New pass to perform DSA based optimizations. Initially we just support turning

globals into constants if we can prove it's safe


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@6941 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/DataStructure/DataStructureOpt.cpp

@@ -0,0 +1,76 @@
+//===- DataStructureOpt.cpp - Data Structure Analysis Based Optimizations -===//
+//
+// This pass uses DSA to a series of simple optimizations, like marking
+// unwritten global variables 'constant'.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/DataStructure.h"
+#include "llvm/Analysis/DSGraph.h"
+#include "llvm/Module.h"
+#include "llvm/Constant.h"
+#include "Support/Statistic.h"
+
+namespace {
+  Statistic<>
+  NumGlobalsConstanted("ds-opt", "Number of globals marked constant");
+
+  class DSOpt : public Pass {
+    TDDataStructures *TD;
+  public:
+    bool run(Module &M) {
+      TD = &getAnalysis<TDDataStructures>();
+      bool Changed = OptimizeGlobals(M);
+      return Changed;
+    }
+
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.addRequired<TDDataStructures>();      // Uses TD Datastructures
+      AU.addPreserved<LocalDataStructures>();  // Preserves local...
+      AU.addPreserved<TDDataStructures>();     // Preserves bu...
+      AU.addPreserved<BUDataStructures>();     // Preserves td...
+    }
+
+  private:
+    bool OptimizeGlobals(Module &M);
+  };
+
+  RegisterOpt<DSOpt> X("ds-opt", "DSA-based simple optimizations");
+}
+
+
+/// OptimizeGlobals - This method uses information taken from DSA to optimize
+/// global variables.
+///
+bool DSOpt::OptimizeGlobals(Module &M) {
+  DSGraph &GG = TD->getGlobalsGraph();
+  const DSGraph::ScalarMapTy &SM = GG.getScalarMap();
+  bool Changed = false;
+
+  for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I)
+    if (!I->isExternal()) { // Loop over all of the non-external globals...
+      // Look up the node corresponding to this global, if it exists.
+      DSNode *GNode = 0;
+      DSGraph::ScalarMapTy::const_iterator SMI = SM.find(I);
+      if (SMI != SM.end()) GNode = SMI->second.getNode();
+    
+      if (GNode == 0 && I->hasInternalLinkage()) {
+        // If there is no entry in the scalar map for this global, it was never
+        // referenced in the program.  If it has internal linkage, that means we
+        // can delete it.  We don't ACTUALLY want to delete the global, just
+        // remove anything that references the global: later passes will take
+        // care of nuking it.
+        I->replaceAllUsesWith(Constant::getNullValue((Type*)I->getType()));
+      } else if (GNode && GNode->isComplete()) {
+        // We expect that there will almost always be a node for this global.
+        // If there is, and the node doesn't have the M bit set, we can set the
+        // 'constant' bit on the global.
+        if (!GNode->isModified() && !I->isConstant()) {
+          I->setConstant(true);
+          ++NumGlobalsConstanted;
+          Changed = true;
+        }
+      }
+    }
+  return Changed;
+}