//===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
//
-// This file implements tail recursion elimination.
+// The LLVM Compiler Infrastructure
//
-// Caveats: The algorithm implemented is trivially simple. There are several
-// improvements that could be made:
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
//
-// 1. If the function has any alloca instructions, these instructions will not
-// remain in the entry block of the function. Doing this requires analysis
-// to prove that the alloca is not reachable by the recursively invoked
-// function call.
+//===----------------------------------------------------------------------===//
+//
+// This file transforms calls of the current function (self recursion) followed
+// by a return instruction with a branch to the entry of the function, creating
+// a loop. This pass also implements the following extensions to the basic
+// algorithm:
+//
+// 1. Trivial instructions between the call and return do not prevent the
+// transformation from taking place, though currently the analysis cannot
+// support moving any really useful instructions (only dead ones).
+// 2. This pass transforms functions that are prevented from being tail
+// recursive by an associative and commutative expression to use an
+// accumulator variable, thus compiling the typical naive factorial or
+// 'fib' implementation into efficient code.
+// 3. TRE is performed if the function returns void, if the return
+// returns the result returned by the call, or if the function returns a
+// run-time constant on all exits from the function. It is possible, though
+// unlikely, that the return returns something else (like constant 0), and
+// can still be TRE'd. It can be TRE'd if ALL OTHER return instructions in
+// the function return the exact same value.
+// 4. If it can prove that callees do not access their caller stack frame,
+// they are marked as eligible for tail call elimination (by the code
+// generator).
+//
+// There are several improvements that could be made:
+//
+// 1. If the function has any alloca instructions, these instructions will be
+// moved out of the entry block of the function, causing them to be
+// evaluated each time through the tail recursion. Safely keeping allocas
+// in the entry block requires analysis to proves that the tail-called
+// function does not read or write the stack object.
// 2. Tail recursion is only performed if the call immediately preceeds the
-// return instruction. Would it be useful to generalize this somehow?
-// 3. TRE is only performed if the function returns void or if the return
-// returns the result returned by the call. It is possible, but unlikely,
-// that the return returns something else (like constant 0), and can still
-// be TRE'd. It can be TRE'd if ALL OTHER return instructions in the
-// function return the exact same value.
+// return instruction. It's possible that there could be a jump between
+// the call and the return.
+// 3. There can be intervening operations between the call and the return that
+// prevent the TRE from occurring. For example, there could be GEP's and
+// stores to memory that will not be read or written by the call. This
+// requires some substantial analysis (such as with DSA) to prove safe to
+// move ahead of the call, but doing so could allow many more TREs to be
+// performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
+// 4. The algorithm we use to detect if callees access their caller stack
+// frames is very primitive.
//
//===----------------------------------------------------------------------===//
+#define DEBUG_TYPE "tailcallelim"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Pass.h"
-#include "Support/Statistic.h"
+#include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/Analysis/InlineCost.h"
+#include "llvm/Analysis/Loads.h"
+#include "llvm/Support/CallSite.h"
+#include "llvm/Support/CFG.h"
+#include "llvm/ADT/Statistic.h"
+using namespace llvm;
-namespace {
- Statistic<> NumEliminated("tailcallelim", "Number of tail calls removed");
+STATISTIC(NumEliminated, "Number of tail calls removed");
+STATISTIC(NumAccumAdded, "Number of accumulators introduced");
+namespace {
struct TailCallElim : public FunctionPass {
+ static char ID; // Pass identification, replacement for typeid
+ TailCallElim() : FunctionPass(ID) {
+ initializeTailCallElimPass(*PassRegistry::getPassRegistry());
+ }
+
virtual bool runOnFunction(Function &F);
+
+ private:
+ bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVector<PHINode*, 8> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail);
+ bool CanMoveAboveCall(Instruction *I, CallInst *CI);
+ Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI);
};
- RegisterOpt<TailCallElim> X("tailcallelim", "Tail Call Elimination");
}
+char TailCallElim::ID = 0;
+INITIALIZE_PASS(TailCallElim, "tailcallelim",
+ "Tail Call Elimination", false, false)
+
+// Public interface to the TailCallElimination pass
+FunctionPass *llvm::createTailCallEliminationPass() {
+ return new TailCallElim();
+}
+
+/// AllocaMightEscapeToCalls - Return true if this alloca may be accessed by
+/// callees of this function. We only do very simple analysis right now, this
+/// could be expanded in the future to use mod/ref information for particular
+/// call sites if desired.
+static bool AllocaMightEscapeToCalls(AllocaInst *AI) {
+ // FIXME: do simple 'address taken' analysis.
+ return true;
+}
+
+/// CheckForEscapingAllocas - Scan the specified basic block for alloca
+/// instructions. If it contains any that might be accessed by calls, return
+/// true.
+static bool CheckForEscapingAllocas(BasicBlock *BB,
+ bool &CannotTCETailMarkedCall) {
+ bool RetVal = false;
+ for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
+ if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
+ RetVal |= AllocaMightEscapeToCalls(AI);
+
+ // If this alloca is in the body of the function, or if it is a variable
+ // sized allocation, we cannot tail call eliminate calls marked 'tail'
+ // with this mechanism.
+ if (BB != &BB->getParent()->getEntryBlock() ||
+ !isa<ConstantInt>(AI->getArraySize()))
+ CannotTCETailMarkedCall = true;
+ }
+ return RetVal;
+}
bool TailCallElim::runOnFunction(Function &F) {
// If this function is a varargs function, we won't be able to PHI the args
if (F.getFunctionType()->isVarArg()) return false;
BasicBlock *OldEntry = 0;
- std::vector<PHINode*> ArgumentPHIs;
+ bool TailCallsAreMarkedTail = false;
+ SmallVector<PHINode*, 8> ArgumentPHIs;
bool MadeChange = false;
- // Loop over the function, looking for any returning blocks...
+ bool FunctionContainsEscapingAllocas = false;
+
+ // CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls
+ // marked with the 'tail' attribute, because doing so would cause the stack
+ // size to increase (real TCE would deallocate variable sized allocas, TCE
+ // doesn't).
+ bool CannotTCETailMarkedCall = false;
+
+ // Loop over the function, looking for any returning blocks, and keeping track
+ // of whether this function has any non-trivially used allocas.
+ for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
+ if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall)
+ break;
+
+ FunctionContainsEscapingAllocas |=
+ CheckForEscapingAllocas(BB, CannotTCETailMarkedCall);
+ }
+
+ /// FIXME: The code generator produces really bad code when an 'escaping
+ /// alloca' is changed from being a static alloca to being a dynamic alloca.
+ /// Until this is resolved, disable this transformation if that would ever
+ /// happen. This bug is PR962.
+ if (FunctionContainsEscapingAllocas)
+ return false;
+
+ // Second pass, change any tail calls to loops.
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator()))
- if (Ret != BB->begin())
- if (CallInst *CI = dyn_cast<CallInst>(Ret->getPrev()))
- // Make sure the tail call is to the current function, and that the
- // return either returns void or returns the value computed by the
- // call.
- if (CI->getCalledFunction() == &F &&
- (Ret->getNumOperands() == 0 || Ret->getReturnValue() == CI)) {
- // Ohh, it looks like we found a tail call, is this the first?
- if (!OldEntry) {
- // Ok, so this is the first tail call we have found in this
- // function. Insert a new entry block into the function, allowing
- // us to branch back to the old entry block.
- OldEntry = &F.getEntryNode();
- BasicBlock *NewEntry = new BasicBlock("tailrecurse", OldEntry);
- NewEntry->getInstList().push_back(new BranchInst(OldEntry));
-
- // Now that we have created a new block, which jumps to the entry
- // block, insert a PHI node for each argument of the function.
- // For now, we initialize each PHI to only have the real arguments
- // which are passed in.
- Instruction *InsertPos = OldEntry->begin();
- for (Function::aiterator I = F.abegin(), E = F.aend(); I!=E; ++I){
- PHINode *PN = new PHINode(I->getType(), I->getName()+".tr",
- InsertPos);
- PN->addIncoming(I, NewEntry);
- ArgumentPHIs.push_back(PN);
- }
- }
-
- // Ok, now that we know we have a pseudo-entry block WITH all of the
- // required PHI nodes, add entries into the PHI node for the actual
- // parameters passed into the tail-recursive call.
- for (unsigned i = 0, e = CI->getNumOperands()-1; i != e; ++i)
- ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB);
-
- // Now that all of the PHI nodes are in place, remove the call and
- // ret instructions, replacing them with an unconditional branch.
- new BranchInst(OldEntry, CI);
- BB->getInstList().pop_back(); // Remove return.
- BB->getInstList().pop_back(); // Remove call.
- MadeChange = true;
- NumEliminated++;
- }
-
+ MadeChange |= ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
+ ArgumentPHIs,CannotTCETailMarkedCall);
+
+ // If we eliminated any tail recursions, it's possible that we inserted some
+ // silly PHI nodes which just merge an initial value (the incoming operand)
+ // with themselves. Check to see if we did and clean up our mess if so. This
+ // occurs when a function passes an argument straight through to its tail
+ // call.
+ if (!ArgumentPHIs.empty()) {
+ for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
+ PHINode *PN = ArgumentPHIs[i];
+
+ // If the PHI Node is a dynamic constant, replace it with the value it is.
+ if (Value *PNV = PN->hasConstantValue()) {
+ PN->replaceAllUsesWith(PNV);
+ PN->eraseFromParent();
+ }
+ }
+ }
+
+ // Finally, if this function contains no non-escaping allocas, mark all calls
+ // in the function as eligible for tail calls (there is no stack memory for
+ // them to access).
+ if (!FunctionContainsEscapingAllocas)
+ for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
+ for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
+ if (CallInst *CI = dyn_cast<CallInst>(I)) {
+ CI->setTailCall();
+ MadeChange = true;
+ }
+
return MadeChange;
}
+
+/// CanMoveAboveCall - Return true if it is safe to move the specified
+/// instruction from after the call to before the call, assuming that all
+/// instructions between the call and this instruction are movable.
+///
+bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
+ // FIXME: We can move load/store/call/free instructions above the call if the
+ // call does not mod/ref the memory location being processed.
+ if (I->mayHaveSideEffects()) // This also handles volatile loads.
+ return false;
+
+ if (LoadInst *L = dyn_cast<LoadInst>(I)) {
+ // Loads may always be moved above calls without side effects.
+ if (CI->mayHaveSideEffects()) {
+ // Non-volatile loads may be moved above a call with side effects if it
+ // does not write to memory and the load provably won't trap.
+ // FIXME: Writes to memory only matter if they may alias the pointer
+ // being loaded from.
+ if (CI->mayWriteToMemory() ||
+ !isSafeToLoadUnconditionally(L->getPointerOperand(), L,
+ L->getAlignment()))
+ return false;
+ }
+ }
+
+ // Otherwise, if this is a side-effect free instruction, check to make sure
+ // that it does not use the return value of the call. If it doesn't use the
+ // return value of the call, it must only use things that are defined before
+ // the call, or movable instructions between the call and the instruction
+ // itself.
+ for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
+ if (I->getOperand(i) == CI)
+ return false;
+ return true;
+}
+
+// isDynamicConstant - Return true if the specified value is the same when the
+// return would exit as it was when the initial iteration of the recursive
+// function was executed.
+//
+// We currently handle static constants and arguments that are not modified as
+// part of the recursion.
+//
+static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) {
+ if (isa<Constant>(V)) return true; // Static constants are always dyn consts
+
+ // Check to see if this is an immutable argument, if so, the value
+ // will be available to initialize the accumulator.
+ if (Argument *Arg = dyn_cast<Argument>(V)) {
+ // Figure out which argument number this is...
+ unsigned ArgNo = 0;
+ Function *F = CI->getParent()->getParent();
+ for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI)
+ ++ArgNo;
+
+ // If we are passing this argument into call as the corresponding
+ // argument operand, then the argument is dynamically constant.
+ // Otherwise, we cannot transform this function safely.
+ if (CI->getArgOperand(ArgNo) == Arg)
+ return true;
+ }
+
+ // Switch cases are always constant integers. If the value is being switched
+ // on and the return is only reachable from one of its cases, it's
+ // effectively constant.
+ if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor())
+ if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator()))
+ if (SI->getCondition() == V)
+ return SI->getDefaultDest() != RI->getParent();
+
+ // Not a constant or immutable argument, we can't safely transform.
+ return false;
+}
+
+// getCommonReturnValue - Check to see if the function containing the specified
+// tail call consistently returns the same runtime-constant value at all exit
+// points except for IgnoreRI. If so, return the returned value.
+//
+static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) {
+ Function *F = CI->getParent()->getParent();
+ Value *ReturnedValue = 0;
+
+ for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) {
+ ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator());
+ if (RI == 0 || RI == IgnoreRI) continue;
+
+ // We can only perform this transformation if the value returned is
+ // evaluatable at the start of the initial invocation of the function,
+ // instead of at the end of the evaluation.
+ //
+ Value *RetOp = RI->getOperand(0);
+ if (!isDynamicConstant(RetOp, CI, RI))
+ return 0;
+
+ if (ReturnedValue && RetOp != ReturnedValue)
+ return 0; // Cannot transform if differing values are returned.
+ ReturnedValue = RetOp;
+ }
+ return ReturnedValue;
+}
+
+/// CanTransformAccumulatorRecursion - If the specified instruction can be
+/// transformed using accumulator recursion elimination, return the constant
+/// which is the start of the accumulator value. Otherwise return null.
+///
+Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
+ CallInst *CI) {
+ if (!I->isAssociative() || !I->isCommutative()) return 0;
+ assert(I->getNumOperands() == 2 &&
+ "Associative/commutative operations should have 2 args!");
+
+ // Exactly one operand should be the result of the call instruction.
+ if ((I->getOperand(0) == CI && I->getOperand(1) == CI) ||
+ (I->getOperand(0) != CI && I->getOperand(1) != CI))
+ return 0;
+
+ // The only user of this instruction we allow is a single return instruction.
+ if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back()))
+ return 0;
+
+ // Ok, now we have to check all of the other return instructions in this
+ // function. If they return non-constants or differing values, then we cannot
+ // transform the function safely.
+ return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI);
+}
+
+bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
+ bool &TailCallsAreMarkedTail,
+ SmallVector<PHINode*, 8> &ArgumentPHIs,
+ bool CannotTailCallElimCallsMarkedTail) {
+ BasicBlock *BB = Ret->getParent();
+ Function *F = BB->getParent();
+
+ if (&BB->front() == Ret) // Make sure there is something before the ret...
+ return false;
+
+ // Scan backwards from the return, checking to see if there is a tail call in
+ // this block. If so, set CI to it.
+ CallInst *CI;
+ BasicBlock::iterator BBI = Ret;
+ while (1) {
+ CI = dyn_cast<CallInst>(BBI);
+ if (CI && CI->getCalledFunction() == F)
+ break;
+
+ if (BBI == BB->begin())
+ return false; // Didn't find a potential tail call.
+ --BBI;
+ }
+
+ // If this call is marked as a tail call, and if there are dynamic allocas in
+ // the function, we cannot perform this optimization.
+ if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
+ return false;
+
+ // As a special case, detect code like this:
+ // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
+ // and disable this xform in this case, because the code generator will
+ // lower the call to fabs into inline code.
+ if (BB == &F->getEntryBlock() &&
+ &BB->front() == CI && &*++BB->begin() == Ret &&
+ callIsSmall(F)) {
+ // A single-block function with just a call and a return. Check that
+ // the arguments match.
+ CallSite::arg_iterator I = CallSite(CI).arg_begin(),
+ E = CallSite(CI).arg_end();
+ Function::arg_iterator FI = F->arg_begin(),
+ FE = F->arg_end();
+ for (; I != E && FI != FE; ++I, ++FI)
+ if (*I != &*FI) break;
+ if (I == E && FI == FE)
+ return false;
+ }
+
+ // If we are introducing accumulator recursion to eliminate operations after
+ // the call instruction that are both associative and commutative, the initial
+ // value for the accumulator is placed in this variable. If this value is set
+ // then we actually perform accumulator recursion elimination instead of
+ // simple tail recursion elimination. If the operation is an LLVM instruction
+ // (eg: "add") then it is recorded in AccumulatorRecursionInstr. If not, then
+ // we are handling the case when the return instruction returns a constant C
+ // which is different to the constant returned by other return instructions
+ // (which is recorded in AccumulatorRecursionEliminationInitVal). This is a
+ // special case of accumulator recursion, the operation being "return C".
+ Value *AccumulatorRecursionEliminationInitVal = 0;
+ Instruction *AccumulatorRecursionInstr = 0;
+
+ // Ok, we found a potential tail call. We can currently only transform the
+ // tail call if all of the instructions between the call and the return are
+ // movable to above the call itself, leaving the call next to the return.
+ // Check that this is the case now.
+ for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI) {
+ if (CanMoveAboveCall(BBI, CI)) continue;
+
+ // If we can't move the instruction above the call, it might be because it
+ // is an associative and commutative operation that could be tranformed
+ // using accumulator recursion elimination. Check to see if this is the
+ // case, and if so, remember the initial accumulator value for later.
+ if ((AccumulatorRecursionEliminationInitVal =
+ CanTransformAccumulatorRecursion(BBI, CI))) {
+ // Yes, this is accumulator recursion. Remember which instruction
+ // accumulates.
+ AccumulatorRecursionInstr = BBI;
+ } else {
+ return false; // Otherwise, we cannot eliminate the tail recursion!
+ }
+ }
+
+ // We can only transform call/return pairs that either ignore the return value
+ // of the call and return void, ignore the value of the call and return a
+ // constant, return the value returned by the tail call, or that are being
+ // accumulator recursion variable eliminated.
+ if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
+ !isa<UndefValue>(Ret->getReturnValue()) &&
+ AccumulatorRecursionEliminationInitVal == 0 &&
+ !getCommonReturnValue(0, CI)) {
+ // One case remains that we are able to handle: the current return
+ // instruction returns a constant, and all other return instructions
+ // return a different constant.
+ if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret))
+ return false; // Current return instruction does not return a constant.
+ // Check that all other return instructions return a common constant. If
+ // so, record it in AccumulatorRecursionEliminationInitVal.
+ AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI);
+ if (!AccumulatorRecursionEliminationInitVal)
+ return false;
+ }
+
+ // OK! We can transform this tail call. If this is the first one found,
+ // create the new entry block, allowing us to branch back to the old entry.
+ if (OldEntry == 0) {
+ OldEntry = &F->getEntryBlock();
+ BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry);
+ NewEntry->takeName(OldEntry);
+ OldEntry->setName("tailrecurse");
+ BranchInst::Create(OldEntry, NewEntry);
+
+ // If this tail call is marked 'tail' and if there are any allocas in the
+ // entry block, move them up to the new entry block.
+ TailCallsAreMarkedTail = CI->isTailCall();
+ if (TailCallsAreMarkedTail)
+ // Move all fixed sized allocas from OldEntry to NewEntry.
+ for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(),
+ NEBI = NewEntry->begin(); OEBI != E; )
+ if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
+ if (isa<ConstantInt>(AI->getArraySize()))
+ AI->moveBefore(NEBI);
+
+ // Now that we have created a new block, which jumps to the entry
+ // block, insert a PHI node for each argument of the function.
+ // For now, we initialize each PHI to only have the real arguments
+ // which are passed in.
+ Instruction *InsertPos = OldEntry->begin();
+ for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
+ I != E; ++I) {
+ PHINode *PN = PHINode::Create(I->getType(),
+ I->getName() + ".tr", InsertPos);
+ I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
+ PN->addIncoming(I, NewEntry);
+ ArgumentPHIs.push_back(PN);
+ }
+ }
+
+ // If this function has self recursive calls in the tail position where some
+ // are marked tail and some are not, only transform one flavor or another. We
+ // have to choose whether we move allocas in the entry block to the new entry
+ // block or not, so we can't make a good choice for both. NOTE: We could do
+ // slightly better here in the case that the function has no entry block
+ // allocas.
+ if (TailCallsAreMarkedTail && !CI->isTailCall())
+ return false;
+
+ // Ok, now that we know we have a pseudo-entry block WITH all of the
+ // required PHI nodes, add entries into the PHI node for the actual
+ // parameters passed into the tail-recursive call.
+ for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i)
+ ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), BB);
+
+ // If we are introducing an accumulator variable to eliminate the recursion,
+ // do so now. Note that we _know_ that no subsequent tail recursion
+ // eliminations will happen on this function because of the way the
+ // accumulator recursion predicate is set up.
+ //
+ if (AccumulatorRecursionEliminationInitVal) {
+ Instruction *AccRecInstr = AccumulatorRecursionInstr;
+ // Start by inserting a new PHI node for the accumulator.
+ PHINode *AccPN =
+ PHINode::Create(AccumulatorRecursionEliminationInitVal->getType(),
+ "accumulator.tr", OldEntry->begin());
+
+ // Loop over all of the predecessors of the tail recursion block. For the
+ // real entry into the function we seed the PHI with the initial value,
+ // computed earlier. For any other existing branches to this block (due to
+ // other tail recursions eliminated) the accumulator is not modified.
+ // Because we haven't added the branch in the current block to OldEntry yet,
+ // it will not show up as a predecessor.
+ for (pred_iterator PI = pred_begin(OldEntry), PE = pred_end(OldEntry);
+ PI != PE; ++PI) {
+ BasicBlock *P = *PI;
+ if (P == &F->getEntryBlock())
+ AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P);
+ else
+ AccPN->addIncoming(AccPN, P);
+ }
+
+ if (AccRecInstr) {
+ // Add an incoming argument for the current block, which is computed by
+ // our associative and commutative accumulator instruction.
+ AccPN->addIncoming(AccRecInstr, BB);
+
+ // Next, rewrite the accumulator recursion instruction so that it does not
+ // use the result of the call anymore, instead, use the PHI node we just
+ // inserted.
+ AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
+ } else {
+ // Add an incoming argument for the current block, which is just the
+ // constant returned by the current return instruction.
+ AccPN->addIncoming(Ret->getReturnValue(), BB);
+ }
+
+ // Finally, rewrite any return instructions in the program to return the PHI
+ // node instead of the "initval" that they do currently. This loop will
+ // actually rewrite the return value we are destroying, but that's ok.
+ for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
+ if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
+ RI->setOperand(0, AccPN);
+ ++NumAccumAdded;
+ }
+
+ // Now that all of the PHI nodes are in place, remove the call and
+ // ret instructions, replacing them with an unconditional branch.
+ BranchInst::Create(OldEntry, Ret);
+ BB->getInstList().erase(Ret); // Remove return.
+ BB->getInstList().erase(CI); // Remove call.
+ ++NumEliminated;
+ return true;
+}
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