[lld] r243496 - [ELF2] Devirtualize SymbolBody.

[Michael Spencer]

On Tue, Jul 28, 2015 at 4:22 PM, Chandler Carruth <chandlerc at google.com> wrote:
> On Tue, Jul 28, 2015 at 4:01 PM Michael J. Spencer <bigcheesegs at gmail.com>
> wrote:
>>
>> Author: mspencer
>> Date: Tue Jul 28 17:58:25 2015
>> New Revision: 243496
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=243496&view=rev
>> Log:
>> [ELF2] Devirtualize SymbolBody.
>
>
> Could you outline the specific benefits you're hoping to achieve here?
> Memory reduction, speed, even just readability?
>
> I just think a bit more context would help folks reading the patches.

The goal here is to reduce divergence from the COFF linker. It is also
expected that the same performance issues addressed by this change
(r241001) in the COFF linker will be encountered by the ELF linker,
and it's easier to build it right the first time.

- Michael Spencer

>>
>>
>> Modified:
>>     lld/trunk/ELF/InputFiles.cpp
>>     lld/trunk/ELF/Symbols.cpp
>>     lld/trunk/ELF/Symbols.h
>>
>> Modified: lld/trunk/ELF/InputFiles.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputFiles.cpp?rev=243496&r1=243495&r2=243496&view=diff
>>
>> ==============================================================================
>> --- lld/trunk/ELF/InputFiles.cpp (original)
>> +++ lld/trunk/ELF/InputFiles.cpp Tue Jul 28 17:58:25 2015
>> @@ -65,7 +65,7 @@ SymbolBody *elf2::ObjectFile<ELFT>::crea
>>    StringRef Name = *NameOrErr;
>>    if (Sym->isUndefined())
>>      return new (Alloc) Undefined(Name);
>> -  return new (Alloc) DefinedRegular<ELFT>(Name);
>> +  return new (Alloc) DefinedRegular<ELFT>(this, Sym);
>>  }
>>
>>  namespace lld {
>>
>> Modified: lld/trunk/ELF/Symbols.cpp
>> URL:
>> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Symbols.cpp?rev=243496&r1=243495&r2=243496&view=diff
>>
>>
>> ==============================================================================
>> --- lld/trunk/ELF/Symbols.cpp (original)
>> +++ lld/trunk/ELF/Symbols.cpp Tue Jul 28 17:58:25 2015
>> @@ -9,12 +9,30 @@
>>
>>  #include "Symbols.h"
>>  #include "Chunks.h"
>> +#include "InputFiles.h"
>>
>>  using namespace llvm::object;
>>
>>  using namespace lld;
>>  using namespace lld::elf2;
>>
>> +template <class ELFT>
>> +StringRef
>> +getSymbolName(const llvm::object::ELFFile<ELFT> *F,
>> +  const typename llvm::object::ELFFile<ELFT>::Elf_Sym *S) {
>> +  ErrorOr<StringRef> StrTab =
>> F->getStringTableForSymtab(*F->getDotSymtabSec());
>> +  if (!StrTab || S->st_name >= StrTab->size())
>> +    llvm::report_fatal_error("Invalid string table.");
>> +  return StrTab->data() + S->st_name;
>> +}
>> +
>> +template <class ELFT>
>> +DefinedRegular<ELFT>::DefinedRegular(ObjectFile<ELFT> *F, const Elf_Sym
>> *S)
>> +    : Defined(DefinedRegularKind, getSymbolName<ELFT>(F->getObj(), S)),
>> File(F),
>> +      Sym(S) {
>> +  IsExternal = S->isExternal();
>> +}
>> +
>>  // Returns 1, 0 or -1 if this symbol should take precedence
>>  // over the Other, tie or lose, respectively.
>>  int SymbolBody::compare(SymbolBody *Other) {
>>
>> Modified: lld/trunk/ELF/Symbols.h
>> URL:
>> http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Symbols.h?rev=243496&r1=243495&r2=243496&view=diff
>>
>>
>> ==============================================================================
>> --- lld/trunk/ELF/Symbols.h (original)
>> +++ lld/trunk/ELF/Symbols.h Tue Jul 28 17:58:25 2015
>> @@ -21,6 +21,7 @@ using llvm::object::ELFFile;
>>  class Chunk;
>>  class InputFile;
>>  class SymbolBody;
>> +template <class ELFT> class ObjectFile;
>>
>>  // A real symbol object, SymbolBody, is usually accessed indirectly
>>  // through a Symbol. There's always one Symbol for each symbol name.
>> @@ -40,14 +41,13 @@ public:
>>      UndefinedKind,
>>    };
>>
>> -  Kind kind() const { return SymbolKind; }
>> -  virtual ~SymbolBody() {}
>> +  Kind kind() const { return static_cast<Kind>(SymbolKind); }
>>
>>    // Returns true if this is an external symbol.
>> -  virtual bool isExternal() { return true; }
>> +  bool isExternal() const { return true; }
>>
>>    // Returns the symbol name.
>> -  virtual StringRef getName() = 0;
>> +  StringRef getName() const { return Name; }
>>
>>    // A SymbolBody has a backreference to a Symbol. Originally they are
>>    // doubly-linked. A backreference will never change. But the pointer
>> @@ -64,10 +64,13 @@ public:
>>    int compare(SymbolBody *Other);
>>
>>  protected:
>> -  SymbolBody(Kind K) : SymbolKind(K) {}
>> +  SymbolBody(Kind K, StringRef N = "")
>> +      : SymbolKind(K), IsExternal(true), Name(N) {}
>>
>> -private:
>> -  const Kind SymbolKind;
>> +protected:
>> +  const unsigned SymbolKind : 8;
>> +  unsigned IsExternal : 1;
>> +  StringRef Name;
>>    Symbol *Backref = nullptr;
>>  };
>>
>> @@ -75,7 +78,7 @@ private:
>>  // etc.
>>  class Defined : public SymbolBody {
>>  public:
>> -  Defined(Kind K) : SymbolBody(K) {}
>> +  Defined(Kind K, StringRef N = "") : SymbolBody(K, N) {}
>>
>>    static bool classof(const SymbolBody *S) {
>>      Kind K = S->kind();
>> @@ -85,31 +88,28 @@ public:
>>
>>  // Regular defined symbols read from object file symbol tables.
>>  template <class ELFT> class DefinedRegular : public Defined {
>> +  typedef typename llvm::object::ELFFile<ELFT>::Elf_Sym Elf_Sym;
>> +
>>  public:
>> -  DefinedRegular(StringRef Name) : Defined(DefinedRegularKind),
>> Name(Name) {}
>> +  DefinedRegular(ObjectFile<ELFT> *F, const Elf_Sym *S);
>>
>>    static bool classof(const SymbolBody *S) {
>>      return S->kind() == DefinedRegularKind;
>>    }
>>
>> -  StringRef getName() override { return Name; }
>> -
>>  private:
>> -  StringRef Name;
>> +  ObjectFile<ELFT> *File;
>> +  const Elf_Sym *Sym;
>>  };
>>
>>  // Undefined symbols.
>>  class Undefined : public SymbolBody {
>>  public:
>> -  explicit Undefined(StringRef N) : SymbolBody(UndefinedKind), Name(N) {}
>> +  explicit Undefined(StringRef N) : SymbolBody(UndefinedKind, N) {}
>>
>>    static bool classof(const SymbolBody *S) {
>>      return S->kind() == UndefinedKind;
>>    }
>> -  StringRef getName() override { return Name; }
>> -
>> -private:
>> -  StringRef Name;
>>  };
>>
>>  } // namespace elf2
>>
>>
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