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 transforms calls of the current function (self recursion) followed
11 // by a return instruction with a branch to the entry of the function, creating
12 // a loop. This pass also implements the following extensions to the basic
15 // 1. Trivial instructions between the call and return do not prevent the
16 // transformation from taking place, though currently the analysis cannot
17 // support moving any really useful instructions (only dead ones).
18 // 2. This pass transforms functions that are prevented from being tail
19 // recursive by an associative expression to use an accumulator variable,
20 // thus compiling the typical naive factorial or 'fib' implementation into
22 // 3. TRE is performed if the function returns void, if the return
23 // returns the result returned by the call, or if the function returns a
24 // run-time constant on all exits from the function. It is possible, though
25 // unlikely, that the return returns something else (like constant 0), and
26 // can still be TRE'd. It can be TRE'd if ALL OTHER return instructions in
27 // the function return the exact same value.
29 // There are several improvements that could be made:
31 // 1. If the function has any alloca instructions, these instructions will be
32 // moved out of the entry block of the function, causing them to be
33 // evaluated each time through the tail recursion. Safely keeping allocas
34 // in the entry block requires analysis to proves that the tail-called
35 // function does not read or write the stack object.
36 // 2. Tail recursion is only performed if the call immediately preceeds the
37 // return instruction. It's possible that there could be a jump between
38 // the call and the return.
39 // 3. There can be intervening operations between the call and the return that
40 // prevent the TRE from occurring. For example, there could be GEP's and
41 // stores to memory that will not be read or written by the call. This
42 // requires some substantial analysis (such as with DSA) to prove safe to
43 // move ahead of the call, but doing so could allow many more TREs to be
44 // performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
46 //===----------------------------------------------------------------------===//
48 #include "llvm/Transforms/Scalar.h"
49 #include "llvm/DerivedTypes.h"
50 #include "llvm/Function.h"
51 #include "llvm/Instructions.h"
52 #include "llvm/Pass.h"
53 #include "llvm/Support/CFG.h"
54 #include "llvm/ADT/Statistic.h"
58 Statistic<> NumEliminated("tailcallelim", "Number of tail calls removed");
59 Statistic<> NumAccumAdded("tailcallelim","Number of accumulators introduced");
61 struct TailCallElim : public FunctionPass {
62 virtual bool runOnFunction(Function &F);
65 bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
66 std::vector<PHINode*> &ArgumentPHIs);
67 bool CanMoveAboveCall(Instruction *I, CallInst *CI);
68 Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI);
70 RegisterOpt<TailCallElim> X("tailcallelim", "Tail Call Elimination");
73 // Public interface to the TailCallElimination pass
74 FunctionPass *llvm::createTailCallEliminationPass() {
75 return new TailCallElim();
79 bool TailCallElim::runOnFunction(Function &F) {
80 // If this function is a varargs function, we won't be able to PHI the args
81 // right, so don't even try to convert it...
82 if (F.getFunctionType()->isVarArg()) return false;
84 BasicBlock *OldEntry = 0;
85 std::vector<PHINode*> ArgumentPHIs;
86 bool MadeChange = false;
88 // Loop over the function, looking for any returning blocks...
89 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
90 if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator()))
91 MadeChange |= ProcessReturningBlock(Ret, OldEntry, ArgumentPHIs);
93 // If we eliminated any tail recursions, it's possible that we inserted some
94 // silly PHI nodes which just merge an initial value (the incoming operand)
95 // with themselves. Check to see if we did and clean up our mess if so. This
96 // occurs when a function passes an argument straight through to its tail
98 if (!ArgumentPHIs.empty()) {
99 unsigned NumIncoming = ArgumentPHIs[0]->getNumIncomingValues();
100 for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
101 PHINode *PN = ArgumentPHIs[i];
103 for (unsigned op = 0, e = NumIncoming; op != e; ++op) {
104 Value *Op = PN->getIncomingValue(op);
107 V = Op; // First value seen?
108 } else if (V != Op) {
115 // If the PHI Node is a dynamic constant, replace it with the value it is.
117 PN->replaceAllUsesWith(V);
118 PN->getParent()->getInstList().erase(PN);
127 /// CanMoveAboveCall - Return true if it is safe to move the specified
128 /// instruction from after the call to before the call, assuming that all
129 /// instructions between the call and this instruction are movable.
131 bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
132 // FIXME: We can move load/store/call/free instructions above the call if the
133 // call does not mod/ref the memory location being processed.
134 if (I->mayWriteToMemory() || isa<LoadInst>(I))
137 // Otherwise, if this is a side-effect free instruction, check to make sure
138 // that it does not use the return value of the call. If it doesn't use the
139 // return value of the call, it must only use things that are defined before
140 // the call, or movable instructions between the call and the instruction
142 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
143 if (I->getOperand(i) == CI)
148 // isDynamicConstant - Return true if the specified value is the same when the
149 // return would exit as it was when the initial iteration of the recursive
150 // function was executed.
152 // We currently handle static constants and arguments that are not modified as
153 // part of the recursion.
155 static bool isDynamicConstant(Value *V, CallInst *CI) {
156 if (isa<Constant>(V)) return true; // Static constants are always dyn consts
158 // Check to see if this is an immutable argument, if so, the value
159 // will be available to initialize the accumulator.
160 if (Argument *Arg = dyn_cast<Argument>(V)) {
161 // Figure out which argument number this is...
163 Function *F = CI->getParent()->getParent();
164 for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI)
167 // If we are passing this argument into call as the corresponding
168 // argument operand, then the argument is dynamically constant.
169 // Otherwise, we cannot transform this function safely.
170 if (CI->getOperand(ArgNo+1) == Arg)
173 // Not a constant or immutable argument, we can't safely transform.
177 // getCommonReturnValue - Check to see if the function containing the specified
178 // return instruction and tail call consistently returns the same
179 // runtime-constant value at all exit points. If so, return the returned value.
181 static Value *getCommonReturnValue(ReturnInst *TheRI, CallInst *CI) {
182 Function *F = TheRI->getParent()->getParent();
183 Value *ReturnedValue = 0;
185 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
186 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
188 Value *RetOp = RI->getOperand(0);
190 // We can only perform this transformation if the value returned is
191 // evaluatable at the start of the initial invocation of the function,
192 // instead of at the end of the evaluation.
194 if (!isDynamicConstant(RetOp, CI))
197 if (ReturnedValue && RetOp != ReturnedValue)
198 return 0; // Cannot transform if differing values are returned.
199 ReturnedValue = RetOp;
201 return ReturnedValue;
204 /// CanTransformAccumulatorRecursion - If the specified instruction can be
205 /// transformed using accumulator recursion elimination, return the constant
206 /// which is the start of the accumulator value. Otherwise return null.
208 Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
210 if (!I->isAssociative()) return 0;
211 assert(I->getNumOperands() == 2 &&
212 "Associative operations should have 2 args!");
214 // Exactly one operand should be the result of the call instruction...
215 if (I->getOperand(0) == CI && I->getOperand(1) == CI ||
216 I->getOperand(0) != CI && I->getOperand(1) != CI)
219 // The only user of this instruction we allow is a single return instruction.
220 if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back()))
223 // Ok, now we have to check all of the other return instructions in this
224 // function. If they return non-constants or differing values, then we cannot
225 // transform the function safely.
226 return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI);
229 bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
230 std::vector<PHINode*> &ArgumentPHIs) {
231 BasicBlock *BB = Ret->getParent();
232 Function *F = BB->getParent();
234 if (&BB->front() == Ret) // Make sure there is something before the ret...
237 // Scan backwards from the return, checking to see if there is a tail call in
238 // this block. If so, set CI to it.
240 BasicBlock::iterator BBI = Ret;
242 CI = dyn_cast<CallInst>(BBI);
243 if (CI && CI->getCalledFunction() == F)
246 if (BBI == BB->begin())
247 return false; // Didn't find a potential tail call.
251 // If we are introducing accumulator recursion to eliminate associative
252 // operations after the call instruction, this variable contains the initial
253 // value for the accumulator. If this value is set, we actually perform
254 // accumulator recursion elimination instead of simple tail recursion
256 Value *AccumulatorRecursionEliminationInitVal = 0;
257 Instruction *AccumulatorRecursionInstr = 0;
259 // Ok, we found a potential tail call. We can currently only transform the
260 // tail call if all of the instructions between the call and the return are
261 // movable to above the call itself, leaving the call next to the return.
262 // Check that this is the case now.
263 for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI)
264 if (!CanMoveAboveCall(BBI, CI)) {
265 // If we can't move the instruction above the call, it might be because it
266 // is an associative operation that could be tranformed using accumulator
267 // recursion elimination. Check to see if this is the case, and if so,
268 // remember the initial accumulator value for later.
269 if ((AccumulatorRecursionEliminationInitVal =
270 CanTransformAccumulatorRecursion(BBI, CI))) {
271 // Yes, this is accumulator recursion. Remember which instruction
273 AccumulatorRecursionInstr = BBI;
275 return false; // Otherwise, we cannot eliminate the tail recursion!
279 // We can only transform call/return pairs that either ignore the return value
280 // of the call and return void, ignore the value of the call and return a
281 // constant, return the value returned by the tail call, or that are being
282 // accumulator recursion variable eliminated.
283 if (Ret->getNumOperands() != 0 && Ret->getReturnValue() != CI &&
284 AccumulatorRecursionEliminationInitVal == 0 &&
285 !getCommonReturnValue(Ret, CI))
288 // OK! We can transform this tail call. If this is the first one found,
289 // create the new entry block, allowing us to branch back to the old entry.
291 OldEntry = &F->getEntryBlock();
292 std::string OldName = OldEntry->getName(); OldEntry->setName("tailrecurse");
293 BasicBlock *NewEntry = new BasicBlock(OldName, F, OldEntry);
294 new BranchInst(OldEntry, NewEntry);
296 // Now that we have created a new block, which jumps to the entry
297 // block, insert a PHI node for each argument of the function.
298 // For now, we initialize each PHI to only have the real arguments
299 // which are passed in.
300 Instruction *InsertPos = OldEntry->begin();
301 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I) {
302 PHINode *PN = new PHINode(I->getType(), I->getName()+".tr", InsertPos);
303 I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
304 PN->addIncoming(I, NewEntry);
305 ArgumentPHIs.push_back(PN);
309 // Ok, now that we know we have a pseudo-entry block WITH all of the
310 // required PHI nodes, add entries into the PHI node for the actual
311 // parameters passed into the tail-recursive call.
312 for (unsigned i = 0, e = CI->getNumOperands()-1; i != e; ++i)
313 ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB);
315 // If we are introducing an accumulator variable to eliminate the recursion,
316 // do so now. Note that we _know_ that no subsequent tail recursion
317 // eliminations will happen on this function because of the way the
318 // accumulator recursion predicate is set up.
320 if (AccumulatorRecursionEliminationInitVal) {
321 Instruction *AccRecInstr = AccumulatorRecursionInstr;
322 // Start by inserting a new PHI node for the accumulator.
323 PHINode *AccPN = new PHINode(AccRecInstr->getType(), "accumulator.tr",
326 // Loop over all of the predecessors of the tail recursion block. For the
327 // real entry into the function we seed the PHI with the initial value,
328 // computed earlier. For any other existing branches to this block (due to
329 // other tail recursions eliminated) the accumulator is not modified.
330 // Because we haven't added the branch in the current block to OldEntry yet,
331 // it will not show up as a predecessor.
332 for (pred_iterator PI = pred_begin(OldEntry), PE = pred_end(OldEntry);
334 if (*PI == &F->getEntryBlock())
335 AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, *PI);
337 AccPN->addIncoming(AccPN, *PI);
340 // Add an incoming argument for the current block, which is computed by our
341 // associative accumulator instruction.
342 AccPN->addIncoming(AccRecInstr, BB);
344 // Next, rewrite the accumulator recursion instruction so that it does not
345 // use the result of the call anymore, instead, use the PHI node we just
347 AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
349 // Finally, rewrite any return instructions in the program to return the PHI
350 // node instead of the "initval" that they do currently. This loop will
351 // actually rewrite the return value we are destroying, but that's ok.
352 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
353 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
354 RI->setOperand(0, AccPN);
358 // Now that all of the PHI nodes are in place, remove the call and
359 // ret instructions, replacing them with an unconditional branch.
360 new BranchInst(OldEntry, Ret);
361 BB->getInstList().erase(Ret); // Remove return.
362 BB->getInstList().erase(CI); // Remove call.