3 files changed, 125 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/Utils/CMakeLists.txt b/llvm/lib/Transforms/Utils/CMakeLists.txt
index 716e655affb..8308a9b6914 100644
--- a/llvm/lib/Transforms/Utils/CMakeLists.txt
+++ b/llvm/lib/Transforms/Utils/CMakeLists.txt
@@ -34,6 +34,7 @@ add_llvm_library(LLVMTransformUtils
   SimplifyIndVar.cpp
   SimplifyInstructions.cpp
   SimplifyLibCalls.cpp
+  SplitModule.cpp
   SymbolRewriter.cpp
   UnifyFunctionExitNodes.cpp
   Utils.cpp
diff --git a/llvm/lib/Transforms/Utils/CloneModule.cpp b/llvm/lib/Transforms/Utils/CloneModule.cpp
index 61f1811e7b4..5e7cae5364a 100644
--- a/llvm/lib/Transforms/Utils/CloneModule.cpp
+++ b/llvm/lib/Transforms/Utils/CloneModule.cpp
@@ -33,6 +33,12 @@ Module *llvm::CloneModule(const Module *M) {
 }
 
 Module *llvm::CloneModule(const Module *M, ValueToValueMapTy &VMap) {
+  return CloneModule(M, VMap, [](const GlobalValue *GV) { return true; });
+}
+
+Module *llvm::CloneModule(
+    const Module *M, ValueToValueMapTy &VMap,
+    std::function<bool(const GlobalValue *)> ShouldCloneDefinition) {
   // First off, we need to create the new module.
   Module *New = new Module(M->getModuleIdentifier(), M->getContext());
   New->setDataLayout(M->getDataLayout());
@@ -68,6 +74,26 @@ Module *llvm::CloneModule(const Module *M, ValueToValueMapTy &VMap) {
   // Loop over the aliases in the module
   for (Module::const_alias_iterator I = M->alias_begin(), E = M->alias_end();
        I != E; ++I) {
+    if (!ShouldCloneDefinition(I)) {
+      // An alias cannot act as an external reference, so we need to create
+      // either a function or a global variable depending on the value type.
+      // FIXME: Once pointee types are gone we can probably pick one or the
+      // other.
+      GlobalValue *GV;
+      if (I->getValueType()->isFunctionTy())
+        GV = Function::Create(cast<FunctionType>(I->getValueType()),
+                              GlobalValue::ExternalLinkage, I->getName(), New);
+      else
+        GV = new GlobalVariable(
+            *New, I->getValueType(), false, GlobalValue::ExternalLinkage,
+            (Constant *)nullptr, I->getName(), (GlobalVariable *)nullptr,
+            I->getThreadLocalMode(), I->getType()->getAddressSpace());
+      VMap[I] = GV;
+      // We do not copy attributes (mainly because copying between different
+      // kinds of globals is forbidden), but this is generally not required for
+      // correctness.
+      continue;
+    }
     auto *PTy = cast<PointerType>(I->getType());
     auto *GA = GlobalAlias::create(PTy, I->getLinkage(), I->getName(), New);
     GA->copyAttributesFrom(I);
@@ -81,6 +107,11 @@ Module *llvm::CloneModule(const Module *M, ValueToValueMapTy &VMap) {
   for (Module::const_global_iterator I = M->global_begin(), E = M->global_end();
        I != E; ++I) {
     GlobalVariable *GV = cast<GlobalVariable>(VMap[I]);
+    if (!ShouldCloneDefinition(I)) {
+      // Skip after setting the correct linkage for an external reference.
+      GV->setLinkage(GlobalValue::ExternalLinkage);
+      continue;
+    }
     if (I->hasInitializer())
       GV->setInitializer(MapValue(I->getInitializer(), VMap));
   }
@@ -89,6 +120,11 @@ Module *llvm::CloneModule(const Module *M, ValueToValueMapTy &VMap) {
   //
   for (Module::const_iterator I = M->begin(), E = M->end(); I != E; ++I) {
     Function *F = cast<Function>(VMap[I]);
+    if (!ShouldCloneDefinition(I)) {
+      // Skip after setting the correct linkage for an external reference.
+      F->setLinkage(GlobalValue::ExternalLinkage);
+      continue;
+    }
     if (!I->isDeclaration()) {
       Function::arg_iterator DestI = F->arg_begin();
       for (Function::const_arg_iterator J = I->arg_begin(); J != I->arg_end();
@@ -109,6 +145,9 @@ Module *llvm::CloneModule(const Module *M, ValueToValueMapTy &VMap) {
   // And aliases
   for (Module::const_alias_iterator I = M->alias_begin(), E = M->alias_end();
        I != E; ++I) {
+    // We already dealt with undefined aliases above.
+    if (!ShouldCloneDefinition(I))
+      continue;
     GlobalAlias *GA = cast<GlobalAlias>(VMap[I]);
     if (const Constant *C = I->getAliasee())
       GA->setAliasee(MapValue(C, VMap));
diff --git a/llvm/lib/Transforms/Utils/SplitModule.cpp b/llvm/lib/Transforms/Utils/SplitModule.cpp
new file mode 100644
index 00000000000..ad6b782caf8
--- /dev/null
+++ b/llvm/lib/Transforms/Utils/SplitModule.cpp
@@ -0,0 +1,85 @@
+//===- SplitModule.cpp - Split a module into partitions -------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the function llvm::SplitModule, which splits a module
+// into multiple linkable partitions. It can be used to implement parallel code
+// generation for link-time optimization.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/SplitModule.h"
+#include "llvm/ADT/Hashing.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/GlobalAlias.h"
+#include "llvm/IR/GlobalObject.h"
+#include "llvm/IR/GlobalValue.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/MD5.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Utils/Cloning.h"
+
+using namespace llvm;
+
+static void externalize(GlobalValue *GV) {
+  if (GV->hasLocalLinkage()) {
+    GV->setLinkage(GlobalValue::ExternalLinkage);
+    GV->setVisibility(GlobalValue::HiddenVisibility);
+  }
+
+  // Unnamed entities must be named consistently between modules. setName will
+  // give a distinct name to each such entity.
+  if (!GV->hasName())
+    GV->setName("__llvmsplit_unnamed");
+}
+
+// Returns whether GV should be in partition (0-based) I of N.
+static bool isInPartition(const GlobalValue *GV, unsigned I, unsigned N) {
+  if (auto GA = dyn_cast<GlobalAlias>(GV))
+    if (const GlobalObject *Base = GA->getBaseObject())
+      GV = Base;
+
+  StringRef Name;
+  if (const Comdat *C = GV->getComdat())
+    Name = C->getName();
+  else
+    Name = GV->getName();
+
+  // Partition by MD5 hash. We only need a few bits for evenness as the number
+  // of partitions will generally be in the 1-2 figure range; the low 16 bits
+  // are enough.
+  MD5 H;
+  MD5::MD5Result R;
+  H.update(Name);
+  H.final(R);
+  return (R[0] | (R[1] << 8)) % N == I;
+}
+
+void llvm::SplitModule(
+    std::unique_ptr<Module> M, unsigned N,
+    std::function<void(std::unique_ptr<Module> MPart)> ModuleCallback) {
+  for (Function &F : *M)
+    externalize(&F);
+  for (GlobalVariable &GV : M->globals())
+    externalize(&GV);
+  for (GlobalAlias &GA : M->aliases())
+    externalize(&GA);
+
+  // FIXME: We should be able to reuse M as the last partition instead of
+  // cloning it.
+  for (unsigned I = 0; I != N; ++I) {
+    ValueToValueMapTy VMap;
+    std::unique_ptr<Module> MPart(
+        CloneModule(M.get(), VMap, [=](const GlobalValue *GV) {
+          return isInPartition(GV, I, N);
+        }));
+    if (I != 0)
+      MPart->setModuleInlineAsm("");
+    ModuleCallback(std::move(MPart));
+  }
+}