[DebugInfo] Reduce debug_str_offsets section size
Summary:
The accelerator tables use the debug_str section to store their strings.
However, they do not support the indirect method of access that is
available for the debug_info section (DW_FORM_strx et al.).
Currently our code is assuming that all strings can/will be referenced
indirectly, and puts all of them into the debug_str_offsets section.
This is generally true for regular (unsplit) dwarf, but in the DWO case,
most of the strings in the debug_str section will only be used from the
accelerator tables. Therefore the contents of the debug_str_offsets
section will be largely unused and bloating the main executable.
This patch rectifies this by teaching the DwarfStringPool to
differentiate between strings accessed directly and indirectly. When a
user inserts a string into the pool it has to declare whether that
string will be referenced directly or not. If at least one user requsts
indirect access, that string will be assigned an index ID and put into
debug_str_offsets table. Otherwise, the offset table is skipped.
This approach reduces the overall binary size (when compiled with
-gdwarf-5 -gsplit-dwarf) in my tests by about 2% (debug_str_offsets is
shrunk by 99%).
Reviewers: probinson, dblaikie, JDevlieghere
Subscribers: aprantl, mgrang, llvm-commits
Differential Revision: https://reviews.llvm.org/D49493
llvm-svn: 339122
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 549bf74..388fe8a 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -2437,8 +2437,7 @@
return;
DwarfFile &Holder = useSplitDwarf() ? SkeletonHolder : InfoHolder;
- DwarfStringPoolEntryRef Ref =
- Holder.getStringPool().getEntry(*Asm, Name);
+ DwarfStringPoolEntryRef Ref = Holder.getStringPool().getEntry(*Asm, Name);
switch (getAccelTableKind()) {
case AccelTableKind::Apple:
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp
index a61fa83..83c07d1 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.cpp
@@ -24,25 +24,39 @@
: Pool(A), Prefix(Prefix),
ShouldCreateSymbols(Asm.MAI->doesDwarfUseRelocationsAcrossSections()) {}
-DwarfStringPool::EntryRef DwarfStringPool::getEntry(AsmPrinter &Asm,
- StringRef Str) {
+StringMapEntry<DwarfStringPool::EntryTy> &
+DwarfStringPool::getEntryImpl(AsmPrinter &Asm, StringRef Str) {
auto I = Pool.insert(std::make_pair(Str, EntryTy()));
+ auto &Entry = I.first->second;
if (I.second) {
- auto &Entry = I.first->second;
- Entry.Index = Pool.size() - 1;
+ Entry.Index = EntryTy::NotIndexed;
Entry.Offset = NumBytes;
Entry.Symbol = ShouldCreateSymbols ? Asm.createTempSymbol(Prefix) : nullptr;
NumBytes += Str.size() + 1;
assert(NumBytes > Entry.Offset && "Unexpected overflow");
}
- return EntryRef(*I.first);
+ return *I.first;
+}
+
+DwarfStringPool::EntryRef DwarfStringPool::getEntry(AsmPrinter &Asm,
+ StringRef Str) {
+ auto &MapEntry = getEntryImpl(Asm, Str);
+ return EntryRef(MapEntry, false);
+}
+
+DwarfStringPool::EntryRef DwarfStringPool::getIndexedEntry(AsmPrinter &Asm,
+ StringRef Str) {
+ auto &MapEntry = getEntryImpl(Asm, Str);
+ if (!MapEntry.getValue().isIndexed())
+ MapEntry.getValue().Index = NumIndexedStrings++;
+ return EntryRef(MapEntry, true);
}
void DwarfStringPool::emitStringOffsetsTableHeader(AsmPrinter &Asm,
MCSection *Section,
MCSymbol *StartSym) {
- if (empty())
+ if (getNumIndexedStrings() == 0)
return;
Asm.OutStreamer->SwitchSection(Section);
unsigned EntrySize = 4;
@@ -51,7 +65,7 @@
// table. The header consists of an entry with the contribution's
// size (not including the size of the length field), the DWARF version and
// 2 bytes of padding.
- Asm.emitInt32(size() * EntrySize + 4);
+ Asm.emitInt32(getNumIndexedStrings() * EntrySize + 4);
Asm.emitInt16(Asm.getDwarfVersion());
Asm.emitInt16(0);
// Define the symbol that marks the start of the contribution. It is
@@ -69,12 +83,18 @@
// Start the dwarf str section.
Asm.OutStreamer->SwitchSection(StrSection);
- // Get all of the string pool entries and put them in an array by their ID so
- // we can sort them.
- SmallVector<const StringMapEntry<EntryTy> *, 64> Entries(Pool.size());
+ // Get all of the string pool entries and sort them by their offset.
+ SmallVector<const StringMapEntry<EntryTy> *, 64> Entries;
+ Entries.reserve(Pool.size());
for (const auto &E : Pool)
- Entries[E.getValue().Index] = &E;
+ Entries.push_back(&E);
+
+ llvm::sort(
+ Entries.begin(), Entries.end(),
+ [](const StringMapEntry<EntryTy> *A, const StringMapEntry<EntryTy> *B) {
+ return A->getValue().Offset < B->getValue().Offset;
+ });
for (const auto &Entry : Entries) {
assert(ShouldCreateSymbols == static_cast<bool>(Entry->getValue().Symbol) &&
@@ -93,6 +113,14 @@
// If we've got an offset section go ahead and emit that now as well.
if (OffsetSection) {
+ // Now only take the indexed entries and put them in an array by their ID so
+ // we can emit them in order.
+ Entries.resize(NumIndexedStrings);
+ for (const auto &Entry : Pool) {
+ if (Entry.getValue().isIndexed())
+ Entries[Entry.getValue().Index] = &Entry;
+ }
+
Asm.OutStreamer->SwitchSection(OffsetSection);
unsigned size = 4; // FIXME: DWARF64 is 8.
for (const auto &Entry : Entries)
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.h b/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.h
index 6e6988e..f484540 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.h
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfStringPool.h
@@ -30,8 +30,11 @@
StringMap<EntryTy, BumpPtrAllocator &> Pool;
StringRef Prefix;
unsigned NumBytes = 0;
+ unsigned NumIndexedStrings = 0;
bool ShouldCreateSymbols;
+ StringMapEntry<EntryTy> &getEntryImpl(AsmPrinter &Asm, StringRef Str);
+
public:
using EntryRef = DwarfStringPoolEntryRef;
@@ -48,8 +51,15 @@
unsigned size() const { return Pool.size(); }
+ unsigned getNumIndexedStrings() const { return NumIndexedStrings; }
+
/// Get a reference to an entry in the string pool.
EntryRef getEntry(AsmPrinter &Asm, StringRef Str);
+
+ /// Same as getEntry, except that you can use EntryRef::getIndex to obtain a
+ /// unique ID of this entry (e.g., for use in indexed forms like
+ /// DW_FORM_strx).
+ EntryRef getIndexedEntry(AsmPrinter &Asm, StringRef Str);
};
} // end namespace llvm
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index df67c5f..e38c854 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -243,9 +243,14 @@
DIEInlineString(String, DIEValueAllocator));
return;
}
- auto StringPoolEntry = DU->getStringPool().getEntry(*Asm, String);
dwarf::Form IxForm =
isDwoUnit() ? dwarf::DW_FORM_GNU_str_index : dwarf::DW_FORM_strp;
+
+ auto StringPoolEntry =
+ useSegmentedStringOffsetsTable() || IxForm == dwarf::DW_FORM_GNU_str_index
+ ? DU->getStringPool().getIndexedEntry(*Asm, String)
+ : DU->getStringPool().getEntry(*Asm, String);
+
// For DWARF v5 and beyond, use the smallest strx? form possible.
if (useSegmentedStringOffsetsTable()) {
IxForm = dwarf::DW_FORM_strx1;