Library-ize the dyld components of llvm-rtdyld.

Move the dynamic linking functionality of the llvm-rtdyld program into an
ExecutionEngine support library. Update llvm-rtdyld to just load an object
file into memory, use the library to process it, then run the _main()
function, if one is found.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@128031 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ExecutionEngine/RuntimeDyld.h

@@ -0,0 +1,45 @@
+//===-- RuntimeDyld.h - Run-time dynamic linker for MC-JIT ------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Interface for the runtime dynamic linker facilities of the MC-JIT.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_RUNTIME_DYLD_H
+#define LLVM_RUNTIME_DYLD_H
+
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Memory.h"
+
+namespace llvm {
+
+class RuntimeDyldImpl;
+class MemoryBuffer;
+
+class RuntimeDyld {
+  RuntimeDyld(const RuntimeDyld &);     // DO NOT IMPLEMENT
+  void operator=(const RuntimeDyld &);  // DO NOT IMPLEMENT
+
+  // RuntimeDyldImpl is the actual class. RuntimeDyld is just the public
+  // interface.
+  RuntimeDyldImpl *Dyld;
+public:
+  RuntimeDyld();
+  ~RuntimeDyld();
+
+  bool loadObject(MemoryBuffer *InputBuffer);
+  void *getSymbolAddress(StringRef Name);
+  // FIXME: Should be parameterized to get the memory block associated with
+  // a particular loaded object.
+  sys::MemoryBlock getMemoryBlock();
+};
+
+} // end namespace llvm
+
+#endif

lib/ExecutionEngine/Makefile

@@ -8,6 +8,6 @@
 ##===----------------------------------------------------------------------===##
 LEVEL = ../..
 LIBRARYNAME = LLVMExecutionEngine
-PARALLEL_DIRS = Interpreter JIT MCJIT
+PARALLEL_DIRS = Interpreter JIT MCJIT RuntimeDyld
 
 include $(LEVEL)/Makefile.common

lib/ExecutionEngine/RuntimeDyld/CMakeLists.txt

@@ -0,0 +1,3 @@
+add_llvm_library(LLVMRuntimeDyld
+  RuntimeDyld.cpp
+  )

lib/ExecutionEngine/RuntimeDyld/Makefile

@@ -0,0 +1,13 @@
+##===- lib/ExecutionEngine/MCJIT/Makefile ------------------*- Makefile -*-===##
+#
+#                     The LLVM Compiler Infrastructure
+#
+# This file is distributed under the University of Illinois Open Source
+# License. See LICENSE.TXT for details.
+#
+##===----------------------------------------------------------------------===##
+
+LEVEL = ../../..
+LIBRARYNAME = LLVMRuntimeDyld
+
+include $(LEVEL)/Makefile.common

lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp

@@ -0,0 +1,329 @@
+//===-- RuntimeDyld.h - Run-time dynamic linker for MC-JIT ------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Implementation of the MC-JIT runtime dynamic linker.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/OwningPtr.h"
+#include "llvm/ADT/StringMap.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/Twine.h"
+#include "llvm/ExecutionEngine/RuntimeDyld.h"
+#include "llvm/Object/MachOObject.h"
+#include "llvm/Support/Memory.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/system_error.h"
+using namespace llvm;
+using namespace llvm::object;
+
+namespace llvm {
+class RuntimeDyldImpl {
+  // Master symbol table. As modules are loaded and external symbols are
+  // resolved, their addresses are stored here.
+  StringMap<void*> SymbolTable;
+
+  // FIXME: Should have multiple data blocks, one for each loaded chunk of
+  //        compiled code.
+  sys::MemoryBlock Data;
+
+  bool HasError;
+  std::string ErrorStr;
+
+  // Set the error state and record an error string.
+  bool Error(const Twine &Msg) {
+    ErrorStr = Msg.str();
+    HasError = true;
+    return true;
+  }
+
+  bool loadSegment32(const MachOObject *Obj,
+                     const MachOObject::LoadCommandInfo *SegmentLCI,
+                     const InMemoryStruct<macho::SymtabLoadCommand> &SymtabLC);
+  bool loadSegment64(const MachOObject *Obj,
+                     const MachOObject::LoadCommandInfo *SegmentLCI,
+                     const InMemoryStruct<macho::SymtabLoadCommand> &SymtabLC);
+
+public:
+  bool loadObject(MemoryBuffer *InputBuffer);
+
+  void *getSymbolAddress(StringRef Name) {
+    // Use lookup() rather than [] because we don't want to add an entry
+    // if there isn't one already, which the [] operator does.
+    return SymbolTable.lookup(Name);
+  }
+
+  sys::MemoryBlock getMemoryBlock() { return Data; }
+
+  // Is the linker in an error state?
+  bool hasError() { return HasError; }
+
+  // Mark the error condition as handled and continue.
+  void clearError() { HasError = false; }
+
+  // Get the error message.
+  StringRef getErrorString() { return ErrorStr; }
+};
+
+
+
+bool RuntimeDyldImpl::
+loadSegment32(const MachOObject *Obj,
+              const MachOObject::LoadCommandInfo *SegmentLCI,
+              const InMemoryStruct<macho::SymtabLoadCommand> &SymtabLC) {
+  InMemoryStruct<macho::SegmentLoadCommand> Segment32LC;
+  Obj->ReadSegmentLoadCommand(*SegmentLCI, Segment32LC);
+  if (!Segment32LC)
+    return Error("unable to load segment load command");
+
+  // Map the segment into memory.
+  std::string ErrorStr;
+  Data = sys::Memory::AllocateRWX(Segment32LC->VMSize, 0, &ErrorStr);
+  if (!Data.base())
+    return Error("unable to allocate memory block: '" + ErrorStr + "'");
+  memcpy(Data.base(), Obj->getData(Segment32LC->FileOffset,
+                                   Segment32LC->FileSize).data(),
+         Segment32LC->FileSize);
+  memset((char*)Data.base() + Segment32LC->FileSize, 0,
+         Segment32LC->VMSize - Segment32LC->FileSize);
+
+  // Bind the section indices to address.
+  void **SectionBases = new void*[Segment32LC->NumSections];
+  for (unsigned i = 0; i != Segment32LC->NumSections; ++i) {
+    InMemoryStruct<macho::Section> Sect;
+    Obj->ReadSection(*SegmentLCI, i, Sect);
+    if (!Sect)
+      return Error("unable to load section: '" + Twine(i) + "'");
+
+    // FIXME: We don't support relocations yet.
+    if (Sect->NumRelocationTableEntries != 0)
+      return Error("not yet implemented: relocations!");
+
+    // FIXME: Improve check.
+    if (Sect->Flags != 0x80000400)
+      return Error("unsupported section type!");
+
+    SectionBases[i] = (char*) Data.base() + Sect->Address;
+  }
+
+  // Bind all the symbols to address.
+  for (unsigned i = 0; i != SymtabLC->NumSymbolTableEntries; ++i) {
+    InMemoryStruct<macho::SymbolTableEntry> STE;
+    Obj->ReadSymbolTableEntry(SymtabLC->SymbolTableOffset, i, STE);
+    if (!STE)
+      return Error("unable to read symbol: '" + Twine(i) + "'");
+    if (STE->SectionIndex == 0)
+      return Error("unexpected undefined symbol!");
+
+    unsigned Index = STE->SectionIndex - 1;
+    if (Index >= Segment32LC->NumSections)
+      return Error("invalid section index for symbol: '" + Twine() + "'");
+
+    // Get the symbol name.
+    StringRef Name = Obj->getStringAtIndex(STE->StringIndex);
+
+    // Get the section base address.
+    void *SectionBase = SectionBases[Index];
+
+    // Get the symbol address.
+    void *Address = (char*) SectionBase + STE->Value;
+
+    // FIXME: Check the symbol type and flags.
+    if (STE->Type != 0xF)
+      return Error("unexpected symbol type!");
+    if (STE->Flags != 0x0)
+      return Error("unexpected symbol type!");
+
+    SymbolTable[Name] = Address;
+  }
+
+  delete SectionBases;
+  return false;
+}
+
+
+bool RuntimeDyldImpl::
+loadSegment64(const MachOObject *Obj,
+              const MachOObject::LoadCommandInfo *SegmentLCI,
+              const InMemoryStruct<macho::SymtabLoadCommand> &SymtabLC) {
+  InMemoryStruct<macho::Segment64LoadCommand> Segment64LC;
+  Obj->ReadSegment64LoadCommand(*SegmentLCI, Segment64LC);
+  if (!Segment64LC)
+    return Error("unable to load segment load command");
+
+  // Map the segment into memory.
+  std::string ErrorStr;
+  Data = sys::Memory::AllocateRWX(Segment64LC->VMSize, 0, &ErrorStr);
+  if (!Data.base())
+    return Error("unable to allocate memory block: '" + ErrorStr + "'");
+  memcpy(Data.base(), Obj->getData(Segment64LC->FileOffset,
+                                   Segment64LC->FileSize).data(),
+         Segment64LC->FileSize);
+  memset((char*)Data.base() + Segment64LC->FileSize, 0,
+         Segment64LC->VMSize - Segment64LC->FileSize);
+
+  // Bind the section indices to address.
+  void **SectionBases = new void*[Segment64LC->NumSections];
+  for (unsigned i = 0; i != Segment64LC->NumSections; ++i) {
+    InMemoryStruct<macho::Section64> Sect;
+    Obj->ReadSection64(*SegmentLCI, i, Sect);
+    if (!Sect)
+      return Error("unable to load section: '" + Twine(i) + "'");
+
+    // FIXME: We don't support relocations yet.
+    if (Sect->NumRelocationTableEntries != 0)
+      return Error("not yet implemented: relocations!");
+
+    // FIXME: Improve check.
+    if (Sect->Flags != 0x80000400)
+      return Error("unsupported section type!");
+
+    SectionBases[i] = (char*) Data.base() + Sect->Address;
+  }
+
+  // Bind all the symbols to address.
+  for (unsigned i = 0; i != SymtabLC->NumSymbolTableEntries; ++i) {
+    InMemoryStruct<macho::Symbol64TableEntry> STE;
+    Obj->ReadSymbol64TableEntry(SymtabLC->SymbolTableOffset, i, STE);
+    if (!STE)
+      return Error("unable to read symbol: '" + Twine(i) + "'");
+    if (STE->SectionIndex == 0)
+      return Error("unexpected undefined symbol!");
+
+    unsigned Index = STE->SectionIndex - 1;
+    if (Index >= Segment64LC->NumSections)
+      return Error("invalid section index for symbol: '" + Twine() + "'");
+
+    // Get the symbol name.
+    StringRef Name = Obj->getStringAtIndex(STE->StringIndex);
+
+    // Get the section base address.
+    void *SectionBase = SectionBases[Index];
+
+    // Get the symbol address.
+    void *Address = (char*) SectionBase + STE->Value;
+
+    // FIXME: Check the symbol type and flags.
+    if (STE->Type != 0xF)
+      return Error("unexpected symbol type!");
+    if (STE->Flags != 0x0)
+      return Error("unexpected symbol type!");
+
+    SymbolTable[Name] = Address;
+  }
+
+  delete SectionBases;
+  return false;
+}
+
+
+
+bool RuntimeDyldImpl::loadObject(MemoryBuffer *InputBuffer) {
+  // If the linker is in an error state, don't do anything.
+  if (hasError())
+    return true;
+  // Load the Mach-O wrapper object.
+  std::string ErrorStr;
+  OwningPtr<MachOObject> Obj(
+    MachOObject::LoadFromBuffer(InputBuffer, &ErrorStr));
+  if (!Obj)
+    return Error("unable to load object: '" + ErrorStr + "'");
+
+  // Validate that the load commands match what we expect.
+  const MachOObject::LoadCommandInfo *SegmentLCI = 0, *SymtabLCI = 0,
+    *DysymtabLCI = 0;
+  for (unsigned i = 0; i != Obj->getHeader().NumLoadCommands; ++i) {
+    const MachOObject::LoadCommandInfo &LCI = Obj->getLoadCommandInfo(i);
+    switch (LCI.Command.Type) {
+    case macho::LCT_Segment:
+    case macho::LCT_Segment64:
+      if (SegmentLCI)
+        return Error("unexpected input object (multiple segments)");
+      SegmentLCI = &LCI;
+      break;
+    case macho::LCT_Symtab:
+      if (SymtabLCI)
+        return Error("unexpected input object (multiple symbol tables)");
+      SymtabLCI = &LCI;
+      break;
+    case macho::LCT_Dysymtab:
+      if (DysymtabLCI)
+        return Error("unexpected input object (multiple symbol tables)");
+      DysymtabLCI = &LCI;
+      break;
+    default:
+      return Error("unexpected input object (unexpected load command");
+    }
+  }
+
+  if (!SymtabLCI)
+    return Error("no symbol table found in object");
+  if (!SegmentLCI)
+    return Error("no symbol table found in object");
+
+  // Read and register the symbol table data.
+  InMemoryStruct<macho::SymtabLoadCommand> SymtabLC;
+  Obj->ReadSymtabLoadCommand(*SymtabLCI, SymtabLC);
+  if (!SymtabLC)
+    return Error("unable to load symbol table load command");
+  Obj->RegisterStringTable(*SymtabLC);
+
+  // Read the dynamic link-edit information, if present (not present in static
+  // objects).
+  if (DysymtabLCI) {
+    InMemoryStruct<macho::DysymtabLoadCommand> DysymtabLC;
+    Obj->ReadDysymtabLoadCommand(*DysymtabLCI, DysymtabLC);
+    if (!DysymtabLC)
+      return Error("unable to load dynamic link-exit load command");
+
+    // FIXME: We don't support anything interesting yet.
+    if (DysymtabLC->LocalSymbolsIndex != 0)
+      return Error("NOT YET IMPLEMENTED: local symbol entries");
+    if (DysymtabLC->ExternalSymbolsIndex != 0)
+      return Error("NOT YET IMPLEMENTED: non-external symbol entries");
+    if (DysymtabLC->UndefinedSymbolsIndex != SymtabLC->NumSymbolTableEntries)
+      return Error("NOT YET IMPLEMENTED: undefined symbol entries");
+  }
+
+  // Load the segment load command.
+  if (SegmentLCI->Command.Type == macho::LCT_Segment) {
+    if (loadSegment32(Obj.get(), SegmentLCI, SymtabLC))
+      return true;
+  } else {
+    if (loadSegment64(Obj.get(), SegmentLCI, SymtabLC))
+      return true;
+  }
+
+  return false;
+}
+
+
+//===----------------------------------------------------------------------===//
+// RuntimeDyld class implementation
+RuntimeDyld::RuntimeDyld() {
+  Dyld = new RuntimeDyldImpl;
+}
+
+RuntimeDyld::~RuntimeDyld() {
+  delete Dyld;
+}
+
+bool RuntimeDyld::loadObject(MemoryBuffer *InputBuffer) {
+  return Dyld->loadObject(InputBuffer);
+}
+
+void *RuntimeDyld::getSymbolAddress(StringRef Name) {
+  return Dyld->getSymbolAddress(Name);
+}
+
+sys::MemoryBlock RuntimeDyld::getMemoryBlock() {
+  return Dyld->getMemoryBlock();
+}
+
+} // end namespace llvm

tools/llvm-rtdyld/Makefile

@@ -18,6 +18,6 @@ TOOL_NO_EXPORTS = 1
 # early so we can set up LINK_COMPONENTS before including Makefile.rules
 include $(LEVEL)/Makefile.config
 
-LINK_COMPONENTS := $(TARGETS_TO_BUILD) support MC object
+LINK_COMPONENTS := $(TARGETS_TO_BUILD) support MC object RuntimeDyld
 
 include $(LLVM_SRC_ROOT)/Makefile.rules

tools/llvm-rtdyld/llvm-rtdyld.cpp

@@ -13,6 +13,7 @@
 
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/OwningPtr.h"
+#include "llvm/ExecutionEngine/RuntimeDyld.h"
 #include "llvm/Object/MachOObject.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ManagedStatic.h"
@@ -51,157 +52,6 @@ static int Error(const Twine &Msg) {
 }
 
 /* *** */
-static bool
-loadSegment32(const MachOObject *Obj,
-              sys::MemoryBlock &Data,
-              const MachOObject::LoadCommandInfo *SegmentLCI,
-              const InMemoryStruct<macho::SymtabLoadCommand> &SymtabLC,
-              StringMap<void*> &SymbolTable) {
-  InMemoryStruct<macho::SegmentLoadCommand> Segment32LC;
-  Obj->ReadSegmentLoadCommand(*SegmentLCI, Segment32LC);
-  if (!Segment32LC)
-    return Error("unable to load segment load command");
-
-  // Map the segment into memory.
-  std::string ErrorStr;
-  Data = sys::Memory::AllocateRWX(Segment32LC->VMSize, 0, &ErrorStr);
-  if (!Data.base())
-    return Error("unable to allocate memory block: '" + ErrorStr + "'");
-  memcpy(Data.base(), Obj->getData(Segment32LC->FileOffset,
-                                   Segment32LC->FileSize).data(),
-         Segment32LC->FileSize);
-  memset((char*)Data.base() + Segment32LC->FileSize, 0,
-         Segment32LC->VMSize - Segment32LC->FileSize);
-
-  // Bind the section indices to address.
-  void **SectionBases = new void*[Segment32LC->NumSections];
-  for (unsigned i = 0; i != Segment32LC->NumSections; ++i) {
-    InMemoryStruct<macho::Section> Sect;
-    Obj->ReadSection(*SegmentLCI, i, Sect);
-    if (!Sect)
-      return Error("unable to load section: '" + Twine(i) + "'");
-
-    // FIXME: We don't support relocations yet.
-    if (Sect->NumRelocationTableEntries != 0)
-      return Error("not yet implemented: relocations!");
-
-    // FIXME: Improve check.
-    if (Sect->Flags != 0x80000400)
-      return Error("unsupported section type!");
-
-    SectionBases[i] = (char*) Data.base() + Sect->Address;
-  }
-
-  // Bind all the symbols to address.
-  for (unsigned i = 0; i != SymtabLC->NumSymbolTableEntries; ++i) {
-    InMemoryStruct<macho::SymbolTableEntry> STE;
-    Obj->ReadSymbolTableEntry(SymtabLC->SymbolTableOffset, i, STE);
-    if (!STE)
-      return Error("unable to read symbol: '" + Twine(i) + "'");
-    if (STE->SectionIndex == 0)
-      return Error("unexpected undefined symbol!");
-
-    unsigned Index = STE->SectionIndex - 1;
-    if (Index >= Segment32LC->NumSections)
-      return Error("invalid section index for symbol: '" + Twine() + "'");
-
-    // Get the symbol name.
-    StringRef Name = Obj->getStringAtIndex(STE->StringIndex);
-
-    // Get the section base address.
-    void *SectionBase = SectionBases[Index];
-
-    // Get the symbol address.
-    void *Address = (char*) SectionBase + STE->Value;
-
-    // FIXME: Check the symbol type and flags.
-    if (STE->Type != 0xF)
-      return Error("unexpected symbol type!");
-    if (STE->Flags != 0x0)
-      return Error("unexpected symbol type!");
-
-    SymbolTable[Name] = Address;
-  }
-
-  delete SectionBases;
-  return false;
-}
-
-static bool
-loadSegment64(const MachOObject *Obj,
-              sys::MemoryBlock &Data,
-              const MachOObject::LoadCommandInfo *SegmentLCI,
-              const InMemoryStruct<macho::SymtabLoadCommand> &SymtabLC,
-              StringMap<void*> &SymbolTable) {
-  InMemoryStruct<macho::Segment64LoadCommand> Segment64LC;
-  Obj->ReadSegment64LoadCommand(*SegmentLCI, Segment64LC);
-  if (!Segment64LC)
-    return Error("unable to load segment load command");
-
-  // Map the segment into memory.
-  std::string ErrorStr;
-  Data = sys::Memory::AllocateRWX(Segment64LC->VMSize, 0, &ErrorStr);
-  if (!Data.base())
-    return Error("unable to allocate memory block: '" + ErrorStr + "'");
-  memcpy(Data.base(), Obj->getData(Segment64LC->FileOffset,
-                                   Segment64LC->FileSize).data(),
-         Segment64LC->FileSize);
-  memset((char*)Data.base() + Segment64LC->FileSize, 0,
-         Segment64LC->VMSize - Segment64LC->FileSize);
-
-  // Bind the section indices to address.
-  void **SectionBases = new void*[Segment64LC->NumSections];
-  for (unsigned i = 0; i != Segment64LC->NumSections; ++i) {
-    InMemoryStruct<macho::Section64> Sect;
-    Obj->ReadSection64(*SegmentLCI, i, Sect);
-    if (!Sect)
-      return Error("unable to load section: '" + Twine(i) + "'");
-
-    // FIXME: We don't support relocations yet.
-    if (Sect->NumRelocationTableEntries != 0)
-      return Error("not yet implemented: relocations!");
-
-    // FIXME: Improve check.
-    if (Sect->Flags != 0x80000400)
-      return Error("unsupported section type!");
-
-    SectionBases[i] = (char*) Data.base() + Sect->Address;
-  }
-
-  // Bind all the symbols to address.
-  for (unsigned i = 0; i != SymtabLC->NumSymbolTableEntries; ++i) {
-    InMemoryStruct<macho::Symbol64TableEntry> STE;
-    Obj->ReadSymbol64TableEntry(SymtabLC->SymbolTableOffset, i, STE);
-    if (!STE)
-      return Error("unable to read symbol: '" + Twine(i) + "'");
-    if (STE->SectionIndex == 0)
-      return Error("unexpected undefined symbol!");
-
-    unsigned Index = STE->SectionIndex - 1;
-    if (Index >= Segment64LC->NumSections)
-      return Error("invalid section index for symbol: '" + Twine() + "'");
-
-    // Get the symbol name.
-    StringRef Name = Obj->getStringAtIndex(STE->StringIndex);
-
-    // Get the section base address.
-    void *SectionBase = SectionBases[Index];
-
-    // Get the symbol address.
-    void *Address = (char*) SectionBase + STE->Value;
-
-    // FIXME: Check the symbol type and flags.
-    if (STE->Type != 0xF)
-      return Error("unexpected symbol type!");
-    if (STE->Flags != 0x0)
-      return Error("unexpected symbol type!");
-
-    SymbolTable[Name] = Address;
-  }
-
-  delete SectionBases;
-  return false;
-}
 
 static int executeInput() {
   // Load the input memory buffer.
@@ -209,94 +59,28 @@ static int executeInput() {
   if (error_code ec = MemoryBuffer::getFileOrSTDIN(InputFile, InputBuffer))
     return Error("unable to read input: '" + ec.message() + "'");
 
-  // Load the Mach-O wrapper object.
+  // Instantiate a dynamic linker.
-  std::string ErrorStr;
+  RuntimeDyld Dyld;
-  OwningPtr<MachOObject> Obj(
-    MachOObject::LoadFromBuffer(InputBuffer.take(), &ErrorStr));
-  if (!Obj)
-    return Error("unable to load object: '" + ErrorStr + "'");
 
-  // Validate that the load commands match what we expect.
+  // Load the object file into it.
-  const MachOObject::LoadCommandInfo *SegmentLCI = 0, *SymtabLCI = 0,
+  if (Dyld.loadObject(InputBuffer.take()))
-    *DysymtabLCI = 0;
+    return true;
-  for (unsigned i = 0; i != Obj->getHeader().NumLoadCommands; ++i) {
-    const MachOObject::LoadCommandInfo &LCI = Obj->getLoadCommandInfo(i);
-    switch (LCI.Command.Type) {
-    case macho::LCT_Segment:
-    case macho::LCT_Segment64:
-      if (SegmentLCI)
-        return Error("unexpected input object (multiple segments)");
-      SegmentLCI = &LCI;
-      break;
-    case macho::LCT_Symtab:
-      if (SymtabLCI)
-        return Error("unexpected input object (multiple symbol tables)");
-      SymtabLCI = &LCI;
-      break;
-    case macho::LCT_Dysymtab:
-      if (DysymtabLCI)
-        return Error("unexpected input object (multiple symbol tables)");
-      DysymtabLCI = &LCI;
-      break;
-    default:
-      return Error("unexpected input object (unexpected load command");
-    }
-  }
-
-  if (!SymtabLCI)
-    return Error("no symbol table found in object");
-  if (!SegmentLCI)
-    return Error("no symbol table found in object");
-
-  // Read and register the symbol table data.
-  InMemoryStruct<macho::SymtabLoadCommand> SymtabLC;
-  Obj->ReadSymtabLoadCommand(*SymtabLCI, SymtabLC);
-  if (!SymtabLC)
-    return Error("unable to load symbol table load command");
-  Obj->RegisterStringTable(*SymtabLC);
-
-  // Read the dynamic link-edit information, if present (not present in static
-  // objects).
-  if (DysymtabLCI) {
-    InMemoryStruct<macho::DysymtabLoadCommand> DysymtabLC;
-    Obj->ReadDysymtabLoadCommand(*DysymtabLCI, DysymtabLC);
-    if (!DysymtabLC)
-      return Error("unable to load dynamic link-exit load command");
-
-    // FIXME: We don't support anything interesting yet.
-    if (DysymtabLC->LocalSymbolsIndex != 0)
-      return Error("NOT YET IMPLEMENTED: local symbol entries");
-    if (DysymtabLC->ExternalSymbolsIndex != 0)
-      return Error("NOT YET IMPLEMENTED: non-external symbol entries");
-    if (DysymtabLC->UndefinedSymbolsIndex != SymtabLC->NumSymbolTableEntries)
-      return Error("NOT YET IMPLEMENTED: undefined symbol entries");
-  }
-
-  // Load the segment load command.
-  sys::MemoryBlock Data;
-  StringMap<void*> SymbolTable;
-  if (SegmentLCI->Command.Type == macho::LCT_Segment) {
-    if (loadSegment32(Obj.get(), Data, SegmentLCI, SymtabLC, SymbolTable))
-      return true;
-  } else {
-    if (loadSegment64(Obj.get(), Data, SegmentLCI, SymtabLC, SymbolTable))
-      return true;
-  }
 
   // Get the address of "_main".
-  StringMap<void*>::iterator it = SymbolTable.find("_main");
+  void *MainAddress = Dyld.getSymbolAddress("_main");
-  if (it == SymbolTable.end())
+  if (MainAddress == 0)
     return Error("no definition for '_main'");
 
   // Invalidate the instruction cache.
+  sys::MemoryBlock Data = Dyld.getMemoryBlock();
   sys::Memory::InvalidateInstructionCache(Data.base(), Data.size());
 
   // Make sure the memory is executable.
+  std::string ErrorStr;
   if (!sys::Memory::setExecutable(Data, &ErrorStr))
     return Error("unable to mark function executable: '" + ErrorStr + "'");
 
   // Dispatch to _main().
-  void *MainAddress = it->second;
   errs() << "loaded '_main' at: " << MainAddress << "\n";
 
   int (*Main)(int, const char**) =