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 // If the trip count of a loop is computable, this pass also makes the following
16 // 1. The exit condition for the loop is canonicalized to compare the
17 // induction value against the exit value. This turns loops like:
18 // 'for (i = 7; i*i < 1000; ++i)' into 'for (i = 0; i != 25; ++i)'
19 // 2. Any use outside of the loop of an expression derived from the indvar
20 // is changed to compute the derived value outside of the loop, eliminating
21 // the dependence on the exit value of the induction variable. If the only
22 // purpose of the loop is to compute the exit value of some derived
23 // expression, this transformation will make the loop dead.
25 //===----------------------------------------------------------------------===//
27 #include "llvm/Transforms/Scalar.h"
28 #include "llvm/ADT/DenseMap.h"
29 #include "llvm/ADT/SmallVector.h"
30 #include "llvm/ADT/Statistic.h"
31 #include "llvm/Analysis/LoopInfo.h"
32 #include "llvm/Analysis/LoopPass.h"
33 #include "llvm/Analysis/ScalarEvolutionExpander.h"
34 #include "llvm/Analysis/TargetLibraryInfo.h"
35 #include "llvm/Analysis/TargetTransformInfo.h"
36 #include "llvm/IR/BasicBlock.h"
37 #include "llvm/IR/CFG.h"
38 #include "llvm/IR/Constants.h"
39 #include "llvm/IR/DataLayout.h"
40 #include "llvm/IR/Dominators.h"
41 #include "llvm/IR/Instructions.h"
42 #include "llvm/IR/IntrinsicInst.h"
43 #include "llvm/IR/LLVMContext.h"
44 #include "llvm/IR/PatternMatch.h"
45 #include "llvm/IR/Type.h"
46 #include "llvm/Support/CommandLine.h"
47 #include "llvm/Support/Debug.h"
48 #include "llvm/Support/raw_ostream.h"
49 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
50 #include "llvm/Transforms/Utils/Local.h"
51 #include "llvm/Transforms/Utils/SimplifyIndVar.h"
54 #define DEBUG_TYPE "indvars"
56 STATISTIC(NumWidened , "Number of indvars widened");
57 STATISTIC(NumReplaced , "Number of exit values replaced");
58 STATISTIC(NumLFTR , "Number of loop exit tests replaced");
59 STATISTIC(NumElimExt , "Number of IV sign/zero extends eliminated");
60 STATISTIC(NumElimIV , "Number of congruent IVs eliminated");
62 // Trip count verification can be enabled by default under NDEBUG if we
63 // implement a strong expression equivalence checker in SCEV. Until then, we
64 // use the verify-indvars flag, which may assert in some cases.
65 static cl::opt<bool> VerifyIndvars(
66 "verify-indvars", cl::Hidden,
67 cl::desc("Verify the ScalarEvolution result after running indvars"));
69 static cl::opt<bool> ReduceLiveIVs("liv-reduce", cl::Hidden,
70 cl::desc("Reduce live induction variables."));
72 enum ReplaceExitVal { NeverRepl, OnlyCheapRepl, AlwaysRepl };
74 static cl::opt<ReplaceExitVal> ReplaceExitValue(
75 "replexitval", cl::Hidden, cl::init(OnlyCheapRepl),
76 cl::desc("Choose the strategy to replace exit value in IndVarSimplify"),
77 cl::values(clEnumValN(NeverRepl, "never", "never replace exit value"),
78 clEnumValN(OnlyCheapRepl, "cheap",
79 "only replace exit value when the cost is cheap"),
80 clEnumValN(AlwaysRepl, "always",
81 "always replace exit value whenever possible"),
89 class IndVarSimplify : public LoopPass {
93 TargetLibraryInfo *TLI;
94 const TargetTransformInfo *TTI;
96 SmallVector<WeakVH, 16> DeadInsts;
100 static char ID; // Pass identification, replacement for typeid
102 : LoopPass(ID), LI(nullptr), SE(nullptr), DT(nullptr), Changed(false) {
103 initializeIndVarSimplifyPass(*PassRegistry::getPassRegistry());
106 bool runOnLoop(Loop *L, LPPassManager &LPM) override;
108 void getAnalysisUsage(AnalysisUsage &AU) const override {
109 AU.addRequired<DominatorTreeWrapperPass>();
110 AU.addRequired<LoopInfoWrapperPass>();
111 AU.addRequired<ScalarEvolution>();
112 AU.addRequiredID(LoopSimplifyID);
113 AU.addRequiredID(LCSSAID);
114 AU.addPreserved<ScalarEvolution>();
115 AU.addPreservedID(LoopSimplifyID);
116 AU.addPreservedID(LCSSAID);
117 AU.setPreservesCFG();
121 void releaseMemory() override {
125 bool isValidRewrite(Value *FromVal, Value *ToVal);
127 void HandleFloatingPointIV(Loop *L, PHINode *PH);
128 void RewriteNonIntegerIVs(Loop *L);
130 void SimplifyAndExtend(Loop *L, SCEVExpander &Rewriter, LPPassManager &LPM);
132 bool CanLoopBeDeleted(Loop *L, SmallVector<RewritePhi, 8> &RewritePhiSet);
133 void RewriteLoopExitValues(Loop *L, SCEVExpander &Rewriter);
135 Value *LinearFunctionTestReplace(Loop *L, const SCEV *BackedgeTakenCount,
136 PHINode *IndVar, SCEVExpander &Rewriter);
138 void SinkUnusedInvariants(Loop *L);
140 Value *ExpandSCEVIfNeeded(SCEVExpander &Rewriter, const SCEV *S, Loop *L,
141 Instruction *InsertPt, Type *Ty,
142 bool &IsHighCostExpansion);
146 char IndVarSimplify::ID = 0;
147 INITIALIZE_PASS_BEGIN(IndVarSimplify, "indvars",
148 "Induction Variable Simplification", false, false)
149 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
150 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
151 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
152 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
153 INITIALIZE_PASS_DEPENDENCY(LCSSA)
154 INITIALIZE_PASS_END(IndVarSimplify, "indvars",
155 "Induction Variable Simplification", false, false)
157 Pass *llvm::createIndVarSimplifyPass() {
158 return new IndVarSimplify();
161 /// isValidRewrite - Return true if the SCEV expansion generated by the
162 /// rewriter can replace the original value. SCEV guarantees that it
163 /// produces the same value, but the way it is produced may be illegal IR.
164 /// Ideally, this function will only be called for verification.
165 bool IndVarSimplify::isValidRewrite(Value *FromVal, Value *ToVal) {
166 // If an SCEV expression subsumed multiple pointers, its expansion could
167 // reassociate the GEP changing the base pointer. This is illegal because the
168 // final address produced by a GEP chain must be inbounds relative to its
169 // underlying object. Otherwise basic alias analysis, among other things,
170 // could fail in a dangerous way. Ultimately, SCEV will be improved to avoid
171 // producing an expression involving multiple pointers. Until then, we must
174 // Retrieve the pointer operand of the GEP. Don't use GetUnderlyingObject
175 // because it understands lcssa phis while SCEV does not.
176 Value *FromPtr = FromVal;
177 Value *ToPtr = ToVal;
178 if (GEPOperator *GEP = dyn_cast<GEPOperator>(FromVal)) {
179 FromPtr = GEP->getPointerOperand();
181 if (GEPOperator *GEP = dyn_cast<GEPOperator>(ToVal)) {
182 ToPtr = GEP->getPointerOperand();
184 if (FromPtr != FromVal || ToPtr != ToVal) {
185 // Quickly check the common case
186 if (FromPtr == ToPtr)
189 // SCEV may have rewritten an expression that produces the GEP's pointer
190 // operand. That's ok as long as the pointer operand has the same base
191 // pointer. Unlike GetUnderlyingObject(), getPointerBase() will find the
192 // base of a recurrence. This handles the case in which SCEV expansion
193 // converts a pointer type recurrence into a nonrecurrent pointer base
194 // indexed by an integer recurrence.
196 // If the GEP base pointer is a vector of pointers, abort.
197 if (!FromPtr->getType()->isPointerTy() || !ToPtr->getType()->isPointerTy())
200 const SCEV *FromBase = SE->getPointerBase(SE->getSCEV(FromPtr));
201 const SCEV *ToBase = SE->getPointerBase(SE->getSCEV(ToPtr));
202 if (FromBase == ToBase)
205 DEBUG(dbgs() << "INDVARS: GEP rewrite bail out "
206 << *FromBase << " != " << *ToBase << "\n");
213 /// Determine the insertion point for this user. By default, insert immediately
214 /// before the user. SCEVExpander or LICM will hoist loop invariants out of the
215 /// loop. For PHI nodes, there may be multiple uses, so compute the nearest
216 /// common dominator for the incoming blocks.
217 static Instruction *getInsertPointForUses(Instruction *User, Value *Def,
219 PHINode *PHI = dyn_cast<PHINode>(User);
223 Instruction *InsertPt = nullptr;
224 for (unsigned i = 0, e = PHI->getNumIncomingValues(); i != e; ++i) {
225 if (PHI->getIncomingValue(i) != Def)
228 BasicBlock *InsertBB = PHI->getIncomingBlock(i);
230 InsertPt = InsertBB->getTerminator();
233 InsertBB = DT->findNearestCommonDominator(InsertPt->getParent(), InsertBB);
234 InsertPt = InsertBB->getTerminator();
236 assert(InsertPt && "Missing phi operand");
237 assert((!isa<Instruction>(Def) ||
238 DT->dominates(cast<Instruction>(Def), InsertPt)) &&
239 "def does not dominate all uses");
243 //===----------------------------------------------------------------------===//
244 // RewriteNonIntegerIVs and helpers. Prefer integer IVs.
245 //===----------------------------------------------------------------------===//
247 /// ConvertToSInt - Convert APF to an integer, if possible.
248 static bool ConvertToSInt(const APFloat &APF, int64_t &IntVal) {
249 bool isExact = false;
250 // See if we can convert this to an int64_t
252 if (APF.convertToInteger(&UIntVal, 64, true, APFloat::rmTowardZero,
253 &isExact) != APFloat::opOK || !isExact)
259 /// HandleFloatingPointIV - If the loop has floating induction variable
260 /// then insert corresponding integer induction variable if possible.
262 /// for(double i = 0; i < 10000; ++i)
264 /// is converted into
265 /// for(int i = 0; i < 10000; ++i)
268 void IndVarSimplify::HandleFloatingPointIV(Loop *L, PHINode *PN) {
269 unsigned IncomingEdge = L->contains(PN->getIncomingBlock(0));
270 unsigned BackEdge = IncomingEdge^1;
272 // Check incoming value.
273 ConstantFP *InitValueVal =
274 dyn_cast<ConstantFP>(PN->getIncomingValue(IncomingEdge));
277 if (!InitValueVal || !ConvertToSInt(InitValueVal->getValueAPF(), InitValue))
280 // Check IV increment. Reject this PN if increment operation is not
281 // an add or increment value can not be represented by an integer.
282 BinaryOperator *Incr =
283 dyn_cast<BinaryOperator>(PN->getIncomingValue(BackEdge));
284 if (Incr == nullptr || Incr->getOpcode() != Instruction::FAdd) return;
286 // If this is not an add of the PHI with a constantfp, or if the constant fp
287 // is not an integer, bail out.
288 ConstantFP *IncValueVal = dyn_cast<ConstantFP>(Incr->getOperand(1));
290 if (IncValueVal == nullptr || Incr->getOperand(0) != PN ||
291 !ConvertToSInt(IncValueVal->getValueAPF(), IncValue))
294 // Check Incr uses. One user is PN and the other user is an exit condition
295 // used by the conditional terminator.
296 Value::user_iterator IncrUse = Incr->user_begin();
297 Instruction *U1 = cast<Instruction>(*IncrUse++);
298 if (IncrUse == Incr->user_end()) return;
299 Instruction *U2 = cast<Instruction>(*IncrUse++);
300 if (IncrUse != Incr->user_end()) return;
302 // Find exit condition, which is an fcmp. If it doesn't exist, or if it isn't
303 // only used by a branch, we can't transform it.
304 FCmpInst *Compare = dyn_cast<FCmpInst>(U1);
306 Compare = dyn_cast<FCmpInst>(U2);
307 if (!Compare || !Compare->hasOneUse() ||
308 !isa<BranchInst>(Compare->user_back()))
311 BranchInst *TheBr = cast<BranchInst>(Compare->user_back());
313 // We need to verify that the branch actually controls the iteration count
314 // of the loop. If not, the new IV can overflow and no one will notice.
315 // The branch block must be in the loop and one of the successors must be out
317 assert(TheBr->isConditional() && "Can't use fcmp if not conditional");
318 if (!L->contains(TheBr->getParent()) ||
319 (L->contains(TheBr->getSuccessor(0)) &&
320 L->contains(TheBr->getSuccessor(1))))
324 // If it isn't a comparison with an integer-as-fp (the exit value), we can't
326 ConstantFP *ExitValueVal = dyn_cast<ConstantFP>(Compare->getOperand(1));
328 if (ExitValueVal == nullptr ||
329 !ConvertToSInt(ExitValueVal->getValueAPF(), ExitValue))
332 // Find new predicate for integer comparison.
333 CmpInst::Predicate NewPred = CmpInst::BAD_ICMP_PREDICATE;
334 switch (Compare->getPredicate()) {
335 default: return; // Unknown comparison.
336 case CmpInst::FCMP_OEQ:
337 case CmpInst::FCMP_UEQ: NewPred = CmpInst::ICMP_EQ; break;
338 case CmpInst::FCMP_ONE:
339 case CmpInst::FCMP_UNE: NewPred = CmpInst::ICMP_NE; break;
340 case CmpInst::FCMP_OGT:
341 case CmpInst::FCMP_UGT: NewPred = CmpInst::ICMP_SGT; break;
342 case CmpInst::FCMP_OGE:
343 case CmpInst::FCMP_UGE: NewPred = CmpInst::ICMP_SGE; break;
344 case CmpInst::FCMP_OLT:
345 case CmpInst::FCMP_ULT: NewPred = CmpInst::ICMP_SLT; break;
346 case CmpInst::FCMP_OLE:
347 case CmpInst::FCMP_ULE: NewPred = CmpInst::ICMP_SLE; break;
350 // We convert the floating point induction variable to a signed i32 value if
351 // we can. This is only safe if the comparison will not overflow in a way
352 // that won't be trapped by the integer equivalent operations. Check for this
354 // TODO: We could use i64 if it is native and the range requires it.
356 // The start/stride/exit values must all fit in signed i32.
357 if (!isInt<32>(InitValue) || !isInt<32>(IncValue) || !isInt<32>(ExitValue))
360 // If not actually striding (add x, 0.0), avoid touching the code.
364 // Positive and negative strides have different safety conditions.
366 // If we have a positive stride, we require the init to be less than the
368 if (InitValue >= ExitValue)
371 uint32_t Range = uint32_t(ExitValue-InitValue);
372 // Check for infinite loop, either:
373 // while (i <= Exit) or until (i > Exit)
374 if (NewPred == CmpInst::ICMP_SLE || NewPred == CmpInst::ICMP_SGT) {
375 if (++Range == 0) return; // Range overflows.
378 unsigned Leftover = Range % uint32_t(IncValue);
380 // If this is an equality comparison, we require that the strided value
381 // exactly land on the exit value, otherwise the IV condition will wrap
382 // around and do things the fp IV wouldn't.
383 if ((NewPred == CmpInst::ICMP_EQ || NewPred == CmpInst::ICMP_NE) &&
387 // If the stride would wrap around the i32 before exiting, we can't
389 if (Leftover != 0 && int32_t(ExitValue+IncValue) < ExitValue)
393 // If we have a negative stride, we require the init to be greater than the
395 if (InitValue <= ExitValue)
398 uint32_t Range = uint32_t(InitValue-ExitValue);
399 // Check for infinite loop, either:
400 // while (i >= Exit) or until (i < Exit)
401 if (NewPred == CmpInst::ICMP_SGE || NewPred == CmpInst::ICMP_SLT) {
402 if (++Range == 0) return; // Range overflows.
405 unsigned Leftover = Range % uint32_t(-IncValue);
407 // If this is an equality comparison, we require that the strided value
408 // exactly land on the exit value, otherwise the IV condition will wrap
409 // around and do things the fp IV wouldn't.
410 if ((NewPred == CmpInst::ICMP_EQ || NewPred == CmpInst::ICMP_NE) &&
414 // If the stride would wrap around the i32 before exiting, we can't
416 if (Leftover != 0 && int32_t(ExitValue+IncValue) > ExitValue)
420 IntegerType *Int32Ty = Type::getInt32Ty(PN->getContext());
422 // Insert new integer induction variable.
423 PHINode *NewPHI = PHINode::Create(Int32Ty, 2, PN->getName()+".int", PN);
424 NewPHI->addIncoming(ConstantInt::get(Int32Ty, InitValue),
425 PN->getIncomingBlock(IncomingEdge));
428 BinaryOperator::CreateAdd(NewPHI, ConstantInt::get(Int32Ty, IncValue),
429 Incr->getName()+".int", Incr);
430 NewPHI->addIncoming(NewAdd, PN->getIncomingBlock(BackEdge));
432 ICmpInst *NewCompare = new ICmpInst(TheBr, NewPred, NewAdd,
433 ConstantInt::get(Int32Ty, ExitValue),
436 // In the following deletions, PN may become dead and may be deleted.
437 // Use a WeakVH to observe whether this happens.
440 // Delete the old floating point exit comparison. The branch starts using the
442 NewCompare->takeName(Compare);
443 Compare->replaceAllUsesWith(NewCompare);
444 RecursivelyDeleteTriviallyDeadInstructions(Compare, TLI);
446 // Delete the old floating point increment.
447 Incr->replaceAllUsesWith(UndefValue::get(Incr->getType()));
448 RecursivelyDeleteTriviallyDeadInstructions(Incr, TLI);
450 // If the FP induction variable still has uses, this is because something else
451 // in the loop uses its value. In order to canonicalize the induction
452 // variable, we chose to eliminate the IV and rewrite it in terms of an
455 // We give preference to sitofp over uitofp because it is faster on most
458 Value *Conv = new SIToFPInst(NewPHI, PN->getType(), "indvar.conv",
459 PN->getParent()->getFirstInsertionPt());
460 PN->replaceAllUsesWith(Conv);
461 RecursivelyDeleteTriviallyDeadInstructions(PN, TLI);
466 void IndVarSimplify::RewriteNonIntegerIVs(Loop *L) {
467 // First step. Check to see if there are any floating-point recurrences.
468 // If there are, change them into integer recurrences, permitting analysis by
469 // the SCEV routines.
471 BasicBlock *Header = L->getHeader();
473 SmallVector<WeakVH, 8> PHIs;
474 for (BasicBlock::iterator I = Header->begin();
475 PHINode *PN = dyn_cast<PHINode>(I); ++I)
478 for (unsigned i = 0, e = PHIs.size(); i != e; ++i)
479 if (PHINode *PN = dyn_cast_or_null<PHINode>(&*PHIs[i]))
480 HandleFloatingPointIV(L, PN);
482 // If the loop previously had floating-point IV, ScalarEvolution
483 // may not have been able to compute a trip count. Now that we've done some
484 // re-writing, the trip count may be computable.
490 // Collect information about PHI nodes which can be transformed in
491 // RewriteLoopExitValues.
494 unsigned Ith; // Ith incoming value.
495 Value *Val; // Exit value after expansion.
496 bool HighCost; // High Cost when expansion.
497 bool SafePhi; // LCSSASafePhiForRAUW.
499 RewritePhi(PHINode *P, unsigned I, Value *V, bool H, bool S)
500 : PN(P), Ith(I), Val(V), HighCost(H), SafePhi(S) {}
504 Value *IndVarSimplify::ExpandSCEVIfNeeded(SCEVExpander &Rewriter, const SCEV *S,
505 Loop *L, Instruction *InsertPt,
507 bool &IsHighCostExpansion) {
508 using namespace llvm::PatternMatch;
510 if (!Rewriter.isHighCostExpansion(S, L)) {
511 IsHighCostExpansion = false;
512 return Rewriter.expandCodeFor(S, ResultTy, InsertPt);
515 // Before expanding S into an expensive LLVM expression, see if we can use an
516 // already existing value as the expansion for S. There is potential to make
517 // this significantly smarter, but this simple heuristic already gets some
518 // interesting cases.
520 SmallVector<BasicBlock *, 4> Latches;
521 L->getLoopLatches(Latches);
523 for (BasicBlock *BB : Latches) {
524 ICmpInst::Predicate Pred;
525 Instruction *LHS, *RHS;
526 BasicBlock *TrueBB, *FalseBB;
528 if (!match(BB->getTerminator(),
529 m_Br(m_ICmp(Pred, m_Instruction(LHS), m_Instruction(RHS)),
533 if (SE->getSCEV(LHS) == S && DT->dominates(LHS, InsertPt)) {
534 IsHighCostExpansion = false;
538 if (SE->getSCEV(RHS) == S && DT->dominates(RHS, InsertPt)) {
539 IsHighCostExpansion = false;
544 // We didn't find anything, fall back to using SCEVExpander.
545 assert(Rewriter.isHighCostExpansion(S, L) && "this should not have changed!");
546 IsHighCostExpansion = true;
547 return Rewriter.expandCodeFor(S, ResultTy, InsertPt);
550 //===----------------------------------------------------------------------===//
551 // RewriteLoopExitValues - Optimize IV users outside the loop.
552 // As a side effect, reduces the amount of IV processing within the loop.
553 //===----------------------------------------------------------------------===//
555 /// RewriteLoopExitValues - Check to see if this loop has a computable
556 /// loop-invariant execution count. If so, this means that we can compute the
557 /// final value of any expressions that are recurrent in the loop, and
558 /// substitute the exit values from the loop into any instructions outside of
559 /// the loop that use the final values of the current expressions.
561 /// This is mostly redundant with the regular IndVarSimplify activities that
562 /// happen later, except that it's more powerful in some cases, because it's
563 /// able to brute-force evaluate arbitrary instructions as long as they have
564 /// constant operands at the beginning of the loop.
565 void IndVarSimplify::RewriteLoopExitValues(Loop *L, SCEVExpander &Rewriter) {
566 // Verify the input to the pass in already in LCSSA form.
567 assert(L->isLCSSAForm(*DT));
569 SmallVector<BasicBlock*, 8> ExitBlocks;
570 L->getUniqueExitBlocks(ExitBlocks);
572 SmallVector<RewritePhi, 8> RewritePhiSet;
573 // Find all values that are computed inside the loop, but used outside of it.
574 // Because of LCSSA, these values will only occur in LCSSA PHI Nodes. Scan
575 // the exit blocks of the loop to find them.
576 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
577 BasicBlock *ExitBB = ExitBlocks[i];
579 // If there are no PHI nodes in this exit block, then no values defined
580 // inside the loop are used on this path, skip it.
581 PHINode *PN = dyn_cast<PHINode>(ExitBB->begin());
584 unsigned NumPreds = PN->getNumIncomingValues();
586 // We would like to be able to RAUW single-incoming value PHI nodes. We
587 // have to be certain this is safe even when this is an LCSSA PHI node.
588 // While the computed exit value is no longer varying in *this* loop, the
589 // exit block may be an exit block for an outer containing loop as well,
590 // the exit value may be varying in the outer loop, and thus it may still
591 // require an LCSSA PHI node. The safe case is when this is
592 // single-predecessor PHI node (LCSSA) and the exit block containing it is
593 // part of the enclosing loop, or this is the outer most loop of the nest.
594 // In either case the exit value could (at most) be varying in the same
595 // loop body as the phi node itself. Thus if it is in turn used outside of
596 // an enclosing loop it will only be via a separate LCSSA node.
597 bool LCSSASafePhiForRAUW =
599 (!L->getParentLoop() || L->getParentLoop() == LI->getLoopFor(ExitBB));
601 // Iterate over all of the PHI nodes.
602 BasicBlock::iterator BBI = ExitBB->begin();
603 while ((PN = dyn_cast<PHINode>(BBI++))) {
605 continue; // dead use, don't replace it
607 // SCEV only supports integer expressions for now.
608 if (!PN->getType()->isIntegerTy() && !PN->getType()->isPointerTy())
611 // It's necessary to tell ScalarEvolution about this explicitly so that
612 // it can walk the def-use list and forget all SCEVs, as it may not be
613 // watching the PHI itself. Once the new exit value is in place, there
614 // may not be a def-use connection between the loop and every instruction
615 // which got a SCEVAddRecExpr for that loop.
618 // Iterate over all of the values in all the PHI nodes.
619 for (unsigned i = 0; i != NumPreds; ++i) {
620 // If the value being merged in is not integer or is not defined
621 // in the loop, skip it.
622 Value *InVal = PN->getIncomingValue(i);
623 if (!isa<Instruction>(InVal))
626 // If this pred is for a subloop, not L itself, skip it.
627 if (LI->getLoopFor(PN->getIncomingBlock(i)) != L)
628 continue; // The Block is in a subloop, skip it.
630 // Check that InVal is defined in the loop.
631 Instruction *Inst = cast<Instruction>(InVal);
632 if (!L->contains(Inst))
635 // Okay, this instruction has a user outside of the current loop
636 // and varies predictably *inside* the loop. Evaluate the value it
637 // contains when the loop exits, if possible.
638 const SCEV *ExitValue = SE->getSCEVAtScope(Inst, L->getParentLoop());
639 if (!SE->isLoopInvariant(ExitValue, L) ||
640 !isSafeToExpand(ExitValue, *SE))
643 // Computing the value outside of the loop brings no benefit if :
644 // - it is definitely used inside the loop in a way which can not be
646 // - no use outside of the loop can take advantage of hoisting the
647 // computation out of the loop
648 if (ExitValue->getSCEVType()>=scMulExpr) {
649 unsigned NumHardInternalUses = 0;
650 unsigned NumSoftExternalUses = 0;
651 unsigned NumUses = 0;
652 for (auto IB = Inst->user_begin(), IE = Inst->user_end();
653 IB != IE && NumUses <= 6; ++IB) {
654 Instruction *UseInstr = cast<Instruction>(*IB);
655 unsigned Opc = UseInstr->getOpcode();
657 if (L->contains(UseInstr)) {
658 if (Opc == Instruction::Call || Opc == Instruction::Ret)
659 NumHardInternalUses++;
661 if (Opc == Instruction::PHI) {
662 // Do not count the Phi as a use. LCSSA may have inserted
663 // plenty of trivial ones.
665 for (auto PB = UseInstr->user_begin(),
666 PE = UseInstr->user_end();
667 PB != PE && NumUses <= 6; ++PB, ++NumUses) {
668 unsigned PhiOpc = cast<Instruction>(*PB)->getOpcode();
669 if (PhiOpc != Instruction::Call && PhiOpc != Instruction::Ret)
670 NumSoftExternalUses++;
674 if (Opc != Instruction::Call && Opc != Instruction::Ret)
675 NumSoftExternalUses++;
678 if (NumUses <= 6 && NumHardInternalUses && !NumSoftExternalUses)
682 bool HighCost = false;
683 Value *ExitVal = ExpandSCEVIfNeeded(Rewriter, ExitValue, L, Inst,
684 PN->getType(), HighCost);
686 DEBUG(dbgs() << "INDVARS: RLEV: AfterLoopVal = " << *ExitVal << '\n'
687 << " LoopVal = " << *Inst << "\n");
689 if (!isValidRewrite(Inst, ExitVal)) {
690 DeadInsts.push_back(ExitVal);
694 // Collect all the candidate PHINodes to be rewritten.
695 RewritePhiSet.push_back(
696 RewritePhi(PN, i, ExitVal, HighCost, LCSSASafePhiForRAUW));
701 bool LoopCanBeDel = CanLoopBeDeleted(L, RewritePhiSet);
704 for (const RewritePhi &Phi : RewritePhiSet) {
705 PHINode *PN = Phi.PN;
706 Value *ExitVal = Phi.Val;
708 // Only do the rewrite when the ExitValue can be expanded cheaply.
709 // If LoopCanBeDel is true, rewrite exit value aggressively.
710 if (ReplaceExitValue == OnlyCheapRepl && !LoopCanBeDel && Phi.HighCost) {
711 DeadInsts.push_back(ExitVal);
717 Instruction *Inst = cast<Instruction>(PN->getIncomingValue(Phi.Ith));
718 PN->setIncomingValue(Phi.Ith, ExitVal);
720 // If this instruction is dead now, delete it. Don't do it now to avoid
721 // invalidating iterators.
722 if (isInstructionTriviallyDead(Inst, TLI))
723 DeadInsts.push_back(Inst);
725 // If we determined that this PHI is safe to replace even if an LCSSA
728 PN->replaceAllUsesWith(ExitVal);
729 PN->eraseFromParent();
733 // The insertion point instruction may have been deleted; clear it out
734 // so that the rewriter doesn't trip over it later.
735 Rewriter.clearInsertPoint();
738 /// CanLoopBeDeleted - Check whether it is possible to delete the loop after
739 /// rewriting exit value. If it is possible, ignore ReplaceExitValue and
740 /// do rewriting aggressively.
741 bool IndVarSimplify::CanLoopBeDeleted(
742 Loop *L, SmallVector<RewritePhi, 8> &RewritePhiSet) {
744 BasicBlock *Preheader = L->getLoopPreheader();
745 // If there is no preheader, the loop will not be deleted.
749 // In LoopDeletion pass Loop can be deleted when ExitingBlocks.size() > 1.
750 // We obviate multiple ExitingBlocks case for simplicity.
751 // TODO: If we see testcase with multiple ExitingBlocks can be deleted
752 // after exit value rewriting, we can enhance the logic here.
753 SmallVector<BasicBlock *, 4> ExitingBlocks;
754 L->getExitingBlocks(ExitingBlocks);
755 SmallVector<BasicBlock *, 8> ExitBlocks;
756 L->getUniqueExitBlocks(ExitBlocks);
757 if (ExitBlocks.size() > 1 || ExitingBlocks.size() > 1)
760 BasicBlock *ExitBlock = ExitBlocks[0];
761 BasicBlock::iterator BI = ExitBlock->begin();
762 while (PHINode *P = dyn_cast<PHINode>(BI)) {
763 Value *Incoming = P->getIncomingValueForBlock(ExitingBlocks[0]);
765 // If the Incoming value of P is found in RewritePhiSet, we know it
766 // could be rewritten to use a loop invariant value in transformation
767 // phase later. Skip it in the loop invariant check below.
769 for (const RewritePhi &Phi : RewritePhiSet) {
770 unsigned i = Phi.Ith;
771 if (Phi.PN == P && (Phi.PN)->getIncomingValue(i) == Incoming) {
778 if (!found && (I = dyn_cast<Instruction>(Incoming)))
779 if (!L->hasLoopInvariantOperands(I))
785 for (Loop::block_iterator LI = L->block_begin(), LE = L->block_end();
787 for (BasicBlock::iterator BI = (*LI)->begin(), BE = (*LI)->end(); BI != BE;
789 if (BI->mayHaveSideEffects())
797 //===----------------------------------------------------------------------===//
798 // IV Widening - Extend the width of an IV to cover its widest uses.
799 //===----------------------------------------------------------------------===//
802 // Collect information about induction variables that are used by sign/zero
803 // extend operations. This information is recorded by CollectExtend and
804 // provides the input to WidenIV.
807 Type *WidestNativeType; // Widest integer type created [sz]ext
808 bool IsSigned; // Was a sext user seen before a zext?
810 WideIVInfo() : NarrowIV(nullptr), WidestNativeType(nullptr),
815 /// visitCast - Update information about the induction variable that is
816 /// extended by this sign or zero extend operation. This is used to determine
817 /// the final width of the IV before actually widening it.
818 static void visitIVCast(CastInst *Cast, WideIVInfo &WI, ScalarEvolution *SE,
819 const TargetTransformInfo *TTI) {
820 bool IsSigned = Cast->getOpcode() == Instruction::SExt;
821 if (!IsSigned && Cast->getOpcode() != Instruction::ZExt)
824 Type *Ty = Cast->getType();
825 uint64_t Width = SE->getTypeSizeInBits(Ty);
826 if (!Cast->getModule()->getDataLayout().isLegalInteger(Width))
829 // Cast is either an sext or zext up to this point.
830 // We should not widen an indvar if arithmetics on the wider indvar are more
831 // expensive than those on the narrower indvar. We check only the cost of ADD
832 // because at least an ADD is required to increment the induction variable. We
833 // could compute more comprehensively the cost of all instructions on the
834 // induction variable when necessary.
836 TTI->getArithmeticInstrCost(Instruction::Add, Ty) >
837 TTI->getArithmeticInstrCost(Instruction::Add,
838 Cast->getOperand(0)->getType())) {
842 if (!WI.WidestNativeType) {
843 WI.WidestNativeType = SE->getEffectiveSCEVType(Ty);
844 WI.IsSigned = IsSigned;
848 // We extend the IV to satisfy the sign of its first user, arbitrarily.
849 if (WI.IsSigned != IsSigned)
852 if (Width > SE->getTypeSizeInBits(WI.WidestNativeType))
853 WI.WidestNativeType = SE->getEffectiveSCEVType(Ty);
858 /// NarrowIVDefUse - Record a link in the Narrow IV def-use chain along with the
859 /// WideIV that computes the same value as the Narrow IV def. This avoids
860 /// caching Use* pointers.
861 struct NarrowIVDefUse {
862 Instruction *NarrowDef;
863 Instruction *NarrowUse;
864 Instruction *WideDef;
866 NarrowIVDefUse(): NarrowDef(nullptr), NarrowUse(nullptr), WideDef(nullptr) {}
868 NarrowIVDefUse(Instruction *ND, Instruction *NU, Instruction *WD):
869 NarrowDef(ND), NarrowUse(NU), WideDef(WD) {}
872 /// WidenIV - The goal of this transform is to remove sign and zero extends
873 /// without creating any new induction variables. To do this, it creates a new
874 /// phi of the wider type and redirects all users, either removing extends or
875 /// inserting truncs whenever we stop propagating the type.
891 Instruction *WideInc;
892 const SCEV *WideIncExpr;
893 SmallVectorImpl<WeakVH> &DeadInsts;
895 SmallPtrSet<Instruction*,16> Widened;
896 SmallVector<NarrowIVDefUse, 8> NarrowIVUsers;
899 WidenIV(const WideIVInfo &WI, LoopInfo *LInfo,
900 ScalarEvolution *SEv, DominatorTree *DTree,
901 SmallVectorImpl<WeakVH> &DI) :
902 OrigPhi(WI.NarrowIV),
903 WideType(WI.WidestNativeType),
904 IsSigned(WI.IsSigned),
906 L(LI->getLoopFor(OrigPhi->getParent())),
911 WideIncExpr(nullptr),
913 assert(L->getHeader() == OrigPhi->getParent() && "Phi must be an IV");
916 PHINode *CreateWideIV(SCEVExpander &Rewriter);
919 Value *getExtend(Value *NarrowOper, Type *WideType, bool IsSigned,
922 Instruction *CloneIVUser(NarrowIVDefUse DU);
924 const SCEVAddRecExpr *GetWideRecurrence(Instruction *NarrowUse);
926 const SCEVAddRecExpr* GetExtendedOperandRecurrence(NarrowIVDefUse DU);
928 const SCEV *GetSCEVByOpCode(const SCEV *LHS, const SCEV *RHS,
929 unsigned OpCode) const;
931 Instruction *WidenIVUse(NarrowIVDefUse DU, SCEVExpander &Rewriter);
933 bool WidenLoopCompare(NarrowIVDefUse DU);
935 void pushNarrowIVUsers(Instruction *NarrowDef, Instruction *WideDef);
937 } // anonymous namespace
939 /// isLoopInvariant - Perform a quick domtree based check for loop invariance
940 /// assuming that V is used within the loop. LoopInfo::isLoopInvariant() seems
941 /// gratuitous for this purpose.
942 static bool isLoopInvariant(Value *V, const Loop *L, const DominatorTree *DT) {
943 Instruction *Inst = dyn_cast<Instruction>(V);
947 return DT->properlyDominates(Inst->getParent(), L->getHeader());
950 Value *WidenIV::getExtend(Value *NarrowOper, Type *WideType, bool IsSigned,
952 // Set the debug location and conservative insertion point.
953 IRBuilder<> Builder(Use);
954 // Hoist the insertion point into loop preheaders as far as possible.
955 for (const Loop *L = LI->getLoopFor(Use->getParent());
956 L && L->getLoopPreheader() && isLoopInvariant(NarrowOper, L, DT);
957 L = L->getParentLoop())
958 Builder.SetInsertPoint(L->getLoopPreheader()->getTerminator());
960 return IsSigned ? Builder.CreateSExt(NarrowOper, WideType) :
961 Builder.CreateZExt(NarrowOper, WideType);
964 /// CloneIVUser - Instantiate a wide operation to replace a narrow
965 /// operation. This only needs to handle operations that can evaluation to
966 /// SCEVAddRec. It can safely return 0 for any operation we decide not to clone.
967 Instruction *WidenIV::CloneIVUser(NarrowIVDefUse DU) {
968 unsigned Opcode = DU.NarrowUse->getOpcode();
972 case Instruction::Add:
973 case Instruction::Mul:
974 case Instruction::UDiv:
975 case Instruction::Sub:
976 case Instruction::And:
977 case Instruction::Or:
978 case Instruction::Xor:
979 case Instruction::Shl:
980 case Instruction::LShr:
981 case Instruction::AShr:
982 DEBUG(dbgs() << "Cloning IVUser: " << *DU.NarrowUse << "\n");
984 // Replace NarrowDef operands with WideDef. Otherwise, we don't know
985 // anything about the narrow operand yet so must insert a [sz]ext. It is
986 // probably loop invariant and will be folded or hoisted. If it actually
987 // comes from a widened IV, it should be removed during a future call to
989 Value *LHS = (DU.NarrowUse->getOperand(0) == DU.NarrowDef) ? DU.WideDef :
990 getExtend(DU.NarrowUse->getOperand(0), WideType, IsSigned, DU.NarrowUse);
991 Value *RHS = (DU.NarrowUse->getOperand(1) == DU.NarrowDef) ? DU.WideDef :
992 getExtend(DU.NarrowUse->getOperand(1), WideType, IsSigned, DU.NarrowUse);
994 BinaryOperator *NarrowBO = cast<BinaryOperator>(DU.NarrowUse);
995 BinaryOperator *WideBO = BinaryOperator::Create(NarrowBO->getOpcode(),
997 NarrowBO->getName());
998 IRBuilder<> Builder(DU.NarrowUse);
999 Builder.Insert(WideBO);
1000 if (const OverflowingBinaryOperator *OBO =
1001 dyn_cast<OverflowingBinaryOperator>(NarrowBO)) {
1002 if (OBO->hasNoUnsignedWrap()) WideBO->setHasNoUnsignedWrap();
1003 if (OBO->hasNoSignedWrap()) WideBO->setHasNoSignedWrap();
1009 const SCEV *WidenIV::GetSCEVByOpCode(const SCEV *LHS, const SCEV *RHS,
1010 unsigned OpCode) const {
1011 if (OpCode == Instruction::Add)
1012 return SE->getAddExpr(LHS, RHS);
1013 if (OpCode == Instruction::Sub)
1014 return SE->getMinusSCEV(LHS, RHS);
1015 if (OpCode == Instruction::Mul)
1016 return SE->getMulExpr(LHS, RHS);
1018 llvm_unreachable("Unsupported opcode.");
1021 /// No-wrap operations can transfer sign extension of their result to their
1022 /// operands. Generate the SCEV value for the widened operation without
1023 /// actually modifying the IR yet. If the expression after extending the
1024 /// operands is an AddRec for this loop, return it.
1025 const SCEVAddRecExpr* WidenIV::GetExtendedOperandRecurrence(NarrowIVDefUse DU) {
1027 // Handle the common case of add<nsw/nuw>
1028 const unsigned OpCode = DU.NarrowUse->getOpcode();
1029 // Only Add/Sub/Mul instructions supported yet.
1030 if (OpCode != Instruction::Add && OpCode != Instruction::Sub &&
1031 OpCode != Instruction::Mul)
1034 // One operand (NarrowDef) has already been extended to WideDef. Now determine
1035 // if extending the other will lead to a recurrence.
1036 const unsigned ExtendOperIdx =
1037 DU.NarrowUse->getOperand(0) == DU.NarrowDef ? 1 : 0;
1038 assert(DU.NarrowUse->getOperand(1-ExtendOperIdx) == DU.NarrowDef && "bad DU");
1040 const SCEV *ExtendOperExpr = nullptr;
1041 const OverflowingBinaryOperator *OBO =
1042 cast<OverflowingBinaryOperator>(DU.NarrowUse);
1043 if (IsSigned && OBO->hasNoSignedWrap())
1044 ExtendOperExpr = SE->getSignExtendExpr(
1045 SE->getSCEV(DU.NarrowUse->getOperand(ExtendOperIdx)), WideType);
1046 else if(!IsSigned && OBO->hasNoUnsignedWrap())
1047 ExtendOperExpr = SE->getZeroExtendExpr(
1048 SE->getSCEV(DU.NarrowUse->getOperand(ExtendOperIdx)), WideType);
1052 // When creating this SCEV expr, don't apply the current operations NSW or NUW
1053 // flags. This instruction may be guarded by control flow that the no-wrap
1054 // behavior depends on. Non-control-equivalent instructions can be mapped to
1055 // the same SCEV expression, and it would be incorrect to transfer NSW/NUW
1056 // semantics to those operations.
1057 const SCEV *lhs = SE->getSCEV(DU.WideDef);
1058 const SCEV *rhs = ExtendOperExpr;
1060 // Let's swap operands to the initial order for the case of non-commutative
1061 // operations, like SUB. See PR21014.
1062 if (ExtendOperIdx == 0)
1063 std::swap(lhs, rhs);
1064 const SCEVAddRecExpr *AddRec =
1065 dyn_cast<SCEVAddRecExpr>(GetSCEVByOpCode(lhs, rhs, OpCode));
1067 if (!AddRec || AddRec->getLoop() != L)
1072 /// GetWideRecurrence - Is this instruction potentially interesting for further
1073 /// simplification after widening it's type? In other words, can the
1074 /// extend be safely hoisted out of the loop with SCEV reducing the value to a
1075 /// recurrence on the same loop. If so, return the sign or zero extended
1076 /// recurrence. Otherwise return NULL.
1077 const SCEVAddRecExpr *WidenIV::GetWideRecurrence(Instruction *NarrowUse) {
1078 if (!SE->isSCEVable(NarrowUse->getType()))
1081 const SCEV *NarrowExpr = SE->getSCEV(NarrowUse);
1082 if (SE->getTypeSizeInBits(NarrowExpr->getType())
1083 >= SE->getTypeSizeInBits(WideType)) {
1084 // NarrowUse implicitly widens its operand. e.g. a gep with a narrow
1085 // index. So don't follow this use.
1089 const SCEV *WideExpr = IsSigned ?
1090 SE->getSignExtendExpr(NarrowExpr, WideType) :
1091 SE->getZeroExtendExpr(NarrowExpr, WideType);
1092 const SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(WideExpr);
1093 if (!AddRec || AddRec->getLoop() != L)
1098 /// This IV user cannot be widen. Replace this use of the original narrow IV
1099 /// with a truncation of the new wide IV to isolate and eliminate the narrow IV.
1100 static void truncateIVUse(NarrowIVDefUse DU, DominatorTree *DT) {
1101 DEBUG(dbgs() << "INDVARS: Truncate IV " << *DU.WideDef
1102 << " for user " << *DU.NarrowUse << "\n");
1103 IRBuilder<> Builder(getInsertPointForUses(DU.NarrowUse, DU.NarrowDef, DT));
1104 Value *Trunc = Builder.CreateTrunc(DU.WideDef, DU.NarrowDef->getType());
1105 DU.NarrowUse->replaceUsesOfWith(DU.NarrowDef, Trunc);
1108 /// If the narrow use is a compare instruction, then widen the compare
1109 // (and possibly the other operand). The extend operation is hoisted into the
1110 // loop preheader as far as possible.
1111 bool WidenIV::WidenLoopCompare(NarrowIVDefUse DU) {
1112 ICmpInst *Cmp = dyn_cast<ICmpInst>(DU.NarrowUse);
1116 // Sign of IV user and compare must match.
1117 if (IsSigned != CmpInst::isSigned(Cmp->getPredicate()))
1120 Value *Op = Cmp->getOperand(Cmp->getOperand(0) == DU.NarrowDef ? 1 : 0);
1121 unsigned CastWidth = SE->getTypeSizeInBits(Op->getType());
1122 unsigned IVWidth = SE->getTypeSizeInBits(WideType);
1123 assert (CastWidth <= IVWidth && "Unexpected width while widening compare.");
1125 // Widen the compare instruction.
1126 IRBuilder<> Builder(getInsertPointForUses(DU.NarrowUse, DU.NarrowDef, DT));
1127 DU.NarrowUse->replaceUsesOfWith(DU.NarrowDef, DU.WideDef);
1129 // Widen the other operand of the compare, if necessary.
1130 if (CastWidth < IVWidth) {
1131 Value *ExtOp = getExtend(Op, WideType, IsSigned, Cmp);
1132 DU.NarrowUse->replaceUsesOfWith(Op, ExtOp);
1137 /// WidenIVUse - Determine whether an individual user of the narrow IV can be
1138 /// widened. If so, return the wide clone of the user.
1139 Instruction *WidenIV::WidenIVUse(NarrowIVDefUse DU, SCEVExpander &Rewriter) {
1141 // Stop traversing the def-use chain at inner-loop phis or post-loop phis.
1142 if (PHINode *UsePhi = dyn_cast<PHINode>(DU.NarrowUse)) {
1143 if (LI->getLoopFor(UsePhi->getParent()) != L) {
1144 // For LCSSA phis, sink the truncate outside the loop.
1145 // After SimplifyCFG most loop exit targets have a single predecessor.
1146 // Otherwise fall back to a truncate within the loop.
1147 if (UsePhi->getNumOperands() != 1)
1148 truncateIVUse(DU, DT);
1151 PHINode::Create(DU.WideDef->getType(), 1, UsePhi->getName() + ".wide",
1153 WidePhi->addIncoming(DU.WideDef, UsePhi->getIncomingBlock(0));
1154 IRBuilder<> Builder(WidePhi->getParent()->getFirstInsertionPt());
1155 Value *Trunc = Builder.CreateTrunc(WidePhi, DU.NarrowDef->getType());
1156 UsePhi->replaceAllUsesWith(Trunc);
1157 DeadInsts.emplace_back(UsePhi);
1158 DEBUG(dbgs() << "INDVARS: Widen lcssa phi " << *UsePhi
1159 << " to " << *WidePhi << "\n");
1164 // Our raison d'etre! Eliminate sign and zero extension.
1165 if (IsSigned ? isa<SExtInst>(DU.NarrowUse) : isa<ZExtInst>(DU.NarrowUse)) {
1166 Value *NewDef = DU.WideDef;
1167 if (DU.NarrowUse->getType() != WideType) {
1168 unsigned CastWidth = SE->getTypeSizeInBits(DU.NarrowUse->getType());
1169 unsigned IVWidth = SE->getTypeSizeInBits(WideType);
1170 if (CastWidth < IVWidth) {
1171 // The cast isn't as wide as the IV, so insert a Trunc.
1172 IRBuilder<> Builder(DU.NarrowUse);
1173 NewDef = Builder.CreateTrunc(DU.WideDef, DU.NarrowUse->getType());
1176 // A wider extend was hidden behind a narrower one. This may induce
1177 // another round of IV widening in which the intermediate IV becomes
1178 // dead. It should be very rare.
1179 DEBUG(dbgs() << "INDVARS: New IV " << *WidePhi
1180 << " not wide enough to subsume " << *DU.NarrowUse << "\n");
1181 DU.NarrowUse->replaceUsesOfWith(DU.NarrowDef, DU.WideDef);
1182 NewDef = DU.NarrowUse;
1185 if (NewDef != DU.NarrowUse) {
1186 DEBUG(dbgs() << "INDVARS: eliminating " << *DU.NarrowUse
1187 << " replaced by " << *DU.WideDef << "\n");
1189 DU.NarrowUse->replaceAllUsesWith(NewDef);
1190 DeadInsts.emplace_back(DU.NarrowUse);
1192 // Now that the extend is gone, we want to expose it's uses for potential
1193 // further simplification. We don't need to directly inform SimplifyIVUsers
1194 // of the new users, because their parent IV will be processed later as a
1195 // new loop phi. If we preserved IVUsers analysis, we would also want to
1196 // push the uses of WideDef here.
1198 // No further widening is needed. The deceased [sz]ext had done it for us.
1202 // Does this user itself evaluate to a recurrence after widening?
1203 const SCEVAddRecExpr *WideAddRec = GetWideRecurrence(DU.NarrowUse);
1205 WideAddRec = GetExtendedOperandRecurrence(DU);
1208 // If use is a loop condition, try to promote the condition instead of
1209 // truncating the IV first.
1210 if (WidenLoopCompare(DU))
1213 // This user does not evaluate to a recurence after widening, so don't
1214 // follow it. Instead insert a Trunc to kill off the original use,
1215 // eventually isolating the original narrow IV so it can be removed.
1216 truncateIVUse(DU, DT);
1219 // Assume block terminators cannot evaluate to a recurrence. We can't to
1220 // insert a Trunc after a terminator if there happens to be a critical edge.
1221 assert(DU.NarrowUse != DU.NarrowUse->getParent()->getTerminator() &&
1222 "SCEV is not expected to evaluate a block terminator");
1224 // Reuse the IV increment that SCEVExpander created as long as it dominates
1226 Instruction *WideUse = nullptr;
1227 if (WideAddRec == WideIncExpr
1228 && Rewriter.hoistIVInc(WideInc, DU.NarrowUse))
1231 WideUse = CloneIVUser(DU);
1235 // Evaluation of WideAddRec ensured that the narrow expression could be
1236 // extended outside the loop without overflow. This suggests that the wide use
1237 // evaluates to the same expression as the extended narrow use, but doesn't
1238 // absolutely guarantee it. Hence the following failsafe check. In rare cases
1239 // where it fails, we simply throw away the newly created wide use.
1240 if (WideAddRec != SE->getSCEV(WideUse)) {
1241 DEBUG(dbgs() << "Wide use expression mismatch: " << *WideUse
1242 << ": " << *SE->getSCEV(WideUse) << " != " << *WideAddRec << "\n");
1243 DeadInsts.emplace_back(WideUse);
1247 // Returning WideUse pushes it on the worklist.
1251 /// pushNarrowIVUsers - Add eligible users of NarrowDef to NarrowIVUsers.
1253 void WidenIV::pushNarrowIVUsers(Instruction *NarrowDef, Instruction *WideDef) {
1254 for (User *U : NarrowDef->users()) {
1255 Instruction *NarrowUser = cast<Instruction>(U);
1257 // Handle data flow merges and bizarre phi cycles.
1258 if (!Widened.insert(NarrowUser).second)
1261 NarrowIVUsers.push_back(NarrowIVDefUse(NarrowDef, NarrowUser, WideDef));
1265 /// CreateWideIV - Process a single induction variable. First use the
1266 /// SCEVExpander to create a wide induction variable that evaluates to the same
1267 /// recurrence as the original narrow IV. Then use a worklist to forward
1268 /// traverse the narrow IV's def-use chain. After WidenIVUse has processed all
1269 /// interesting IV users, the narrow IV will be isolated for removal by
1272 /// It would be simpler to delete uses as they are processed, but we must avoid
1273 /// invalidating SCEV expressions.
1275 PHINode *WidenIV::CreateWideIV(SCEVExpander &Rewriter) {
1276 // Is this phi an induction variable?
1277 const SCEVAddRecExpr *AddRec = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(OrigPhi));
1281 // Widen the induction variable expression.
1282 const SCEV *WideIVExpr = IsSigned ?
1283 SE->getSignExtendExpr(AddRec, WideType) :
1284 SE->getZeroExtendExpr(AddRec, WideType);
1286 assert(SE->getEffectiveSCEVType(WideIVExpr->getType()) == WideType &&
1287 "Expect the new IV expression to preserve its type");
1289 // Can the IV be extended outside the loop without overflow?
1290 AddRec = dyn_cast<SCEVAddRecExpr>(WideIVExpr);
1291 if (!AddRec || AddRec->getLoop() != L)
1294 // An AddRec must have loop-invariant operands. Since this AddRec is
1295 // materialized by a loop header phi, the expression cannot have any post-loop
1296 // operands, so they must dominate the loop header.
1297 assert(SE->properlyDominates(AddRec->getStart(), L->getHeader()) &&
1298 SE->properlyDominates(AddRec->getStepRecurrence(*SE), L->getHeader())
1299 && "Loop header phi recurrence inputs do not dominate the loop");
1301 // The rewriter provides a value for the desired IV expression. This may
1302 // either find an existing phi or materialize a new one. Either way, we
1303 // expect a well-formed cyclic phi-with-increments. i.e. any operand not part
1304 // of the phi-SCC dominates the loop entry.
1305 Instruction *InsertPt = L->getHeader()->begin();
1306 WidePhi = cast<PHINode>(Rewriter.expandCodeFor(AddRec, WideType, InsertPt));
1308 // Remembering the WideIV increment generated by SCEVExpander allows
1309 // WidenIVUse to reuse it when widening the narrow IV's increment. We don't
1310 // employ a general reuse mechanism because the call above is the only call to
1311 // SCEVExpander. Henceforth, we produce 1-to-1 narrow to wide uses.
1312 if (BasicBlock *LatchBlock = L->getLoopLatch()) {
1314 cast<Instruction>(WidePhi->getIncomingValueForBlock(LatchBlock));
1315 WideIncExpr = SE->getSCEV(WideInc);
1318 DEBUG(dbgs() << "Wide IV: " << *WidePhi << "\n");
1321 // Traverse the def-use chain using a worklist starting at the original IV.
1322 assert(Widened.empty() && NarrowIVUsers.empty() && "expect initial state" );
1324 Widened.insert(OrigPhi);
1325 pushNarrowIVUsers(OrigPhi, WidePhi);
1327 while (!NarrowIVUsers.empty()) {
1328 NarrowIVDefUse DU = NarrowIVUsers.pop_back_val();
1330 // Process a def-use edge. This may replace the use, so don't hold a
1331 // use_iterator across it.
1332 Instruction *WideUse = WidenIVUse(DU, Rewriter);
1334 // Follow all def-use edges from the previous narrow use.
1336 pushNarrowIVUsers(DU.NarrowUse, WideUse);
1338 // WidenIVUse may have removed the def-use edge.
1339 if (DU.NarrowDef->use_empty())
1340 DeadInsts.emplace_back(DU.NarrowDef);
1345 //===----------------------------------------------------------------------===//
1346 // Live IV Reduction - Minimize IVs live across the loop.
1347 //===----------------------------------------------------------------------===//
1350 //===----------------------------------------------------------------------===//
1351 // Simplification of IV users based on SCEV evaluation.
1352 //===----------------------------------------------------------------------===//
1355 class IndVarSimplifyVisitor : public IVVisitor {
1356 ScalarEvolution *SE;
1357 const TargetTransformInfo *TTI;
1363 IndVarSimplifyVisitor(PHINode *IV, ScalarEvolution *SCEV,
1364 const TargetTransformInfo *TTI,
1365 const DominatorTree *DTree)
1366 : SE(SCEV), TTI(TTI), IVPhi(IV) {
1368 WI.NarrowIV = IVPhi;
1370 setSplitOverflowIntrinsics();
1373 // Implement the interface used by simplifyUsersOfIV.
1374 void visitCast(CastInst *Cast) override { visitIVCast(Cast, WI, SE, TTI); }
1378 /// SimplifyAndExtend - Iteratively perform simplification on a worklist of IV
1379 /// users. Each successive simplification may push more users which may
1380 /// themselves be candidates for simplification.
1382 /// Sign/Zero extend elimination is interleaved with IV simplification.
1384 void IndVarSimplify::SimplifyAndExtend(Loop *L,
1385 SCEVExpander &Rewriter,
1386 LPPassManager &LPM) {
1387 SmallVector<WideIVInfo, 8> WideIVs;
1389 SmallVector<PHINode*, 8> LoopPhis;
1390 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
1391 LoopPhis.push_back(cast<PHINode>(I));
1393 // Each round of simplification iterates through the SimplifyIVUsers worklist
1394 // for all current phis, then determines whether any IVs can be
1395 // widened. Widening adds new phis to LoopPhis, inducing another round of
1396 // simplification on the wide IVs.
1397 while (!LoopPhis.empty()) {
1398 // Evaluate as many IV expressions as possible before widening any IVs. This
1399 // forces SCEV to set no-wrap flags before evaluating sign/zero
1400 // extension. The first time SCEV attempts to normalize sign/zero extension,
1401 // the result becomes final. So for the most predictable results, we delay
1402 // evaluation of sign/zero extend evaluation until needed, and avoid running
1403 // other SCEV based analysis prior to SimplifyAndExtend.
1405 PHINode *CurrIV = LoopPhis.pop_back_val();
1407 // Information about sign/zero extensions of CurrIV.
1408 IndVarSimplifyVisitor Visitor(CurrIV, SE, TTI, DT);
1410 Changed |= simplifyUsersOfIV(CurrIV, SE, &LPM, DeadInsts, &Visitor);
1412 if (Visitor.WI.WidestNativeType) {
1413 WideIVs.push_back(Visitor.WI);
1415 } while(!LoopPhis.empty());
1417 for (; !WideIVs.empty(); WideIVs.pop_back()) {
1418 WidenIV Widener(WideIVs.back(), LI, SE, DT, DeadInsts);
1419 if (PHINode *WidePhi = Widener.CreateWideIV(Rewriter)) {
1421 LoopPhis.push_back(WidePhi);
1427 //===----------------------------------------------------------------------===//
1428 // LinearFunctionTestReplace and its kin. Rewrite the loop exit condition.
1429 //===----------------------------------------------------------------------===//
1431 /// canExpandBackedgeTakenCount - Return true if this loop's backedge taken
1432 /// count expression can be safely and cheaply expanded into an instruction
1433 /// sequence that can be used by LinearFunctionTestReplace.
1435 /// TODO: This fails for pointer-type loop counters with greater than one byte
1436 /// strides, consequently preventing LFTR from running. For the purpose of LFTR
1437 /// we could skip this check in the case that the LFTR loop counter (chosen by
1438 /// FindLoopCounter) is also pointer type. Instead, we could directly convert
1439 /// the loop test to an inequality test by checking the target data's alignment
1440 /// of element types (given that the initial pointer value originates from or is
1441 /// used by ABI constrained operation, as opposed to inttoptr/ptrtoint).
1442 /// However, we don't yet have a strong motivation for converting loop tests
1443 /// into inequality tests.
1444 static bool canExpandBackedgeTakenCount(Loop *L, ScalarEvolution *SE,
1445 SCEVExpander &Rewriter) {
1446 const SCEV *BackedgeTakenCount = SE->getBackedgeTakenCount(L);
1447 if (isa<SCEVCouldNotCompute>(BackedgeTakenCount) ||
1448 BackedgeTakenCount->isZero())
1451 if (!L->getExitingBlock())
1454 // Can't rewrite non-branch yet.
1455 if (!isa<BranchInst>(L->getExitingBlock()->getTerminator()))
1458 if (Rewriter.isHighCostExpansion(BackedgeTakenCount, L))
1464 /// getLoopPhiForCounter - Return the loop header phi IFF IncV adds a loop
1465 /// invariant value to the phi.
1466 static PHINode *getLoopPhiForCounter(Value *IncV, Loop *L, DominatorTree *DT) {
1467 Instruction *IncI = dyn_cast<Instruction>(IncV);
1471 switch (IncI->getOpcode()) {
1472 case Instruction::Add:
1473 case Instruction::Sub:
1475 case Instruction::GetElementPtr:
1476 // An IV counter must preserve its type.
1477 if (IncI->getNumOperands() == 2)
1483 PHINode *Phi = dyn_cast<PHINode>(IncI->getOperand(0));
1484 if (Phi && Phi->getParent() == L->getHeader()) {
1485 if (isLoopInvariant(IncI->getOperand(1), L, DT))
1489 if (IncI->getOpcode() == Instruction::GetElementPtr)
1492 // Allow add/sub to be commuted.
1493 Phi = dyn_cast<PHINode>(IncI->getOperand(1));
1494 if (Phi && Phi->getParent() == L->getHeader()) {
1495 if (isLoopInvariant(IncI->getOperand(0), L, DT))
1501 /// Return the compare guarding the loop latch, or NULL for unrecognized tests.
1502 static ICmpInst *getLoopTest(Loop *L) {
1503 assert(L->getExitingBlock() && "expected loop exit");
1505 BasicBlock *LatchBlock = L->getLoopLatch();
1506 // Don't bother with LFTR if the loop is not properly simplified.
1510 BranchInst *BI = dyn_cast<BranchInst>(L->getExitingBlock()->getTerminator());
1511 assert(BI && "expected exit branch");
1513 return dyn_cast<ICmpInst>(BI->getCondition());
1516 /// needsLFTR - LinearFunctionTestReplace policy. Return true unless we can show
1517 /// that the current exit test is already sufficiently canonical.
1518 static bool needsLFTR(Loop *L, DominatorTree *DT) {
1519 // Do LFTR to simplify the exit condition to an ICMP.
1520 ICmpInst *Cond = getLoopTest(L);
1524 // Do LFTR to simplify the exit ICMP to EQ/NE
1525 ICmpInst::Predicate Pred = Cond->getPredicate();
1526 if (Pred != ICmpInst::ICMP_NE && Pred != ICmpInst::ICMP_EQ)
1529 // Look for a loop invariant RHS
1530 Value *LHS = Cond->getOperand(0);
1531 Value *RHS = Cond->getOperand(1);
1532 if (!isLoopInvariant(RHS, L, DT)) {
1533 if (!isLoopInvariant(LHS, L, DT))
1535 std::swap(LHS, RHS);
1537 // Look for a simple IV counter LHS
1538 PHINode *Phi = dyn_cast<PHINode>(LHS);
1540 Phi = getLoopPhiForCounter(LHS, L, DT);
1545 // Do LFTR if PHI node is defined in the loop, but is *not* a counter.
1546 int Idx = Phi->getBasicBlockIndex(L->getLoopLatch());
1550 // Do LFTR if the exit condition's IV is *not* a simple counter.
1551 Value *IncV = Phi->getIncomingValue(Idx);
1552 return Phi != getLoopPhiForCounter(IncV, L, DT);
1555 /// Recursive helper for hasConcreteDef(). Unfortunately, this currently boils
1556 /// down to checking that all operands are constant and listing instructions
1557 /// that may hide undef.
1558 static bool hasConcreteDefImpl(Value *V, SmallPtrSetImpl<Value*> &Visited,
1560 if (isa<Constant>(V))
1561 return !isa<UndefValue>(V);
1566 // Conservatively handle non-constant non-instructions. For example, Arguments
1568 Instruction *I = dyn_cast<Instruction>(V);
1572 // Load and return values may be undef.
1573 if(I->mayReadFromMemory() || isa<CallInst>(I) || isa<InvokeInst>(I))
1576 // Optimistically handle other instructions.
1577 for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI) {
1578 if (!Visited.insert(*OI).second)
1580 if (!hasConcreteDefImpl(*OI, Visited, Depth+1))
1586 /// Return true if the given value is concrete. We must prove that undef can
1589 /// TODO: If we decide that this is a good approach to checking for undef, we
1590 /// may factor it into a common location.
1591 static bool hasConcreteDef(Value *V) {
1592 SmallPtrSet<Value*, 8> Visited;
1594 return hasConcreteDefImpl(V, Visited, 0);
1597 /// AlmostDeadIV - Return true if this IV has any uses other than the (soon to
1598 /// be rewritten) loop exit test.
1599 static bool AlmostDeadIV(PHINode *Phi, BasicBlock *LatchBlock, Value *Cond) {
1600 int LatchIdx = Phi->getBasicBlockIndex(LatchBlock);
1601 Value *IncV = Phi->getIncomingValue(LatchIdx);
1603 for (User *U : Phi->users())
1604 if (U != Cond && U != IncV) return false;
1606 for (User *U : IncV->users())
1607 if (U != Cond && U != Phi) return false;
1611 /// FindLoopCounter - Find an affine IV in canonical form.
1613 /// BECount may be an i8* pointer type. The pointer difference is already
1614 /// valid count without scaling the address stride, so it remains a pointer
1615 /// expression as far as SCEV is concerned.
1617 /// Currently only valid for LFTR. See the comments on hasConcreteDef below.
1619 /// FIXME: Accept -1 stride and set IVLimit = IVInit - BECount
1621 /// FIXME: Accept non-unit stride as long as SCEV can reduce BECount * Stride.
1622 /// This is difficult in general for SCEV because of potential overflow. But we
1623 /// could at least handle constant BECounts.
1624 static PHINode *FindLoopCounter(Loop *L, const SCEV *BECount,
1625 ScalarEvolution *SE, DominatorTree *DT) {
1626 uint64_t BCWidth = SE->getTypeSizeInBits(BECount->getType());
1629 cast<BranchInst>(L->getExitingBlock()->getTerminator())->getCondition();
1631 // Loop over all of the PHI nodes, looking for a simple counter.
1632 PHINode *BestPhi = nullptr;
1633 const SCEV *BestInit = nullptr;
1634 BasicBlock *LatchBlock = L->getLoopLatch();
1635 assert(LatchBlock && "needsLFTR should guarantee a loop latch");
1637 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
1638 PHINode *Phi = cast<PHINode>(I);
1639 if (!SE->isSCEVable(Phi->getType()))
1642 // Avoid comparing an integer IV against a pointer Limit.
1643 if (BECount->getType()->isPointerTy() && !Phi->getType()->isPointerTy())
1646 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(Phi));
1647 if (!AR || AR->getLoop() != L || !AR->isAffine())
1650 // AR may be a pointer type, while BECount is an integer type.
1651 // AR may be wider than BECount. With eq/ne tests overflow is immaterial.
1652 // AR may not be a narrower type, or we may never exit.
1653 uint64_t PhiWidth = SE->getTypeSizeInBits(AR->getType());
1654 if (PhiWidth < BCWidth ||
1655 !L->getHeader()->getModule()->getDataLayout().isLegalInteger(PhiWidth))
1658 const SCEV *Step = dyn_cast<SCEVConstant>(AR->getStepRecurrence(*SE));
1659 if (!Step || !Step->isOne())
1662 int LatchIdx = Phi->getBasicBlockIndex(LatchBlock);
1663 Value *IncV = Phi->getIncomingValue(LatchIdx);
1664 if (getLoopPhiForCounter(IncV, L, DT) != Phi)
1667 // Avoid reusing a potentially undef value to compute other values that may
1668 // have originally had a concrete definition.
1669 if (!hasConcreteDef(Phi)) {
1670 // We explicitly allow unknown phis as long as they are already used by
1671 // the loop test. In this case we assume that performing LFTR could not
1672 // increase the number of undef users.
1673 if (ICmpInst *Cond = getLoopTest(L)) {
1674 if (Phi != getLoopPhiForCounter(Cond->getOperand(0), L, DT)
1675 && Phi != getLoopPhiForCounter(Cond->getOperand(1), L, DT)) {
1680 const SCEV *Init = AR->getStart();
1682 if (BestPhi && !AlmostDeadIV(BestPhi, LatchBlock, Cond)) {
1683 // Don't force a live loop counter if another IV can be used.
1684 if (AlmostDeadIV(Phi, LatchBlock, Cond))
1687 // Prefer to count-from-zero. This is a more "canonical" counter form. It
1688 // also prefers integer to pointer IVs.
1689 if (BestInit->isZero() != Init->isZero()) {
1690 if (BestInit->isZero())
1693 // If two IVs both count from zero or both count from nonzero then the
1694 // narrower is likely a dead phi that has been widened. Use the wider phi
1695 // to allow the other to be eliminated.
1696 else if (PhiWidth <= SE->getTypeSizeInBits(BestPhi->getType()))
1705 /// genLoopLimit - Help LinearFunctionTestReplace by generating a value that
1706 /// holds the RHS of the new loop test.
1707 static Value *genLoopLimit(PHINode *IndVar, const SCEV *IVCount, Loop *L,
1708 SCEVExpander &Rewriter, ScalarEvolution *SE) {
1709 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(IndVar));
1710 assert(AR && AR->getLoop() == L && AR->isAffine() && "bad loop counter");
1711 const SCEV *IVInit = AR->getStart();
1713 // IVInit may be a pointer while IVCount is an integer when FindLoopCounter
1714 // finds a valid pointer IV. Sign extend BECount in order to materialize a
1715 // GEP. Avoid running SCEVExpander on a new pointer value, instead reusing
1716 // the existing GEPs whenever possible.
1717 if (IndVar->getType()->isPointerTy()
1718 && !IVCount->getType()->isPointerTy()) {
1720 // IVOffset will be the new GEP offset that is interpreted by GEP as a
1721 // signed value. IVCount on the other hand represents the loop trip count,
1722 // which is an unsigned value. FindLoopCounter only allows induction
1723 // variables that have a positive unit stride of one. This means we don't
1724 // have to handle the case of negative offsets (yet) and just need to zero
1726 Type *OfsTy = SE->getEffectiveSCEVType(IVInit->getType());
1727 const SCEV *IVOffset = SE->getTruncateOrZeroExtend(IVCount, OfsTy);
1729 // Expand the code for the iteration count.
1730 assert(SE->isLoopInvariant(IVOffset, L) &&
1731 "Computed iteration count is not loop invariant!");
1732 BranchInst *BI = cast<BranchInst>(L->getExitingBlock()->getTerminator());
1733 Value *GEPOffset = Rewriter.expandCodeFor(IVOffset, OfsTy, BI);
1735 Value *GEPBase = IndVar->getIncomingValueForBlock(L->getLoopPreheader());
1736 assert(AR->getStart() == SE->getSCEV(GEPBase) && "bad loop counter");
1737 // We could handle pointer IVs other than i8*, but we need to compensate for
1738 // gep index scaling. See canExpandBackedgeTakenCount comments.
1739 assert(SE->getSizeOfExpr(IntegerType::getInt64Ty(IndVar->getContext()),
1740 cast<PointerType>(GEPBase->getType())->getElementType())->isOne()
1741 && "unit stride pointer IV must be i8*");
1743 IRBuilder<> Builder(L->getLoopPreheader()->getTerminator());
1744 return Builder.CreateGEP(nullptr, GEPBase, GEPOffset, "lftr.limit");
1747 // In any other case, convert both IVInit and IVCount to integers before
1748 // comparing. This may result in SCEV expension of pointers, but in practice
1749 // SCEV will fold the pointer arithmetic away as such:
1750 // BECount = (IVEnd - IVInit - 1) => IVLimit = IVInit (postinc).
1752 // Valid Cases: (1) both integers is most common; (2) both may be pointers
1753 // for simple memset-style loops.
1755 // IVInit integer and IVCount pointer would only occur if a canonical IV
1756 // were generated on top of case #2, which is not expected.
1758 const SCEV *IVLimit = nullptr;
1759 // For unit stride, IVCount = Start + BECount with 2's complement overflow.
1760 // For non-zero Start, compute IVCount here.
1761 if (AR->getStart()->isZero())
1764 assert(AR->getStepRecurrence(*SE)->isOne() && "only handles unit stride");
1765 const SCEV *IVInit = AR->getStart();
1767 // For integer IVs, truncate the IV before computing IVInit + BECount.
1768 if (SE->getTypeSizeInBits(IVInit->getType())
1769 > SE->getTypeSizeInBits(IVCount->getType()))
1770 IVInit = SE->getTruncateExpr(IVInit, IVCount->getType());
1772 IVLimit = SE->getAddExpr(IVInit, IVCount);
1774 // Expand the code for the iteration count.
1775 BranchInst *BI = cast<BranchInst>(L->getExitingBlock()->getTerminator());
1776 IRBuilder<> Builder(BI);
1777 assert(SE->isLoopInvariant(IVLimit, L) &&
1778 "Computed iteration count is not loop invariant!");
1779 // Ensure that we generate the same type as IndVar, or a smaller integer
1780 // type. In the presence of null pointer values, we have an integer type
1781 // SCEV expression (IVInit) for a pointer type IV value (IndVar).
1782 Type *LimitTy = IVCount->getType()->isPointerTy() ?
1783 IndVar->getType() : IVCount->getType();
1784 return Rewriter.expandCodeFor(IVLimit, LimitTy, BI);
1788 /// LinearFunctionTestReplace - This method rewrites the exit condition of the
1789 /// loop to be a canonical != comparison against the incremented loop induction
1790 /// variable. This pass is able to rewrite the exit tests of any loop where the
1791 /// SCEV analysis can determine a loop-invariant trip count of the loop, which
1792 /// is actually a much broader range than just linear tests.
1793 Value *IndVarSimplify::
1794 LinearFunctionTestReplace(Loop *L,
1795 const SCEV *BackedgeTakenCount,
1797 SCEVExpander &Rewriter) {
1798 assert(canExpandBackedgeTakenCount(L, SE, Rewriter) && "precondition");
1800 // Initialize CmpIndVar and IVCount to their preincremented values.
1801 Value *CmpIndVar = IndVar;
1802 const SCEV *IVCount = BackedgeTakenCount;
1804 // If the exiting block is the same as the backedge block, we prefer to
1805 // compare against the post-incremented value, otherwise we must compare
1806 // against the preincremented value.
1807 if (L->getExitingBlock() == L->getLoopLatch()) {
1808 // Add one to the "backedge-taken" count to get the trip count.
1809 // This addition may overflow, which is valid as long as the comparison is
1810 // truncated to BackedgeTakenCount->getType().
1811 IVCount = SE->getAddExpr(BackedgeTakenCount,
1812 SE->getConstant(BackedgeTakenCount->getType(), 1));
1813 // The BackedgeTaken expression contains the number of times that the
1814 // backedge branches to the loop header. This is one less than the
1815 // number of times the loop executes, so use the incremented indvar.
1816 CmpIndVar = IndVar->getIncomingValueForBlock(L->getExitingBlock());
1819 Value *ExitCnt = genLoopLimit(IndVar, IVCount, L, Rewriter, SE);
1820 assert(ExitCnt->getType()->isPointerTy() == IndVar->getType()->isPointerTy()
1821 && "genLoopLimit missed a cast");
1823 // Insert a new icmp_ne or icmp_eq instruction before the branch.
1824 BranchInst *BI = cast<BranchInst>(L->getExitingBlock()->getTerminator());
1825 ICmpInst::Predicate P;
1826 if (L->contains(BI->getSuccessor(0)))
1827 P = ICmpInst::ICMP_NE;
1829 P = ICmpInst::ICMP_EQ;
1831 DEBUG(dbgs() << "INDVARS: Rewriting loop exit condition to:\n"
1832 << " LHS:" << *CmpIndVar << '\n'
1834 << (P == ICmpInst::ICMP_NE ? "!=" : "==") << "\n"
1835 << " RHS:\t" << *ExitCnt << "\n"
1836 << " IVCount:\t" << *IVCount << "\n");
1838 IRBuilder<> Builder(BI);
1840 // LFTR can ignore IV overflow and truncate to the width of
1841 // BECount. This avoids materializing the add(zext(add)) expression.
1842 unsigned CmpIndVarSize = SE->getTypeSizeInBits(CmpIndVar->getType());
1843 unsigned ExitCntSize = SE->getTypeSizeInBits(ExitCnt->getType());
1844 if (CmpIndVarSize > ExitCntSize) {
1845 const SCEVAddRecExpr *AR = cast<SCEVAddRecExpr>(SE->getSCEV(IndVar));
1846 const SCEV *ARStart = AR->getStart();
1847 const SCEV *ARStep = AR->getStepRecurrence(*SE);
1848 // For constant IVCount, avoid truncation.
1849 if (isa<SCEVConstant>(ARStart) && isa<SCEVConstant>(IVCount)) {
1850 const APInt &Start = cast<SCEVConstant>(ARStart)->getValue()->getValue();
1851 APInt Count = cast<SCEVConstant>(IVCount)->getValue()->getValue();
1852 // Note that the post-inc value of BackedgeTakenCount may have overflowed
1853 // above such that IVCount is now zero.
1854 if (IVCount != BackedgeTakenCount && Count == 0) {
1855 Count = APInt::getMaxValue(Count.getBitWidth()).zext(CmpIndVarSize);
1859 Count = Count.zext(CmpIndVarSize);
1861 if (cast<SCEVConstant>(ARStep)->getValue()->isNegative())
1862 NewLimit = Start - Count;
1864 NewLimit = Start + Count;
1865 ExitCnt = ConstantInt::get(CmpIndVar->getType(), NewLimit);
1867 DEBUG(dbgs() << " Widen RHS:\t" << *ExitCnt << "\n");
1869 CmpIndVar = Builder.CreateTrunc(CmpIndVar, ExitCnt->getType(),
1873 Value *Cond = Builder.CreateICmp(P, CmpIndVar, ExitCnt, "exitcond");
1874 Value *OrigCond = BI->getCondition();
1875 // It's tempting to use replaceAllUsesWith here to fully replace the old
1876 // comparison, but that's not immediately safe, since users of the old
1877 // comparison may not be dominated by the new comparison. Instead, just
1878 // update the branch to use the new comparison; in the common case this
1879 // will make old comparison dead.
1880 BI->setCondition(Cond);
1881 DeadInsts.push_back(OrigCond);
1888 //===----------------------------------------------------------------------===//
1889 // SinkUnusedInvariants. A late subpass to cleanup loop preheaders.
1890 //===----------------------------------------------------------------------===//
1892 /// If there's a single exit block, sink any loop-invariant values that
1893 /// were defined in the preheader but not used inside the loop into the
1894 /// exit block to reduce register pressure in the loop.
1895 void IndVarSimplify::SinkUnusedInvariants(Loop *L) {
1896 BasicBlock *ExitBlock = L->getExitBlock();
1897 if (!ExitBlock) return;
1899 BasicBlock *Preheader = L->getLoopPreheader();
1900 if (!Preheader) return;
1902 Instruction *InsertPt = ExitBlock->getFirstInsertionPt();
1903 BasicBlock::iterator I = Preheader->getTerminator();
1904 while (I != Preheader->begin()) {
1906 // New instructions were inserted at the end of the preheader.
1907 if (isa<PHINode>(I))
1910 // Don't move instructions which might have side effects, since the side
1911 // effects need to complete before instructions inside the loop. Also don't
1912 // move instructions which might read memory, since the loop may modify
1913 // memory. Note that it's okay if the instruction might have undefined
1914 // behavior: LoopSimplify guarantees that the preheader dominates the exit
1916 if (I->mayHaveSideEffects() || I->mayReadFromMemory())
1919 // Skip debug info intrinsics.
1920 if (isa<DbgInfoIntrinsic>(I))
1923 // Skip landingpad instructions.
1924 if (isa<LandingPadInst>(I))
1927 // Don't sink alloca: we never want to sink static alloca's out of the
1928 // entry block, and correctly sinking dynamic alloca's requires
1929 // checks for stacksave/stackrestore intrinsics.
1930 // FIXME: Refactor this check somehow?
1931 if (isa<AllocaInst>(I))
1934 // Determine if there is a use in or before the loop (direct or
1936 bool UsedInLoop = false;
1937 for (Use &U : I->uses()) {
1938 Instruction *User = cast<Instruction>(U.getUser());
1939 BasicBlock *UseBB = User->getParent();
1940 if (PHINode *P = dyn_cast<PHINode>(User)) {
1942 PHINode::getIncomingValueNumForOperand(U.getOperandNo());
1943 UseBB = P->getIncomingBlock(i);
1945 if (UseBB == Preheader || L->contains(UseBB)) {
1951 // If there is, the def must remain in the preheader.
1955 // Otherwise, sink it to the exit block.
1956 Instruction *ToMove = I;
1959 if (I != Preheader->begin()) {
1960 // Skip debug info intrinsics.
1963 } while (isa<DbgInfoIntrinsic>(I) && I != Preheader->begin());
1965 if (isa<DbgInfoIntrinsic>(I) && I == Preheader->begin())
1971 ToMove->moveBefore(InsertPt);
1977 //===----------------------------------------------------------------------===//
1978 // IndVarSimplify driver. Manage several subpasses of IV simplification.
1979 //===----------------------------------------------------------------------===//
1981 bool IndVarSimplify::runOnLoop(Loop *L, LPPassManager &LPM) {
1982 if (skipOptnoneFunction(L))
1985 // If LoopSimplify form is not available, stay out of trouble. Some notes:
1986 // - LSR currently only supports LoopSimplify-form loops. Indvars'
1987 // canonicalization can be a pessimization without LSR to "clean up"
1989 // - We depend on having a preheader; in particular,
1990 // Loop::getCanonicalInductionVariable only supports loops with preheaders,
1991 // and we're in trouble if we can't find the induction variable even when
1992 // we've manually inserted one.
1993 if (!L->isLoopSimplifyForm())
1996 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
1997 SE = &getAnalysis<ScalarEvolution>();
1998 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
1999 auto *TLIP = getAnalysisIfAvailable<TargetLibraryInfoWrapperPass>();
2000 TLI = TLIP ? &TLIP->getTLI() : nullptr;
2001 auto *TTIP = getAnalysisIfAvailable<TargetTransformInfoWrapperPass>();
2002 TTI = TTIP ? &TTIP->getTTI(*L->getHeader()->getParent()) : nullptr;
2003 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
2008 // If there are any floating-point recurrences, attempt to
2009 // transform them to use integer recurrences.
2010 RewriteNonIntegerIVs(L);
2012 const SCEV *BackedgeTakenCount = SE->getBackedgeTakenCount(L);
2014 // Create a rewriter object which we'll use to transform the code with.
2015 SCEVExpander Rewriter(*SE, DL, "indvars");
2017 Rewriter.setDebugType(DEBUG_TYPE);
2020 // Eliminate redundant IV users.
2022 // Simplification works best when run before other consumers of SCEV. We
2023 // attempt to avoid evaluating SCEVs for sign/zero extend operations until
2024 // other expressions involving loop IVs have been evaluated. This helps SCEV
2025 // set no-wrap flags before normalizing sign/zero extension.
2026 Rewriter.disableCanonicalMode();
2027 SimplifyAndExtend(L, Rewriter, LPM);
2029 // Check to see if this loop has a computable loop-invariant execution count.
2030 // If so, this means that we can compute the final value of any expressions
2031 // that are recurrent in the loop, and substitute the exit values from the
2032 // loop into any instructions outside of the loop that use the final values of
2033 // the current expressions.
2035 if (ReplaceExitValue != NeverRepl &&
2036 !isa<SCEVCouldNotCompute>(BackedgeTakenCount))
2037 RewriteLoopExitValues(L, Rewriter);
2039 // Eliminate redundant IV cycles.
2040 NumElimIV += Rewriter.replaceCongruentIVs(L, DT, DeadInsts);
2042 // If we have a trip count expression, rewrite the loop's exit condition
2043 // using it. We can currently only handle loops with a single exit.
2044 if (canExpandBackedgeTakenCount(L, SE, Rewriter) && needsLFTR(L, DT)) {
2045 PHINode *IndVar = FindLoopCounter(L, BackedgeTakenCount, SE, DT);
2047 // Check preconditions for proper SCEVExpander operation. SCEV does not
2048 // express SCEVExpander's dependencies, such as LoopSimplify. Instead any
2049 // pass that uses the SCEVExpander must do it. This does not work well for
2050 // loop passes because SCEVExpander makes assumptions about all loops,
2051 // while LoopPassManager only forces the current loop to be simplified.
2053 // FIXME: SCEV expansion has no way to bail out, so the caller must
2054 // explicitly check any assumptions made by SCEV. Brittle.
2055 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(BackedgeTakenCount);
2056 if (!AR || AR->getLoop()->getLoopPreheader())
2057 (void)LinearFunctionTestReplace(L, BackedgeTakenCount, IndVar,
2061 // Clear the rewriter cache, because values that are in the rewriter's cache
2062 // can be deleted in the loop below, causing the AssertingVH in the cache to
2066 // Now that we're done iterating through lists, clean up any instructions
2067 // which are now dead.
2068 while (!DeadInsts.empty())
2069 if (Instruction *Inst =
2070 dyn_cast_or_null<Instruction>(DeadInsts.pop_back_val()))
2071 RecursivelyDeleteTriviallyDeadInstructions(Inst, TLI);
2073 // The Rewriter may not be used from this point on.
2075 // Loop-invariant instructions in the preheader that aren't used in the
2076 // loop may be sunk below the loop to reduce register pressure.
2077 SinkUnusedInvariants(L);
2079 // Clean up dead instructions.
2080 Changed |= DeleteDeadPHIs(L->getHeader(), TLI);
2081 // Check a post-condition.
2082 assert(L->isLCSSAForm(*DT) &&
2083 "Indvars did not leave the loop in lcssa form!");
2085 // Verify that LFTR, and any other change have not interfered with SCEV's
2086 // ability to compute trip count.
2088 if (VerifyIndvars && !isa<SCEVCouldNotCompute>(BackedgeTakenCount)) {
2090 const SCEV *NewBECount = SE->getBackedgeTakenCount(L);
2091 if (SE->getTypeSizeInBits(BackedgeTakenCount->getType()) <
2092 SE->getTypeSizeInBits(NewBECount->getType()))
2093 NewBECount = SE->getTruncateOrNoop(NewBECount,
2094 BackedgeTakenCount->getType());
2096 BackedgeTakenCount = SE->getTruncateOrNoop(BackedgeTakenCount,
2097 NewBECount->getType());
2098 assert(BackedgeTakenCount == NewBECount && "indvars must preserve SCEV");