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