1 files changed, 227 insertions, 0 deletions
diff --git a/lib/Transforms/IPO/GlobalDCE.cpp b/lib/Transforms/IPO/GlobalDCE.cpp
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+++ b/lib/Transforms/IPO/GlobalDCE.cpp
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+//===-- GlobalDCE.cpp - DCE unreachable internal functions ----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This transform is designed to eliminate unreachable internal globals from the
+// program.  It uses an aggressive algorithm, searching out globals that are
+// known to be alive.  After it finds all of the globals which are needed, it
+// deletes whatever is left over.  This allows it to delete recursive chunks of
+// the program which are unreachable.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "globaldce"
+#include "llvm/Transforms/IPO.h"
+#include "llvm/Constants.h"
+#include "llvm/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Support/Compiler.h"
+#include <set>
+using namespace llvm;
+
+STATISTIC(NumAliases  , "Number of global aliases removed");
+STATISTIC(NumFunctions, "Number of functions removed");
+STATISTIC(NumVariables, "Number of global variables removed");
+
+namespace {
+  struct VISIBILITY_HIDDEN GlobalDCE : public ModulePass {
+    static char ID; // Pass identification, replacement for typeid
+    GlobalDCE() : ModulePass(&ID) {}
+
+    // run - Do the GlobalDCE pass on the specified module, optionally updating
+    // the specified callgraph to reflect the changes.
+    //
+    bool runOnModule(Module &M);
+
+  private:
+    std::set<GlobalValue*> AliveGlobals;
+
+    /// GlobalIsNeeded - mark the specific global value as needed, and
+    /// recursively mark anything that it uses as also needed.
+    void GlobalIsNeeded(GlobalValue *GV);
+    void MarkUsedGlobalsAsNeeded(Constant *C);
+
+    bool SafeToDestroyConstant(Constant* C);
+    bool RemoveUnusedGlobalValue(GlobalValue &GV);
+  };
+}
+
+char GlobalDCE::ID = 0;
+static RegisterPass<GlobalDCE> X("globaldce", "Dead Global Elimination");
+
+ModulePass *llvm::createGlobalDCEPass() { return new GlobalDCE(); }
+
+bool GlobalDCE::runOnModule(Module &M) {
+  bool Changed = false;
+  // Loop over the module, adding globals which are obviously necessary.
+  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
+    Changed |= RemoveUnusedGlobalValue(*I);
+    // Functions with external linkage are needed if they have a body
+    if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
+        !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
+      GlobalIsNeeded(I);
+  }
+
+  for (Module::global_iterator I = M.global_begin(), E = M.global_end();
+       I != E; ++I) {
+    Changed |= RemoveUnusedGlobalValue(*I);
+    // Externally visible & appending globals are needed, if they have an
+    // initializer.
+    if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage() &&
+        !I->isDeclaration() && !I->hasAvailableExternallyLinkage())
+      GlobalIsNeeded(I);
+  }
+
+  for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end();
+       I != E; ++I) {
+    Changed |= RemoveUnusedGlobalValue(*I);
+    // Externally visible aliases are needed.
+    if (!I->hasLocalLinkage() && !I->hasLinkOnceLinkage())
+      GlobalIsNeeded(I);
+  }
+
+  // Now that all globals which are needed are in the AliveGlobals set, we loop
+  // through the program, deleting those which are not alive.
+  //
+
+  // The first pass is to drop initializers of global variables which are dead.
+  std::vector<GlobalVariable*> DeadGlobalVars;   // Keep track of dead globals
+  for (Module::global_iterator I = M.global_begin(), E = M.global_end(); I != E; ++I)
+    if (!AliveGlobals.count(I)) {
+      DeadGlobalVars.push_back(I);         // Keep track of dead globals
+      I->setInitializer(0);
+    }
+
+  // The second pass drops the bodies of functions which are dead...
+  std::vector<Function*> DeadFunctions;
+  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
+    if (!AliveGlobals.count(I)) {
+      DeadFunctions.push_back(I);         // Keep track of dead globals
+      if (!I->isDeclaration())
+        I->deleteBody();
+    }
+
+  // The third pass drops targets of aliases which are dead...
+  std::vector<GlobalAlias*> DeadAliases;
+  for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
+       ++I)
+    if (!AliveGlobals.count(I)) {
+      DeadAliases.push_back(I);
+      I->setAliasee(0);
+    }
+
+  if (!DeadFunctions.empty()) {
+    // Now that all interferences have been dropped, delete the actual objects
+    // themselves.
+    for (unsigned i = 0, e = DeadFunctions.size(); i != e; ++i) {
+      RemoveUnusedGlobalValue(*DeadFunctions[i]);
+      M.getFunctionList().erase(DeadFunctions[i]);
+    }
+    NumFunctions += DeadFunctions.size();
+    Changed = true;
+  }
+
+  if (!DeadGlobalVars.empty()) {
+    for (unsigned i = 0, e = DeadGlobalVars.size(); i != e; ++i) {
+      RemoveUnusedGlobalValue(*DeadGlobalVars[i]);
+      M.getGlobalList().erase(DeadGlobalVars[i]);
+    }
+    NumVariables += DeadGlobalVars.size();
+    Changed = true;
+  }
+
+  // Now delete any dead aliases.
+  if (!DeadAliases.empty()) {
+    for (unsigned i = 0, e = DeadAliases.size(); i != e; ++i) {
+      RemoveUnusedGlobalValue(*DeadAliases[i]);
+      M.getAliasList().erase(DeadAliases[i]);
+    }
+    NumAliases += DeadAliases.size();
+    Changed = true;
+  }
+
+  // Make sure that all memory is released
+  AliveGlobals.clear();
+  return Changed;
+}
+
+/// GlobalIsNeeded - the specific global value as needed, and
+/// recursively mark anything that it uses as also needed.
+void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
+  std::set<GlobalValue*>::iterator I = AliveGlobals.find(G);
+
+  // If the global is already in the set, no need to reprocess it.
+  if (I != AliveGlobals.end()) return;
+
+  // Otherwise insert it now, so we do not infinitely recurse
+  AliveGlobals.insert(I, G);
+
+  if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
+    // If this is a global variable, we must make sure to add any global values
+    // referenced by the initializer to the alive set.
+    if (GV->hasInitializer())
+      MarkUsedGlobalsAsNeeded(GV->getInitializer());
+  } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
+    // The target of a global alias is needed.
+    MarkUsedGlobalsAsNeeded(GA->getAliasee());
+  } else {
+    // Otherwise this must be a function object.  We have to scan the body of
+    // the function looking for constants and global values which are used as
+    // operands.  Any operands of these types must be processed to ensure that
+    // any globals used will be marked as needed.
+    Function *F = cast<Function>(G);
+    // For all basic blocks...
+    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
+      // For all instructions...
+      for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
+        // For all operands...
+        for (User::op_iterator U = I->op_begin(), E = I->op_end(); U != E; ++U)
+          if (GlobalValue *GV = dyn_cast<GlobalValue>(*U))
+            GlobalIsNeeded(GV);
+          else if (Constant *C = dyn_cast<Constant>(*U))
+            MarkUsedGlobalsAsNeeded(C);
+  }
+}
+
+void GlobalDCE::MarkUsedGlobalsAsNeeded(Constant *C) {
+  if (GlobalValue *GV = dyn_cast<GlobalValue>(C))
+    GlobalIsNeeded(GV);
+  else {
+    // Loop over all of the operands of the constant, adding any globals they
+    // use to the list of needed globals.
+    for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E; ++I)
+      MarkUsedGlobalsAsNeeded(cast<Constant>(*I));
+  }
+}
+
+// RemoveUnusedGlobalValue - 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::RemoveUnusedGlobalValue(GlobalValue &GV) {
+  if (GV.use_empty()) return false;
+  GV.removeDeadConstantUsers();
+  return GV.use_empty();
+}
+
+// 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;
+}