[GlobalMerge] Allow targets to enable merging of extern variables, NFC.

Adjust the GlobalMergeOnExternal option so that the default behaviour is to
do whatever the Target thinks is best. Explicitly enabled or disabling the
option will override this default.

Differential Revision: http://reviews.llvm.org/D10965

llvm-svn: 243873

include/llvm/Transforms/Scalar.h

@@ -161,7 +161,8 @@ Pass *createLoopStrengthReducePass();
 // It can also be configured to focus on size optimizations only.
 //
 Pass *createGlobalMergePass(const TargetMachine *TM, unsigned MaximalOffset,
-                            bool OnlyOptimizeForSize = false);
+                            bool OnlyOptimizeForSize = false,
+                            bool MergeExternalByDefault = false);
 
 //===----------------------------------------------------------------------===//
 //

lib/CodeGen/GlobalMerge.cpp

@@ -108,10 +108,9 @@ EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
 
 // 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>
+static cl::opt<cl::boolOrDefault>
 EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
-     cl::desc("Enable global merge pass on external linkage"),
-     cl::init(false));
+     cl::desc("Enable global merge pass on external linkage"));
 
 STATISTIC(NumMerged, "Number of globals merged");
 namespace {
@@ -129,6 +128,9 @@ namespace {
     /// FIXME: This could learn about optsize, and be used in the cost model.
     bool OnlyOptimizeForSize;
 
+    /// Whether we should merge global variables that have external linkage.
+    bool MergeExternalGlobals;
+
     bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
                  Module &M, bool isConst, unsigned AddrSpace) const;
     /// \brief Merge everything in \p Globals for which the corresponding bit
@@ -158,9 +160,11 @@ namespace {
     static char ID;             // Pass identification, replacement for typeid.
     explicit GlobalMerge(const TargetMachine *TM = nullptr,
                          unsigned MaximalOffset = 0,
-                         bool OnlyOptimizeForSize = false)
+                         bool OnlyOptimizeForSize = false,
+                         bool MergeExternalGlobals = false)
         : FunctionPass(ID), TM(TM), MaxOffset(MaximalOffset),
-          OnlyOptimizeForSize(OnlyOptimizeForSize) {
+          OnlyOptimizeForSize(OnlyOptimizeForSize),
+          MergeExternalGlobals(MergeExternalGlobals) {
       initializeGlobalMergePass(*PassRegistry::getPassRegistry());
     }
 
@@ -541,7 +545,7 @@ bool GlobalMerge::doInitialization(Module &M) {
     if (I->isDeclaration() || I->isThreadLocal() || I->hasSection())
       continue;
 
-    if (!(EnableGlobalMergeOnExternal && I->hasExternalLinkage()) &&
+    if (!(MergeExternalGlobals && I->hasExternalLinkage()) &&
         !I->hasInternalLinkage())
       continue;
 
@@ -604,6 +608,9 @@ bool GlobalMerge::doFinalization(Module &M) {
 }
 
 Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset,
-                                  bool OnlyOptimizeForSize) {
-  return new GlobalMerge(TM, Offset, OnlyOptimizeForSize);
+                                  bool OnlyOptimizeForSize,
+                                  bool MergeExternalByDefault) {
+  bool MergeExternal = (EnableGlobalMergeOnExternal == cl::BOU_UNSET) ?
+    MergeExternalByDefault : (EnableGlobalMergeOnExternal == cl::BOU_TRUE);
+  return new GlobalMerge(TM, Offset, OnlyOptimizeForSize, MergeExternal);
 }