New LLVM pass: argument promotion. This version only handles simple scalar

variables.

llvm-svn: 12193

llvm/lib/Transforms/IPO/ArgumentPromotion.cpp

+//===-- ArgumentPromotion.cpp - Promote 'by reference' arguments ----------===//
+// 
+//                     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 promotes "by reference" arguments to be "by value" arguments.  In
+// practice, this means looking for internal functions that have pointer
+// arguments.  If we can prove, through the use of alias analysis, that that an
+// argument is *only* loaded, then we can pass the value into the function
+// instead of the address of the value.  This can cause recursive simplification
+// of code, and lead to the elimination of allocas, especially in C++ template
+// code like the STL.
+//
+// Note that this transformation could also be done for arguments that are only
+// stored to (returning the value instead), but we do not currently handle that
+// case.
+//
+// Note that we should be able to promote pointers to structures that are only
+// loaded from as well.  The danger is creating way to many arguments, so this
+// transformation should be limited to 3 element structs or something.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO.h"
+#include "llvm/Constants.h"
+#include "llvm/DerivedTypes.h"
+#include "llvm/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Instructions.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/Support/CallSite.h"
+#include "llvm/Support/CFG.h"
+#include "Support/Debug.h"
+#include "Support/DepthFirstIterator.h"
+#include "Support/Statistic.h"
+#include <set>
+using namespace llvm;
+
+namespace {
+  Statistic<> NumArgumentsPromoted("argpromotion",
+                                   "Number of pointer arguments promoted");
+  Statistic<> NumArgumentsDead("argpromotion",
+                               "Number of dead pointer args eliminated");
+
+  /// ArgPromotion - The 'by reference' to 'by value' argument promotion pass.
+  ///
+  class ArgPromotion : public Pass {
+    // WorkList - The set of internal functions that we have yet to process.  As
+    // we eliminate arguments from a function, we push all callers into this set
+    // so that the by reference argument can be bubbled out as far as possible.
+    // This set contains only internal functions.
+    std::set<Function*> WorkList;
+  public:
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.addRequired<AliasAnalysis>();
+      AU.addRequired<TargetData>();
+    }
+
+    virtual bool run(Module &M);
+  private:
+    bool PromoteArguments(Function *F);
+    bool isSafeToPromoteArgument(Argument *Arg) const;  
+    void DoPromotion(Function *F, std::vector<Argument*> &ArgsToPromote);
+  };
+
+  RegisterOpt<ArgPromotion> X("argpromotion",
+                              "Promote 'by reference' arguments to scalars");
+}
+
+Pass *llvm::createArgumentPromotionPass() {
+  return new ArgPromotion();
+}
+
+bool ArgPromotion::run(Module &M) {
+  bool Changed = false;
+  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
+    if (I->hasInternalLinkage()) {
+      WorkList.insert(I);
+
+      // If there are any constant pointer refs pointing to this function,
+      // eliminate them now if possible.
+      ConstantPointerRef *CPR = 0;
+      for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
+           ++UI)
+        if ((CPR = dyn_cast<ConstantPointerRef>(*UI)))
+          break;  // Found one!
+      if (CPR) {
+        // See if we can transform all users to use the function directly.
+        while (!CPR->use_empty()) {
+          User *TheUser = CPR->use_back();
+          if (!isa<Constant>(TheUser)) {
+            Changed = true;
+            TheUser->replaceUsesOfWith(CPR, I);
+          } else {
+            // We won't be able to eliminate all users.  :(
+            WorkList.erase(I);  // Minor efficiency win.
+            break;
+          }
+        }
+
+        // If we nuked all users of the CPR, kill the CPR now!
+        if (CPR->use_empty()) {
+          CPR->destroyConstant();
+          Changed = true;
+        }
+      }
+    }
+  
+  while (!WorkList.empty()) {
+    Function *F = *WorkList.begin();
+    WorkList.erase(WorkList.begin());
+
+    if (PromoteArguments(F))    // Attempt to promote an argument.
+      Changed = true;           // Remember that we changed something.
+  }
+  
+  return Changed;
+}
+
+
+bool ArgPromotion::PromoteArguments(Function *F) {
+  assert(F->hasInternalLinkage() && "We can only process internal functions!");
+
+  // First check: see if there are any pointer arguments!  If not, quick exit.
+  std::vector<Argument*> PointerArgs;
+  for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
+    if (isa<PointerType>(I->getType()))
+      PointerArgs.push_back(I);
+  if (PointerArgs.empty()) return false;
+
+  // Second check: make sure that all callers are direct callers.  We can't
+  // transform functions that have indirect callers.
+  for (Value::use_iterator UI = F->use_begin(), E = F->use_end();
+       UI != E; ++UI)
+    // What about CPRs?
+    if (!CallSite::get(*UI).getInstruction())
+      return false;  // Cannot promote an indirect call!
+
+  // Check to see which arguments are promotable.  If an argument is not
+  // promotable, remove it from the PointerArgs vector.
+  for (unsigned i = 0; i != PointerArgs.size(); ++i)
+    if (!isSafeToPromoteArgument(PointerArgs[i])) {
+      std::swap(PointerArgs[i--], PointerArgs.back());
+      PointerArgs.pop_back();
+    }
+
+  // No promotable pointer arguments.
+  if (PointerArgs.empty()) return false;
+
+  // Okay, promote all of the arguments are rewrite the callees!
+  DoPromotion(F, PointerArgs);
+  return true;
+}
+
+bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const {
+  // We can only promote this argument if all of the uses are loads...
+  std::vector<LoadInst*> Loads;
+  for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end();
+       UI != E; ++UI)
+    if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
+      if (LI->isVolatile()) return false;  // Don't hack volatile loads
+      Loads.push_back(LI);
+    } else
+      return false;
+
+  if (Loads.empty()) return true;  // No users, dead argument.
+
+  const Type *LoadTy = cast<PointerType>(Arg->getType())->getElementType();
+  unsigned LoadSize = getAnalysis<TargetData>().getTypeSize(LoadTy);
+
+  // Okay, now we know that the argument is only used by load instructions.
+  // Check to see if the pointer is guaranteed to not be modified from entry of
+  // the function to each of the load instructions.
+  Function &F = *Arg->getParent();
+
+  // Because there could be several/many load instructions, remember which
+  // blocks we know to be transparent to the load.
+  std::set<BasicBlock*> TranspBlocks;
+
+  AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
+
+  for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
+    // Check to see if the load is invalidated from the start of the block to
+    // the load itself.
+    LoadInst *Load = Loads[i];
+    BasicBlock *BB = Load->getParent();
+    if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
+      return false;  // Pointer is invalidated!
+
+    // Now check every path from the entry block to the load for transparency.
+    // To do this, we perform a depth first search on the inverse CFG from the
+    // loading block.
+    for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
+      for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks),
+             E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
+        if (AA.canBasicBlockModify(**I, Arg, LoadSize))
+          return false;
+  }
+
+  // If the path from the entry of the function to each load is free of
+  // instructions that potentially invalidate the load, we can make the
+  // transformation!
+  return true;
+}
+
+
+void ArgPromotion::DoPromotion(Function *F, std::vector<Argument*> &Args2Prom) {
+  std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end());
+  
+  // Start by computing a new prototype for the function, which is the same as
+  // the old function, but has modified arguments.
+  const FunctionType *FTy = F->getFunctionType();
+  std::vector<const Type*> Params;
+
+  for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I)
+    if (!ArgsToPromote.count(I)) {
+      Params.push_back(I->getType());
+    } else if (!I->use_empty()) {
+      Params.push_back(cast<PointerType>(I->getType())->getElementType());
+      ++NumArgumentsPromoted;
+    } else {
+      ++NumArgumentsDead;
+    }
+
+  const Type *RetTy = FTy->getReturnType();
+
+  // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
+  // have zero fixed arguments.
+  bool ExtraArgHack = false;
+  if (Params.empty() && FTy->isVarArg()) {
+    ExtraArgHack = true;
+    Params.push_back(Type::IntTy);
+  }
+  FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
+  
+   // Create the new function body and insert it into the module...
+  Function *NF = new Function(NFTy, F->getLinkage(), F->getName());
+  F->getParent()->getFunctionList().insert(F, NF);
+  
+  // Loop over all of the callers of the function, transforming the call sites
+  // to pass in the loaded pointers.
+  //
+  std::vector<Value*> Args;
+  while (!F->use_empty()) {
+    CallSite CS = CallSite::get(F->use_back());
+    Instruction *Call = CS.getInstruction();
+
+    // Make sure the caller of this function is revisited.
+    if (Call->getParent()->getParent()->hasInternalLinkage())
+      WorkList.insert(Call->getParent()->getParent());
+    
+    // Loop over the operands, deleting dead ones...
+    CallSite::arg_iterator AI = CS.arg_begin();
+    for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I, ++AI)
+      if (!ArgsToPromote.count(I))
+        Args.push_back(*AI);          // Unmodified argument
+      else if (!I->use_empty()) {
+        // Non-dead instruction
+        Args.push_back(new LoadInst(*AI, (*AI)->getName()+".val", Call));
+      }
+
+    if (ExtraArgHack)
+      Args.push_back(Constant::getNullValue(Type::IntTy));
+
+    // Push any varargs arguments on the list
+    for (; AI != CS.arg_end(); ++AI)
+      Args.push_back(*AI);
+
+    Instruction *New;
+    if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
+      New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(),
+                           Args, "", Call);
+    } else {
+      New = new CallInst(NF, Args, "", Call);
+    }
+    Args.clear();
+
+    if (!Call->use_empty()) {
+      Call->replaceAllUsesWith(New);
+      std::string Name = Call->getName();
+      Call->setName("");
+      New->setName(Name);
+    }
+    
+    // Finally, remove the old call from the program, reducing the use-count of
+    // F.
+    Call->getParent()->getInstList().erase(Call);
+  }
+
+  // Since we have now created the new function, splice the body of the old
+  // function right into the new function, leaving the old rotting hulk of the
+  // function empty.
+  NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
+
+  // Loop over the argument list, transfering uses of the old arguments over to
+  // the new arguments, also transfering over the names as well.
+  //
+  for (Function::aiterator I = F->abegin(), E = F->aend(), I2 = NF->abegin();
+       I != E; ++I)
+    if (!ArgsToPromote.count(I)) {
+      // If this is an unmodified argument, move the name and users over to the
+      // new version.
+      I->replaceAllUsesWith(I2);
+      I2->setName(I->getName());
+      ++I2;
+    } else if (!I->use_empty()) {
+      // Otherwise, if we promoted this argument, then all users are load
+      // instructions, and all loads should be using the new argument that we
+      // added.
+      /*DEBUG*/(std::cerr << "*** Promoted argument '" << I->getName()
+                      << "' of function '" << F->getName() << "'\n");
+      I2->setName(I->getName()+".val");
+      while (!I->use_empty()) {
+        LoadInst *LI = cast<LoadInst>(I->use_back());
+        LI->replaceAllUsesWith(I2);
+        LI->getParent()->getInstList().erase(LI);
+      }
+      ++I2;
+    }
+
+  // Now that the old function is dead, delete it.
+  F->getParent()->getFunctionList().erase(F);
+}