1 //===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
3 // This file implements tail recursion elimination.
5 // Caveats: The algorithm implemented is trivially simple. There are several
6 // improvements that could be made:
8 // 1. If the function has any alloca instructions, these instructions will not
9 // remain in the entry block of the function. Doing this requires analysis
10 // to prove that the alloca is not reachable by the recursively invoked
12 // 2. Tail recursion is only performed if the call immediately preceeds the
13 // return instruction. Would it be useful to generalize this somehow?
14 // 3. TRE is only performed if the function returns void or if the return
15 // returns the result returned by the call. It is possible, but unlikely,
16 // that the return returns something else (like constant 0), and can still
17 // be TRE'd. It can be TRE'd if ALL OTHER return instructions in the
18 // function return the exact same value.
20 //===----------------------------------------------------------------------===//
22 #include "llvm/Transforms/Scalar.h"
23 #include "llvm/DerivedTypes.h"
24 #include "llvm/Function.h"
25 #include "llvm/Instructions.h"
26 #include "llvm/Pass.h"
27 #include "Support/Statistic.h"
30 Statistic<> NumEliminated("tailcallelim", "Number of tail calls removed");
32 struct TailCallElim : public FunctionPass {
33 virtual bool runOnFunction(Function &F);
35 RegisterOpt<TailCallElim> X("tailcallelim", "Tail Call Elimination");
38 FunctionPass *createTailCallEliminationPass() { return new TailCallElim(); }
41 bool TailCallElim::runOnFunction(Function &F) {
42 // If this function is a varargs function, we won't be able to PHI the args
43 // right, so don't even try to convert it...
44 if (F.getFunctionType()->isVarArg()) return false;
46 BasicBlock *OldEntry = 0;
47 std::vector<PHINode*> ArgumentPHIs;
48 bool MadeChange = false;
50 // Loop over the function, looking for any returning blocks...
51 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
52 if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator()))
53 if (Ret != BB->begin()) // Make sure there is something before the ret...
54 if (CallInst *CI = dyn_cast<CallInst>(Ret->getPrev()))
55 // Make sure the tail call is to the current function, and that the
56 // return either returns void or returns the value computed by the
58 if (CI->getCalledFunction() == &F &&
59 (Ret->getNumOperands() == 0 || Ret->getReturnValue() == CI)) {
60 // Ohh, it looks like we found a tail call, is this the first?
62 // Ok, so this is the first tail call we have found in this
63 // function. Insert a new entry block into the function, allowing
64 // us to branch back to the old entry block.
65 OldEntry = &F.getEntryNode();
66 BasicBlock *NewEntry = new BasicBlock("tailrecurse", OldEntry);
67 NewEntry->getInstList().push_back(new BranchInst(OldEntry));
69 // Now that we have created a new block, which jumps to the entry
70 // block, insert a PHI node for each argument of the function.
71 // For now, we initialize each PHI to only have the real arguments
72 // which are passed in.
73 Instruction *InsertPos = OldEntry->begin();
74 for (Function::aiterator I = F.abegin(), E = F.aend(); I!=E; ++I){
75 PHINode *PN = new PHINode(I->getType(), I->getName()+".tr",
77 I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
78 PN->addIncoming(I, NewEntry);
79 ArgumentPHIs.push_back(PN);
83 // Ok, now that we know we have a pseudo-entry block WITH all of the
84 // required PHI nodes, add entries into the PHI node for the actual
85 // parameters passed into the tail-recursive call.
86 for (unsigned i = 0, e = CI->getNumOperands()-1; i != e; ++i)
87 ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB);
89 // Now that all of the PHI nodes are in place, remove the call and
90 // ret instructions, replacing them with an unconditional branch.
91 new BranchInst(OldEntry, CI);
92 BB->getInstList().pop_back(); // Remove return.
93 BB->getInstList().pop_back(); // Remove call.