[ThinLTO] Move global processing from Linker to TransformUtils (NFC)

Summary:
As discussed on IRC, move the ThinLTOGlobalProcessing code out of
the linker, and into TransformUtils. The name of the class is changed
to FunctionImportGlobalProcessing.

Reviewers: joker.eph, rafael

Subscribers: joker.eph, llvm-commits

Differential Revision: http://reviews.llvm.org/D17081

llvm-svn: 260395

2 files changed, 238 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/CMakeLists.txt b/llvm/lib/Transforms/Utils/CMakeLists.txt
index c3c5cdb4a93..04d2aced9dc 100644
--- a/llvm/lib/Transforms/Utils/CMakeLists.txt
+++ b/llvm/lib/Transforms/Utils/CMakeLists.txt
@@ -13,6 +13,7 @@ add_llvm_library(LLVMTransformUtils
   DemoteRegToStack.cpp
   Evaluator.cpp
   FlattenCFG.cpp
+  FunctionImportUtils.cpp
   GlobalStatus.cpp
   InlineFunction.cpp
   InstructionNamer.cpp
diff --git a/llvm/lib/Transforms/Utils/FunctionImportUtils.cpp b/llvm/lib/Transforms/Utils/FunctionImportUtils.cpp
new file mode 100644
index 00000000000..b114ad094a1
--- /dev/null
+++ b/llvm/lib/Transforms/Utils/FunctionImportUtils.cpp
@@ -0,0 +1,237 @@
+//===- lib/Transforms/Utils/FunctionImportUtils.cpp - Importing utilities -===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the FunctionImportGlobalProcessing class, used
+// to perform the necessary global value handling for function importing.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/FunctionImportUtils.h"
+using namespace llvm;
+
+/// Checks if we should import SGV as a definition, otherwise import as a
+/// declaration.
+bool FunctionImportGlobalProcessing::doImportAsDefinition(
+    const GlobalValue *SGV, DenseSet<const GlobalValue *> *FunctionsToImport) {
+  auto *GA = dyn_cast<GlobalAlias>(SGV);
+  if (GA) {
+    if (GA->hasWeakAnyLinkage())
+      return false;
+    const GlobalObject *GO = GA->getBaseObject();
+    if (!GO->hasLinkOnceODRLinkage())
+      return false;
+    return FunctionImportGlobalProcessing::doImportAsDefinition(
+        GO, FunctionsToImport);
+  }
+  // Always import GlobalVariable definitions, except for the special
+  // case of WeakAny which are imported as ExternalWeak declarations
+  // (see comments in FunctionImportGlobalProcessing::getLinkage). The linkage
+  // changes described in FunctionImportGlobalProcessing::getLinkage ensure the
+  // correct behavior (e.g. global variables with external linkage are
+  // transformed to available_externally definitions, which are ultimately
+  // turned into declarations after the EliminateAvailableExternally pass).
+  if (isa<GlobalVariable>(SGV) && !SGV->isDeclaration() &&
+      !SGV->hasWeakAnyLinkage())
+    return true;
+  // Only import the function requested for importing.
+  auto *SF = dyn_cast<Function>(SGV);
+  if (SF && FunctionsToImport->count(SF))
+    return true;
+  // Otherwise no.
+  return false;
+}
+
+bool FunctionImportGlobalProcessing::doImportAsDefinition(
+    const GlobalValue *SGV) {
+  if (!isPerformingImport())
+    return false;
+  return FunctionImportGlobalProcessing::doImportAsDefinition(
+      SGV, FunctionsToImport);
+}
+
+bool FunctionImportGlobalProcessing::doPromoteLocalToGlobal(
+    const GlobalValue *SGV) {
+  assert(SGV->hasLocalLinkage());
+  // Both the imported references and the original local variable must
+  // be promoted.
+  if (!isPerformingImport() && !isModuleExporting())
+    return false;
+
+  // Local const variables never need to be promoted unless they are address
+  // taken. The imported uses can simply use the clone created in this module.
+  // For now we are conservative in determining which variables are not
+  // address taken by checking the unnamed addr flag. To be more aggressive,
+  // the address taken information must be checked earlier during parsing
+  // of the module and recorded in the function index for use when importing
+  // from that module.
+  auto *GVar = dyn_cast<GlobalVariable>(SGV);
+  if (GVar && GVar->isConstant() && GVar->hasUnnamedAddr())
+    return false;
+
+  // Eventually we only need to promote functions in the exporting module that
+  // are referenced by a potentially exported function (i.e. one that is in the
+  // function index).
+  return true;
+}
+
+std::string FunctionImportGlobalProcessing::getName(const GlobalValue *SGV) {
+  // For locals that must be promoted to global scope, ensure that
+  // the promoted name uniquely identifies the copy in the original module,
+  // using the ID assigned during combined index creation. When importing,
+  // we rename all locals (not just those that are promoted) in order to
+  // avoid naming conflicts between locals imported from different modules.
+  if (SGV->hasLocalLinkage() &&
+      (doPromoteLocalToGlobal(SGV) || isPerformingImport()))
+    return FunctionInfoIndex::getGlobalNameForLocal(
+        SGV->getName(),
+        ImportIndex->getModuleId(SGV->getParent()->getModuleIdentifier()));
+  return SGV->getName();
+}
+
+GlobalValue::LinkageTypes
+FunctionImportGlobalProcessing::getLinkage(const GlobalValue *SGV) {
+  // Any local variable that is referenced by an exported function needs
+  // to be promoted to global scope. Since we don't currently know which
+  // functions reference which local variables/functions, we must treat
+  // all as potentially exported if this module is exporting anything.
+  if (isModuleExporting()) {
+    if (SGV->hasLocalLinkage() && doPromoteLocalToGlobal(SGV))
+      return GlobalValue::ExternalLinkage;
+    return SGV->getLinkage();
+  }
+
+  // Otherwise, if we aren't importing, no linkage change is needed.
+  if (!isPerformingImport())
+    return SGV->getLinkage();
+
+  switch (SGV->getLinkage()) {
+  case GlobalValue::ExternalLinkage:
+    // External defnitions are converted to available_externally
+    // definitions upon import, so that they are available for inlining
+    // and/or optimization, but are turned into declarations later
+    // during the EliminateAvailableExternally pass.
+    if (doImportAsDefinition(SGV) && !dyn_cast<GlobalAlias>(SGV))
+      return GlobalValue::AvailableExternallyLinkage;
+    // An imported external declaration stays external.
+    return SGV->getLinkage();
+
+  case GlobalValue::AvailableExternallyLinkage:
+    // An imported available_externally definition converts
+    // to external if imported as a declaration.
+    if (!doImportAsDefinition(SGV))
+      return GlobalValue::ExternalLinkage;
+    // An imported available_externally declaration stays that way.
+    return SGV->getLinkage();
+
+  case GlobalValue::LinkOnceAnyLinkage:
+  case GlobalValue::LinkOnceODRLinkage:
+    // These both stay the same when importing the definition.
+    // The ThinLTO pass will eventually force-import their definitions.
+    return SGV->getLinkage();
+
+  case GlobalValue::WeakAnyLinkage:
+    // Can't import weak_any definitions correctly, or we might change the
+    // program semantics, since the linker will pick the first weak_any
+    // definition and importing would change the order they are seen by the
+    // linker. The module linking caller needs to enforce this.
+    assert(!doImportAsDefinition(SGV));
+    // If imported as a declaration, it becomes external_weak.
+    return GlobalValue::ExternalWeakLinkage;
+
+  case GlobalValue::WeakODRLinkage:
+    // For weak_odr linkage, there is a guarantee that all copies will be
+    // equivalent, so the issue described above for weak_any does not exist,
+    // and the definition can be imported. It can be treated similarly
+    // to an imported externally visible global value.
+    if (doImportAsDefinition(SGV) && !dyn_cast<GlobalAlias>(SGV))
+      return GlobalValue::AvailableExternallyLinkage;
+    else
+      return GlobalValue::ExternalLinkage;
+
+  case GlobalValue::AppendingLinkage:
+    // It would be incorrect to import an appending linkage variable,
+    // since it would cause global constructors/destructors to be
+    // executed multiple times. This should have already been handled
+    // by linkIfNeeded, and we will assert in shouldLinkFromSource
+    // if we try to import, so we simply return AppendingLinkage.
+    return GlobalValue::AppendingLinkage;
+
+  case GlobalValue::InternalLinkage:
+  case GlobalValue::PrivateLinkage:
+    // If we are promoting the local to global scope, it is handled
+    // similarly to a normal externally visible global.
+    if (doPromoteLocalToGlobal(SGV)) {
+      if (doImportAsDefinition(SGV) && !dyn_cast<GlobalAlias>(SGV))
+        return GlobalValue::AvailableExternallyLinkage;
+      else
+        return GlobalValue::ExternalLinkage;
+    }
+    // A non-promoted imported local definition stays local.
+    // The ThinLTO pass will eventually force-import their definitions.
+    return SGV->getLinkage();
+
+  case GlobalValue::ExternalWeakLinkage:
+    // External weak doesn't apply to definitions, must be a declaration.
+    assert(!doImportAsDefinition(SGV));
+    // Linkage stays external_weak.
+    return SGV->getLinkage();
+
+  case GlobalValue::CommonLinkage:
+    // Linkage stays common on definitions.
+    // The ThinLTO pass will eventually force-import their definitions.
+    return SGV->getLinkage();
+  }
+
+  llvm_unreachable("unknown linkage type");
+}
+
+void FunctionImportGlobalProcessing::processGlobalForThinLTO(GlobalValue &GV) {
+  if (GV.hasLocalLinkage() &&
+      (doPromoteLocalToGlobal(&GV) || isPerformingImport())) {
+    GV.setName(getName(&GV));
+    GV.setLinkage(getLinkage(&GV));
+    if (!GV.hasLocalLinkage())
+      GV.setVisibility(GlobalValue::HiddenVisibility);
+    if (isModuleExporting())
+      NewExportedValues.insert(&GV);
+  } else
+    GV.setLinkage(getLinkage(&GV));
+
+  // Remove functions imported as available externally defs from comdats,
+  // as this is a declaration for the linker, and will be dropped eventually.
+  // It is illegal for comdats to contain declarations.
+  auto *GO = dyn_cast_or_null<GlobalObject>(&GV);
+  if (GO && GO->isDeclarationForLinker() && GO->hasComdat()) {
+    // The IRMover should not have placed any imported declarations in
+    // a comdat, so the only declaration that should be in a comdat
+    // at this point would be a definition imported as available_externally.
+    assert(GO->hasAvailableExternallyLinkage() &&
+           "Expected comdat on definition (possibly available external)");
+    GO->setComdat(nullptr);
+  }
+}
+
+void FunctionImportGlobalProcessing::processGlobalsForThinLTO() {
+  for (GlobalVariable &GV : M.globals())
+    processGlobalForThinLTO(GV);
+  for (Function &SF : M)
+    processGlobalForThinLTO(SF);
+  for (GlobalAlias &GA : M.aliases())
+    processGlobalForThinLTO(GA);
+}
+
+bool FunctionImportGlobalProcessing::run() {
+  processGlobalsForThinLTO();
+  return false;
+}
+
+bool llvm::renameModuleForThinLTO(Module &M, const FunctionInfoIndex *Index) {
+  FunctionImportGlobalProcessing ThinLTOProcessing(M, Index);
+  return ThinLTOProcessing.run();
+}