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TailRecursionElimination.cpp

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00001 //===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
00002 // 
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file was developed by the LLVM research group and is distributed under
00006 // the University of Illinois Open Source License. See LICENSE.TXT for details.
00007 // 
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This file transforms calls of the current function (self recursion) followed
00011 // by a return instruction with a branch to the entry of the function, creating
00012 // a loop.  This pass also implements the following extensions to the basic
00013 // algorithm:
00014 //
00015 //  1. Trivial instructions between the call and return do not prevent the
00016 //     transformation from taking place, though currently the analysis cannot
00017 //     support moving any really useful instructions (only dead ones).
00018 //  2. This pass transforms functions that are prevented from being tail
00019 //     recursive by an associative expression to use an accumulator variable,
00020 //     thus compiling the typical naive factorial or 'fib' implementation into
00021 //     efficient code.
00022 //  3. TRE is performed if the function returns void, if the return
00023 //     returns the result returned by the call, or if the function returns a
00024 //     run-time constant on all exits from the function.  It is possible, though
00025 //     unlikely, that the return returns something else (like constant 0), and
00026 //     can still be TRE'd.  It can be TRE'd if ALL OTHER return instructions in
00027 //     the function return the exact same value.
00028 //
00029 // There are several improvements that could be made:
00030 //
00031 //  1. If the function has any alloca instructions, these instructions will be
00032 //     moved out of the entry block of the function, causing them to be
00033 //     evaluated each time through the tail recursion.  Safely keeping allocas
00034 //     in the entry block requires analysis to proves that the tail-called
00035 //     function does not read or write the stack object.
00036 //  2. Tail recursion is only performed if the call immediately preceeds the
00037 //     return instruction.  It's possible that there could be a jump between
00038 //     the call and the return.
00039 //  3. There can be intervening operations between the call and the return that
00040 //     prevent the TRE from occurring.  For example, there could be GEP's and
00041 //     stores to memory that will not be read or written by the call.  This
00042 //     requires some substantial analysis (such as with DSA) to prove safe to
00043 //     move ahead of the call, but doing so could allow many more TREs to be
00044 //     performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
00045 //
00046 //===----------------------------------------------------------------------===//
00047 
00048 #include "llvm/Transforms/Scalar.h"
00049 #include "llvm/DerivedTypes.h"
00050 #include "llvm/Function.h"
00051 #include "llvm/Instructions.h"
00052 #include "llvm/Pass.h"
00053 #include "llvm/Support/CFG.h"
00054 #include "llvm/ADT/Statistic.h"
00055 using namespace llvm;
00056 
00057 namespace {
00058   Statistic<> NumEliminated("tailcallelim", "Number of tail calls removed");
00059   Statistic<> NumAccumAdded("tailcallelim","Number of accumulators introduced");
00060 
00061   struct TailCallElim : public FunctionPass {
00062     virtual bool runOnFunction(Function &F);
00063 
00064   private:
00065     bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
00066                                std::vector<PHINode*> &ArgumentPHIs);
00067     bool CanMoveAboveCall(Instruction *I, CallInst *CI);
00068     Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI);
00069   };
00070   RegisterOpt<TailCallElim> X("tailcallelim", "Tail Call Elimination");
00071 }
00072 
00073 // Public interface to the TailCallElimination pass
00074 FunctionPass *llvm::createTailCallEliminationPass() {
00075   return new TailCallElim();
00076 }
00077 
00078 
00079 bool TailCallElim::runOnFunction(Function &F) {
00080   // If this function is a varargs function, we won't be able to PHI the args
00081   // right, so don't even try to convert it...
00082   if (F.getFunctionType()->isVarArg()) return false;
00083 
00084   BasicBlock *OldEntry = 0;
00085   std::vector<PHINode*> ArgumentPHIs;
00086   bool MadeChange = false;
00087 
00088   // Loop over the function, looking for any returning blocks...
00089   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
00090     if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator()))
00091       MadeChange |= ProcessReturningBlock(Ret, OldEntry, ArgumentPHIs);
00092   
00093   // If we eliminated any tail recursions, it's possible that we inserted some
00094   // silly PHI nodes which just merge an initial value (the incoming operand)
00095   // with themselves.  Check to see if we did and clean up our mess if so.  This
00096   // occurs when a function passes an argument straight through to its tail
00097   // call.
00098   if (!ArgumentPHIs.empty()) {
00099     unsigned NumIncoming = ArgumentPHIs[0]->getNumIncomingValues();
00100     for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
00101       PHINode *PN = ArgumentPHIs[i];
00102       Value *V = 0;
00103       for (unsigned op = 0, e = NumIncoming; op != e; ++op) {
00104         Value *Op = PN->getIncomingValue(op);
00105         if (Op != PN) {
00106           if (V == 0) {
00107             V = Op;     // First value seen?
00108           } else if (V != Op) {
00109             V = 0;
00110             break;
00111           }
00112         }
00113       }
00114 
00115       // If the PHI Node is a dynamic constant, replace it with the value it is.
00116       if (V) {
00117         PN->replaceAllUsesWith(V);
00118         PN->getParent()->getInstList().erase(PN);
00119       }
00120     }
00121   }
00122 
00123   return MadeChange;
00124 }
00125 
00126 
00127 /// CanMoveAboveCall - Return true if it is safe to move the specified
00128 /// instruction from after the call to before the call, assuming that all
00129 /// instructions between the call and this instruction are movable.
00130 ///
00131 bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
00132   // FIXME: We can move load/store/call/free instructions above the call if the
00133   // call does not mod/ref the memory location being processed.
00134   if (I->mayWriteToMemory() || isa<LoadInst>(I))
00135     return false;
00136 
00137   // Otherwise, if this is a side-effect free instruction, check to make sure
00138   // that it does not use the return value of the call.  If it doesn't use the
00139   // return value of the call, it must only use things that are defined before
00140   // the call, or movable instructions between the call and the instruction
00141   // itself.
00142   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
00143     if (I->getOperand(i) == CI)
00144       return false;
00145   return true;
00146 }
00147 
00148 // isDynamicConstant - Return true if the specified value is the same when the
00149 // return would exit as it was when the initial iteration of the recursive
00150 // function was executed.
00151 //
00152 // We currently handle static constants and arguments that are not modified as
00153 // part of the recursion.
00154 //
00155 static bool isDynamicConstant(Value *V, CallInst *CI) {
00156   if (isa<Constant>(V)) return true; // Static constants are always dyn consts
00157 
00158   // Check to see if this is an immutable argument, if so, the value
00159   // will be available to initialize the accumulator.
00160   if (Argument *Arg = dyn_cast<Argument>(V)) {
00161     // Figure out which argument number this is...
00162     unsigned ArgNo = 0;
00163     Function *F = CI->getParent()->getParent();
00164     for (Function::aiterator AI = F->abegin(); &*AI != Arg; ++AI)
00165       ++ArgNo;
00166     
00167     // If we are passing this argument into call as the corresponding
00168     // argument operand, then the argument is dynamically constant.
00169     // Otherwise, we cannot transform this function safely.
00170     if (CI->getOperand(ArgNo+1) == Arg)
00171       return true;
00172   }
00173   // Not a constant or immutable argument, we can't safely transform.
00174   return false;
00175 }
00176 
00177 // getCommonReturnValue - Check to see if the function containing the specified
00178 // return instruction and tail call consistently returns the same
00179 // runtime-constant value at all exit points.  If so, return the returned value.
00180 //
00181 static Value *getCommonReturnValue(ReturnInst *TheRI, CallInst *CI) {
00182   Function *F = TheRI->getParent()->getParent();
00183   Value *ReturnedValue = 0;
00184 
00185   for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
00186     if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
00187       if (RI != TheRI) {
00188         Value *RetOp = RI->getOperand(0);
00189 
00190         // We can only perform this transformation if the value returned is
00191         // evaluatable at the start of the initial invocation of the function,
00192         // instead of at the end of the evaluation.
00193         //
00194         if (!isDynamicConstant(RetOp, CI))
00195           return 0;
00196 
00197         if (ReturnedValue && RetOp != ReturnedValue)
00198           return 0;     // Cannot transform if differing values are returned.
00199         ReturnedValue = RetOp;
00200       }
00201   return ReturnedValue;
00202 }
00203 
00204 /// CanTransformAccumulatorRecursion - If the specified instruction can be
00205 /// transformed using accumulator recursion elimination, return the constant
00206 /// which is the start of the accumulator value.  Otherwise return null.
00207 ///
00208 Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
00209                                                       CallInst *CI) {
00210   if (!I->isAssociative()) return 0;
00211   assert(I->getNumOperands() == 2 &&
00212          "Associative operations should have 2 args!");
00213 
00214   // Exactly one operand should be the result of the call instruction...
00215   if (I->getOperand(0) == CI && I->getOperand(1) == CI ||
00216       I->getOperand(0) != CI && I->getOperand(1) != CI)
00217     return 0;
00218 
00219   // The only user of this instruction we allow is a single return instruction.
00220   if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back()))
00221     return 0;
00222 
00223   // Ok, now we have to check all of the other return instructions in this
00224   // function.  If they return non-constants or differing values, then we cannot
00225   // transform the function safely.
00226   return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI);
00227 }
00228 
00229 bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
00230                                          std::vector<PHINode*> &ArgumentPHIs) {
00231   BasicBlock *BB = Ret->getParent();
00232   Function *F = BB->getParent();
00233 
00234   if (&BB->front() == Ret) // Make sure there is something before the ret...
00235     return false;
00236 
00237   // Scan backwards from the return, checking to see if there is a tail call in
00238   // this block.  If so, set CI to it.
00239   CallInst *CI;
00240   BasicBlock::iterator BBI = Ret;
00241   while (1) {
00242     CI = dyn_cast<CallInst>(BBI);
00243     if (CI && CI->getCalledFunction() == F)
00244       break;
00245 
00246     if (BBI == BB->begin())
00247       return false;          // Didn't find a potential tail call.
00248     --BBI;
00249   }
00250 
00251   // If we are introducing accumulator recursion to eliminate associative
00252   // operations after the call instruction, this variable contains the initial
00253   // value for the accumulator.  If this value is set, we actually perform
00254   // accumulator recursion elimination instead of simple tail recursion
00255   // elimination.
00256   Value *AccumulatorRecursionEliminationInitVal = 0;
00257   Instruction *AccumulatorRecursionInstr = 0;
00258 
00259   // Ok, we found a potential tail call.  We can currently only transform the
00260   // tail call if all of the instructions between the call and the return are
00261   // movable to above the call itself, leaving the call next to the return.
00262   // Check that this is the case now.
00263   for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI)
00264     if (!CanMoveAboveCall(BBI, CI)) {
00265       // If we can't move the instruction above the call, it might be because it
00266       // is an associative operation that could be tranformed using accumulator
00267       // recursion elimination.  Check to see if this is the case, and if so,
00268       // remember the initial accumulator value for later.
00269       if ((AccumulatorRecursionEliminationInitVal =
00270                              CanTransformAccumulatorRecursion(BBI, CI))) {
00271         // Yes, this is accumulator recursion.  Remember which instruction
00272         // accumulates.
00273         AccumulatorRecursionInstr = BBI;
00274       } else {
00275         return false;   // Otherwise, we cannot eliminate the tail recursion!
00276       }
00277     }
00278 
00279   // We can only transform call/return pairs that either ignore the return value
00280   // of the call and return void, ignore the value of the call and return a
00281   // constant, return the value returned by the tail call, or that are being
00282   // accumulator recursion variable eliminated.
00283   if (Ret->getNumOperands() != 0 && Ret->getReturnValue() != CI &&
00284       AccumulatorRecursionEliminationInitVal == 0 &&
00285       !getCommonReturnValue(Ret, CI))
00286     return false;
00287 
00288   // OK! We can transform this tail call.  If this is the first one found,
00289   // create the new entry block, allowing us to branch back to the old entry.
00290   if (OldEntry == 0) {
00291     OldEntry = &F->getEntryBlock();
00292     std::string OldName = OldEntry->getName(); OldEntry->setName("tailrecurse");
00293     BasicBlock *NewEntry = new BasicBlock(OldName, F, OldEntry);
00294     new BranchInst(OldEntry, NewEntry);
00295     
00296     // Now that we have created a new block, which jumps to the entry
00297     // block, insert a PHI node for each argument of the function.
00298     // For now, we initialize each PHI to only have the real arguments
00299     // which are passed in.
00300     Instruction *InsertPos = OldEntry->begin();
00301     for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I) {
00302       PHINode *PN = new PHINode(I->getType(), I->getName()+".tr", InsertPos);
00303       I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
00304       PN->addIncoming(I, NewEntry);
00305       ArgumentPHIs.push_back(PN);
00306     }
00307   }
00308   
00309   // Ok, now that we know we have a pseudo-entry block WITH all of the
00310   // required PHI nodes, add entries into the PHI node for the actual
00311   // parameters passed into the tail-recursive call.
00312   for (unsigned i = 0, e = CI->getNumOperands()-1; i != e; ++i)
00313     ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB);
00314   
00315   // If we are introducing an accumulator variable to eliminate the recursion,
00316   // do so now.  Note that we _know_ that no subsequent tail recursion
00317   // eliminations will happen on this function because of the way the
00318   // accumulator recursion predicate is set up.
00319   //
00320   if (AccumulatorRecursionEliminationInitVal) {
00321     Instruction *AccRecInstr = AccumulatorRecursionInstr;
00322     // Start by inserting a new PHI node for the accumulator.
00323     PHINode *AccPN = new PHINode(AccRecInstr->getType(), "accumulator.tr",
00324                                  OldEntry->begin());
00325 
00326     // Loop over all of the predecessors of the tail recursion block.  For the
00327     // real entry into the function we seed the PHI with the initial value,
00328     // computed earlier.  For any other existing branches to this block (due to
00329     // other tail recursions eliminated) the accumulator is not modified.
00330     // Because we haven't added the branch in the current block to OldEntry yet,
00331     // it will not show up as a predecessor.
00332     for (pred_iterator PI = pred_begin(OldEntry), PE = pred_end(OldEntry);
00333          PI != PE; ++PI) {
00334       if (*PI == &F->getEntryBlock())
00335         AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, *PI);
00336       else
00337         AccPN->addIncoming(AccPN, *PI);
00338     }
00339 
00340     // Add an incoming argument for the current block, which is computed by our
00341     // associative accumulator instruction.
00342     AccPN->addIncoming(AccRecInstr, BB);
00343 
00344     // Next, rewrite the accumulator recursion instruction so that it does not
00345     // use the result of the call anymore, instead, use the PHI node we just
00346     // inserted.
00347     AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
00348 
00349     // Finally, rewrite any return instructions in the program to return the PHI
00350     // node instead of the "initval" that they do currently.  This loop will
00351     // actually rewrite the return value we are destroying, but that's ok.
00352     for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
00353       if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
00354         RI->setOperand(0, AccPN);
00355     ++NumAccumAdded;
00356   }
00357 
00358   // Now that all of the PHI nodes are in place, remove the call and
00359   // ret instructions, replacing them with an unconditional branch.
00360   new BranchInst(OldEntry, Ret);
00361   BB->getInstList().erase(Ret);  // Remove return.
00362   BB->getInstList().erase(CI);   // Remove call.
00363   ++NumEliminated;
00364   return true;
00365 }