6 files changed, 121 insertions, 7 deletions
diff --git a/llvm/lib/Passes/PassBuilder.cpp b/llvm/lib/Passes/PassBuilder.cpp
index 2f567262fbb..e1d0a47fb6d 100644
--- a/llvm/lib/Passes/PassBuilder.cpp
+++ b/llvm/lib/Passes/PassBuilder.cpp
@@ -153,6 +153,7 @@
 #include "llvm/Transforms/Scalar/WarnMissedTransforms.h"
 #include "llvm/Transforms/Utils/AddDiscriminators.h"
 #include "llvm/Transforms/Utils/BreakCriticalEdges.h"
+#include "llvm/Transforms/Utils/CanonicalizeAliases.h"
 #include "llvm/Transforms/Utils/EntryExitInstrumenter.h"
 #include "llvm/Transforms/Utils/LCSSA.h"
 #include "llvm/Transforms/Utils/LibCallsShrinkWrap.h"
diff --git a/llvm/lib/Passes/PassRegistry.def b/llvm/lib/Passes/PassRegistry.def
index 2d5fc2e99b1..f20b597a8f0 100644
--- a/llvm/lib/Passes/PassRegistry.def
+++ b/llvm/lib/Passes/PassRegistry.def
@@ -42,6 +42,7 @@ MODULE_ALIAS_ANALYSIS("globals-aa", GlobalsAA())
 #endif
 MODULE_PASS("always-inline", AlwaysInlinerPass())
 MODULE_PASS("called-value-propagation", CalledValuePropagationPass())
+MODULE_PASS("canonicalize-aliases", CanonicalizeAliasesPass())
 MODULE_PASS("cg-profile", CGProfilePass())
 MODULE_PASS("constmerge", ConstantMergePass())
 MODULE_PASS("cross-dso-cfi", CrossDSOCFIPass())
diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
index 60bd6def6ce..9764944dc33 100644
--- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -462,12 +462,14 @@ void PassManagerBuilder::populateModulePassManager(
 
     addExtensionsToPM(EP_EnabledOnOptLevel0, MPM);
 
-    // Rename anon globals to be able to export them in the summary.
-    // This has to be done after we add the extensions to the pass manager
-    // as there could be passes (e.g. Adddress sanitizer) which introduce
-    // new unnamed globals.
-    if (PrepareForLTO || PrepareForThinLTO)
+    if (PrepareForLTO || PrepareForThinLTO) {
+      MPM.add(createCanonicalizeAliasesPass());
+      // Rename anon globals to be able to export them in the summary.
+      // This has to be done after we add the extensions to the pass manager
+      // as there could be passes (e.g. Adddress sanitizer) which introduce
+      // new unnamed globals.
       MPM.add(createNameAnonGlobalPass());
+    }
     return;
   }
 
@@ -585,6 +587,7 @@ void PassManagerBuilder::populateModulePassManager(
     // Ensure we perform any last passes, but do so before renaming anonymous
     // globals in case the passes add any.
     addExtensionsToPM(EP_OptimizerLast, MPM);
+    MPM.add(createCanonicalizeAliasesPass());
     // Rename anon globals to be able to export them in the summary.
     MPM.add(createNameAnonGlobalPass());
     return;
@@ -744,9 +747,11 @@ void PassManagerBuilder::populateModulePassManager(
 
   addExtensionsToPM(EP_OptimizerLast, MPM);
 
-  // Rename anon globals to be able to handle them in the summary
-  if (PrepareForLTO)
+  if (PrepareForLTO) {
+    MPM.add(createCanonicalizeAliasesPass());
+    // Rename anon globals to be able to handle them in the summary
     MPM.add(createNameAnonGlobalPass());
+  }
 }
 
 void PassManagerBuilder::addLTOOptimizationPasses(legacy::PassManagerBase &PM) {
diff --git a/llvm/lib/Transforms/Utils/CMakeLists.txt b/llvm/lib/Transforms/Utils/CMakeLists.txt
index eb3bad79129..cb3dc17c03a 100644
--- a/llvm/lib/Transforms/Utils/CMakeLists.txt
+++ b/llvm/lib/Transforms/Utils/CMakeLists.txt
@@ -6,6 +6,7 @@ add_llvm_library(LLVMTransformUtils
   BuildLibCalls.cpp
   BypassSlowDivision.cpp
   CallPromotionUtils.cpp
+  CanonicalizeAliases.cpp
   CloneFunction.cpp
   CloneModule.cpp
   CodeExtractor.cpp
diff --git a/llvm/lib/Transforms/Utils/CanonicalizeAliases.cpp b/llvm/lib/Transforms/Utils/CanonicalizeAliases.cpp
new file mode 100644
index 00000000000..cf41fd2e14c
--- /dev/null
+++ b/llvm/lib/Transforms/Utils/CanonicalizeAliases.cpp
@@ -0,0 +1,105 @@
+//===- CanonicalizeAliases.cpp - ThinLTO Support: Canonicalize Aliases ----===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// Currently this file implements partial alias canonicalization, to
+// flatten chains of aliases (also done by GlobalOpt, but not on for
+// O0 compiles). E.g.
+//  @a = alias i8, i8 *@b
+//  @b = alias i8, i8 *@g
+//
+// will be converted to:
+//  @a = alias i8, i8 *@g  <-- @a is now an alias to base object @g
+//  @b = alias i8, i8 *@g
+//
+// Eventually this file will implement full alias canonicalation, so that
+// all aliasees are private anonymous values. E.g.
+//  @a = alias i8, i8 *@g
+//  @g = global i8 0
+//
+// will be converted to:
+//  @0 = private global
+//  @a = alias i8, i8* @0
+//  @g = alias i8, i8* @0
+//
+// This simplifies optimization and ThinLTO linking of the original symbols.
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/CanonicalizeAliases.h"
+
+#include "llvm/IR/Operator.h"
+#include "llvm/IR/ValueHandle.h"
+
+using namespace llvm;
+
+namespace {
+
+static Constant *canonicalizeAlias(Constant *C, bool &Changed) {
+  if (auto *GA = dyn_cast<GlobalAlias>(C)) {
+    auto *NewAliasee = canonicalizeAlias(GA->getAliasee(), Changed);
+    if (NewAliasee != GA->getAliasee()) {
+      GA->setAliasee(NewAliasee);
+      Changed = true;
+    }
+    return NewAliasee;
+  }
+
+  auto *CE = dyn_cast<ConstantExpr>(C);
+  if (!CE)
+    return C;
+
+  std::vector<Constant *> Ops;
+  for (Use &U : CE->operands())
+    Ops.push_back(canonicalizeAlias(cast<Constant>(U), Changed));
+  return CE->getWithOperands(Ops);
+}
+
+/// Convert aliases to canonical form.
+static bool canonicalizeAliases(Module &M) {
+  bool Changed = false;
+  for (auto &GA : M.aliases())
+    canonicalizeAlias(&GA, Changed);
+  return Changed;
+}
+
+// Legacy pass that canonicalizes aliases.
+class CanonicalizeAliasesLegacyPass : public ModulePass {
+
+public:
+  /// Pass identification, replacement for typeid
+  static char ID;
+
+  /// Specify pass name for debug output
+  StringRef getPassName() const override { return "Canonicalize Aliases"; }
+
+  explicit CanonicalizeAliasesLegacyPass() : ModulePass(ID) {}
+
+  bool runOnModule(Module &M) override { return canonicalizeAliases(M); }
+};
+char CanonicalizeAliasesLegacyPass::ID = 0;
+
+} // anonymous namespace
+
+PreservedAnalyses CanonicalizeAliasesPass::run(Module &M,
+                                               ModuleAnalysisManager &AM) {
+  if (!canonicalizeAliases(M))
+    return PreservedAnalyses::all();
+
+  return PreservedAnalyses::none();
+}
+
+INITIALIZE_PASS_BEGIN(CanonicalizeAliasesLegacyPass, "canonicalize-aliases",
+                      "Canonicalize aliases", false, false)
+INITIALIZE_PASS_END(CanonicalizeAliasesLegacyPass, "canonicalize-aliases",
+                    "Canonicalize aliases", false, false)
+
+namespace llvm {
+ModulePass *createCanonicalizeAliasesPass() {
+  return new CanonicalizeAliasesLegacyPass();
+}
+} // namespace llvm
diff --git a/llvm/lib/Transforms/Utils/Utils.cpp b/llvm/lib/Transforms/Utils/Utils.cpp
index afd842f5991..95416de0743 100644
--- a/llvm/lib/Transforms/Utils/Utils.cpp
+++ b/llvm/lib/Transforms/Utils/Utils.cpp
@@ -26,6 +26,7 @@ using namespace llvm;
 void llvm::initializeTransformUtils(PassRegistry &Registry) {
   initializeAddDiscriminatorsLegacyPassPass(Registry);
   initializeBreakCriticalEdgesPass(Registry);
+  initializeCanonicalizeAliasesLegacyPassPass(Registry);
   initializeInstNamerPass(Registry);
   initializeLCSSAWrapperPassPass(Registry);
   initializeLibCallsShrinkWrapLegacyPassPass(Registry);