Global merge for global symbols.

This commit is to improve global merge pass and support global symbol merge.
The global symbol merge is not enabled by default. For aarch64, we need some
more back-end fix to make it really benifit ADRP CSE.

llvm-svn: 210640

2 files changed, 72 insertions, 10 deletions

diff --git a/llvm/lib/Transforms/Scalar/GlobalMerge.cpp b/llvm/lib/Transforms/Scalar/GlobalMerge.cpp
index 52227126316..ecf9be861e7 100644
--- a/llvm/lib/Transforms/Scalar/GlobalMerge.cpp
+++ b/llvm/lib/Transforms/Scalar/GlobalMerge.cpp
@@ -81,6 +81,13 @@ EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
                          cl::desc("Enable global merge pass on constants"),
                          cl::init(false));
 
+// FIXME: this could be a transitional option, and we probably need to remove
+// it if only we are sure this optimization could always benefit all targets.
+static cl::opt<bool>
+EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
+     cl::desc("Enable global merge pass on external linkage"),
+     cl::init(false));
+
 STATISTIC(NumMerged      , "Number of globals merged");
 namespace {
   class GlobalMerge : public FunctionPass {
@@ -129,9 +136,19 @@ namespace {
 } // end anonymous namespace
 
 char GlobalMerge::ID = 0;
-INITIALIZE_PASS(GlobalMerge, "global-merge",
-                "Global Merge", false, false)
 
+static void *initializeGlobalMergePassOnce(PassRegistry &Registry) {
+  PassInfo *PI = new PassInfo(
+      "Merge global variables", "global-merge", &GlobalMerge::ID,
+      PassInfo::NormalCtor_t(callDefaultCtor<GlobalMerge>), false, false,
+      PassInfo::TargetMachineCtor_t(callTargetMachineCtor<GlobalMerge>));
+  Registry.registerPass(*PI, true);
+  return PI;
+}
+
+void llvm::initializeGlobalMergePass(PassRegistry &Registry) {
+  CALL_ONCE_INITIALIZATION(initializeGlobalMergePassOnce)
+}
 
 bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
                           Module &M, bool isConst, unsigned AddrSpace) const {
@@ -154,11 +171,23 @@ bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
 
   Type *Int32Ty = Type::getInt32Ty(M.getContext());
 
+  assert(Globals.size() > 1);
+
+  // FIXME: This simple solution merges globals all together as maximum as
+  // possible. However, with this solution it would be hard to remove dead
+  // global symbols at link-time. An alternative solution could be checking
+  // global symbols references function by function, and make the symbols
+  // being referred in the same function merged and we would probably need
+  // to introduce heuristic algorithm to solve the merge conflict from
+  // different functions.
   for (size_t i = 0, e = Globals.size(); i != e; ) {
     size_t j = 0;
     uint64_t MergedSize = 0;
     std::vector<Type*> Tys;
     std::vector<Constant*> Inits;
+
+    bool HasExternal = false;
+    GlobalVariable *TheFirstExternal = 0;
     for (j = i; j != e; ++j) {
       Type *Ty = Globals[j]->getType()->getElementType();
       MergedSize += DL->getTypeAllocSize(Ty);
@@ -167,17 +196,37 @@ bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
       }
       Tys.push_back(Ty);
       Inits.push_back(Globals[j]->getInitializer());
+
+      if (Globals[j]->hasExternalLinkage() && !HasExternal) {
+        HasExternal = true;
+        TheFirstExternal = Globals[j];
+      }
     }
 
+    // If merged variables doesn't have external linkage, we needn't to expose
+    // the symbol after merging.
+    GlobalValue::LinkageTypes Linkage = HasExternal
+                                            ? GlobalValue::ExternalLinkage
+                                            : GlobalValue::InternalLinkage;
+
+    // If merged variables have external linkage, we use symbol name of the
+    // first variable merged as the suffix of global symbol name. This would
+    // be able to avoid the link-time naming conflict for globalm symbols.
+    Twine MergedGVName = HasExternal
+                             ? "_MergedGlobals_" + TheFirstExternal->getName()
+                             : "_MergedGlobals";
+
     StructType *MergedTy = StructType::get(M.getContext(), Tys);
     Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
-    GlobalVariable *MergedGV = new GlobalVariable(M, MergedTy, isConst,
-                                                  GlobalValue::InternalLinkage,
-                                                  MergedInit, "_MergedGlobals",
-                                                  nullptr,
-                                                  GlobalVariable::NotThreadLocal,
-                                                  AddrSpace);
+
+    GlobalVariable *MergedGV = new GlobalVariable(
+        M, MergedTy, isConst, Linkage, MergedInit, MergedGVName, nullptr,
+        GlobalVariable::NotThreadLocal, AddrSpace);
+
     for (size_t k = i; k < j; ++k) {
+      GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
+      std::string Name = Globals[k]->getName();
+
       Constant *Idx[2] = {
         ConstantInt::get(Int32Ty, 0),
         ConstantInt::get(Int32Ty, k-i)
@@ -185,6 +234,14 @@ bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
       Constant *GEP = ConstantExpr::getInBoundsGetElementPtr(MergedGV, Idx);
       Globals[k]->replaceAllUsesWith(GEP);
       Globals[k]->eraseFromParent();
+
+      if (Linkage != GlobalValue::InternalLinkage) {
+        // Generate a new alias...
+        auto *PTy = cast<PointerType>(GEP->getType());
+        GlobalAlias::create(PTy->getElementType(), PTy->getAddressSpace(),
+                            Linkage, Name, GEP, &M);
+      }
+
       NumMerged++;
     }
     i = j;
@@ -245,8 +302,12 @@ bool GlobalMerge::doInitialization(Module &M) {
   // Grab all non-const globals.
   for (Module::global_iterator I = M.global_begin(),
          E = M.global_end(); I != E; ++I) {
-    // Merge is safe for "normal" internal globals only
-    if (!I->hasLocalLinkage() || I->isThreadLocal() || I->hasSection())
+    // Merge is safe for "normal" internal or external globals only
+    if (I->isDeclaration() || I->isThreadLocal() || I->hasSection())
+      continue;
+
+    if (!(EnableGlobalMergeOnExternal && I->hasExternalLinkage()) &&
+        !I->hasInternalLinkage())
       continue;
 
     PointerType *PT = dyn_cast<PointerType>(I->getType());
diff --git a/llvm/lib/Transforms/Scalar/Scalar.cpp b/llvm/lib/Transforms/Scalar/Scalar.cpp
index edf012d8117..5c7db5b16e3 100644
--- a/llvm/lib/Transforms/Scalar/Scalar.cpp
+++ b/llvm/lib/Transforms/Scalar/Scalar.cpp
@@ -38,6 +38,7 @@ void llvm::initializeScalarOpts(PassRegistry &Registry) {
   initializeDSEPass(Registry);
   initializeGVNPass(Registry);
   initializeEarlyCSEPass(Registry);
+  initializeGlobalMergePass(Registry);
   initializeIndVarSimplifyPass(Registry);
   initializeJumpThreadingPass(Registry);
   initializeLICMPass(Registry);