Move symbol resolution code out of SymbolTable class.

This is the last patch of the series of patches to make it possible to
resolve symbols without asking SymbolTable to do so.

The main point of this patch is the introduction of
`elf::resolveSymbol(Symbol *Old, Symbol *New)`. That function resolves
or merges given symbols by examining symbol types and call
replaceSymbol (which memcpy's New to Old) if necessary.

With the new function, we have now separated symbol resolution from
symbol lookup. If you already have a Symbol pointer, you can directly
resolve the symbol without asking SymbolTable to do that.

Now that the nice abstraction become available, I can start working on
performance improvement of the linker. As a starter, I'm thinking of
making --{start,end}-lib faster.

--{start,end}-lib is currently unnecessarily slow because it looks up
the symbol table twice for each symbol.

 - The first hash table lookup/insertion occurs when we instantiate a
   LazyObject file to insert LazyObject symbols.

 - The second hash table lookup/insertion occurs when we create an
   ObjFile from LazyObject file. That overwrites LazyObject symbols
   with Defined symbols.

I think it is not too hard to see how we can now eliminate the second
hash table lookup. We can keep LazyObject symbols in Step 1, and then
call elf::resolveSymbol() to do Step 2.

Differential Revision: https://reviews.llvm.org/D61898

llvm-svn: 360975

1 files changed, 89 insertions, 104 deletions

diff --git a/lld/ELF/SymbolTable.cpp b/lld/ELF/SymbolTable.cpp
index 38cc4d5f38d..9135e543407 100644
--- a/lld/ELF/SymbolTable.cpp
+++ b/lld/ELF/SymbolTable.cpp
@@ -86,15 +86,14 @@ static uint8_t getMinVisibility(uint8_t VA, uint8_t VB) {
   return std::min(VA, VB);
 }
 
-// Find an existing symbol or create and insert a new one.
-Symbol *SymbolTable::insert(const Symbol &New) {
+// Find an existing symbol or create a new one.
+Symbol *SymbolTable::insert(StringRef Name) {
   // <name>@@<version> means the symbol is the default version. In that
   // case <name>@@<version> will be used to resolve references to <name>.
   //
   // Since this is a hot path, the following string search code is
   // optimized for speed. StringRef::find(char) is much faster than
   // StringRef::find(StringRef).
-  StringRef Name = New.getName();
   size_t Pos = Name.find('@');
   if (Pos != StringRef::npos && Pos + 1 < Name.size() && Name[Pos + 1] == '@')
     Name = Name.take_front(Pos);
@@ -110,55 +109,35 @@ Symbol *SymbolTable::insert(const Symbol &New) {
     Traced = true;
   }
 
-  Symbol *Old;
-  if (IsNew) {
-    Old = reinterpret_cast<Symbol *>(make<SymbolUnion>());
-    SymVector.push_back(Old);
-
-    Old->SymbolKind = Symbol::PlaceholderKind;
-    Old->VersionId = Config->DefaultSymbolVersion;
-    Old->Visibility = STV_DEFAULT;
-    Old->IsUsedInRegularObj = false;
-    Old->ExportDynamic = false;
-    Old->CanInline = true;
-    Old->Traced = Traced;
-    Old->ScriptDefined = false;
-  } else {
-    Old = SymVector[SymIndex];
-  }
-
-  return Old;
+  if (!IsNew)
+    return SymVector[SymIndex];
+
+  Symbol *Sym = reinterpret_cast<Symbol *>(make<SymbolUnion>());
+  SymVector.push_back(Sym);
+
+  Sym->SymbolKind = Symbol::PlaceholderKind;
+  Sym->VersionId = Config->DefaultSymbolVersion;
+  Sym->Visibility = STV_DEFAULT;
+  Sym->IsUsedInRegularObj = false;
+  Sym->ExportDynamic = false;
+  Sym->CanInline = true;
+  Sym->Traced = Traced;
+  Sym->ScriptDefined = false;
+  return Sym;
 }
 
-// Merge symbol properties.
-//
-// When we have many symbols of the same name, we choose one of them,
-// and that's the result of symbol resolution. However, symbols that
-// were not chosen still affect some symbol properteis.
-void SymbolTable::mergeProperties(Symbol *Old, const Symbol &New) {
-  // Merge symbol properties.
-  Old->ExportDynamic = Old->ExportDynamic || New.ExportDynamic;
-  Old->IsUsedInRegularObj = Old->IsUsedInRegularObj || New.IsUsedInRegularObj;
-
-  // DSO symbols do not affect visibility in the output.
-  if (!isa<SharedSymbol>(New))
-    Old->Visibility = getMinVisibility(Old->Visibility, New.Visibility);
+Symbol *SymbolTable::addSymbol(const Symbol &New) {
+  Symbol *Old = Symtab->insert(New.getName());
+  resolveSymbol(Old, New);
+  return Old;
 }
 
-Symbol *SymbolTable::addUndefined(const Undefined &New) {
-  Symbol *Old = insert(New);
-  mergeProperties(Old, New);
-
-  if (Old->isPlaceholder()) {
-    replaceSymbol(Old, &New);
-    return Old;
-  }
-
+static void addUndefined(Symbol *Old, const Undefined &New) {
   // An undefined symbol with non default visibility must be satisfied
   // in the same DSO.
   if (Old->isShared() && New.Visibility != STV_DEFAULT) {
-    replaceSymbol(Old, &New);
-    return Old;
+    replaceSymbol(Old, New);
+    return;
   }
 
   if (Old->isShared() || Old->isLazy() ||
@@ -170,7 +149,7 @@ Symbol *SymbolTable::addUndefined(const Undefined &New) {
     // Symbols.h for the details.
     if (New.Binding == STB_WEAK) {
       Old->Type = New.Type;
-      return Old;
+      return;
     }
 
     // Do extra check for --warn-backrefs.
@@ -225,7 +204,7 @@ Symbol *SymbolTable::addUndefined(const Undefined &New) {
     // group assignment rule simulates the traditional linker's semantics.
     bool Backref = Config->WarnBackrefs && New.File &&
                    Old->File->GroupId < New.File->GroupId;
-    fetchLazy(Old);
+    Symtab->fetchLazy(Old);
 
     // We don't report backward references to weak symbols as they can be
     // overridden later.
@@ -233,7 +212,6 @@ Symbol *SymbolTable::addUndefined(const Undefined &New) {
       warn("backward reference detected: " + New.getName() + " in " +
            toString(New.File) + " refers to " + toString(Old->File));
   }
-  return Old;
 }
 
 // Using .symver foo,foo@@VER unfortunately creates two symbols: foo and
@@ -300,22 +278,14 @@ static int compare(const Symbol *Old, const Symbol *New) {
   return 0;
 }
 
-Symbol *SymbolTable::addCommon(const CommonSymbol &New) {
-  Symbol *Old = insert(New);
-  mergeProperties(Old, New);
-
-  if (Old->isPlaceholder()) {
-    replaceSymbol(Old, &New);
-    return Old;
-  }
-
+static void addCommon(Symbol *Old, const CommonSymbol &New) {
   int Cmp = compare(Old, &New);
   if (Cmp < 0)
-    return Old;
+    return;
 
   if (Cmp > 0) {
-    replaceSymbol(Old, &New);
-    return Old;
+    replaceSymbol(Old, New);
+    return;
   }
 
   CommonSymbol *OldSym = cast<CommonSymbol>(Old);
@@ -325,7 +295,6 @@ Symbol *SymbolTable::addCommon(const CommonSymbol &New) {
     OldSym->File = New.File;
     OldSym->Size = New.Size;
   }
-  return OldSym;
 }
 
 static void reportDuplicate(Symbol *Sym, InputFile *NewFile,
@@ -363,45 +332,23 @@ static void reportDuplicate(Symbol *Sym, InputFile *NewFile,
   error(Msg);
 }
 
-Symbol *SymbolTable::addDefined(const Defined &New) {
-  Symbol *Old = insert(New);
-  mergeProperties(Old, New);
-
-  if (Old->isPlaceholder()) {
-    replaceSymbol(Old, &New);
-    return Old;
-  }
-
+static void addDefined(Symbol *Old, const Defined &New) {
   int Cmp = compare(Old, &New);
   if (Cmp > 0)
-    replaceSymbol(Old, &New);
+    replaceSymbol(Old, New);
   else if (Cmp == 0)
     reportDuplicate(Old, New.File,
                     dyn_cast_or_null<InputSectionBase>(New.Section), New.Value);
-  return Old;
 }
 
-Symbol *SymbolTable::addShared(const SharedSymbol &New) {
-  Symbol *Old = insert(New);
-  mergeProperties(Old, New);
-
-  // Make sure we preempt DSO symbols with default visibility.
-  if (New.Visibility == STV_DEFAULT)
-    Old->ExportDynamic = true;
-
-  if (Old->isPlaceholder()) {
-    replaceSymbol(Old, &New);
-    return Old;
-  }
-
+static void addShared(Symbol *Old, const SharedSymbol &New) {
   if (Old->Visibility == STV_DEFAULT && (Old->isUndefined() || Old->isLazy())) {
     // An undefined symbol with non default visibility must be satisfied
     // in the same DSO.
     uint8_t Binding = Old->Binding;
-    replaceSymbol(Old, &New);
+    replaceSymbol(Old, New);
     Old->Binding = Binding;
   }
-  return Old;
 }
 
 Symbol *SymbolTable::find(StringRef Name) {
@@ -413,39 +360,30 @@ Symbol *SymbolTable::find(StringRef Name) {
   return SymVector[It->second];
 }
 
-template <class LazyT> Symbol *SymbolTable::addLazy(const LazyT &New) {
-  Symbol *Old = insert(New);
-  mergeProperties(Old, New);
-
-  if (Old->isPlaceholder()) {
-    replaceSymbol(Old, &New);
-    return Old;
-  }
-
+template <class LazyT> static void addLazy(Symbol *Old, const LazyT &New) {
   if (!Old->isUndefined())
-    return Old;
+    return;
 
   // An undefined weak will not fetch archive members. See comment on Lazy in
   // Symbols.h for the details.
   if (Old->isWeak()) {
     uint8_t Type = Old->Type;
-    replaceSymbol(Old, &New);
+    replaceSymbol(Old, New);
     Old->Type = Type;
     Old->Binding = STB_WEAK;
-    return Old;
+    return;
   }
 
   if (InputFile *F = New.fetch())
     parseFile(F);
-  return Old;
 }
 
-Symbol *SymbolTable::addLazyArchive(const LazyArchive &New) {
-  return addLazy(New);
+static void addLazyArchive(Symbol *Old, const LazyArchive &New) {
+  addLazy(Old, New);
 }
 
-Symbol *SymbolTable::addLazyObject(const LazyObject &New) {
-  return addLazy(New);
+static void addLazyObject(Symbol *Old, const LazyObject &New) {
+  addLazy(Old, New);
 }
 
 void SymbolTable::fetchLazy(Symbol *Sym) {
@@ -621,6 +559,53 @@ void SymbolTable::scanVersionScript() {
     Sym->parseSymbolVersion();
 }
 
+// Merge symbol properties.
+//
+// When we have many symbols of the same name, we choose one of them,
+// and that's the result of symbol resolution. However, symbols that
+// were not chosen still affect some symbol properties.
+void elf::mergeSymbolProperties(Symbol *Old, const Symbol &New) {
+  // Merge symbol properties.
+  Old->ExportDynamic = Old->ExportDynamic || New.ExportDynamic;
+  Old->IsUsedInRegularObj = Old->IsUsedInRegularObj || New.IsUsedInRegularObj;
+
+  // DSO symbols do not affect visibility in the output.
+  if (!New.isShared())
+    Old->Visibility = getMinVisibility(Old->Visibility, New.Visibility);
+}
+
+void elf::resolveSymbol(Symbol *Old, const Symbol &New) {
+  mergeSymbolProperties(Old, New);
+
+  if (Old->isPlaceholder()) {
+    replaceSymbol(Old, New);
+    return;
+  }
+
+  switch (New.kind()) {
+  case Symbol::UndefinedKind:
+    addUndefined(Old, cast<Undefined>(New));
+    break;
+  case Symbol::CommonKind:
+    addCommon(Old, cast<CommonSymbol>(New));
+    break;
+  case Symbol::DefinedKind:
+    addDefined(Old, cast<Defined>(New));
+    break;
+  case Symbol::LazyArchiveKind:
+    addLazyArchive(Old, cast<LazyArchive>(New));
+    break;
+  case Symbol::LazyObjectKind:
+    addLazyObject(Old, cast<LazyObject>(New));
+    break;
+  case Symbol::SharedKind:
+    addShared(Old, cast<SharedSymbol>(New));
+    break;
+  case Symbol::PlaceholderKind:
+    llvm_unreachable("bad symbol kind");
+  }
+}
+
 template void SymbolTable::addCombinedLTOObject<ELF32LE>();
 template void SymbolTable::addCombinedLTOObject<ELF32BE>();
 template void SymbolTable::addCombinedLTOObject<ELF64LE>();