1 //===-- PPCCTRLoops.cpp - Identify and generate CTR loops -----------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass identifies loops where we can generate the PPC branch instructions
11 // that decrement and test the count register (CTR) (bdnz and friends).
12 //
13 // The pattern that defines the induction variable can changed depending on
14 // prior optimizations.  For example, the IndVarSimplify phase run by 'opt'
15 // normalizes induction variables, and the Loop Strength Reduction pass
16 // run by 'llc' may also make changes to the induction variable.
17 //
18 // Criteria for CTR loops:
19 //  - Countable loops (w/ ind. var for a trip count)
20 //  - Try inner-most loops first
21 //  - No nested CTR loops.
22 //  - No function calls in loops.
23 //
24 //===----------------------------------------------------------------------===//
25 
26 #define DEBUG_TYPE "ctrloops"
27 
28 #include "llvm/Transforms/Scalar.h"
29 #include "llvm/ADT/Statistic.h"
30 #include "llvm/ADT/STLExtras.h"
31 #include "llvm/Analysis/Dominators.h"
32 #include "llvm/Analysis/LoopInfo.h"
33 #include "llvm/Analysis/ScalarEvolutionExpander.h"
34 #include "llvm/IR/Constants.h"
35 #include "llvm/IR/DerivedTypes.h"
36 #include "llvm/IR/InlineAsm.h"
37 #include "llvm/IR/Instructions.h"
38 #include "llvm/IR/IntrinsicInst.h"
39 #include "llvm/IR/Module.h"
40 #include "llvm/PassSupport.h"
41 #include "llvm/Support/CommandLine.h"
42 #include "llvm/Support/Debug.h"
43 #include "llvm/Support/ValueHandle.h"
44 #include "llvm/Support/raw_ostream.h"
45 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
46 #include "llvm/Transforms/Utils/Local.h"
47 #include "llvm/Transforms/Utils/LoopUtils.h"
48 #include "llvm/Target/TargetLibraryInfo.h"
49 #include "PPCTargetMachine.h"
50 #include "PPC.h"
51 
52 #ifndef NDEBUG
53 #include "llvm/CodeGen/MachineDominators.h"
54 #include "llvm/CodeGen/MachineFunction.h"
55 #include "llvm/CodeGen/MachineFunctionPass.h"
56 #include "llvm/CodeGen/MachineRegisterInfo.h"
57 #endif
58 
59 #include <algorithm>
60 #include <vector>
61 
62 using namespace llvm;
63 
64 #ifndef NDEBUG
65 static cl::opt<int> CTRLoopLimit("ppc-max-ctrloop", cl::Hidden, cl::init(-1));
66 #endif
67 
68 STATISTIC(NumCTRLoops, "Number of loops converted to CTR loops");
69 
70 namespace llvm {
71   void initializePPCCTRLoopsPass(PassRegistry&);
72 #ifndef NDEBUG
73   void initializePPCCTRLoopsVerifyPass(PassRegistry&);
74 #endif
75 }
76 
77 namespace {
78   struct PPCCTRLoops : public FunctionPass {
79 
80 #ifndef NDEBUG
81     static int Counter;
82 #endif
83 
84   public:
85     static char ID;
86 
87     PPCCTRLoops() : FunctionPass(ID), TM(0) {
88       initializePPCCTRLoopsPass(*PassRegistry::getPassRegistry());
89     }
90     PPCCTRLoops(PPCTargetMachine &TM) : FunctionPass(ID), TM(&TM) {
91       initializePPCCTRLoopsPass(*PassRegistry::getPassRegistry());
92     }
93 
94     virtual bool runOnFunction(Function &F);
95 
96     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
97       AU.addRequired<LoopInfo>();
98       AU.addPreserved<LoopInfo>();
99       AU.addRequired<DominatorTree>();
100       AU.addPreserved<DominatorTree>();
101       AU.addRequired<ScalarEvolution>();
102     }
103 
104   private:
105     bool mightUseCTR(const Triple &TT, BasicBlock *BB);
106     bool convertToCTRLoop(Loop *L);
107 
108   private:
109     PPCTargetMachine *TM;
110     LoopInfo *LI;
111     ScalarEvolution *SE;
112     DataLayout *TD;
113     DominatorTree *DT;
114     const TargetLibraryInfo *LibInfo;
115   };
116 
117   char PPCCTRLoops::ID = 0;
118 #ifndef NDEBUG
119   int PPCCTRLoops::Counter = 0;
120 #endif
121 
122 #ifndef NDEBUG
123   struct PPCCTRLoopsVerify : public MachineFunctionPass {
124   public:
125     static char ID;
126 
127     PPCCTRLoopsVerify() : MachineFunctionPass(ID) {
128       initializePPCCTRLoopsVerifyPass(*PassRegistry::getPassRegistry());
129     }
130 
131     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
132       AU.addRequired<MachineDominatorTree>();
133       MachineFunctionPass::getAnalysisUsage(AU);
134     }
135 
136     virtual bool runOnMachineFunction(MachineFunction &MF);
137 
138   private:
139     MachineDominatorTree *MDT;
140   };
141 
142   char PPCCTRLoopsVerify::ID = 0;
143 #endif // NDEBUG
144 } // end anonymous namespace
145 
146 INITIALIZE_PASS_BEGIN(PPCCTRLoops, "ppc-ctr-loops", "PowerPC CTR Loops",
147                       false, false)
148 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
149 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
150 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
151 INITIALIZE_PASS_END(PPCCTRLoops, "ppc-ctr-loops", "PowerPC CTR Loops",
152                     false, false)
153 
154 FunctionPass *llvm::createPPCCTRLoops(PPCTargetMachine &TM) {
155   return new PPCCTRLoops(TM);
156 }
157 
158 #ifndef NDEBUG
159 INITIALIZE_PASS_BEGIN(PPCCTRLoopsVerify, "ppc-ctr-loops-verify",
160                       "PowerPC CTR Loops Verify", false, false)
161 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
162 INITIALIZE_PASS_END(PPCCTRLoopsVerify, "ppc-ctr-loops-verify",
163                     "PowerPC CTR Loops Verify", false, false)
164 
165 FunctionPass *llvm::createPPCCTRLoopsVerify() {
166   return new PPCCTRLoopsVerify();
167 }
168 #endif // NDEBUG
169 
170 bool PPCCTRLoops::runOnFunction(Function &F) {
171   LI = &getAnalysis<LoopInfo>();
172   SE = &getAnalysis<ScalarEvolution>();
173   DT = &getAnalysis<DominatorTree>();
174   TD = getAnalysisIfAvailable<DataLayout>();
175   LibInfo = getAnalysisIfAvailable<TargetLibraryInfo>();
176 
177   bool MadeChange = false;
178 
179   for (LoopInfo::iterator I = LI->begin(), E = LI->end();
180        I != E; ++I) {
181     Loop *L = *I;
182     if (!L->getParentLoop())
183       MadeChange |= convertToCTRLoop(L);
184   }
185 
186   return MadeChange;
187 }
188 
189 bool PPCCTRLoops::mightUseCTR(const Triple &TT, BasicBlock *BB) {
190   for (BasicBlock::iterator J = BB->begin(), JE = BB->end();
191        J != JE; ++J) {
192     if (CallInst *CI = dyn_cast<CallInst>(J)) {
193       if (InlineAsm *IA = dyn_cast<InlineAsm>(CI->getCalledValue())) {
194         // Inline ASM is okay, unless it clobbers the ctr register.
195         InlineAsm::ConstraintInfoVector CIV = IA->ParseConstraints();
196         for (unsigned i = 0, ie = CIV.size(); i < ie; ++i) {
197           InlineAsm::ConstraintInfo &C = CIV[i];
198           if (C.Type != InlineAsm::isInput)
199             for (unsigned j = 0, je = C.Codes.size(); j < je; ++j)
200               if (StringRef(C.Codes[j]).equals_lower("{ctr}"))
201                 return true;
202         }
203 
204         continue;
205       }
206 
207       if (!TM)
208         return true;
209       const TargetLowering *TLI = TM->getTargetLowering();
210 
211       if (Function *F = CI->getCalledFunction()) {
212         // Most intrinsics don't become function calls, but some might.
213         // sin, cos, exp and log are always calls.
214         unsigned Opcode;
215         if (F->getIntrinsicID() != Intrinsic::not_intrinsic) {
216           switch (F->getIntrinsicID()) {
217           default: continue;
218 
219 // VisualStudio defines setjmp as _setjmp
220 #if defined(_MSC_VER) && defined(setjmp) && \
221                        !defined(setjmp_undefined_for_msvc)
222 #  pragma push_macro("setjmp")
223 #  undef setjmp
224 #  define setjmp_undefined_for_msvc
225 #endif
226 
227           case Intrinsic::setjmp:
228 
229 #if defined(_MSC_VER) && defined(setjmp_undefined_for_msvc)
230  // let's return it to _setjmp state
231 #  pragma pop_macro("setjmp")
232 #  undef setjmp_undefined_for_msvc
233 #endif
234 
235           case Intrinsic::longjmp:
236 
237           // Exclude eh_sjlj_setjmp; we don't need to exclude eh_sjlj_longjmp
238           // because, although it does clobber the counter register, the
239           // control can't then return to inside the loop unless there is also
240           // an eh_sjlj_setjmp.
241           case Intrinsic::eh_sjlj_setjmp:
242 
243           case Intrinsic::memcpy:
244           case Intrinsic::memmove:
245           case Intrinsic::memset:
246           case Intrinsic::powi:
247           case Intrinsic::log:
248           case Intrinsic::log2:
249           case Intrinsic::log10:
250           case Intrinsic::exp:
251           case Intrinsic::exp2:
252           case Intrinsic::pow:
253           case Intrinsic::sin:
254           case Intrinsic::cos:
255             return true;
256           case Intrinsic::sqrt:      Opcode = ISD::FSQRT;      break;
257           case Intrinsic::floor:     Opcode = ISD::FFLOOR;     break;
258           case Intrinsic::ceil:      Opcode = ISD::FCEIL;      break;
259           case Intrinsic::trunc:     Opcode = ISD::FTRUNC;     break;
260           case Intrinsic::rint:      Opcode = ISD::FRINT;      break;
261           case Intrinsic::nearbyint: Opcode = ISD::FNEARBYINT; break;
262           case Intrinsic::round:     Opcode = ISD::FROUND;     break;
263           }
264         }
265 
266         // PowerPC does not use [US]DIVREM or other library calls for
267         // operations on regular types which are not otherwise library calls
268         // (i.e. soft float or atomics). If adapting for targets that do,
269         // additional care is required here.
270 
271         LibFunc::Func Func;
272         if (!F->hasLocalLinkage() && F->hasName() && LibInfo &&
273             LibInfo->getLibFunc(F->getName(), Func) &&
274             LibInfo->hasOptimizedCodeGen(Func)) {
275           // Non-read-only functions are never treated as intrinsics.
276           if (!CI->onlyReadsMemory())
277             return true;
278 
279           // Conversion happens only for FP calls.
280           if (!CI->getArgOperand(0)->getType()->isFloatingPointTy())
281             return true;
282 
283           switch (Func) {
284           default: return true;
285           case LibFunc::copysign:
286           case LibFunc::copysignf:
287           case LibFunc::copysignl:
288             continue; // ISD::FCOPYSIGN is never a library call.
289           case LibFunc::fabs:
290           case LibFunc::fabsf:
291           case LibFunc::fabsl:
292             continue; // ISD::FABS is never a library call.
293           case LibFunc::sqrt:
294           case LibFunc::sqrtf:
295           case LibFunc::sqrtl:
296             Opcode = ISD::FSQRT; break;
297           case LibFunc::floor:
298           case LibFunc::floorf:
299           case LibFunc::floorl:
300             Opcode = ISD::FFLOOR; break;
301           case LibFunc::nearbyint:
302           case LibFunc::nearbyintf:
303           case LibFunc::nearbyintl:
304             Opcode = ISD::FNEARBYINT; break;
305           case LibFunc::ceil:
306           case LibFunc::ceilf:
307           case LibFunc::ceill:
308             Opcode = ISD::FCEIL; break;
309           case LibFunc::rint:
310           case LibFunc::rintf:
311           case LibFunc::rintl:
312             Opcode = ISD::FRINT; break;
313           case LibFunc::round:
314           case LibFunc::roundf:
315           case LibFunc::roundl:
316             Opcode = ISD::FROUND; break;
317           case LibFunc::trunc:
318           case LibFunc::truncf:
319           case LibFunc::truncl:
320             Opcode = ISD::FTRUNC; break;
321           }
322 
323           MVT VTy =
324             TLI->getSimpleValueType(CI->getArgOperand(0)->getType(), true);
325           if (VTy == MVT::Other)
326             return true;
327 
328           if (TLI->isOperationLegalOrCustom(Opcode, VTy))
329             continue;
330           else if (VTy.isVector() &&
331                    TLI->isOperationLegalOrCustom(Opcode, VTy.getScalarType()))
332             continue;
333 
334           return true;
335         }
336       }
337 
338       return true;
339     } else if (isa<BinaryOperator>(J) &&
340                J->getType()->getScalarType()->isPPC_FP128Ty()) {
341       // Most operations on ppc_f128 values become calls.
342       return true;
343     } else if (isa<UIToFPInst>(J) || isa<SIToFPInst>(J) ||
344                isa<FPToUIInst>(J) || isa<FPToSIInst>(J)) {
345       CastInst *CI = cast<CastInst>(J);
346       if (CI->getSrcTy()->getScalarType()->isPPC_FP128Ty() ||
347           CI->getDestTy()->getScalarType()->isPPC_FP128Ty() ||
348           (TT.isArch32Bit() &&
349            (CI->getSrcTy()->getScalarType()->isIntegerTy(64) ||
350             CI->getDestTy()->getScalarType()->isIntegerTy(64))
351           ))
352         return true;
353     } else if (TT.isArch32Bit() &&
354                J->getType()->getScalarType()->isIntegerTy(64) &&
355                (J->getOpcode() == Instruction::UDiv ||
356                 J->getOpcode() == Instruction::SDiv ||
357                 J->getOpcode() == Instruction::URem ||
358                 J->getOpcode() == Instruction::SRem)) {
359       return true;
360     } else if (isa<IndirectBrInst>(J) || isa<InvokeInst>(J)) {
361       // On PowerPC, indirect jumps use the counter register.
362       return true;
363     } else if (SwitchInst *SI = dyn_cast<SwitchInst>(J)) {
364       if (!TM)
365         return true;
366       const TargetLowering *TLI = TM->getTargetLowering();
367 
368       if (TLI->supportJumpTables() &&
369           SI->getNumCases()+1 >= (unsigned) TLI->getMinimumJumpTableEntries())
370         return true;
371     }
372   }
373 
374   return false;
375 }
376 
377 bool PPCCTRLoops::convertToCTRLoop(Loop *L) {
378   bool MadeChange = false;
379 
380   Triple TT = Triple(L->getHeader()->getParent()->getParent()->
381                      getTargetTriple());
382   if (!TT.isArch32Bit() && !TT.isArch64Bit())
383     return MadeChange; // Unknown arch. type.
384 
385   // Process nested loops first.
386   for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) {
387     MadeChange |= convertToCTRLoop(*I);
388   }
389 
390   // If a nested loop has been converted, then we can't convert this loop.
391   if (MadeChange)
392     return MadeChange;
393 
394 #ifndef NDEBUG
395   // Stop trying after reaching the limit (if any).
396   int Limit = CTRLoopLimit;
397   if (Limit >= 0) {
398     if (Counter >= CTRLoopLimit)
399       return false;
400     Counter++;
401   }
402 #endif
403 
404   // We don't want to spill/restore the counter register, and so we don't
405   // want to use the counter register if the loop contains calls.
406   for (Loop::block_iterator I = L->block_begin(), IE = L->block_end();
407        I != IE; ++I)
408     if (mightUseCTR(TT, *I))
409       return MadeChange;
410 
411   SmallVector<BasicBlock*, 4> ExitingBlocks;
412   L->getExitingBlocks(ExitingBlocks);
413 
414   BasicBlock *CountedExitBlock = 0;
415   const SCEV *ExitCount = 0;
416   BranchInst *CountedExitBranch = 0;
417   for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
418        IE = ExitingBlocks.end(); I != IE; ++I) {
419     const SCEV *EC = SE->getExitCount(L, *I);
420     DEBUG(dbgs() << "Exit Count for " << *L << " from block " <<
421                     (*I)->getName() << ": " << *EC << "\n");
422     if (isa<SCEVCouldNotCompute>(EC))
423       continue;
424     if (const SCEVConstant *ConstEC = dyn_cast<SCEVConstant>(EC)) {
425       if (ConstEC->getValue()->isZero())
426         continue;
427     } else if (!SE->isLoopInvariant(EC, L))
428       continue;
429 
430     if (SE->getTypeSizeInBits(EC->getType()) > (TT.isArch64Bit() ? 64 : 32))
431       continue;
432 
433     // We now have a loop-invariant count of loop iterations (which is not the
434     // constant zero) for which we know that this loop will not exit via this
435     // exisiting block.
436 
437     // We need to make sure that this block will run on every loop iteration.
438     // For this to be true, we must dominate all blocks with backedges. Such
439     // blocks are in-loop predecessors to the header block.
440     bool NotAlways = false;
441     for (pred_iterator PI = pred_begin(L->getHeader()),
442          PIE = pred_end(L->getHeader()); PI != PIE; ++PI) {
443       if (!L->contains(*PI))
444         continue;
445 
446       if (!DT->dominates(*I, *PI)) {
447         NotAlways = true;
448         break;
449       }
450     }
451 
452     if (NotAlways)
453       continue;
454 
455     // Make sure this blocks ends with a conditional branch.
456     Instruction *TI = (*I)->getTerminator();
457     if (!TI)
458       continue;
459 
460     if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
461       if (!BI->isConditional())
462         continue;
463 
464       CountedExitBranch = BI;
465     } else
466       continue;
467 
468     // Note that this block may not be the loop latch block, even if the loop
469     // has a latch block.
470     CountedExitBlock = *I;
471     ExitCount = EC;
472     break;
473   }
474 
475   if (!CountedExitBlock)
476     return MadeChange;
477 
478   BasicBlock *Preheader = L->getLoopPreheader();
479 
480   // If we don't have a preheader, then insert one. If we already have a
481   // preheader, then we can use it (except if the preheader contains a use of
482   // the CTR register because some such uses might be reordered by the
483   // selection DAG after the mtctr instruction).
484   if (!Preheader || mightUseCTR(TT, Preheader))
485     Preheader = InsertPreheaderForLoop(L, this);
486   if (!Preheader)
487     return MadeChange;
488 
489   DEBUG(dbgs() << "Preheader for exit count: " << Preheader->getName() << "\n");
490 
491   // Insert the count into the preheader and replace the condition used by the
492   // selected branch.
493   MadeChange = true;
494 
495   SCEVExpander SCEVE(*SE, "loopcnt");
496   LLVMContext &C = SE->getContext();
497   Type *CountType = TT.isArch64Bit() ? Type::getInt64Ty(C) :
498                                        Type::getInt32Ty(C);
499   if (!ExitCount->getType()->isPointerTy() &&
500       ExitCount->getType() != CountType)
501     ExitCount = SE->getZeroExtendExpr(ExitCount, CountType);
502   ExitCount = SE->getAddExpr(ExitCount,
503                              SE->getConstant(CountType, 1));
504   Value *ECValue = SCEVE.expandCodeFor(ExitCount, CountType,
505                                        Preheader->getTerminator());
506 
507   IRBuilder<> CountBuilder(Preheader->getTerminator());
508   Module *M = Preheader->getParent()->getParent();
509   Value *MTCTRFunc = Intrinsic::getDeclaration(M, Intrinsic::ppc_mtctr,
510                                                CountType);
511   CountBuilder.CreateCall(MTCTRFunc, ECValue);
512 
513   IRBuilder<> CondBuilder(CountedExitBranch);
514   Value *DecFunc =
515     Intrinsic::getDeclaration(M, Intrinsic::ppc_is_decremented_ctr_nonzero);
516   Value *NewCond = CondBuilder.CreateCall(DecFunc);
517   Value *OldCond = CountedExitBranch->getCondition();
518   CountedExitBranch->setCondition(NewCond);
519 
520   // The false branch must exit the loop.
521   if (!L->contains(CountedExitBranch->getSuccessor(0)))
522     CountedExitBranch->swapSuccessors();
523 
524   // The old condition may be dead now, and may have even created a dead PHI
525   // (the original induction variable).
526   RecursivelyDeleteTriviallyDeadInstructions(OldCond);
527   DeleteDeadPHIs(CountedExitBlock);
528 
529   ++NumCTRLoops;
530   return MadeChange;
531 }
532 
533 #ifndef NDEBUG
534 static bool clobbersCTR(const MachineInstr *MI) {
535   for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
536     const MachineOperand &MO = MI->getOperand(i);
537     if (MO.isReg()) {
538       if (MO.isDef() && (MO.getReg() == PPC::CTR || MO.getReg() == PPC::CTR8))
539         return true;
540     } else if (MO.isRegMask()) {
541       if (MO.clobbersPhysReg(PPC::CTR) || MO.clobbersPhysReg(PPC::CTR8))
542         return true;
543     }
544   }
545 
546   return false;
547 }
548 
549 static bool verifyCTRBranch(MachineBasicBlock *MBB,
550                             MachineBasicBlock::iterator I) {
551   MachineBasicBlock::iterator BI = I;
552   SmallSet<MachineBasicBlock *, 16>   Visited;
553   SmallVector<MachineBasicBlock *, 8> Preds;
554   bool CheckPreds;
555 
556   if (I == MBB->begin()) {
557     Visited.insert(MBB);
558     goto queue_preds;
559   } else
560     --I;
561 
562 check_block:
563   Visited.insert(MBB);
564   if (I == MBB->end())
565     goto queue_preds;
566 
567   CheckPreds = true;
568   for (MachineBasicBlock::iterator IE = MBB->begin();; --I) {
569     unsigned Opc = I->getOpcode();
570     if (Opc == PPC::MTCTRloop || Opc == PPC::MTCTR8loop) {
571       CheckPreds = false;
572       break;
573     }
574 
575     if (I != BI && clobbersCTR(I)) {
576       DEBUG(dbgs() << "BB#" << MBB->getNumber() << " (" <<
577                       MBB->getFullName() << ") instruction " << *I <<
578                       " clobbers CTR, invalidating " << "BB#" <<
579                       BI->getParent()->getNumber() << " (" <<
580                       BI->getParent()->getFullName() << ") instruction " <<
581                       *BI << "\n");
582       return false;
583     }
584 
585     if (I == IE)
586       break;
587   }
588 
589   if (!CheckPreds && Preds.empty())
590     return true;
591 
592   if (CheckPreds) {
593 queue_preds:
594     if (MachineFunction::iterator(MBB) == MBB->getParent()->begin()) {
595       DEBUG(dbgs() << "Unable to find a MTCTR instruction for BB#" <<
596                       BI->getParent()->getNumber() << " (" <<
597                       BI->getParent()->getFullName() << ") instruction " <<
598                       *BI << "\n");
599       return false;
600     }
601 
602     for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
603          PIE = MBB->pred_end(); PI != PIE; ++PI)
604       Preds.push_back(*PI);
605   }
606 
607   do {
608     MBB = Preds.pop_back_val();
609     if (!Visited.count(MBB)) {
610       I = MBB->getLastNonDebugInstr();
611       goto check_block;
612     }
613   } while (!Preds.empty());
614 
615   return true;
616 }
617 
618 bool PPCCTRLoopsVerify::runOnMachineFunction(MachineFunction &MF) {
619   MDT = &getAnalysis<MachineDominatorTree>();
620 
621   // Verify that all bdnz/bdz instructions are dominated by a loop mtctr before
622   // any other instructions that might clobber the ctr register.
623   for (MachineFunction::iterator I = MF.begin(), IE = MF.end();
624        I != IE; ++I) {
625     MachineBasicBlock *MBB = I;
626     if (!MDT->isReachableFromEntry(MBB))
627       continue;
628 
629     for (MachineBasicBlock::iterator MII = MBB->getFirstTerminator(),
630       MIIE = MBB->end(); MII != MIIE; ++MII) {
631       unsigned Opc = MII->getOpcode();
632       if (Opc == PPC::BDNZ8 || Opc == PPC::BDNZ ||
633           Opc == PPC::BDZ8  || Opc == PPC::BDZ)
634         if (!verifyCTRBranch(MBB, MII))
635           llvm_unreachable("Invalid PPC CTR loop!");
636     }
637   }
638 
639   return false;
640 }
641 #endif // NDEBUG
642 
643