[PATCH] D38129: [ELF] - Speedup -r and --emit-relocs

[George Rimar via Phabricator via llvm-commits]

grimar created this revision.
Herald added a subscriber: emaste.

This is "Bug 34688 - lld much slower than bfd when linking the linux kernel"

Inside `copyRelocations()` we have O(N*M) algorithm, where N - amount of
relocations and M - amount of symbols in symbol table. It isincredibly slow for linking linux kernel.

Patch creates local search tables to speedup.
With this fix link time goes for me from 12.95s to 0.55s what is almost 23x faster. (used release LLD).


https://reviews.llvm.org/D38129

Files:
  ELF/InputSection.cpp
  ELF/SyntheticSections.cpp


Index: ELF/SyntheticSections.cpp
===================================================================
--- ELF/SyntheticSections.cpp
+++ ELF/SyntheticSections.cpp
@@ -1354,15 +1354,8 @@
 }
 
 size_t SymbolTableBaseSection::getSymbolIndex(SymbolBody *Body) {
-  auto I = llvm::find_if(Symbols, [&](const SymbolTableEntry &E) {
-    if (E.Symbol == Body)
-      return true;
-    // This is used for -r, so we have to handle multiple section
-    // symbols being combined.
-    if (Body->Type == STT_SECTION && E.Symbol->Type == STT_SECTION)
-      return Body->getOutputSection() == E.Symbol->getOutputSection();
-    return false;
-  });
+  auto I = llvm::find_if(
+      Symbols, [&](const SymbolTableEntry &E) { return E.Symbol == Body; });
   if (I == Symbols.end())
     return 0;
   return I - Symbols.begin() + 1;
Index: ELF/InputSection.cpp
===================================================================
--- ELF/InputSection.cpp
+++ ELF/InputSection.cpp
@@ -378,17 +378,35 @@
   return Sections[this->Info];
 }
 
+static DenseMap<SymbolBody *, size_t> createSymbolToIndexMap() {
+  DenseMap<SymbolBody *, size_t> Ret(InX::SymTab->getSymbols().size());
+  uint64_t I = 0;
+  for (const SymbolTableEntry &S : InX::SymTab->getSymbols())
+    Ret.insert({S.Symbol, ++I});
+  return Ret;
+}
+
+static DenseMap<OutputSection *, size_t> createSectionToIndexMap() {
+  ArrayRef<SymbolTableEntry> V = InX::SymTab->getSymbols();
+  DenseMap<OutputSection *, size_t> Ret(V.size());
+  for (size_t I = 0, E = V.size(); I != E; ++I)
+    if (V[I].Symbol->Type == STT_SECTION)
+      Ret.insert({V[I].Symbol->getOutputSection(), I + 1});
+  return Ret;
+}
+
 // This is used for -r and --emit-relocs. We can't use memcpy to copy
 // relocations because we need to update symbol table offset and section index
 // for each relocation. So we copy relocations one by one.
 template <class ELFT, class RelTy>
 void InputSection::copyRelocations(uint8_t *Buf, ArrayRef<RelTy> Rels) {
+  // Next mappings used for optimization. We have to create separate mapping for
+  // section symbols, because want to handle case when multiple section symbols
+  // being combined and them are not in symbol table.
+  static DenseMap<SymbolBody *, size_t> SymbolToIndex = createSymbolToIndexMap();
+  static DenseMap<OutputSection *, size_t> SectionToIndex = createSectionToIndexMap();
   InputSectionBase *RelocatedSection = getRelocatedSection();
 
-  // Loop is slow and have complexity O(N*M), where N - amount of
-  // relocations and M - amount of symbols in symbol table.
-  // That happens because getSymbolIndex(...) call below performs
-  // simple linear search.
   for (const RelTy &Rel : Rels) {
     uint32_t Type = Rel.getType(Config->IsMips64EL);
     SymbolBody &Body = this->getFile<ELFT>()->getRelocTargetSym(Rel);
@@ -403,8 +421,11 @@
     // section, but for --emit-relocs it is an virtual address.
     P->r_offset = RelocatedSection->getOutputSection()->Addr +
                   RelocatedSection->getOffset(Rel.r_offset);
-    P->setSymbolAndType(InX::SymTab->getSymbolIndex(&Body), Type,
-                        Config->IsMips64EL);
+
+    size_t Sym = (Body.Type == STT_SECTION)
+                     ? SectionToIndex[Body.getOutputSection()]
+                     : SymbolToIndex[&Body];
+    P->setSymbolAndType(Sym, Type, Config->IsMips64EL);
 
     if (Body.Type == STT_SECTION) {
       // We combine multiple section symbols into only one per


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