Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 1 | //===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===// |
Misha Brukman | fd93908 | 2005-04-21 23:48:37 +0000 | [diff] [blame] | 2 | // |
John Criswell | b576c94 | 2003-10-20 19:43:21 +0000 | [diff] [blame] | 3 | // The LLVM Compiler Infrastructure |
| 4 | // |
| 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. |
Misha Brukman | fd93908 | 2005-04-21 23:48:37 +0000 | [diff] [blame] | 7 | // |
John Criswell | b576c94 | 2003-10-20 19:43:21 +0000 | [diff] [blame] | 8 | //===----------------------------------------------------------------------===// |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 9 | // |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 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 |
| 13 | // algorithm: |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 14 | // |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 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). |
Chris Lattner | 543d622 | 2003-12-08 23:19:26 +0000 | [diff] [blame] | 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 |
| 21 | // efficient code. |
Chris Lattner | d64152a | 2003-12-14 23:57:39 +0000 | [diff] [blame] | 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. |
Chris Lattner | 7f78f21 | 2005-05-09 23:51:13 +0000 | [diff] [blame] | 28 | // 4. If it can prove that callees do not access theier caller stack frame, |
| 29 | // they are marked as eligible for tail call elimination (by the code |
| 30 | // generator). |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 31 | // |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 32 | // There are several improvements that could be made: |
| 33 | // |
| 34 | // 1. If the function has any alloca instructions, these instructions will be |
| 35 | // moved out of the entry block of the function, causing them to be |
| 36 | // evaluated each time through the tail recursion. Safely keeping allocas |
| 37 | // in the entry block requires analysis to proves that the tail-called |
| 38 | // function does not read or write the stack object. |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 39 | // 2. Tail recursion is only performed if the call immediately preceeds the |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 40 | // return instruction. It's possible that there could be a jump between |
| 41 | // the call and the return. |
Chris Lattner | d64152a | 2003-12-14 23:57:39 +0000 | [diff] [blame] | 42 | // 3. There can be intervening operations between the call and the return that |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 43 | // prevent the TRE from occurring. For example, there could be GEP's and |
| 44 | // stores to memory that will not be read or written by the call. This |
| 45 | // requires some substantial analysis (such as with DSA) to prove safe to |
| 46 | // move ahead of the call, but doing so could allow many more TREs to be |
| 47 | // performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark. |
Chris Lattner | 7f78f21 | 2005-05-09 23:51:13 +0000 | [diff] [blame] | 48 | // 4. The algorithm we use to detect if callees access their caller stack |
| 49 | // frames is very primitive. |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 50 | // |
| 51 | //===----------------------------------------------------------------------===// |
| 52 | |
Chris Lattner | 0e5f499 | 2006-12-19 21:40:18 +0000 | [diff] [blame] | 53 | #define DEBUG_TYPE "tailcallelim" |
Chris Lattner | 3fc6ef1 | 2003-09-20 05:14:13 +0000 | [diff] [blame] | 54 | #include "llvm/Transforms/Scalar.h" |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 55 | #include "llvm/Constants.h" |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 56 | #include "llvm/DerivedTypes.h" |
| 57 | #include "llvm/Function.h" |
| 58 | #include "llvm/Instructions.h" |
| 59 | #include "llvm/Pass.h" |
Chris Lattner | 543d622 | 2003-12-08 23:19:26 +0000 | [diff] [blame] | 60 | #include "llvm/Support/CFG.h" |
Reid Spencer | 551ccae | 2004-09-01 22:55:40 +0000 | [diff] [blame] | 61 | #include "llvm/ADT/Statistic.h" |
Reid Spencer | 9133fe2 | 2007-02-05 23:32:05 +0000 | [diff] [blame] | 62 | #include "llvm/Support/Compiler.h" |
Chris Lattner | f8485c6 | 2003-11-20 18:25:24 +0000 | [diff] [blame] | 63 | using namespace llvm; |
Brian Gaeke | d0fde30 | 2003-11-11 22:41:34 +0000 | [diff] [blame] | 64 | |
Chris Lattner | 0e5f499 | 2006-12-19 21:40:18 +0000 | [diff] [blame] | 65 | STATISTIC(NumEliminated, "Number of tail calls removed"); |
| 66 | STATISTIC(NumAccumAdded, "Number of accumulators introduced"); |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 67 | |
Chris Lattner | 0e5f499 | 2006-12-19 21:40:18 +0000 | [diff] [blame] | 68 | namespace { |
Reid Spencer | 9133fe2 | 2007-02-05 23:32:05 +0000 | [diff] [blame] | 69 | struct VISIBILITY_HIDDEN TailCallElim : public FunctionPass { |
Devang Patel | 1997473 | 2007-05-03 01:11:54 +0000 | [diff] [blame^] | 70 | static char ID; // Pass identifcation, replacement for typeid |
Devang Patel | 794fd75 | 2007-05-01 21:15:47 +0000 | [diff] [blame] | 71 | TailCallElim() : FunctionPass((intptr_t)&ID) {} |
| 72 | |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 73 | virtual bool runOnFunction(Function &F); |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 74 | |
| 75 | private: |
| 76 | bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry, |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 77 | bool &TailCallsAreMarkedTail, |
| 78 | std::vector<PHINode*> &ArgumentPHIs, |
| 79 | bool CannotTailCallElimCallsMarkedTail); |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 80 | bool CanMoveAboveCall(Instruction *I, CallInst *CI); |
Chris Lattner | 543d622 | 2003-12-08 23:19:26 +0000 | [diff] [blame] | 81 | Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI); |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 82 | }; |
Devang Patel | 1997473 | 2007-05-03 01:11:54 +0000 | [diff] [blame^] | 83 | char TailCallElim::ID = 0; |
Chris Lattner | 7f8897f | 2006-08-27 22:42:52 +0000 | [diff] [blame] | 84 | RegisterPass<TailCallElim> X("tailcallelim", "Tail Call Elimination"); |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 85 | } |
| 86 | |
Brian Gaeke | d0fde30 | 2003-11-11 22:41:34 +0000 | [diff] [blame] | 87 | // Public interface to the TailCallElimination pass |
Chris Lattner | f8485c6 | 2003-11-20 18:25:24 +0000 | [diff] [blame] | 88 | FunctionPass *llvm::createTailCallEliminationPass() { |
| 89 | return new TailCallElim(); |
| 90 | } |
Chris Lattner | 3fc6ef1 | 2003-09-20 05:14:13 +0000 | [diff] [blame] | 91 | |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 92 | |
Chris Lattner | 7f78f21 | 2005-05-09 23:51:13 +0000 | [diff] [blame] | 93 | /// AllocaMightEscapeToCalls - Return true if this alloca may be accessed by |
| 94 | /// callees of this function. We only do very simple analysis right now, this |
| 95 | /// could be expanded in the future to use mod/ref information for particular |
| 96 | /// call sites if desired. |
| 97 | static bool AllocaMightEscapeToCalls(AllocaInst *AI) { |
| 98 | // FIXME: do simple 'address taken' analysis. |
| 99 | return true; |
| 100 | } |
| 101 | |
| 102 | /// FunctionContainsAllocas - Scan the specified basic block for alloca |
| 103 | /// instructions. If it contains any that might be accessed by calls, return |
| 104 | /// true. |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 105 | static bool CheckForEscapingAllocas(BasicBlock *BB, |
| 106 | bool &CannotTCETailMarkedCall) { |
| 107 | bool RetVal = false; |
Chris Lattner | 7f78f21 | 2005-05-09 23:51:13 +0000 | [diff] [blame] | 108 | for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 109 | if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) { |
| 110 | RetVal |= AllocaMightEscapeToCalls(AI); |
| 111 | |
| 112 | // If this alloca is in the body of the function, or if it is a variable |
| 113 | // sized allocation, we cannot tail call eliminate calls marked 'tail' |
| 114 | // with this mechanism. |
Dan Gohman | ecb7a77 | 2007-03-22 16:38:57 +0000 | [diff] [blame] | 115 | if (BB != &BB->getParent()->getEntryBlock() || |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 116 | !isa<ConstantInt>(AI->getArraySize())) |
| 117 | CannotTCETailMarkedCall = true; |
| 118 | } |
| 119 | return RetVal; |
Chris Lattner | 7f78f21 | 2005-05-09 23:51:13 +0000 | [diff] [blame] | 120 | } |
| 121 | |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 122 | bool TailCallElim::runOnFunction(Function &F) { |
| 123 | // If this function is a varargs function, we won't be able to PHI the args |
| 124 | // right, so don't even try to convert it... |
| 125 | if (F.getFunctionType()->isVarArg()) return false; |
| 126 | |
| 127 | BasicBlock *OldEntry = 0; |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 128 | bool TailCallsAreMarkedTail = false; |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 129 | std::vector<PHINode*> ArgumentPHIs; |
| 130 | bool MadeChange = false; |
| 131 | |
Chris Lattner | 7f78f21 | 2005-05-09 23:51:13 +0000 | [diff] [blame] | 132 | bool FunctionContainsEscapingAllocas = false; |
| 133 | |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 134 | // CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls |
| 135 | // marked with the 'tail' attribute, because doing so would cause the stack |
| 136 | // size to increase (real TCE would deallocate variable sized allocas, TCE |
| 137 | // doesn't). |
| 138 | bool CannotTCETailMarkedCall = false; |
| 139 | |
Chris Lattner | 7f78f21 | 2005-05-09 23:51:13 +0000 | [diff] [blame] | 140 | // Loop over the function, looking for any returning blocks, and keeping track |
| 141 | // of whether this function has any non-trivially used allocas. |
| 142 | for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) { |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 143 | if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall) |
| 144 | break; |
Jeff Cohen | 00b16889 | 2005-07-27 06:12:32 +0000 | [diff] [blame] | 145 | |
Chris Lattner | 4c0e4cd | 2005-08-07 07:00:52 +0000 | [diff] [blame] | 146 | FunctionContainsEscapingAllocas |= |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 147 | CheckForEscapingAllocas(BB, CannotTCETailMarkedCall); |
Chris Lattner | 7f78f21 | 2005-05-09 23:51:13 +0000 | [diff] [blame] | 148 | } |
Chris Lattner | 32b1e87 | 2006-10-22 18:42:26 +0000 | [diff] [blame] | 149 | |
| 150 | /// FIXME: The code generator produces really bad code when an 'escaping |
| 151 | /// alloca' is changed from being a static alloca to being a dynamic alloca. |
| 152 | /// Until this is resolved, disable this transformation if that would ever |
| 153 | /// happen. This bug is PR962. |
| 154 | if (FunctionContainsEscapingAllocas) |
| 155 | return false; |
| 156 | |
Misha Brukman | fd93908 | 2005-04-21 23:48:37 +0000 | [diff] [blame] | 157 | |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 158 | // Second pass, change any tail calls to loops. |
| 159 | for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) |
| 160 | if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) |
| 161 | MadeChange |= ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail, |
| 162 | ArgumentPHIs,CannotTCETailMarkedCall); |
| 163 | |
Chris Lattner | cf2f892 | 2003-12-08 23:37:35 +0000 | [diff] [blame] | 164 | // If we eliminated any tail recursions, it's possible that we inserted some |
| 165 | // silly PHI nodes which just merge an initial value (the incoming operand) |
| 166 | // with themselves. Check to see if we did and clean up our mess if so. This |
| 167 | // occurs when a function passes an argument straight through to its tail |
| 168 | // call. |
| 169 | if (!ArgumentPHIs.empty()) { |
Chris Lattner | cf2f892 | 2003-12-08 23:37:35 +0000 | [diff] [blame] | 170 | for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) { |
| 171 | PHINode *PN = ArgumentPHIs[i]; |
Chris Lattner | cf2f892 | 2003-12-08 23:37:35 +0000 | [diff] [blame] | 172 | |
| 173 | // If the PHI Node is a dynamic constant, replace it with the value it is. |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 174 | if (Value *PNV = PN->hasConstantValue()) { |
| 175 | PN->replaceAllUsesWith(PNV); |
| 176 | PN->eraseFromParent(); |
Chris Lattner | cf2f892 | 2003-12-08 23:37:35 +0000 | [diff] [blame] | 177 | } |
| 178 | } |
| 179 | } |
| 180 | |
Chris Lattner | 7f78f21 | 2005-05-09 23:51:13 +0000 | [diff] [blame] | 181 | // Finally, if this function contains no non-escaping allocas, mark all calls |
| 182 | // in the function as eligible for tail calls (there is no stack memory for |
| 183 | // them to access). |
| 184 | if (!FunctionContainsEscapingAllocas) |
| 185 | for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) |
| 186 | for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) |
| 187 | if (CallInst *CI = dyn_cast<CallInst>(I)) |
| 188 | CI->setTailCall(); |
| 189 | |
Chris Lattner | 2240d2b | 2003-09-20 05:03:31 +0000 | [diff] [blame] | 190 | return MadeChange; |
| 191 | } |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 192 | |
| 193 | |
Chris Lattner | 543d622 | 2003-12-08 23:19:26 +0000 | [diff] [blame] | 194 | /// CanMoveAboveCall - Return true if it is safe to move the specified |
| 195 | /// instruction from after the call to before the call, assuming that all |
| 196 | /// instructions between the call and this instruction are movable. |
| 197 | /// |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 198 | bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) { |
| 199 | // FIXME: We can move load/store/call/free instructions above the call if the |
| 200 | // call does not mod/ref the memory location being processed. |
| 201 | if (I->mayWriteToMemory() || isa<LoadInst>(I)) |
| 202 | return false; |
| 203 | |
| 204 | // Otherwise, if this is a side-effect free instruction, check to make sure |
| 205 | // that it does not use the return value of the call. If it doesn't use the |
| 206 | // return value of the call, it must only use things that are defined before |
| 207 | // the call, or movable instructions between the call and the instruction |
| 208 | // itself. |
| 209 | for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) |
| 210 | if (I->getOperand(i) == CI) |
| 211 | return false; |
| 212 | return true; |
| 213 | } |
| 214 | |
Chris Lattner | d64152a | 2003-12-14 23:57:39 +0000 | [diff] [blame] | 215 | // isDynamicConstant - Return true if the specified value is the same when the |
| 216 | // return would exit as it was when the initial iteration of the recursive |
| 217 | // function was executed. |
| 218 | // |
| 219 | // We currently handle static constants and arguments that are not modified as |
| 220 | // part of the recursion. |
| 221 | // |
| 222 | static bool isDynamicConstant(Value *V, CallInst *CI) { |
| 223 | if (isa<Constant>(V)) return true; // Static constants are always dyn consts |
| 224 | |
| 225 | // Check to see if this is an immutable argument, if so, the value |
| 226 | // will be available to initialize the accumulator. |
| 227 | if (Argument *Arg = dyn_cast<Argument>(V)) { |
| 228 | // Figure out which argument number this is... |
| 229 | unsigned ArgNo = 0; |
| 230 | Function *F = CI->getParent()->getParent(); |
Chris Lattner | e4d5c44 | 2005-03-15 04:54:21 +0000 | [diff] [blame] | 231 | for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI) |
Chris Lattner | d64152a | 2003-12-14 23:57:39 +0000 | [diff] [blame] | 232 | ++ArgNo; |
Misha Brukman | fd93908 | 2005-04-21 23:48:37 +0000 | [diff] [blame] | 233 | |
Chris Lattner | d64152a | 2003-12-14 23:57:39 +0000 | [diff] [blame] | 234 | // If we are passing this argument into call as the corresponding |
| 235 | // argument operand, then the argument is dynamically constant. |
| 236 | // Otherwise, we cannot transform this function safely. |
| 237 | if (CI->getOperand(ArgNo+1) == Arg) |
| 238 | return true; |
| 239 | } |
| 240 | // Not a constant or immutable argument, we can't safely transform. |
| 241 | return false; |
| 242 | } |
| 243 | |
| 244 | // getCommonReturnValue - Check to see if the function containing the specified |
| 245 | // return instruction and tail call consistently returns the same |
| 246 | // runtime-constant value at all exit points. If so, return the returned value. |
| 247 | // |
| 248 | static Value *getCommonReturnValue(ReturnInst *TheRI, CallInst *CI) { |
| 249 | Function *F = TheRI->getParent()->getParent(); |
| 250 | Value *ReturnedValue = 0; |
| 251 | |
| 252 | for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) |
| 253 | if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator())) |
| 254 | if (RI != TheRI) { |
| 255 | Value *RetOp = RI->getOperand(0); |
| 256 | |
| 257 | // We can only perform this transformation if the value returned is |
| 258 | // evaluatable at the start of the initial invocation of the function, |
| 259 | // instead of at the end of the evaluation. |
| 260 | // |
| 261 | if (!isDynamicConstant(RetOp, CI)) |
| 262 | return 0; |
| 263 | |
| 264 | if (ReturnedValue && RetOp != ReturnedValue) |
| 265 | return 0; // Cannot transform if differing values are returned. |
| 266 | ReturnedValue = RetOp; |
| 267 | } |
| 268 | return ReturnedValue; |
| 269 | } |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 270 | |
Chris Lattner | 543d622 | 2003-12-08 23:19:26 +0000 | [diff] [blame] | 271 | /// CanTransformAccumulatorRecursion - If the specified instruction can be |
| 272 | /// transformed using accumulator recursion elimination, return the constant |
| 273 | /// which is the start of the accumulator value. Otherwise return null. |
| 274 | /// |
| 275 | Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I, |
| 276 | CallInst *CI) { |
| 277 | if (!I->isAssociative()) return 0; |
| 278 | assert(I->getNumOperands() == 2 && |
| 279 | "Associative operations should have 2 args!"); |
| 280 | |
| 281 | // Exactly one operand should be the result of the call instruction... |
| 282 | if (I->getOperand(0) == CI && I->getOperand(1) == CI || |
| 283 | I->getOperand(0) != CI && I->getOperand(1) != CI) |
| 284 | return 0; |
| 285 | |
| 286 | // The only user of this instruction we allow is a single return instruction. |
| 287 | if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back())) |
| 288 | return 0; |
| 289 | |
| 290 | // Ok, now we have to check all of the other return instructions in this |
| 291 | // function. If they return non-constants or differing values, then we cannot |
| 292 | // transform the function safely. |
Chris Lattner | d64152a | 2003-12-14 23:57:39 +0000 | [diff] [blame] | 293 | return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI); |
Chris Lattner | 543d622 | 2003-12-08 23:19:26 +0000 | [diff] [blame] | 294 | } |
| 295 | |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 296 | bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 297 | bool &TailCallsAreMarkedTail, |
| 298 | std::vector<PHINode*> &ArgumentPHIs, |
| 299 | bool CannotTailCallElimCallsMarkedTail) { |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 300 | BasicBlock *BB = Ret->getParent(); |
| 301 | Function *F = BB->getParent(); |
| 302 | |
| 303 | if (&BB->front() == Ret) // Make sure there is something before the ret... |
| 304 | return false; |
| 305 | |
| 306 | // Scan backwards from the return, checking to see if there is a tail call in |
| 307 | // this block. If so, set CI to it. |
| 308 | CallInst *CI; |
| 309 | BasicBlock::iterator BBI = Ret; |
| 310 | while (1) { |
| 311 | CI = dyn_cast<CallInst>(BBI); |
| 312 | if (CI && CI->getCalledFunction() == F) |
| 313 | break; |
| 314 | |
| 315 | if (BBI == BB->begin()) |
| 316 | return false; // Didn't find a potential tail call. |
| 317 | --BBI; |
| 318 | } |
| 319 | |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 320 | // If this call is marked as a tail call, and if there are dynamic allocas in |
| 321 | // the function, we cannot perform this optimization. |
| 322 | if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail) |
| 323 | return false; |
| 324 | |
Chris Lattner | 543d622 | 2003-12-08 23:19:26 +0000 | [diff] [blame] | 325 | // If we are introducing accumulator recursion to eliminate associative |
| 326 | // operations after the call instruction, this variable contains the initial |
| 327 | // value for the accumulator. If this value is set, we actually perform |
| 328 | // accumulator recursion elimination instead of simple tail recursion |
| 329 | // elimination. |
| 330 | Value *AccumulatorRecursionEliminationInitVal = 0; |
| 331 | Instruction *AccumulatorRecursionInstr = 0; |
| 332 | |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 333 | // Ok, we found a potential tail call. We can currently only transform the |
| 334 | // tail call if all of the instructions between the call and the return are |
| 335 | // movable to above the call itself, leaving the call next to the return. |
| 336 | // Check that this is the case now. |
| 337 | for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI) |
Chris Lattner | 543d622 | 2003-12-08 23:19:26 +0000 | [diff] [blame] | 338 | if (!CanMoveAboveCall(BBI, CI)) { |
| 339 | // If we can't move the instruction above the call, it might be because it |
| 340 | // is an associative operation that could be tranformed using accumulator |
| 341 | // recursion elimination. Check to see if this is the case, and if so, |
| 342 | // remember the initial accumulator value for later. |
| 343 | if ((AccumulatorRecursionEliminationInitVal = |
| 344 | CanTransformAccumulatorRecursion(BBI, CI))) { |
| 345 | // Yes, this is accumulator recursion. Remember which instruction |
| 346 | // accumulates. |
| 347 | AccumulatorRecursionInstr = BBI; |
| 348 | } else { |
| 349 | return false; // Otherwise, we cannot eliminate the tail recursion! |
| 350 | } |
| 351 | } |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 352 | |
| 353 | // We can only transform call/return pairs that either ignore the return value |
Chris Lattner | d64152a | 2003-12-14 23:57:39 +0000 | [diff] [blame] | 354 | // of the call and return void, ignore the value of the call and return a |
| 355 | // constant, return the value returned by the tail call, or that are being |
| 356 | // accumulator recursion variable eliminated. |
Chris Lattner | 543d622 | 2003-12-08 23:19:26 +0000 | [diff] [blame] | 357 | if (Ret->getNumOperands() != 0 && Ret->getReturnValue() != CI && |
Chris Lattner | 3b5f450 | 2005-11-05 08:21:11 +0000 | [diff] [blame] | 358 | !isa<UndefValue>(Ret->getReturnValue()) && |
Chris Lattner | d64152a | 2003-12-14 23:57:39 +0000 | [diff] [blame] | 359 | AccumulatorRecursionEliminationInitVal == 0 && |
| 360 | !getCommonReturnValue(Ret, CI)) |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 361 | return false; |
| 362 | |
| 363 | // OK! We can transform this tail call. If this is the first one found, |
| 364 | // create the new entry block, allowing us to branch back to the old entry. |
| 365 | if (OldEntry == 0) { |
| 366 | OldEntry = &F->getEntryBlock(); |
Chris Lattner | 6934a04 | 2007-02-11 01:23:03 +0000 | [diff] [blame] | 367 | BasicBlock *NewEntry = new BasicBlock("", F, OldEntry); |
| 368 | NewEntry->takeName(OldEntry); |
| 369 | OldEntry->setName("tailrecurse"); |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 370 | new BranchInst(OldEntry, NewEntry); |
Misha Brukman | fd93908 | 2005-04-21 23:48:37 +0000 | [diff] [blame] | 371 | |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 372 | // If this tail call is marked 'tail' and if there are any allocas in the |
| 373 | // entry block, move them up to the new entry block. |
| 374 | TailCallsAreMarkedTail = CI->isTailCall(); |
| 375 | if (TailCallsAreMarkedTail) |
| 376 | // Move all fixed sized allocas from OldEntry to NewEntry. |
| 377 | for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(), |
| 378 | NEBI = NewEntry->begin(); OEBI != E; ) |
| 379 | if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++)) |
| 380 | if (isa<ConstantInt>(AI->getArraySize())) |
Chris Lattner | 4bc5f80 | 2005-08-08 19:11:57 +0000 | [diff] [blame] | 381 | AI->moveBefore(NEBI); |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 382 | |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 383 | // Now that we have created a new block, which jumps to the entry |
| 384 | // block, insert a PHI node for each argument of the function. |
| 385 | // For now, we initialize each PHI to only have the real arguments |
| 386 | // which are passed in. |
| 387 | Instruction *InsertPos = OldEntry->begin(); |
Chris Lattner | 7f78f21 | 2005-05-09 23:51:13 +0000 | [diff] [blame] | 388 | for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); |
| 389 | I != E; ++I) { |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 390 | PHINode *PN = new PHINode(I->getType(), I->getName()+".tr", InsertPos); |
| 391 | I->replaceAllUsesWith(PN); // Everyone use the PHI node now! |
| 392 | PN->addIncoming(I, NewEntry); |
| 393 | ArgumentPHIs.push_back(PN); |
| 394 | } |
| 395 | } |
Misha Brukman | fd93908 | 2005-04-21 23:48:37 +0000 | [diff] [blame] | 396 | |
Chris Lattner | ce869ee | 2005-08-07 04:27:41 +0000 | [diff] [blame] | 397 | // If this function has self recursive calls in the tail position where some |
| 398 | // are marked tail and some are not, only transform one flavor or another. We |
| 399 | // have to choose whether we move allocas in the entry block to the new entry |
| 400 | // block or not, so we can't make a good choice for both. NOTE: We could do |
| 401 | // slightly better here in the case that the function has no entry block |
| 402 | // allocas. |
| 403 | if (TailCallsAreMarkedTail && !CI->isTailCall()) |
| 404 | return false; |
| 405 | |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 406 | // Ok, now that we know we have a pseudo-entry block WITH all of the |
| 407 | // required PHI nodes, add entries into the PHI node for the actual |
| 408 | // parameters passed into the tail-recursive call. |
| 409 | for (unsigned i = 0, e = CI->getNumOperands()-1; i != e; ++i) |
| 410 | ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB); |
Misha Brukman | fd93908 | 2005-04-21 23:48:37 +0000 | [diff] [blame] | 411 | |
Chris Lattner | 543d622 | 2003-12-08 23:19:26 +0000 | [diff] [blame] | 412 | // If we are introducing an accumulator variable to eliminate the recursion, |
| 413 | // do so now. Note that we _know_ that no subsequent tail recursion |
| 414 | // eliminations will happen on this function because of the way the |
| 415 | // accumulator recursion predicate is set up. |
| 416 | // |
| 417 | if (AccumulatorRecursionEliminationInitVal) { |
| 418 | Instruction *AccRecInstr = AccumulatorRecursionInstr; |
| 419 | // Start by inserting a new PHI node for the accumulator. |
| 420 | PHINode *AccPN = new PHINode(AccRecInstr->getType(), "accumulator.tr", |
| 421 | OldEntry->begin()); |
| 422 | |
| 423 | // Loop over all of the predecessors of the tail recursion block. For the |
| 424 | // real entry into the function we seed the PHI with the initial value, |
| 425 | // computed earlier. For any other existing branches to this block (due to |
| 426 | // other tail recursions eliminated) the accumulator is not modified. |
| 427 | // Because we haven't added the branch in the current block to OldEntry yet, |
| 428 | // it will not show up as a predecessor. |
| 429 | for (pred_iterator PI = pred_begin(OldEntry), PE = pred_end(OldEntry); |
| 430 | PI != PE; ++PI) { |
| 431 | if (*PI == &F->getEntryBlock()) |
| 432 | AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, *PI); |
| 433 | else |
| 434 | AccPN->addIncoming(AccPN, *PI); |
| 435 | } |
| 436 | |
| 437 | // Add an incoming argument for the current block, which is computed by our |
| 438 | // associative accumulator instruction. |
| 439 | AccPN->addIncoming(AccRecInstr, BB); |
| 440 | |
| 441 | // Next, rewrite the accumulator recursion instruction so that it does not |
| 442 | // use the result of the call anymore, instead, use the PHI node we just |
| 443 | // inserted. |
| 444 | AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN); |
| 445 | |
| 446 | // Finally, rewrite any return instructions in the program to return the PHI |
| 447 | // node instead of the "initval" that they do currently. This loop will |
| 448 | // actually rewrite the return value we are destroying, but that's ok. |
| 449 | for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) |
| 450 | if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator())) |
| 451 | RI->setOperand(0, AccPN); |
| 452 | ++NumAccumAdded; |
| 453 | } |
| 454 | |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 455 | // Now that all of the PHI nodes are in place, remove the call and |
| 456 | // ret instructions, replacing them with an unconditional branch. |
| 457 | new BranchInst(OldEntry, Ret); |
| 458 | BB->getInstList().erase(Ret); // Remove return. |
| 459 | BB->getInstList().erase(CI); // Remove call. |
Chris Lattner | 543d622 | 2003-12-08 23:19:26 +0000 | [diff] [blame] | 460 | ++NumEliminated; |
Chris Lattner | 7152da3 | 2003-12-08 05:34:54 +0000 | [diff] [blame] | 461 | return true; |
| 462 | } |