song 2026-03-27 09:31:16 -07:00 committed by Cullen Rhodes
parent 561944ebcb
commit cc87fcfeef
1 changed files with 50 additions and 23 deletions
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73
llvm/lib/CodeGen/AsmPrinter/AccelTable.cpp
View file

@ -21,6 +21,7 @@
#include "llvm/CodeGen/DIE.h"
#include "llvm/MC/MCStreamer.h"
#include "llvm/MC/MCSymbol.h"
#include "llvm/Support/LEB128.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetLoweringObjectFile.h"
#include <cstddef>
@ -225,8 +226,8 @@ class Dwarf5AccelTableWriter : public AccelTableWriter {
void emitAbbrevs() const;
void emitEntry(
const DWARF5AccelTableData &Entry,
const DenseMap<OffsetAndUnitID, MCSymbol *> &DIEOffsetToAccelEntryLabel,
DenseSet<MCSymbol *> &EmittedAccelEntrySymbols);
const DenseMap<OffsetAndUnitID, uint64_t> &DIEOffsetToAccelEntryOffset);
uint64_t getEntrySize(const DWARF5AccelTableData &Entry) const;
void emitData();
public:
@ -523,8 +524,7 @@ void Dwarf5AccelTableWriter::emitAbbrevs() const {
void Dwarf5AccelTableWriter::emitEntry(
const DWARF5AccelTableData &Entry,
const DenseMap<OffsetAndUnitID, MCSymbol *> &DIEOffsetToAccelEntryLabel,
DenseSet<MCSymbol *> &EmittedAccelEntrySymbols) {
const DenseMap<OffsetAndUnitID, uint64_t> &DIEOffsetToAccelEntryOffset) {
unsigned AbbrevIndex = Entry.getAbbrevNumber() - 1;
assert(AbbrevIndex < AbbreviationsVector.size() &&
"Entry abbrev index is outside of abbreviations vector range.");
@ -533,16 +533,6 @@ void Dwarf5AccelTableWriter::emitEntry(
getIndexForEntry(Entry);
std::optional<OffsetAndUnitID> MaybeParentOffset =
Entry.getParentDieOffsetAndUnitID();
auto EntrySymbolIt =
DIEOffsetToAccelEntryLabel.find(Entry.getDieOffsetAndUnitID());
assert(EntrySymbolIt != DIEOffsetToAccelEntryLabel.end());
MCSymbol *EntrySymbol = EntrySymbolIt->getSecond();
// Emit the label for this Entry, so that IDX_parents may refer to it.
// Note: a DIE may have multiple accelerator Entries; this check avoids
// creating/emitting multiple labels for the same DIE.
if (EmittedAccelEntrySymbols.insert(EntrySymbol).second)
Asm->OutStreamer->emitLabel(EntrySymbol);
Asm->emitULEB128(Entry.getAbbrevNumber(), "Abbreviation code");
@ -563,9 +553,9 @@ void Dwarf5AccelTableWriter::emitEntry(
case dwarf::DW_IDX_parent: {
if (AttrEnc.Form == dwarf::Form::DW_FORM_flag_present)
break;
auto ParentSymbolIt = DIEOffsetToAccelEntryLabel.find(*MaybeParentOffset);
assert(ParentSymbolIt != DIEOffsetToAccelEntryLabel.end());
Asm->emitLabelDifference(ParentSymbolIt->getSecond(), EntryPool, 4);
auto It = DIEOffsetToAccelEntryOffset.find(*MaybeParentOffset);
assert(It != DIEOffsetToAccelEntryOffset.end());
Asm->emitInt32(It->second);
break;
}
default:
@ -574,20 +564,57 @@ void Dwarf5AccelTableWriter::emitEntry(
}
}
void Dwarf5AccelTableWriter::emitData() {
DenseMap<OffsetAndUnitID, MCSymbol *> DIEOffsetToAccelEntryLabel;
uint64_t
Dwarf5AccelTableWriter::getEntrySize(const DWARF5AccelTableData &Entry) const {
unsigned AbbrevIndex = Entry.getAbbrevNumber() - 1;
assert(AbbrevIndex < AbbreviationsVector.size());
DebugNamesAbbrev *Abbrev = AbbreviationsVector[AbbrevIndex];
uint64_t Size = getULEB128Size(Entry.getAbbrevNumber());
std::optional<DWARF5AccelTable::UnitIndexAndEncoding> EntryRet =
getIndexForEntry(Entry);
for (const auto &AttrEnc : Abbrev->getAttributes()) {
switch (AttrEnc.Index) {
case dwarf::DW_IDX_compile_unit:
case dwarf::DW_IDX_type_unit:
Size += DIEInteger(EntryRet->Index)
.sizeOf(Asm->getDwarfFormParams(), AttrEnc.Form);
break;
case dwarf::DW_IDX_die_offset:
Size += 4;
break;
case dwarf::DW_IDX_parent:
if (AttrEnc.Form != dwarf::Form::DW_FORM_flag_present)
Size += 4;
break;
default:
llvm_unreachable("Unexpected index attribute!");
}
}
return Size;
}
for (OffsetAndUnitID Offset : IndexedOffsets)
DIEOffsetToAccelEntryLabel.insert({Offset, Asm->createTempSymbol("")});
void Dwarf5AccelTableWriter::emitData() {
// Pre-compute entry pool offsets for DW_IDX_parent references.
DenseMap<OffsetAndUnitID, uint64_t> DIEOffsetToAccelEntryOffset;
uint64_t Offset = 0;
for (auto &Bucket : Contents.getBuckets()) {
for (auto *Hash : Bucket) {
for (const auto *Value : Hash->getValues<DWARF5AccelTableData *>()) {
DIEOffsetToAccelEntryOffset.try_emplace(Value->getDieOffsetAndUnitID(),
Offset);
Offset += getEntrySize(*Value);
}
Offset += 1; // End of list
}
}
Asm->OutStreamer->emitLabel(EntryPool);
DenseSet<MCSymbol *> EmittedAccelEntrySymbols;
for (auto &Bucket : Contents.getBuckets()) {
for (auto *Hash : Bucket) {
// Remember to emit the label for our offset.
Asm->OutStreamer->emitLabel(Hash->Sym);
for (const auto *Value : Hash->getValues<DWARF5AccelTableData *>())
emitEntry(*Value, DIEOffsetToAccelEntryLabel, EmittedAccelEntrySymbols);
emitEntry(*Value, DIEOffsetToAccelEntryOffset);
Asm->OutStreamer->AddComment("End of list: " + Hash->Name.getString());
Asm->emitInt8(0);
}