1 //===- IndVarSimplify.cpp - Induction Variable Elimination ----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This transformation analyzes and transforms the induction variables (and
11 // computations derived from them) into simpler forms suitable for subsequent
12 // analysis and transformation.
14 // This transformation makes the following changes to each loop with an
15 // identifiable induction variable:
16 // 1. All loops are transformed to have a SINGLE canonical induction variable
17 // which starts at zero and steps by one.
18 // 2. The canonical induction variable is guaranteed to be the first PHI node
19 // in the loop header block.
20 // 3. Any pointer arithmetic recurrences are raised to use array subscripts.
22 // If the trip count of a loop is computable, this pass also makes the following
24 // 1. The exit condition for the loop is canonicalized to compare the
25 // induction value against the exit value. This turns loops like:
26 // 'for (i = 7; i*i < 1000; ++i)' into 'for (i = 0; i != 25; ++i)'
27 // 2. Any use outside of the loop of an expression derived from the indvar
28 // is changed to compute the derived value outside of the loop, eliminating
29 // the dependence on the exit value of the induction variable. If the only
30 // purpose of the loop is to compute the exit value of some derived
31 // expression, this transformation will make the loop dead.
33 // This transformation should be followed by strength reduction after all of the
34 // desired loop transformations have been performed. Additionally, on targets
35 // where it is profitable, the loop could be transformed to count down to zero
36 // (the "do loop" optimization).
38 //===----------------------------------------------------------------------===//
40 #define DEBUG_TYPE "indvars"
41 #include "llvm/Transforms/Scalar.h"
42 #include "llvm/BasicBlock.h"
43 #include "llvm/Constants.h"
44 #include "llvm/Instructions.h"
45 #include "llvm/Type.h"
46 #include "llvm/Analysis/ScalarEvolutionExpander.h"
47 #include "llvm/Analysis/LoopInfo.h"
48 #include "llvm/Analysis/LoopPass.h"
49 #include "llvm/Support/CFG.h"
50 #include "llvm/Support/Compiler.h"
51 #include "llvm/Support/Debug.h"
52 #include "llvm/Support/GetElementPtrTypeIterator.h"
53 #include "llvm/Transforms/Utils/Local.h"
54 #include "llvm/Support/CommandLine.h"
55 #include "llvm/ADT/SmallVector.h"
56 #include "llvm/ADT/Statistic.h"
59 STATISTIC(NumRemoved , "Number of aux indvars removed");
60 STATISTIC(NumPointer , "Number of pointer indvars promoted");
61 STATISTIC(NumInserted, "Number of canonical indvars added");
62 STATISTIC(NumReplaced, "Number of exit values replaced");
63 STATISTIC(NumLFTR , "Number of loop exit tests replaced");
66 class VISIBILITY_HIDDEN IndVarSimplify : public LoopPass {
72 static char ID; // Pass identification, replacement for typeid
73 IndVarSimplify() : LoopPass((intptr_t)&ID) {}
75 bool runOnLoop(Loop *L, LPPassManager &LPM);
76 bool doInitialization(Loop *L, LPPassManager &LPM);
77 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
78 AU.addRequired<ScalarEvolution>();
79 AU.addRequiredID(LCSSAID);
80 AU.addRequiredID(LoopSimplifyID);
81 AU.addRequired<LoopInfo>();
82 AU.addPreservedID(LoopSimplifyID);
83 AU.addPreservedID(LCSSAID);
89 void EliminatePointerRecurrence(PHINode *PN, BasicBlock *Preheader,
90 std::set<Instruction*> &DeadInsts);
91 Instruction *LinearFunctionTestReplace(Loop *L, SCEV *IterationCount,
93 void RewriteLoopExitValues(Loop *L);
95 void DeleteTriviallyDeadInstructions(std::set<Instruction*> &Insts);
99 char IndVarSimplify::ID = 0;
100 static RegisterPass<IndVarSimplify>
101 X("indvars", "Canonicalize Induction Variables");
103 LoopPass *llvm::createIndVarSimplifyPass() {
104 return new IndVarSimplify();
107 /// DeleteTriviallyDeadInstructions - If any of the instructions is the
108 /// specified set are trivially dead, delete them and see if this makes any of
109 /// their operands subsequently dead.
110 void IndVarSimplify::
111 DeleteTriviallyDeadInstructions(std::set<Instruction*> &Insts) {
112 while (!Insts.empty()) {
113 Instruction *I = *Insts.begin();
114 Insts.erase(Insts.begin());
115 if (isInstructionTriviallyDead(I)) {
116 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
117 if (Instruction *U = dyn_cast<Instruction>(I->getOperand(i)))
119 SE->deleteValueFromRecords(I);
120 DOUT << "INDVARS: Deleting: " << *I;
121 I->eraseFromParent();
128 /// EliminatePointerRecurrence - Check to see if this is a trivial GEP pointer
129 /// recurrence. If so, change it into an integer recurrence, permitting
130 /// analysis by the SCEV routines.
131 void IndVarSimplify::EliminatePointerRecurrence(PHINode *PN,
132 BasicBlock *Preheader,
133 std::set<Instruction*> &DeadInsts) {
134 assert(PN->getNumIncomingValues() == 2 && "Noncanonicalized loop!");
135 unsigned PreheaderIdx = PN->getBasicBlockIndex(Preheader);
136 unsigned BackedgeIdx = PreheaderIdx^1;
137 if (GetElementPtrInst *GEPI =
138 dyn_cast<GetElementPtrInst>(PN->getIncomingValue(BackedgeIdx)))
139 if (GEPI->getOperand(0) == PN) {
140 assert(GEPI->getNumOperands() == 2 && "GEP types must match!");
141 DOUT << "INDVARS: Eliminating pointer recurrence: " << *GEPI;
143 // Okay, we found a pointer recurrence. Transform this pointer
144 // recurrence into an integer recurrence. Compute the value that gets
145 // added to the pointer at every iteration.
146 Value *AddedVal = GEPI->getOperand(1);
148 // Insert a new integer PHI node into the top of the block.
149 PHINode *NewPhi = PHINode::Create(AddedVal->getType(),
150 PN->getName()+".rec", PN);
151 NewPhi->addIncoming(Constant::getNullValue(NewPhi->getType()), Preheader);
153 // Create the new add instruction.
154 Value *NewAdd = BinaryOperator::CreateAdd(NewPhi, AddedVal,
155 GEPI->getName()+".rec", GEPI);
156 NewPhi->addIncoming(NewAdd, PN->getIncomingBlock(BackedgeIdx));
158 // Update the existing GEP to use the recurrence.
159 GEPI->setOperand(0, PN->getIncomingValue(PreheaderIdx));
161 // Update the GEP to use the new recurrence we just inserted.
162 GEPI->setOperand(1, NewAdd);
164 // If the incoming value is a constant expr GEP, try peeling out the array
165 // 0 index if possible to make things simpler.
166 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(GEPI->getOperand(0)))
167 if (CE->getOpcode() == Instruction::GetElementPtr) {
168 unsigned NumOps = CE->getNumOperands();
169 assert(NumOps > 1 && "CE folding didn't work!");
170 if (CE->getOperand(NumOps-1)->isNullValue()) {
171 // Check to make sure the last index really is an array index.
172 gep_type_iterator GTI = gep_type_begin(CE);
173 for (unsigned i = 1, e = CE->getNumOperands()-1;
176 if (isa<SequentialType>(*GTI)) {
177 // Pull the last index out of the constant expr GEP.
178 SmallVector<Value*, 8> CEIdxs(CE->op_begin()+1, CE->op_end()-1);
179 Constant *NCE = ConstantExpr::getGetElementPtr(CE->getOperand(0),
183 Idx[0] = Constant::getNullValue(Type::Int32Ty);
185 GetElementPtrInst *NGEPI = GetElementPtrInst::Create(
187 GEPI->getName(), GEPI);
188 SE->deleteValueFromRecords(GEPI);
189 GEPI->replaceAllUsesWith(NGEPI);
190 GEPI->eraseFromParent();
197 // Finally, if there are any other users of the PHI node, we must
198 // insert a new GEP instruction that uses the pre-incremented version
199 // of the induction amount.
200 if (!PN->use_empty()) {
201 BasicBlock::iterator InsertPos = PN; ++InsertPos;
202 while (isa<PHINode>(InsertPos)) ++InsertPos;
204 GetElementPtrInst::Create(PN->getIncomingValue(PreheaderIdx),
205 NewPhi, "", InsertPos);
206 PreInc->takeName(PN);
207 PN->replaceAllUsesWith(PreInc);
210 // Delete the old PHI for sure, and the GEP if its otherwise unused.
211 DeadInsts.insert(PN);
218 /// LinearFunctionTestReplace - This method rewrites the exit condition of the
219 /// loop to be a canonical != comparison against the incremented loop induction
220 /// variable. This pass is able to rewrite the exit tests of any loop where the
221 /// SCEV analysis can determine a loop-invariant trip count of the loop, which
222 /// is actually a much broader range than just linear tests.
224 /// This method returns a "potentially dead" instruction whose computation chain
225 /// should be deleted when convenient.
226 Instruction *IndVarSimplify::LinearFunctionTestReplace(Loop *L,
227 SCEV *IterationCount,
229 // Find the exit block for the loop. We can currently only handle loops with
231 SmallVector<BasicBlock*, 8> ExitBlocks;
232 L->getExitBlocks(ExitBlocks);
233 if (ExitBlocks.size() != 1) return 0;
234 BasicBlock *ExitBlock = ExitBlocks[0];
236 // Make sure there is only one predecessor block in the loop.
237 BasicBlock *ExitingBlock = 0;
238 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
240 if (L->contains(*PI)) {
241 if (ExitingBlock == 0)
244 return 0; // Multiple exits from loop to this block.
246 assert(ExitingBlock && "Loop info is broken");
248 if (!isa<BranchInst>(ExitingBlock->getTerminator()))
249 return 0; // Can't rewrite non-branch yet
250 BranchInst *BI = cast<BranchInst>(ExitingBlock->getTerminator());
251 assert(BI->isConditional() && "Must be conditional to be part of loop!");
253 Instruction *PotentiallyDeadInst = dyn_cast<Instruction>(BI->getCondition());
255 // If the exiting block is not the same as the backedge block, we must compare
256 // against the preincremented value, otherwise we prefer to compare against
257 // the post-incremented value.
258 BasicBlock *Header = L->getHeader();
259 pred_iterator HPI = pred_begin(Header);
260 assert(HPI != pred_end(Header) && "Loop with zero preds???");
261 if (!L->contains(*HPI)) ++HPI;
262 assert(HPI != pred_end(Header) && L->contains(*HPI) &&
263 "No backedge in loop?");
265 SCEVHandle TripCount = IterationCount;
267 if (*HPI == ExitingBlock) {
268 // The IterationCount expression contains the number of times that the
269 // backedge actually branches to the loop header. This is one less than the
270 // number of times the loop executes, so add one to it.
271 ConstantInt *OneC = ConstantInt::get(IterationCount->getType(), 1);
272 TripCount = SE->getAddExpr(IterationCount, SE->getConstant(OneC));
273 IndVar = L->getCanonicalInductionVariableIncrement();
275 // We have to use the preincremented value...
276 IndVar = L->getCanonicalInductionVariable();
279 DOUT << "INDVARS: LFTR: TripCount = " << *TripCount
280 << " IndVar = " << *IndVar << "\n";
282 // Expand the code for the iteration count into the preheader of the loop.
283 BasicBlock *Preheader = L->getLoopPreheader();
284 Value *ExitCnt = RW.expandCodeFor(TripCount, Preheader->getTerminator());
286 // Insert a new icmp_ne or icmp_eq instruction before the branch.
287 ICmpInst::Predicate Opcode;
288 if (L->contains(BI->getSuccessor(0)))
289 Opcode = ICmpInst::ICMP_NE;
291 Opcode = ICmpInst::ICMP_EQ;
293 Value *Cond = new ICmpInst(Opcode, IndVar, ExitCnt, "exitcond", BI);
294 BI->setCondition(Cond);
297 return PotentiallyDeadInst;
301 /// RewriteLoopExitValues - Check to see if this loop has a computable
302 /// loop-invariant execution count. If so, this means that we can compute the
303 /// final value of any expressions that are recurrent in the loop, and
304 /// substitute the exit values from the loop into any instructions outside of
305 /// the loop that use the final values of the current expressions.
306 void IndVarSimplify::RewriteLoopExitValues(Loop *L) {
307 BasicBlock *Preheader = L->getLoopPreheader();
309 // Scan all of the instructions in the loop, looking at those that have
310 // extra-loop users and which are recurrences.
311 SCEVExpander Rewriter(*SE, *LI);
313 // We insert the code into the preheader of the loop if the loop contains
314 // multiple exit blocks, or in the exit block if there is exactly one.
315 BasicBlock *BlockToInsertInto;
316 SmallVector<BasicBlock*, 8> ExitBlocks;
317 L->getUniqueExitBlocks(ExitBlocks);
318 if (ExitBlocks.size() == 1)
319 BlockToInsertInto = ExitBlocks[0];
321 BlockToInsertInto = Preheader;
322 BasicBlock::iterator InsertPt = BlockToInsertInto->begin();
323 while (isa<PHINode>(InsertPt)) ++InsertPt;
325 bool HasConstantItCount = isa<SCEVConstant>(SE->getIterationCount(L));
327 std::set<Instruction*> InstructionsToDelete;
328 std::map<Instruction*, Value*> ExitValues;
330 // Find all values that are computed inside the loop, but used outside of it.
331 // Because of LCSSA, these values will only occur in LCSSA PHI Nodes. Scan
332 // the exit blocks of the loop to find them.
333 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
334 BasicBlock *ExitBB = ExitBlocks[i];
336 // If there are no PHI nodes in this exit block, then no values defined
337 // inside the loop are used on this path, skip it.
338 PHINode *PN = dyn_cast<PHINode>(ExitBB->begin());
341 unsigned NumPreds = PN->getNumIncomingValues();
343 // Iterate over all of the PHI nodes.
344 BasicBlock::iterator BBI = ExitBB->begin();
345 while ((PN = dyn_cast<PHINode>(BBI++))) {
347 // Iterate over all of the values in all the PHI nodes.
348 for (unsigned i = 0; i != NumPreds; ++i) {
349 // If the value being merged in is not integer or is not defined
350 // in the loop, skip it.
351 Value *InVal = PN->getIncomingValue(i);
352 if (!isa<Instruction>(InVal) ||
353 // SCEV only supports integer expressions for now.
354 !isa<IntegerType>(InVal->getType()))
357 // If this pred is for a subloop, not L itself, skip it.
358 if (LI->getLoopFor(PN->getIncomingBlock(i)) != L)
359 continue; // The Block is in a subloop, skip it.
361 // Check that InVal is defined in the loop.
362 Instruction *Inst = cast<Instruction>(InVal);
363 if (!L->contains(Inst->getParent()))
366 // We require that this value either have a computable evolution or that
367 // the loop have a constant iteration count. In the case where the loop
368 // has a constant iteration count, we can sometimes force evaluation of
369 // the exit value through brute force.
370 SCEVHandle SH = SE->getSCEV(Inst);
371 if (!SH->hasComputableLoopEvolution(L) && !HasConstantItCount)
372 continue; // Cannot get exit evolution for the loop value.
374 // Okay, this instruction has a user outside of the current loop
375 // and varies predictably *inside* the loop. Evaluate the value it
376 // contains when the loop exits, if possible.
377 SCEVHandle ExitValue = SE->getSCEVAtScope(Inst, L->getParentLoop());
378 if (isa<SCEVCouldNotCompute>(ExitValue) ||
379 !ExitValue->isLoopInvariant(L))
385 // See if we already computed the exit value for the instruction, if so,
387 Value *&ExitVal = ExitValues[Inst];
389 ExitVal = Rewriter.expandCodeFor(ExitValue, InsertPt);
391 DOUT << "INDVARS: RLEV: AfterLoopVal = " << *ExitVal
392 << " LoopVal = " << *Inst << "\n";
394 PN->setIncomingValue(i, ExitVal);
396 // If this instruction is dead now, schedule it to be removed.
397 if (Inst->use_empty())
398 InstructionsToDelete.insert(Inst);
400 // See if this is a single-entry LCSSA PHI node. If so, we can (and
402 // the PHI entirely. This is safe, because the NewVal won't be variant
403 // in the loop, so we don't need an LCSSA phi node anymore.
405 SE->deleteValueFromRecords(PN);
406 PN->replaceAllUsesWith(ExitVal);
407 PN->eraseFromParent();
414 DeleteTriviallyDeadInstructions(InstructionsToDelete);
417 bool IndVarSimplify::doInitialization(Loop *L, LPPassManager &LPM) {
420 // First step. Check to see if there are any trivial GEP pointer recurrences.
421 // If there are, change them into integer recurrences, permitting analysis by
422 // the SCEV routines.
424 BasicBlock *Header = L->getHeader();
425 BasicBlock *Preheader = L->getLoopPreheader();
426 SE = &LPM.getAnalysis<ScalarEvolution>();
428 std::set<Instruction*> DeadInsts;
429 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
430 PHINode *PN = cast<PHINode>(I);
431 if (isa<PointerType>(PN->getType()))
432 EliminatePointerRecurrence(PN, Preheader, DeadInsts);
435 if (!DeadInsts.empty())
436 DeleteTriviallyDeadInstructions(DeadInsts);
441 bool IndVarSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
444 LI = &getAnalysis<LoopInfo>();
445 SE = &getAnalysis<ScalarEvolution>();
448 BasicBlock *Header = L->getHeader();
449 std::set<Instruction*> DeadInsts;
451 // Verify the input to the pass in already in LCSSA form.
452 assert(L->isLCSSAForm());
454 // Check to see if this loop has a computable loop-invariant execution count.
455 // If so, this means that we can compute the final value of any expressions
456 // that are recurrent in the loop, and substitute the exit values from the
457 // loop into any instructions outside of the loop that use the final values of
458 // the current expressions.
460 SCEVHandle IterationCount = SE->getIterationCount(L);
461 if (!isa<SCEVCouldNotCompute>(IterationCount))
462 RewriteLoopExitValues(L);
464 // Next, analyze all of the induction variables in the loop, canonicalizing
465 // auxillary induction variables.
466 std::vector<std::pair<PHINode*, SCEVHandle> > IndVars;
468 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
469 PHINode *PN = cast<PHINode>(I);
470 if (PN->getType()->isInteger()) { // FIXME: when we have fast-math, enable!
471 SCEVHandle SCEV = SE->getSCEV(PN);
472 if (SCEV->hasComputableLoopEvolution(L))
473 // FIXME: It is an extremely bad idea to indvar substitute anything more
474 // complex than affine induction variables. Doing so will put expensive
475 // polynomial evaluations inside of the loop, and the str reduction pass
476 // currently can only reduce affine polynomials. For now just disable
477 // indvar subst on anything more complex than an affine addrec.
478 if (SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(SCEV))
480 IndVars.push_back(std::make_pair(PN, SCEV));
484 // If there are no induction variables in the loop, there is nothing more to
486 if (IndVars.empty()) {
487 // Actually, if we know how many times the loop iterates, lets insert a
488 // canonical induction variable to help subsequent passes.
489 if (!isa<SCEVCouldNotCompute>(IterationCount)) {
490 SCEVExpander Rewriter(*SE, *LI);
491 Rewriter.getOrInsertCanonicalInductionVariable(L,
492 IterationCount->getType());
493 if (Instruction *I = LinearFunctionTestReplace(L, IterationCount,
495 std::set<Instruction*> InstructionsToDelete;
496 InstructionsToDelete.insert(I);
497 DeleteTriviallyDeadInstructions(InstructionsToDelete);
503 // Compute the type of the largest recurrence expression.
505 const Type *LargestType = IndVars[0].first->getType();
506 bool DifferingSizes = false;
507 for (unsigned i = 1, e = IndVars.size(); i != e; ++i) {
508 const Type *Ty = IndVars[i].first->getType();
510 Ty->getPrimitiveSizeInBits() != LargestType->getPrimitiveSizeInBits();
511 if (Ty->getPrimitiveSizeInBits() > LargestType->getPrimitiveSizeInBits())
515 // Create a rewriter object which we'll use to transform the code with.
516 SCEVExpander Rewriter(*SE, *LI);
518 // Now that we know the largest of of the induction variables in this loop,
519 // insert a canonical induction variable of the largest size.
520 Value *IndVar = Rewriter.getOrInsertCanonicalInductionVariable(L,LargestType);
523 DOUT << "INDVARS: New CanIV: " << *IndVar;
525 if (!isa<SCEVCouldNotCompute>(IterationCount)) {
526 if (IterationCount->getType()->getPrimitiveSizeInBits() <
527 LargestType->getPrimitiveSizeInBits())
528 IterationCount = SE->getZeroExtendExpr(IterationCount, LargestType);
529 else if (IterationCount->getType() != LargestType)
530 IterationCount = SE->getTruncateExpr(IterationCount, LargestType);
531 if (Instruction *DI = LinearFunctionTestReplace(L, IterationCount,Rewriter))
532 DeadInsts.insert(DI);
535 // Now that we have a canonical induction variable, we can rewrite any
536 // recurrences in terms of the induction variable. Start with the auxillary
537 // induction variables, and recursively rewrite any of their uses.
538 BasicBlock::iterator InsertPt = Header->begin();
539 while (isa<PHINode>(InsertPt)) ++InsertPt;
541 // If there were induction variables of other sizes, cast the primary
542 // induction variable to the right size for them, avoiding the need for the
543 // code evaluation methods to insert induction variables of different sizes.
544 if (DifferingSizes) {
545 SmallVector<unsigned,4> InsertedSizes;
546 InsertedSizes.push_back(LargestType->getPrimitiveSizeInBits());
547 for (unsigned i = 0, e = IndVars.size(); i != e; ++i) {
548 unsigned ithSize = IndVars[i].first->getType()->getPrimitiveSizeInBits();
549 if (std::find(InsertedSizes.begin(), InsertedSizes.end(), ithSize)
550 == InsertedSizes.end()) {
551 PHINode *PN = IndVars[i].first;
552 InsertedSizes.push_back(ithSize);
553 Instruction *New = new TruncInst(IndVar, PN->getType(), "indvar",
555 Rewriter.addInsertedValue(New, SE->getSCEV(New));
556 DOUT << "INDVARS: Made trunc IV for " << *PN
557 << " NewVal = " << *New << "\n";
562 // Rewrite all induction variables in terms of the canonical induction
564 std::map<unsigned, Value*> InsertedSizes;
565 while (!IndVars.empty()) {
566 PHINode *PN = IndVars.back().first;
567 Value *NewVal = Rewriter.expandCodeFor(IndVars.back().second, InsertPt);
568 DOUT << "INDVARS: Rewrote IV '" << *IndVars.back().second << "' " << *PN
569 << " into = " << *NewVal << "\n";
570 NewVal->takeName(PN);
572 // Replace the old PHI Node with the inserted computation.
573 PN->replaceAllUsesWith(NewVal);
574 DeadInsts.insert(PN);
581 // Now replace all derived expressions in the loop body with simpler
583 for (unsigned i = 0, e = L->getBlocks().size(); i != e; ++i)
584 if (LI->getLoopFor(L->getBlocks()[i]) == L) { // Not in a subloop...
585 BasicBlock *BB = L->getBlocks()[i];
586 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
587 if (I->getType()->isInteger() && // Is an integer instruction
589 !Rewriter.isInsertedInstruction(I)) {
590 SCEVHandle SH = SE->getSCEV(I);
591 Value *V = Rewriter.expandCodeFor(SH, I, I->getType());
593 if (isa<Instruction>(V))
595 I->replaceAllUsesWith(V);
604 DeleteTriviallyDeadInstructions(DeadInsts);
606 assert(L->isLCSSAForm());