Initial checkin of a simple mod/ref analysis for global variables. This is

still overly conservative and uses very simple data structures, but it is a
start, and allows elimination of a lot of loads.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@14462 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/IPA/GlobalsModRef.cpp

@@ -0,0 +1,327 @@
+//===- GlobalsModRef.cpp - Simple Mod/Ref Analysis for Globals ------------===//
+// 
+//                     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 simple pass provides alias and mod/ref information for global values
+// that do not have their address taken.  For this simple (but very common)
+// case, we can provide pretty accurate and useful information.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "globalsmodref"
+#include "llvm/Analysis/Passes.h"
+#include "llvm/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Instructions.h"
+#include "llvm/Constants.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/CallGraph.h"
+#include "Support/Debug.h"
+#include "Support/Statistic.h"
+#include "Support/SCCIterator.h"
+#include <set>
+using namespace llvm;
+
+namespace {
+  Statistic<>
+  NumNonAddrTakenGlobalVars("globalsmodref-aa",
+                            "Number of global vars without address taken");
+  Statistic<>
+  NumNonAddrTakenFunctions("globalsmodref-aa",
+                           "Number of functions without address taken");
+
+  class GlobalsModRef : public Pass, public AliasAnalysis {
+    /// ModRefFns - One instance of this record is kept for each global without
+    /// its address taken.
+    struct ModRefFns {
+      /// RefFns/ModFns - Sets of functions that and write globals.
+      std::set<Function*> RefFns, ModFns;
+    };
+
+    /// NonAddressTakenGlobals - A map of globals that do not have their
+    /// addresses taken to their record.
+    std::map<GlobalValue*, ModRefFns> NonAddressTakenGlobals;
+
+    /// FunctionInfo - For each function, keep track of what globals are
+    /// modified or read.
+    std::map<std::pair<Function*, GlobalValue*>, unsigned> FunctionInfo;
+
+  public:
+    bool run(Module &M) {
+      InitializeAliasAnalysis(this);                 // set up super class
+      AnalyzeGlobals(M);                          // find non-addr taken globals
+      AnalyzeCallGraph(getAnalysis<CallGraph>(), M); // Propagate on CG
+      return false;
+    }
+
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AliasAnalysis::getAnalysisUsage(AU);
+      AU.addRequired<CallGraph>();
+      AU.setPreservesAll();                         // Does not transform code
+    }
+
+    //------------------------------------------------
+    // Implement the AliasAnalysis API
+    //  
+    AliasResult alias(const Value *V1, unsigned V1Size,
+                      const Value *V2, unsigned V2Size);
+    ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
+    bool hasNoModRefInfoForCalls() const { return false; }
+
+    virtual void deleteValue(Value *V);
+    virtual void copyValue(Value *From, Value *To);
+
+  private:
+    void AnalyzeGlobals(Module &M);
+    void AnalyzeCallGraph(CallGraph &CG, Module &M);
+    bool AnalyzeUsesOfGlobal(Value *V, std::vector<Function*> &Readers,
+                             std::vector<Function*> &Writers);
+  };
+  
+  RegisterOpt<GlobalsModRef> X("globalsmodref-aa",
+                               "Simple mod/ref analysis for globals");
+  RegisterAnalysisGroup<AliasAnalysis, GlobalsModRef> Y;
+}
+
+Pass *llvm::createGlobalsModRefPass() { return new GlobalsModRef(); }
+
+
+/// AnalyzeGlobalUses - Scan through the users of all of the internal
+/// GlobalValue's in the program.  If none of them have their "Address taken"
+/// (really, their address passed to something nontrivial), record this fact,
+/// and record the functions that they are used directly in.
+void GlobalsModRef::AnalyzeGlobals(Module &M) {
+  std::vector<Function*> Readers, Writers;
+  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
+    if (I->hasInternalLinkage()) {
+      if (!AnalyzeUsesOfGlobal(I, Readers, Writers)) {
+        // Remember that we are tracking this global, and the mod/ref fns
+        ModRefFns &E = NonAddressTakenGlobals[I];
+        E.RefFns.insert(Readers.begin(), Readers.end());
+        E.ModFns.insert(Writers.begin(), Writers.end());
+        ++NumNonAddrTakenFunctions;
+      }
+      Readers.clear(); Writers.clear();
+    }
+
+  for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I)
+    // FIXME: it is kinda dumb to track aliasing properties for constant
+    // globals, it will never be particularly useful anyways, 'cause they can
+    // never be modified (and the optimizer knows this already)!
+    if (I->hasInternalLinkage()) {
+      if (!AnalyzeUsesOfGlobal(I, Readers, Writers)) {
+        // Remember that we are tracking this global, and the mod/ref fns
+        ModRefFns &E = NonAddressTakenGlobals[I];
+        E.RefFns.insert(Readers.begin(), Readers.end());
+        E.ModFns.insert(Writers.begin(), Writers.end());
+        ++NumNonAddrTakenGlobalVars;
+      }
+      Readers.clear(); Writers.clear();
+    }
+}
+
+/// AnalyzeUsesOfGlobal - Look at all of the users of the specified global value
+/// derived pointer.  If this is used by anything complex (i.e., the address
+/// escapes), return true.  Also, while we are at it, keep track of those
+/// functions that read and write to the value.
+bool GlobalsModRef::AnalyzeUsesOfGlobal(Value *V,
+                                        std::vector<Function*> &Readers,
+                                        std::vector<Function*> &Writers) {
+  //if (!isa<PointerType>(V->getType())) return true;
+
+  for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
+    if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
+      Readers.push_back(LI->getParent()->getParent());
+    } else if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
+      if (V == SI->getOperand(0)) return true;  // Storing the pointer
+      Writers.push_back(SI->getParent()->getParent());
+    } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
+      if (AnalyzeUsesOfGlobal(GEP, Readers, Writers)) return true;
+    } else if (CallInst *CI = dyn_cast<CallInst>(*UI)) {
+      // Make sure that this is just the function being called, not that it is
+      // passing into the function.
+      for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
+        if (CI->getOperand(i) == V) return true;
+    } else if (CallInst *CI = dyn_cast<CallInst>(*UI)) {
+      // Make sure that this is just the function being called, not that it is
+      // passing into the function.
+      for (unsigned i = 1, e = CI->getNumOperands(); i != e; ++i)
+        if (CI->getOperand(i) == V) return true;
+    } else if (InvokeInst *II = dyn_cast<InvokeInst>(*UI)) {
+      // Make sure that this is just the function being called, not that it is
+      // passing into the function.
+      for (unsigned i = 3, e = II->getNumOperands(); i != e; ++i)
+        if (II->getOperand(i) == V) return true;
+    } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(*UI)) {
+      if (CE->getOpcode() == Instruction::GetElementPtr ||
+          CE->getOpcode() == Instruction::Cast) {
+        if (AnalyzeUsesOfGlobal(CE, Readers, Writers))
+          return true;
+      } else {
+        return true;
+      }        
+    } else if (ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(*UI)) {
+      if (AnalyzeUsesOfGlobal(CPR, Readers, Writers)) return true;
+    } else {
+      return true;
+    }
+  return false;
+}
+
+/// AnalyzeCallGraph - At this point, we know the functions where globals are
+/// immediately stored to and read from.  Propagate this information up the call
+/// graph to all callers.
+void GlobalsModRef::AnalyzeCallGraph(CallGraph &CG, Module &M) {
+  if (NonAddressTakenGlobals.empty()) return;  // Don't bother, nothing to do.
+
+  // Invert the NonAddressTakenGlobals map into the FunctionInfo map.
+  for (std::map<GlobalValue*, ModRefFns>::iterator I = 
+         NonAddressTakenGlobals.begin(), E = NonAddressTakenGlobals.end();
+       I != E; ++I) {
+    GlobalValue *GV = I->first;
+    ModRefFns &MRInfo = I->second;
+    for (std::set<Function*>::iterator I = MRInfo.RefFns.begin(), 
+           E = MRInfo.RefFns.begin(); I != E; ++I)
+      FunctionInfo[std::make_pair(*I, GV)] |= Ref;
+    MRInfo.RefFns.clear();
+    for (std::set<Function*>::iterator I = MRInfo.ModFns.begin(), 
+           E = MRInfo.ModFns.begin(); I != E; ++I)
+      FunctionInfo[std::make_pair(*I, GV)] |= Mod;
+    MRInfo.ModFns.clear();
+  }
+
+  // We do a bottom-up SCC traversal of the call graph.  In other words, we
+  // visit all callees before callers (leaf-first).
+  for (scc_iterator<CallGraph*> I = scc_begin(&CG), E = scc_end(&CG);
+       I != E; ++I) {
+    std::map<GlobalValue*, unsigned> ModRefProperties;
+    const std::vector<CallGraphNode *> &SCC = *I;
+
+    // Collect the mod/ref properties due to called functions.
+    for (unsigned i = 0, e = SCC.size(); i != e; ++i)
+      for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
+           CI != E; ++CI) {
+        if (Function *Callee = (*CI)->getFunction()) {
+          // Otherwise, combine the callee properties into our accumulated set.
+          std::map<std::pair<Function*, GlobalValue*>, unsigned>::iterator
+            CI = FunctionInfo.lower_bound(std::make_pair(Callee,
+                                                         (GlobalValue*)0));
+          for (;CI != FunctionInfo.end() && CI->first.first == Callee; ++CI)
+            ModRefProperties[CI->first.second] |= CI->second;
+        } else {
+          // For now assume that external functions could mod/ref anything,
+          // since they could call into an escaping function that mod/refs an
+          // internal.  FIXME: We need better tracking!
+          for (std::map<GlobalValue*, ModRefFns>::iterator GI = 
+                 NonAddressTakenGlobals.begin(),
+                 E = NonAddressTakenGlobals.end(); GI != E; ++GI)
+            ModRefProperties[GI->first] = ModRef;
+          goto Out;
+        }
+      }
+  Out:
+    // Set all functions in the CFG to have these properties.  FIXME: it would
+    // be better to use union find to only store these properties once,
+    // PARTICULARLY if it's the universal set.
+    for (unsigned i = 0, e = SCC.size(); i != e; ++i)
+      if (Function *F = SCC[i]->getFunction()) {
+        for (std::map<GlobalValue*, unsigned>::iterator I =
+               ModRefProperties.begin(), E = ModRefProperties.end();
+             I != E; ++I)
+          FunctionInfo[std::make_pair(F, I->first)] = I->second;
+      }
+  }
+}
+
+
+
+/// getUnderlyingObject - This traverses the use chain to figure out what object
+/// the specified value points to.  If the value points to, or is derived from,
+/// a global object, return it.
+static const GlobalValue *getUnderlyingObject(const Value *V) {
+  //if (!isa<PointerType>(V->getType())) return 0;
+
+  // If we are at some type of object... return it.
+  if (const GlobalValue *GV = dyn_cast<GlobalValue>(V)) return GV;
+  
+  // Traverse through different addressing mechanisms...
+  if (const Instruction *I = dyn_cast<Instruction>(V)) {
+    if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
+      return getUnderlyingObject(I->getOperand(0));
+  } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
+    if (CE->getOpcode() == Instruction::Cast ||
+        CE->getOpcode() == Instruction::GetElementPtr)
+      return getUnderlyingObject(CE->getOperand(0));
+  } else if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V)) {
+    return CPR->getValue();
+  }
+  return 0;
+}
+
+/// alias - If one of the pointers is to a global that we are tracking, and the
+/// other is some random pointer, we know there cannot be an alias, because the
+/// address of the global isn't taken.
+AliasAnalysis::AliasResult
+GlobalsModRef::alias(const Value *V1, unsigned V1Size,
+                     const Value *V2, unsigned V2Size) {
+  GlobalValue *GV1 = const_cast<GlobalValue*>(getUnderlyingObject(V1));
+  GlobalValue *GV2 = const_cast<GlobalValue*>(getUnderlyingObject(V2));
+
+  // If the global's address is taken, pretend we don't know it's a pointer to
+  // the global.
+  if (GV1 && !NonAddressTakenGlobals.count(GV1)) GV1 = 0;
+  if (GV2 && !NonAddressTakenGlobals.count(GV2)) GV2 = 0;
+
+  if ((GV1 || GV2) && GV1 != GV2)
+    return NoAlias;
+
+  return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
+}
+
+AliasAnalysis::ModRefResult
+GlobalsModRef::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
+  unsigned Known = ModRef;
+
+  // If we are asking for mod/ref info of a direct call with a pointer to a
+  // global, return information if we have it.
+  if (GlobalValue *GV = const_cast<GlobalValue*>(getUnderlyingObject(P)))
+    if (GV->hasInternalLinkage())
+      if (Function *F = CS.getCalledFunction()) {
+        std::map<std::pair<Function*, GlobalValue*>, unsigned>::iterator
+          it = FunctionInfo.find(std::make_pair(F, GV));
+        if (it != FunctionInfo.end())
+          Known = it->second;
+      }
+
+  if (Known == NoModRef)
+    return NoModRef; // No need to query other mod/ref analyses
+  return ModRefResult(Known & AliasAnalysis::getModRefInfo(CS, P, Size));
+}
+
+
+//===----------------------------------------------------------------------===//
+// Methods to update the analysis as a result of the client transformation.
+//
+void GlobalsModRef::deleteValue(Value *V) {
+  if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
+    std::map<GlobalValue*, ModRefFns>::iterator I =
+      NonAddressTakenGlobals.find(GV);
+    if (I != NonAddressTakenGlobals.end())
+      NonAddressTakenGlobals.erase(I);
+  }
+}
+
+void GlobalsModRef::copyValue(Value *From, Value *To) {
+  if (GlobalValue *FromGV = dyn_cast<GlobalValue>(From))
+    if (GlobalValue *ToGV = dyn_cast<GlobalValue>(To)) {
+      std::map<GlobalValue*, ModRefFns>::iterator I =
+        NonAddressTakenGlobals.find(FromGV);
+      if (I != NonAddressTakenGlobals.end())
+        NonAddressTakenGlobals[ToGV] = I->second;
+    }
+}