Bug Summary

File:lib/Transforms/Scalar/TailRecursionElimination.cpp
Warning:line 476, column 14
Called C++ object pointer is null

Annotated Source Code

1//===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
2//
3// The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
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:
14//
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 and commutative expression to use an
20// accumulator variable, thus compiling the typical naive factorial or
21// 'fib' implementation into efficient code.
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.
28// 4. If it can prove that callees do not access their caller stack frame,
29// they are marked as eligible for tail call elimination (by the code
30// generator).
31//
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.
39// 2. Tail recursion is only performed if the call immediately precedes the
40// return instruction. It's possible that there could be a jump between
41// the call and the return.
42// 3. There can be intervening operations between the call and the return that
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.
48// 4. The algorithm we use to detect if callees access their caller stack
49// frames is very primitive.
50//
51//===----------------------------------------------------------------------===//
52
53#include "llvm/Transforms/Scalar/TailRecursionElimination.h"
54#include "llvm/Transforms/Scalar.h"
55#include "llvm/ADT/STLExtras.h"
56#include "llvm/ADT/SmallPtrSet.h"
57#include "llvm/ADT/Statistic.h"
58#include "llvm/Analysis/GlobalsModRef.h"
59#include "llvm/Analysis/CFG.h"
60#include "llvm/Analysis/CaptureTracking.h"
61#include "llvm/Analysis/InlineCost.h"
62#include "llvm/Analysis/InstructionSimplify.h"
63#include "llvm/Analysis/Loads.h"
64#include "llvm/Analysis/TargetTransformInfo.h"
65#include "llvm/IR/CFG.h"
66#include "llvm/IR/CallSite.h"
67#include "llvm/IR/Constants.h"
68#include "llvm/IR/DataLayout.h"
69#include "llvm/IR/DerivedTypes.h"
70#include "llvm/IR/DiagnosticInfo.h"
71#include "llvm/IR/Function.h"
72#include "llvm/IR/Instructions.h"
73#include "llvm/IR/IntrinsicInst.h"
74#include "llvm/IR/Module.h"
75#include "llvm/IR/ValueHandle.h"
76#include "llvm/Pass.h"
77#include "llvm/Support/Debug.h"
78#include "llvm/Support/raw_ostream.h"
79#include "llvm/Transforms/Utils/BasicBlockUtils.h"
80#include "llvm/Transforms/Utils/Local.h"
81using namespace llvm;
82
83#define DEBUG_TYPE"tailcallelim" "tailcallelim"
84
85STATISTIC(NumEliminated, "Number of tail calls removed")static llvm::Statistic NumEliminated = {"tailcallelim", "NumEliminated"
, "Number of tail calls removed", {0}, false}
;
86STATISTIC(NumRetDuped, "Number of return duplicated")static llvm::Statistic NumRetDuped = {"tailcallelim", "NumRetDuped"
, "Number of return duplicated", {0}, false}
;
87STATISTIC(NumAccumAdded, "Number of accumulators introduced")static llvm::Statistic NumAccumAdded = {"tailcallelim", "NumAccumAdded"
, "Number of accumulators introduced", {0}, false}
;
88
89/// \brief Scan the specified function for alloca instructions.
90/// If it contains any dynamic allocas, returns false.
91static bool canTRE(Function &F) {
92 // Because of PR962, we don't TRE dynamic allocas.
93 for (auto &BB : F) {
94 for (auto &I : BB) {
95 if (AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
96 if (!AI->isStaticAlloca())
97 return false;
98 }
99 }
100 }
101
102 return true;
103}
104
105namespace {
106struct AllocaDerivedValueTracker {
107 // Start at a root value and walk its use-def chain to mark calls that use the
108 // value or a derived value in AllocaUsers, and places where it may escape in
109 // EscapePoints.
110 void walk(Value *Root) {
111 SmallVector<Use *, 32> Worklist;
112 SmallPtrSet<Use *, 32> Visited;
113
114 auto AddUsesToWorklist = [&](Value *V) {
115 for (auto &U : V->uses()) {
116 if (!Visited.insert(&U).second)
117 continue;
118 Worklist.push_back(&U);
119 }
120 };
121
122 AddUsesToWorklist(Root);
123
124 while (!Worklist.empty()) {
125 Use *U = Worklist.pop_back_val();
126 Instruction *I = cast<Instruction>(U->getUser());
127
128 switch (I->getOpcode()) {
129 case Instruction::Call:
130 case Instruction::Invoke: {
131 CallSite CS(I);
132 bool IsNocapture =
133 CS.isDataOperand(U) && CS.doesNotCapture(CS.getDataOperandNo(U));
134 callUsesLocalStack(CS, IsNocapture);
135 if (IsNocapture) {
136 // If the alloca-derived argument is passed in as nocapture, then it
137 // can't propagate to the call's return. That would be capturing.
138 continue;
139 }
140 break;
141 }
142 case Instruction::Load: {
143 // The result of a load is not alloca-derived (unless an alloca has
144 // otherwise escaped, but this is a local analysis).
145 continue;
146 }
147 case Instruction::Store: {
148 if (U->getOperandNo() == 0)
149 EscapePoints.insert(I);
150 continue; // Stores have no users to analyze.
151 }
152 case Instruction::BitCast:
153 case Instruction::GetElementPtr:
154 case Instruction::PHI:
155 case Instruction::Select:
156 case Instruction::AddrSpaceCast:
157 break;
158 default:
159 EscapePoints.insert(I);
160 break;
161 }
162
163 AddUsesToWorklist(I);
164 }
165 }
166
167 void callUsesLocalStack(CallSite CS, bool IsNocapture) {
168 // Add it to the list of alloca users.
169 AllocaUsers.insert(CS.getInstruction());
170
171 // If it's nocapture then it can't capture this alloca.
172 if (IsNocapture)
173 return;
174
175 // If it can write to memory, it can leak the alloca value.
176 if (!CS.onlyReadsMemory())
177 EscapePoints.insert(CS.getInstruction());
178 }
179
180 SmallPtrSet<Instruction *, 32> AllocaUsers;
181 SmallPtrSet<Instruction *, 32> EscapePoints;
182};
183}
184
185static bool markTails(Function &F, bool &AllCallsAreTailCalls) {
186 if (F.callsFunctionThatReturnsTwice())
187 return false;
188 AllCallsAreTailCalls = true;
189
190 // The local stack holds all alloca instructions and all byval arguments.
191 AllocaDerivedValueTracker Tracker;
192 for (Argument &Arg : F.args()) {
193 if (Arg.hasByValAttr())
194 Tracker.walk(&Arg);
195 }
196 for (auto &BB : F) {
197 for (auto &I : BB)
198 if (AllocaInst *AI = dyn_cast<AllocaInst>(&I))
199 Tracker.walk(AI);
200 }
201
202 bool Modified = false;
203
204 // Track whether a block is reachable after an alloca has escaped. Blocks that
205 // contain the escaping instruction will be marked as being visited without an
206 // escaped alloca, since that is how the block began.
207 enum VisitType {
208 UNVISITED,
209 UNESCAPED,
210 ESCAPED
211 };
212 DenseMap<BasicBlock *, VisitType> Visited;
213
214 // We propagate the fact that an alloca has escaped from block to successor.
215 // Visit the blocks that are propagating the escapedness first. To do this, we
216 // maintain two worklists.
217 SmallVector<BasicBlock *, 32> WorklistUnescaped, WorklistEscaped;
218
219 // We may enter a block and visit it thinking that no alloca has escaped yet,
220 // then see an escape point and go back around a loop edge and come back to
221 // the same block twice. Because of this, we defer setting tail on calls when
222 // we first encounter them in a block. Every entry in this list does not
223 // statically use an alloca via use-def chain analysis, but may find an alloca
224 // through other means if the block turns out to be reachable after an escape
225 // point.
226 SmallVector<CallInst *, 32> DeferredTails;
227
228 BasicBlock *BB = &F.getEntryBlock();
229 VisitType Escaped = UNESCAPED;
230 do {
231 for (auto &I : *BB) {
232 if (Tracker.EscapePoints.count(&I))
233 Escaped = ESCAPED;
234
235 CallInst *CI = dyn_cast<CallInst>(&I);
236 if (!CI || CI->isTailCall())
237 continue;
238
239 bool IsNoTail = CI->isNoTailCall() || CI->hasOperandBundles();
240
241 if (!IsNoTail && CI->doesNotAccessMemory()) {
242 // A call to a readnone function whose arguments are all things computed
243 // outside this function can be marked tail. Even if you stored the
244 // alloca address into a global, a readnone function can't load the
245 // global anyhow.
246 //
247 // Note that this runs whether we know an alloca has escaped or not. If
248 // it has, then we can't trust Tracker.AllocaUsers to be accurate.
249 bool SafeToTail = true;
250 for (auto &Arg : CI->arg_operands()) {
251 if (isa<Constant>(Arg.getUser()))
252 continue;
253 if (Argument *A = dyn_cast<Argument>(Arg.getUser()))
254 if (!A->hasByValAttr())
255 continue;
256 SafeToTail = false;
257 break;
258 }
259 if (SafeToTail) {
260 emitOptimizationRemark(
261 F.getContext(), "tailcallelim", F, CI->getDebugLoc(),
262 "marked this readnone call a tail call candidate");
263 CI->setTailCall();
264 Modified = true;
265 continue;
266 }
267 }
268
269 if (!IsNoTail && Escaped == UNESCAPED && !Tracker.AllocaUsers.count(CI)) {
270 DeferredTails.push_back(CI);
271 } else {
272 AllCallsAreTailCalls = false;
273 }
274 }
275
276 for (auto *SuccBB : make_range(succ_begin(BB), succ_end(BB))) {
277 auto &State = Visited[SuccBB];
278 if (State < Escaped) {
279 State = Escaped;
280 if (State == ESCAPED)
281 WorklistEscaped.push_back(SuccBB);
282 else
283 WorklistUnescaped.push_back(SuccBB);
284 }
285 }
286
287 if (!WorklistEscaped.empty()) {
288 BB = WorklistEscaped.pop_back_val();
289 Escaped = ESCAPED;
290 } else {
291 BB = nullptr;
292 while (!WorklistUnescaped.empty()) {
293 auto *NextBB = WorklistUnescaped.pop_back_val();
294 if (Visited[NextBB] == UNESCAPED) {
295 BB = NextBB;
296 Escaped = UNESCAPED;
297 break;
298 }
299 }
300 }
301 } while (BB);
302
303 for (CallInst *CI : DeferredTails) {
304 if (Visited[CI->getParent()] != ESCAPED) {
305 // If the escape point was part way through the block, calls after the
306 // escape point wouldn't have been put into DeferredTails.
307 emitOptimizationRemark(F.getContext(), "tailcallelim", F,
308 CI->getDebugLoc(),
309 "marked this call a tail call candidate");
310 CI->setTailCall();
311 Modified = true;
312 } else {
313 AllCallsAreTailCalls = false;
314 }
315 }
316
317 return Modified;
318}
319
320/// Return true if it is safe to move the specified
321/// instruction from after the call to before the call, assuming that all
322/// instructions between the call and this instruction are movable.
323///
324static bool canMoveAboveCall(Instruction *I, CallInst *CI) {
325 // FIXME: We can move load/store/call/free instructions above the call if the
326 // call does not mod/ref the memory location being processed.
327 if (I->mayHaveSideEffects()) // This also handles volatile loads.
328 return false;
329
330 if (LoadInst *L = dyn_cast<LoadInst>(I)) {
331 // Loads may always be moved above calls without side effects.
332 if (CI->mayHaveSideEffects()) {
333 // Non-volatile loads may be moved above a call with side effects if it
334 // does not write to memory and the load provably won't trap.
335 // FIXME: Writes to memory only matter if they may alias the pointer
336 // being loaded from.
337 const DataLayout &DL = L->getModule()->getDataLayout();
338 if (CI->mayWriteToMemory() ||
339 !isSafeToLoadUnconditionally(L->getPointerOperand(),
340 L->getAlignment(), DL, L))
341 return false;
342 }
343 }
344
345 // Otherwise, if this is a side-effect free instruction, check to make sure
346 // that it does not use the return value of the call. If it doesn't use the
347 // return value of the call, it must only use things that are defined before
348 // the call, or movable instructions between the call and the instruction
349 // itself.
350 return !is_contained(I->operands(), CI);
351}
352
353/// Return true if the specified value is the same when the return would exit
354/// as it was when the initial iteration of the recursive function was executed.
355///
356/// We currently handle static constants and arguments that are not modified as
357/// part of the recursion.
358static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) {
359 if (isa<Constant>(V)) return true; // Static constants are always dyn consts
360
361 // Check to see if this is an immutable argument, if so, the value
362 // will be available to initialize the accumulator.
363 if (Argument *Arg = dyn_cast<Argument>(V)) {
364 // Figure out which argument number this is...
365 unsigned ArgNo = 0;
366 Function *F = CI->getParent()->getParent();
367 for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI)
368 ++ArgNo;
369
370 // If we are passing this argument into call as the corresponding
371 // argument operand, then the argument is dynamically constant.
372 // Otherwise, we cannot transform this function safely.
373 if (CI->getArgOperand(ArgNo) == Arg)
374 return true;
375 }
376
377 // Switch cases are always constant integers. If the value is being switched
378 // on and the return is only reachable from one of its cases, it's
379 // effectively constant.
380 if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor())
381 if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator()))
382 if (SI->getCondition() == V)
383 return SI->getDefaultDest() != RI->getParent();
384
385 // Not a constant or immutable argument, we can't safely transform.
386 return false;
387}
388
389/// Check to see if the function containing the specified tail call consistently
390/// returns the same runtime-constant value at all exit points except for
391/// IgnoreRI. If so, return the returned value.
392static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) {
393 Function *F = CI->getParent()->getParent();
394 Value *ReturnedValue = nullptr;
395
396 for (BasicBlock &BBI : *F) {
397 ReturnInst *RI = dyn_cast<ReturnInst>(BBI.getTerminator());
398 if (RI == nullptr || RI == IgnoreRI) continue;
399
400 // We can only perform this transformation if the value returned is
401 // evaluatable at the start of the initial invocation of the function,
402 // instead of at the end of the evaluation.
403 //
404 Value *RetOp = RI->getOperand(0);
405 if (!isDynamicConstant(RetOp, CI, RI))
406 return nullptr;
407
408 if (ReturnedValue && RetOp != ReturnedValue)
409 return nullptr; // Cannot transform if differing values are returned.
410 ReturnedValue = RetOp;
411 }
412 return ReturnedValue;
413}
414
415/// If the specified instruction can be transformed using accumulator recursion
416/// elimination, return the constant which is the start of the accumulator
417/// value. Otherwise return null.
418static Value *canTransformAccumulatorRecursion(Instruction *I, CallInst *CI) {
419 if (!I->isAssociative() || !I->isCommutative()) return nullptr;
420 assert(I->getNumOperands() == 2 &&((I->getNumOperands() == 2 && "Associative/commutative operations should have 2 args!"
) ? static_cast<void> (0) : __assert_fail ("I->getNumOperands() == 2 && \"Associative/commutative operations should have 2 args!\""
, "/tmp/buildd/llvm-toolchain-snapshot-5.0~svn303373/lib/Transforms/Scalar/TailRecursionElimination.cpp"
, 421, __PRETTY_FUNCTION__))
421 "Associative/commutative operations should have 2 args!")((I->getNumOperands() == 2 && "Associative/commutative operations should have 2 args!"
) ? static_cast<void> (0) : __assert_fail ("I->getNumOperands() == 2 && \"Associative/commutative operations should have 2 args!\""
, "/tmp/buildd/llvm-toolchain-snapshot-5.0~svn303373/lib/Transforms/Scalar/TailRecursionElimination.cpp"
, 421, __PRETTY_FUNCTION__))
;
422
423 // Exactly one operand should be the result of the call instruction.
424 if ((I->getOperand(0) == CI && I->getOperand(1) == CI) ||
425 (I->getOperand(0) != CI && I->getOperand(1) != CI))
426 return nullptr;
427
428 // The only user of this instruction we allow is a single return instruction.
429 if (!I->hasOneUse() || !isa<ReturnInst>(I->user_back()))
430 return nullptr;
431
432 // Ok, now we have to check all of the other return instructions in this
433 // function. If they return non-constants or differing values, then we cannot
434 // transform the function safely.
435 return getCommonReturnValue(cast<ReturnInst>(I->user_back()), CI);
436}
437
438static Instruction *firstNonDbg(BasicBlock::iterator I) {
439 while (isa<DbgInfoIntrinsic>(I))
440 ++I;
441 return &*I;
442}
443
444static CallInst *findTRECandidate(Instruction *TI,
445 bool CannotTailCallElimCallsMarkedTail,
446 const TargetTransformInfo *TTI) {
447 BasicBlock *BB = TI->getParent();
448 Function *F = BB->getParent();
18
'F' initialized here
449
450 if (&BB->front() == TI) // Make sure there is something before the terminator.
19
Assuming the condition is false
20
Taking false branch
451 return nullptr;
452
453 // Scan backwards from the return, checking to see if there is a tail call in
454 // this block. If so, set CI to it.
455 CallInst *CI = nullptr;
456 BasicBlock::iterator BBI(TI);
457 while (true) {
21
Loop condition is true. Entering loop body
458 CI = dyn_cast<CallInst>(BBI);
459 if (CI && CI->getCalledFunction() == F)
22
Assuming 'CI' is non-null
23
Assuming the condition is true
24
Assuming pointer value is null
25
Taking true branch
460 break;
26
Execution continues on line 469
461
462 if (BBI == BB->begin())
463 return nullptr; // Didn't find a potential tail call.
464 --BBI;
465 }
466
467 // If this call is marked as a tail call, and if there are dynamic allocas in
468 // the function, we cannot perform this optimization.
469 if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
27
Taking false branch
470 return nullptr;
471
472 // As a special case, detect code like this:
473 // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
474 // and disable this xform in this case, because the code generator will
475 // lower the call to fabs into inline code.
476 if (BB == &F->getEntryBlock() &&
28
Called C++ object pointer is null
477 firstNonDbg(BB->front().getIterator()) == CI &&
478 firstNonDbg(std::next(BB->begin())) == TI && CI->getCalledFunction() &&
479 !TTI->isLoweredToCall(CI->getCalledFunction())) {
480 // A single-block function with just a call and a return. Check that
481 // the arguments match.
482 CallSite::arg_iterator I = CallSite(CI).arg_begin(),
483 E = CallSite(CI).arg_end();
484 Function::arg_iterator FI = F->arg_begin(),
485 FE = F->arg_end();
486 for (; I != E && FI != FE; ++I, ++FI)
487 if (*I != &*FI) break;
488 if (I == E && FI == FE)
489 return nullptr;
490 }
491
492 return CI;
493}
494
495static bool
496eliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret, BasicBlock *&OldEntry,
497 bool &TailCallsAreMarkedTail,
498 SmallVectorImpl<PHINode *> &ArgumentPHIs) {
499 // If we are introducing accumulator recursion to eliminate operations after
500 // the call instruction that are both associative and commutative, the initial
501 // value for the accumulator is placed in this variable. If this value is set
502 // then we actually perform accumulator recursion elimination instead of
503 // simple tail recursion elimination. If the operation is an LLVM instruction
504 // (eg: "add") then it is recorded in AccumulatorRecursionInstr. If not, then
505 // we are handling the case when the return instruction returns a constant C
506 // which is different to the constant returned by other return instructions
507 // (which is recorded in AccumulatorRecursionEliminationInitVal). This is a
508 // special case of accumulator recursion, the operation being "return C".
509 Value *AccumulatorRecursionEliminationInitVal = nullptr;
510 Instruction *AccumulatorRecursionInstr = nullptr;
511
512 // Ok, we found a potential tail call. We can currently only transform the
513 // tail call if all of the instructions between the call and the return are
514 // movable to above the call itself, leaving the call next to the return.
515 // Check that this is the case now.
516 BasicBlock::iterator BBI(CI);
517 for (++BBI; &*BBI != Ret; ++BBI) {
518 if (canMoveAboveCall(&*BBI, CI)) continue;
519
520 // If we can't move the instruction above the call, it might be because it
521 // is an associative and commutative operation that could be transformed
522 // using accumulator recursion elimination. Check to see if this is the
523 // case, and if so, remember the initial accumulator value for later.
524 if ((AccumulatorRecursionEliminationInitVal =
525 canTransformAccumulatorRecursion(&*BBI, CI))) {
526 // Yes, this is accumulator recursion. Remember which instruction
527 // accumulates.
528 AccumulatorRecursionInstr = &*BBI;
529 } else {
530 return false; // Otherwise, we cannot eliminate the tail recursion!
531 }
532 }
533
534 // We can only transform call/return pairs that either ignore the return value
535 // of the call and return void, ignore the value of the call and return a
536 // constant, return the value returned by the tail call, or that are being
537 // accumulator recursion variable eliminated.
538 if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
539 !isa<UndefValue>(Ret->getReturnValue()) &&
540 AccumulatorRecursionEliminationInitVal == nullptr &&
541 !getCommonReturnValue(nullptr, CI)) {
542 // One case remains that we are able to handle: the current return
543 // instruction returns a constant, and all other return instructions
544 // return a different constant.
545 if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret))
546 return false; // Current return instruction does not return a constant.
547 // Check that all other return instructions return a common constant. If
548 // so, record it in AccumulatorRecursionEliminationInitVal.
549 AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI);
550 if (!AccumulatorRecursionEliminationInitVal)
551 return false;
552 }
553
554 BasicBlock *BB = Ret->getParent();
555 Function *F = BB->getParent();
556
557 emitOptimizationRemark(F->getContext(), "tailcallelim", *F, CI->getDebugLoc(),
558 "transforming tail recursion to loop");
559
560 // OK! We can transform this tail call. If this is the first one found,
561 // create the new entry block, allowing us to branch back to the old entry.
562 if (!OldEntry) {
563 OldEntry = &F->getEntryBlock();
564 BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry);
565 NewEntry->takeName(OldEntry);
566 OldEntry->setName("tailrecurse");
567 BranchInst::Create(OldEntry, NewEntry);
568
569 // If this tail call is marked 'tail' and if there are any allocas in the
570 // entry block, move them up to the new entry block.
571 TailCallsAreMarkedTail = CI->isTailCall();
572 if (TailCallsAreMarkedTail)
573 // Move all fixed sized allocas from OldEntry to NewEntry.
574 for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(),
575 NEBI = NewEntry->begin(); OEBI != E; )
576 if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
577 if (isa<ConstantInt>(AI->getArraySize()))
578 AI->moveBefore(&*NEBI);
579
580 // Now that we have created a new block, which jumps to the entry
581 // block, insert a PHI node for each argument of the function.
582 // For now, we initialize each PHI to only have the real arguments
583 // which are passed in.
584 Instruction *InsertPos = &OldEntry->front();
585 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
586 I != E; ++I) {
587 PHINode *PN = PHINode::Create(I->getType(), 2,
588 I->getName() + ".tr", InsertPos);
589 I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
590 PN->addIncoming(&*I, NewEntry);
591 ArgumentPHIs.push_back(PN);
592 }
593 }
594
595 // If this function has self recursive calls in the tail position where some
596 // are marked tail and some are not, only transform one flavor or another. We
597 // have to choose whether we move allocas in the entry block to the new entry
598 // block or not, so we can't make a good choice for both. NOTE: We could do
599 // slightly better here in the case that the function has no entry block
600 // allocas.
601 if (TailCallsAreMarkedTail && !CI->isTailCall())
602 return false;
603
604 // Ok, now that we know we have a pseudo-entry block WITH all of the
605 // required PHI nodes, add entries into the PHI node for the actual
606 // parameters passed into the tail-recursive call.
607 for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i)
608 ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), BB);
609
610 // If we are introducing an accumulator variable to eliminate the recursion,
611 // do so now. Note that we _know_ that no subsequent tail recursion
612 // eliminations will happen on this function because of the way the
613 // accumulator recursion predicate is set up.
614 //
615 if (AccumulatorRecursionEliminationInitVal) {
616 Instruction *AccRecInstr = AccumulatorRecursionInstr;
617 // Start by inserting a new PHI node for the accumulator.
618 pred_iterator PB = pred_begin(OldEntry), PE = pred_end(OldEntry);
619 PHINode *AccPN = PHINode::Create(
620 AccumulatorRecursionEliminationInitVal->getType(),
621 std::distance(PB, PE) + 1, "accumulator.tr", &OldEntry->front());
622
623 // Loop over all of the predecessors of the tail recursion block. For the
624 // real entry into the function we seed the PHI with the initial value,
625 // computed earlier. For any other existing branches to this block (due to
626 // other tail recursions eliminated) the accumulator is not modified.
627 // Because we haven't added the branch in the current block to OldEntry yet,
628 // it will not show up as a predecessor.
629 for (pred_iterator PI = PB; PI != PE; ++PI) {
630 BasicBlock *P = *PI;
631 if (P == &F->getEntryBlock())
632 AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P);
633 else
634 AccPN->addIncoming(AccPN, P);
635 }
636
637 if (AccRecInstr) {
638 // Add an incoming argument for the current block, which is computed by
639 // our associative and commutative accumulator instruction.
640 AccPN->addIncoming(AccRecInstr, BB);
641
642 // Next, rewrite the accumulator recursion instruction so that it does not
643 // use the result of the call anymore, instead, use the PHI node we just
644 // inserted.
645 AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
646 } else {
647 // Add an incoming argument for the current block, which is just the
648 // constant returned by the current return instruction.
649 AccPN->addIncoming(Ret->getReturnValue(), BB);
650 }
651
652 // Finally, rewrite any return instructions in the program to return the PHI
653 // node instead of the "initval" that they do currently. This loop will
654 // actually rewrite the return value we are destroying, but that's ok.
655 for (BasicBlock &BBI : *F)
656 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI.getTerminator()))
657 RI->setOperand(0, AccPN);
658 ++NumAccumAdded;
659 }
660
661 // Now that all of the PHI nodes are in place, remove the call and
662 // ret instructions, replacing them with an unconditional branch.
663 BranchInst *NewBI = BranchInst::Create(OldEntry, Ret);
664 NewBI->setDebugLoc(CI->getDebugLoc());
665
666 BB->getInstList().erase(Ret); // Remove return.
667 BB->getInstList().erase(CI); // Remove call.
668 ++NumEliminated;
669 return true;
670}
671
672static bool foldReturnAndProcessPred(BasicBlock *BB, ReturnInst *Ret,
673 BasicBlock *&OldEntry,
674 bool &TailCallsAreMarkedTail,
675 SmallVectorImpl<PHINode *> &ArgumentPHIs,
676 bool CannotTailCallElimCallsMarkedTail,
677 const TargetTransformInfo *TTI) {
678 bool Change = false;
679
680 // If the return block contains nothing but the return and PHI's,
681 // there might be an opportunity to duplicate the return in its
682 // predecessors and perform TRC there. Look for predecessors that end
683 // in unconditional branch and recursive call(s).
684 SmallVector<BranchInst*, 8> UncondBranchPreds;
685 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
686 BasicBlock *Pred = *PI;
687 TerminatorInst *PTI = Pred->getTerminator();
688 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
689 if (BI->isUnconditional())
690 UncondBranchPreds.push_back(BI);
691 }
692
693 while (!UncondBranchPreds.empty()) {
694 BranchInst *BI = UncondBranchPreds.pop_back_val();
695 BasicBlock *Pred = BI->getParent();
696 if (CallInst *CI = findTRECandidate(BI, CannotTailCallElimCallsMarkedTail, TTI)){
697 DEBUG(dbgs() << "FOLDING: " << *BBdo { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("tailcallelim")) { dbgs() << "FOLDING: " << *BB <<
"INTO UNCOND BRANCH PRED: " << *Pred; } } while (false
)
698 << "INTO UNCOND BRANCH PRED: " << *Pred)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("tailcallelim")) { dbgs() << "FOLDING: " << *BB <<
"INTO UNCOND BRANCH PRED: " << *Pred; } } while (false
)
;
699 ReturnInst *RI = FoldReturnIntoUncondBranch(Ret, BB, Pred);
700
701 // Cleanup: if all predecessors of BB have been eliminated by
702 // FoldReturnIntoUncondBranch, delete it. It is important to empty it,
703 // because the ret instruction in there is still using a value which
704 // eliminateRecursiveTailCall will attempt to remove.
705 if (!BB->hasAddressTaken() && pred_begin(BB) == pred_end(BB))
706 BB->eraseFromParent();
707
708 eliminateRecursiveTailCall(CI, RI, OldEntry, TailCallsAreMarkedTail,
709 ArgumentPHIs);
710 ++NumRetDuped;
711 Change = true;
712 }
713 }
714
715 return Change;
716}
717
718static bool processReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
719 bool &TailCallsAreMarkedTail,
720 SmallVectorImpl<PHINode *> &ArgumentPHIs,
721 bool CannotTailCallElimCallsMarkedTail,
722 const TargetTransformInfo *TTI) {
723 CallInst *CI = findTRECandidate(Ret, CannotTailCallElimCallsMarkedTail, TTI);
17
Calling 'findTRECandidate'
724 if (!CI)
725 return false;
726
727 return eliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail,
728 ArgumentPHIs);
729}
730
731static bool eliminateTailRecursion(Function &F, const TargetTransformInfo *TTI) {
732 if (F.getFnAttribute("disable-tail-calls").getValueAsString() == "true")
2
Assuming the condition is false
3
Taking false branch
733 return false;
734
735 bool MadeChange = false;
736 bool AllCallsAreTailCalls = false;
737 MadeChange |= markTails(F, AllCallsAreTailCalls);
738 if (!AllCallsAreTailCalls)
4
Assuming 'AllCallsAreTailCalls' is not equal to 0
5
Taking false branch
739 return MadeChange;
740
741 // If this function is a varargs function, we won't be able to PHI the args
742 // right, so don't even try to convert it...
743 if (F.getFunctionType()->isVarArg())
6
Taking false branch
744 return false;
745
746 BasicBlock *OldEntry = nullptr;
747 bool TailCallsAreMarkedTail = false;
748 SmallVector<PHINode*, 8> ArgumentPHIs;
749
750 // If false, we cannot perform TRE on tail calls marked with the 'tail'
751 // attribute, because doing so would cause the stack size to increase (real
752 // TRE would deallocate variable sized allocas, TRE doesn't).
753 bool CanTRETailMarkedCall = canTRE(F);
754
755 // Change any tail recursive calls to loops.
756 //
757 // FIXME: The code generator produces really bad code when an 'escaping
758 // alloca' is changed from being a static alloca to being a dynamic alloca.
759 // Until this is resolved, disable this transformation if that would ever
760 // happen. This bug is PR962.
761 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; /*in loop*/) {
7
Loop condition is true. Entering loop body
9
Loop condition is true. Entering loop body
11
Loop condition is true. Entering loop body
13
Loop condition is true. Entering loop body
762 BasicBlock *BB = &*BBI++; // foldReturnAndProcessPred may delete BB.
763 if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
8
Taking false branch
10
Taking false branch
12
Taking false branch
14
Assuming 'Ret' is non-null
15
Taking true branch
764 bool Change =
765 processReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
16
Calling 'processReturningBlock'
766 ArgumentPHIs, !CanTRETailMarkedCall, TTI);
767 if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
768 Change =
769 foldReturnAndProcessPred(BB, Ret, OldEntry, TailCallsAreMarkedTail,
770 ArgumentPHIs, !CanTRETailMarkedCall, TTI);
771 MadeChange |= Change;
772 }
773 }
774
775 // If we eliminated any tail recursions, it's possible that we inserted some
776 // silly PHI nodes which just merge an initial value (the incoming operand)
777 // with themselves. Check to see if we did and clean up our mess if so. This
778 // occurs when a function passes an argument straight through to its tail
779 // call.
780 for (PHINode *PN : ArgumentPHIs) {
781 // If the PHI Node is a dynamic constant, replace it with the value it is.
782 if (Value *PNV = SimplifyInstruction(PN, F.getParent()->getDataLayout())) {
783 PN->replaceAllUsesWith(PNV);
784 PN->eraseFromParent();
785 }
786 }
787
788 return MadeChange;
789}
790
791namespace {
792struct TailCallElim : public FunctionPass {
793 static char ID; // Pass identification, replacement for typeid
794 TailCallElim() : FunctionPass(ID) {
795 initializeTailCallElimPass(*PassRegistry::getPassRegistry());
796 }
797
798 void getAnalysisUsage(AnalysisUsage &AU) const override {
799 AU.addRequired<TargetTransformInfoWrapperPass>();
800 AU.addPreserved<GlobalsAAWrapperPass>();
801 }
802
803 bool runOnFunction(Function &F) override {
804 if (skipFunction(F))
805 return false;
806
807 return eliminateTailRecursion(
808 F, &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F));
809 }
810};
811}
812
813char TailCallElim::ID = 0;
814INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim", "Tail Call Elimination",static void *initializeTailCallElimPassOnce(PassRegistry &
Registry) {
815 false, false)static void *initializeTailCallElimPassOnce(PassRegistry &
Registry) {
816INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)initializeTargetTransformInfoWrapperPassPass(Registry);
817INITIALIZE_PASS_END(TailCallElim, "tailcallelim", "Tail Call Elimination",PassInfo *PI = new PassInfo( "Tail Call Elimination", "tailcallelim"
, &TailCallElim::ID, PassInfo::NormalCtor_t(callDefaultCtor
<TailCallElim>), false, false); Registry.registerPass(*
PI, true); return PI; } static llvm::once_flag InitializeTailCallElimPassFlag
; void llvm::initializeTailCallElimPass(PassRegistry &Registry
) { llvm::call_once(InitializeTailCallElimPassFlag, initializeTailCallElimPassOnce
, std::ref(Registry)); }
818 false, false)PassInfo *PI = new PassInfo( "Tail Call Elimination", "tailcallelim"
, &TailCallElim::ID, PassInfo::NormalCtor_t(callDefaultCtor
<TailCallElim>), false, false); Registry.registerPass(*
PI, true); return PI; } static llvm::once_flag InitializeTailCallElimPassFlag
; void llvm::initializeTailCallElimPass(PassRegistry &Registry
) { llvm::call_once(InitializeTailCallElimPassFlag, initializeTailCallElimPassOnce
, std::ref(Registry)); }
819
820// Public interface to the TailCallElimination pass
821FunctionPass *llvm::createTailCallEliminationPass() {
822 return new TailCallElim();
823}
824
825PreservedAnalyses TailCallElimPass::run(Function &F,
826 FunctionAnalysisManager &AM) {
827
828 TargetTransformInfo &TTI = AM.getResult<TargetIRAnalysis>(F);
829
830 bool Changed = eliminateTailRecursion(F, &TTI);
1
Calling 'eliminateTailRecursion'
831
832 if (!Changed)
833 return PreservedAnalyses::all();
834 PreservedAnalyses PA;
835 PA.preserve<GlobalsAA>();
836 return PA;
837}