1 files changed, 82 insertions, 0 deletions
diff --git a/lib/Transforms/IPO/GlobalOpt.cpp b/lib/Transforms/IPO/GlobalOpt.cpp
new file mode 100644
index 0000000000..8f6cbbdfda
--- /dev/null
+++ b/lib/Transforms/IPO/GlobalOpt.cpp
@@ -0,0 +1,82 @@
+//===- GlobalConstifier.cpp - Mark read-only globals constant -------------===//
+// 
+//                     The LLVM Compiler Infrastructure
+//
+// This file was developed by the LLVM research group and is distributed under
+// the University of Illinois Open Source License. See LICENSE.TXT for details.
+// 
+//===----------------------------------------------------------------------===//
+//
+// This pass loops over the non-constant internal global variables in the
+// program.  If it can prove that they are never written to, it marks them
+// constant.
+//
+// NOTE: this should eventually use the alias analysis interfaces to do the
+// transformation, but for now we just stick with a simple solution. DSA in
+// particular could give a much more accurate answer to the mod/ref query, but
+// it's not quite ready for this.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO.h"
+#include "llvm/Constants.h"
+#include "llvm/iMemory.h"
+#include "llvm/Module.h"
+#include "llvm/Pass.h"
+#include "Support/Debug.h"
+#include "Support/Statistic.h"
+using namespace llvm;
+
+namespace {
+  Statistic<> NumMarked("constify", "Number of globals marked constant");
+
+  struct Constifier : public Pass {
+    bool run(Module &M);
+  };
+
+  RegisterOpt<Constifier> X("constify", "Global Constifier");
+}
+
+Pass *llvm::createGlobalConstifierPass() { return new Constifier(); }
+
+/// isStoredThrough - Return false if the specified pointer is provably never
+/// stored through.  If we can't tell, we must conservatively assume it might.
+///
+static bool isStoredThrough(Value *V) {
+  for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
+    if (Constant *C = dyn_cast<Constant>(*UI)) {
+      if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
+        if (isStoredThrough(CE))
+          return true;
+      } else if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(C)) {
+        if (isStoredThrough(CPR)) return true;
+      } else {
+        // Must be an element of a constant array or something.
+        return true;
+      }
+    } else if (Instruction *I = dyn_cast<Instruction>(*UI)) {
+      if (I->getOpcode() == Instruction::GetElementPtr) {
+        if (isStoredThrough(I)) return true;
+      } else if (!isa<LoadInst>(*UI))
+        return true;  // Any other non-load instruction might store!
+    } else {
+      // Otherwise must be a global or some other user.
+      return true;
+    }
+
+  return false;
+}
+
+bool Constifier::run(Module &M) {
+  bool Changed = false;
+  for (Module::giterator GV = M.gbegin(), E = M.gend(); GV != E; ++GV)
+    if (!GV->isConstant() && GV->hasInternalLinkage() && GV->hasInitializer()) {
+      if (!isStoredThrough(GV)) {
+        DEBUG(std::cerr << "MARKING CONSTANT: " << *GV << "\n");
+        GV->setConstant(true);
+        ++NumMarked;
+        Changed = true;
+      }
+    }
+  return Changed;
+}