- Made GlobalDCE worklist driven, making it more successful. Now can handle

cases like: test/Regression/Transforms/GlobalDCE/2002-08-17-WorkListTest.ll

llvm-svn: 3381

llvm/lib/Transforms/IPO/GlobalDCE.cpp

@@ -7,14 +7,18 @@
 
 #include "llvm/Transforms/IPO.h"
 #include "llvm/Module.h"
-#include "llvm/Function.h"
+#include "llvm/Constants.h"
+#include "llvm/DerivedTypes.h"
 #include "llvm/GlobalVariable.h"
 #include "llvm/Analysis/CallGraph.h"
 #include "Support/DepthFirstIterator.h"
 #include "Support/StatisticReporter.h"
+#include <algorithm>
 
 static Statistic<> NumFunctions("globaldce\t- Number of functions removed");
 static Statistic<> NumVariables("globaldce\t- Number of global variables removed");
+static Statistic<> NumCPRs("globaldce\t- Number of const pointer refs removed");
+static Statistic<> NumConsts("globaldce\t- Number of init constants removed");
 
 static bool RemoveUnreachableFunctions(Module &M, CallGraph &CallGraph) {
   // Calculate which functions are reachable from the external functions in the
@@ -52,22 +56,6 @@ static bool RemoveUnreachableFunctions(Module &M, CallGraph &CallGraph) {
   return true;
 }
 
-static bool RemoveUnreachableGlobalVariables(Module &M) {
-  bool Changed = false;
-  // Eliminate all global variables that are unused, and that are internal, or
-  // do not have an initializer.
-  //
-  for (Module::giterator I = M.gbegin(); I != M.gend(); )
-    if (!I->use_empty() || (I->hasExternalLinkage() && I->hasInitializer()))
-      ++I;                     // Cannot eliminate global variable
-    else {
-      I = M.getGlobalList().erase(I);
-      ++NumVariables;
-      Changed = true;
-    }
-  return Changed;
-}
-
 namespace {
   struct GlobalDCE : public Pass {
     // run - Do the GlobalDCE pass on the specified module, optionally updating
@@ -85,8 +73,179 @@ namespace {
     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.addRequired<CallGraph>();
     }
+
+  private:
+    std::vector<GlobalValue*> WorkList;
+
+    inline bool RemoveIfDead(GlobalValue *GV);
+    void DestroyInitializer(Constant *C);
+
+    bool RemoveUnreachableGlobalVariables(Module &M);
+    bool RemoveUnusedConstantPointerRef(GlobalValue &GV);
+    bool SafeToDestroyConstant(Constant *C);
   };
   RegisterOpt<GlobalDCE> X("globaldce", "Dead Global Elimination");
 }
 
 Pass *createGlobalDCEPass() { return new GlobalDCE(); }
+
+
+// RemoveIfDead - If this global value is dead, remove it from the current
+// module and return true.
+//
+bool GlobalDCE::RemoveIfDead(GlobalValue *GV) {
+  // If there is only one use of the global value, it might be a
+  // ConstantPointerRef... which means that this global might actually be
+  // dead.
+  if (GV->use_size() == 1)
+    RemoveUnusedConstantPointerRef(*GV);
+
+  if (!GV->use_empty()) return false;
+
+  if (GlobalVariable *GVar = dyn_cast<GlobalVariable>(GV)) {
+    // Eliminate all global variables that are unused, and that are internal, or
+    // do not have an initializer.
+    //
+    if (!GVar->hasExternalLinkage() || !GVar->hasInitializer()) {
+      Constant *Init = GVar->hasInitializer() ? GVar->getInitializer() : 0;
+      GV->getParent()->getGlobalList().erase(GVar);
+      ++NumVariables;
+
+      // If there was an initializer for the global variable, try to destroy it
+      // now.
+      if (Init) DestroyInitializer(Init);
+
+      // If the global variable is still on the worklist, remove it now.
+      std::vector<GlobalValue*>::iterator I = std::find(WorkList.begin(),
+                                                        WorkList.end(), GV);
+      while (I != WorkList.end())
+        I = std::find(WorkList.erase(I), WorkList.end(), GV);
+
+      return true;
+    }
+  } else {
+    Function *F = cast<Function>(GV);
+    // FIXME: TODO
+
+  }
+  return false;
+}
+
+// DestroyInitializer - A global variable was just destroyed and C is its
+// initializer. If we can, destroy C and all of the constants it refers to.
+//
+void GlobalDCE::DestroyInitializer(Constant *C) {
+  // Cannot destroy constants still being used, and cannot destroy primitive
+  // types.
+  if (!C->use_empty() || C->getType()->isPrimitiveType()) return;
+
+  // If this is a CPR, the global value referred to may be dead now!  Add it to
+  // the worklist.
+  //
+  if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(C)) {
+    WorkList.push_back(CPR->getValue());
+    C->destroyConstant();
+    ++NumCPRs;
+  } else {
+    bool DestroyContents = true;
+
+    // As an optimization to the GlobalDCE algorithm, do attempt to destroy the
+    // contents of an array of primitive types, because we know that this will
+    // never succeed, and there could be a lot of them.
+    //
+    if (ConstantArray *CA = dyn_cast<ConstantArray>(C))
+      if (CA->getType()->getElementType()->isPrimitiveType())
+        DestroyContents = false;    // Nothing we can do with the subcontents
+
+    // All other constants refer to other constants.  Destroy them if possible
+    // as well.
+    //
+    std::vector<Value*> SubConstants;
+    if (DestroyContents) SubConstants.insert(SubConstants.end(),
+                                             C->op_begin(), C->op_end());
+
+    // Destroy the actual constant...
+    C->destroyConstant();
+    ++NumConsts;
+
+    if (DestroyContents) {
+      // Remove duplicates from SubConstants, so that we do not call
+      // DestroyInitializer on the same constant twice (the first call might
+      // delete it, so this would be bad)
+      //
+      std::sort(SubConstants.begin(), SubConstants.end());
+      SubConstants.erase(std::unique(SubConstants.begin(), SubConstants.end()),
+                         SubConstants.end());
+
+      // Loop over the subconstants, destroying them as well.
+      for (unsigned i = 0, e = SubConstants.size(); i != e; ++i)
+        DestroyInitializer(cast<Constant>(SubConstants[i]));
+    }
+  }
+}
+
+bool GlobalDCE::RemoveUnreachableGlobalVariables(Module &M) {
+  bool Changed = false;
+  WorkList.reserve(M.gsize());
+
+  // Insert all of the globals into the WorkList, making sure to run
+  // RemoveUnusedConstantPointerRef at least once on all globals...
+  //
+  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
+    Changed |= RemoveUnusedConstantPointerRef(*I);
+    WorkList.push_back(I);
+  }
+  for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I) {
+    Changed |= RemoveUnusedConstantPointerRef(*I);
+    WorkList.push_back(I);
+  }
+
+  // Loop over the worklist, deleting global objects that we can.  Whenever we
+  // delete something that might make something else dead, it gets added to the
+  // worklist.
+  //
+  while (!WorkList.empty()) {
+    GlobalValue *GV = WorkList.back();
+    WorkList.pop_back();
+
+    Changed |= RemoveIfDead(GV);
+  }
+
+  // Make sure that all memory is free'd from the worklist...
+  std::vector<GlobalValue*>().swap(WorkList);
+  return Changed;
+}
+
+
+// RemoveUnusedConstantPointerRef - Loop over all of the uses of the specified
+// GlobalValue, looking for the constant pointer ref that may be pointing to it.
+// If found, check to see if the constant pointer ref is safe to destroy, and if
+// so, nuke it.  This will reduce the reference count on the global value, which
+// might make it deader.
+//
+bool GlobalDCE::RemoveUnusedConstantPointerRef(GlobalValue &GV) {
+  for (Value::use_iterator I = GV.use_begin(), E = GV.use_end(); I != E; ++I)
+    if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(*I))
+      if (SafeToDestroyConstant(CPR)) {  // Only if unreferenced...
+        CPR->destroyConstant();
+        ++NumCPRs;
+        return true;
+      }
+
+  return false;
+}
+
+// SafeToDestroyConstant - It is safe to destroy a constant iff it is only used
+// by constants itself.  Note that constants cannot be cyclic, so this test is
+// pretty easy to implement recursively.
+//
+bool GlobalDCE::SafeToDestroyConstant(Constant *C) {
+  for (Value::use_iterator I = C->use_begin(), E = C->use_end(); I != E; ++I)
+    if (Constant *User = dyn_cast<Constant>(*I)) {
+      if (!SafeToDestroyConstant(User)) return false;
+    } else {
+      return false;
+    }
+
+  return true;
+}