Bug Summary

File:lib/Transforms/Scalar/LoopUnswitch.cpp
Location:line 420, column 5
Description:Forming reference to null pointer

Annotated Source Code

1//===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
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 transforms loops that contain branches on loop-invariant conditions
11// to have multiple loops. For example, it turns the left into the right code:
12//
13// for (...) if (lic)
14// A for (...)
15// if (lic) A; B; C
16// B else
17// C for (...)
18// A; C
19//
20// This can increase the size of the code exponentially (doubling it every time
21// a loop is unswitched) so we only unswitch if the resultant code will be
22// smaller than a threshold.
23//
24// This pass expects LICM to be run before it to hoist invariant conditions out
25// of the loop, to make the unswitching opportunity obvious.
26//
27//===----------------------------------------------------------------------===//
28
29#include "llvm/Transforms/Scalar.h"
30#include "llvm/ADT/STLExtras.h"
31#include "llvm/ADT/SmallPtrSet.h"
32#include "llvm/ADT/Statistic.h"
33#include "llvm/Analysis/AssumptionCache.h"
34#include "llvm/Analysis/CodeMetrics.h"
35#include "llvm/Analysis/InstructionSimplify.h"
36#include "llvm/Analysis/LoopInfo.h"
37#include "llvm/Analysis/LoopPass.h"
38#include "llvm/Analysis/ScalarEvolution.h"
39#include "llvm/Analysis/TargetTransformInfo.h"
40#include "llvm/IR/Constants.h"
41#include "llvm/IR/DerivedTypes.h"
42#include "llvm/IR/Dominators.h"
43#include "llvm/IR/Function.h"
44#include "llvm/IR/Instructions.h"
45#include "llvm/IR/Module.h"
46#include "llvm/IR/MDBuilder.h"
47#include "llvm/Support/CommandLine.h"
48#include "llvm/Support/Debug.h"
49#include "llvm/Support/raw_ostream.h"
50#include "llvm/Transforms/Utils/BasicBlockUtils.h"
51#include "llvm/Transforms/Utils/Cloning.h"
52#include "llvm/Transforms/Utils/Local.h"
53#include <algorithm>
54#include <map>
55#include <set>
56using namespace llvm;
57
58#define DEBUG_TYPE"loop-unswitch" "loop-unswitch"
59
60STATISTIC(NumBranches, "Number of branches unswitched")static llvm::Statistic NumBranches = { "loop-unswitch", "Number of branches unswitched"
, 0, 0 };
61STATISTIC(NumSwitches, "Number of switches unswitched")static llvm::Statistic NumSwitches = { "loop-unswitch", "Number of switches unswitched"
, 0, 0 };
62STATISTIC(NumSelects , "Number of selects unswitched")static llvm::Statistic NumSelects = { "loop-unswitch", "Number of selects unswitched"
, 0, 0 };
63STATISTIC(NumTrivial , "Number of unswitches that are trivial")static llvm::Statistic NumTrivial = { "loop-unswitch", "Number of unswitches that are trivial"
, 0, 0 };
64STATISTIC(NumSimplify, "Number of simplifications of unswitched code")static llvm::Statistic NumSimplify = { "loop-unswitch", "Number of simplifications of unswitched code"
, 0, 0 };
65STATISTIC(TotalInsts, "Total number of instructions analyzed")static llvm::Statistic TotalInsts = { "loop-unswitch", "Total number of instructions analyzed"
, 0, 0 };
66
67// The specific value of 100 here was chosen based only on intuition and a
68// few specific examples.
69static cl::opt<unsigned>
70Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
71 cl::init(100), cl::Hidden);
72
73namespace {
74
75 class LUAnalysisCache {
76
77 typedef DenseMap<const SwitchInst*, SmallPtrSet<const Value *, 8> >
78 UnswitchedValsMap;
79
80 typedef UnswitchedValsMap::iterator UnswitchedValsIt;
81
82 struct LoopProperties {
83 unsigned CanBeUnswitchedCount;
84 unsigned WasUnswitchedCount;
85 unsigned SizeEstimation;
86 UnswitchedValsMap UnswitchedVals;
87 };
88
89 // Here we use std::map instead of DenseMap, since we need to keep valid
90 // LoopProperties pointer for current loop for better performance.
91 typedef std::map<const Loop*, LoopProperties> LoopPropsMap;
92 typedef LoopPropsMap::iterator LoopPropsMapIt;
93
94 LoopPropsMap LoopsProperties;
95 UnswitchedValsMap *CurLoopInstructions;
96 LoopProperties *CurrentLoopProperties;
97
98 // A loop unswitching with an estimated cost above this threshold
99 // is not performed. MaxSize is turned into unswitching quota for
100 // the current loop, and reduced correspondingly, though note that
101 // the quota is returned by releaseMemory() when the loop has been
102 // processed, so that MaxSize will return to its previous
103 // value. So in most cases MaxSize will equal the Threshold flag
104 // when a new loop is processed. An exception to that is that
105 // MaxSize will have a smaller value while processing nested loops
106 // that were introduced due to loop unswitching of an outer loop.
107 //
108 // FIXME: The way that MaxSize works is subtle and depends on the
109 // pass manager processing loops and calling releaseMemory() in a
110 // specific order. It would be good to find a more straightforward
111 // way of doing what MaxSize does.
112 unsigned MaxSize;
113
114 public:
115 LUAnalysisCache()
116 : CurLoopInstructions(nullptr), CurrentLoopProperties(nullptr),
117 MaxSize(Threshold) {}
118
119 // Analyze loop. Check its size, calculate is it possible to unswitch
120 // it. Returns true if we can unswitch this loop.
121 bool countLoop(const Loop *L, const TargetTransformInfo &TTI,
122 AssumptionCache *AC);
123
124 // Clean all data related to given loop.
125 void forgetLoop(const Loop *L);
126
127 // Mark case value as unswitched.
128 // Since SI instruction can be partly unswitched, in order to avoid
129 // extra unswitching in cloned loops keep track all unswitched values.
130 void setUnswitched(const SwitchInst *SI, const Value *V);
131
132 // Check was this case value unswitched before or not.
133 bool isUnswitched(const SwitchInst *SI, const Value *V);
134
135 // Returns true if another unswitching could be done within the cost
136 // threshold.
137 bool CostAllowsUnswitching();
138
139 // Clone all loop-unswitch related loop properties.
140 // Redistribute unswitching quotas.
141 // Note, that new loop data is stored inside the VMap.
142 void cloneData(const Loop *NewLoop, const Loop *OldLoop,
143 const ValueToValueMapTy &VMap);
144 };
145
146 class LoopUnswitch : public LoopPass {
147 LoopInfo *LI; // Loop information
148 LPPassManager *LPM;
149 AssumptionCache *AC;
150
151 // Used to check if second loop needs processing after
152 // RewriteLoopBodyWithConditionConstant rewrites first loop.
153 std::vector<Loop*> LoopProcessWorklist;
154
155 LUAnalysisCache BranchesInfo;
156
157 bool OptimizeForSize;
158 bool redoLoop;
159
160 Loop *currentLoop;
161 DominatorTree *DT;
162 BasicBlock *loopHeader;
163 BasicBlock *loopPreheader;
164
165 // LoopBlocks contains all of the basic blocks of the loop, including the
166 // preheader of the loop, the body of the loop, and the exit blocks of the
167 // loop, in that order.
168 std::vector<BasicBlock*> LoopBlocks;
169 // NewBlocks contained cloned copy of basic blocks from LoopBlocks.
170 std::vector<BasicBlock*> NewBlocks;
171
172 public:
173 static char ID; // Pass ID, replacement for typeid
174 explicit LoopUnswitch(bool Os = false) :
175 LoopPass(ID), OptimizeForSize(Os), redoLoop(false),
176 currentLoop(nullptr), DT(nullptr), loopHeader(nullptr),
177 loopPreheader(nullptr) {
178 initializeLoopUnswitchPass(*PassRegistry::getPassRegistry());
179 }
180
181 bool runOnLoop(Loop *L, LPPassManager &LPM) override;
182 bool processCurrentLoop();
183
184 /// This transformation requires natural loop information & requires that
185 /// loop preheaders be inserted into the CFG.
186 ///
187 void getAnalysisUsage(AnalysisUsage &AU) const override {
188 AU.addRequired<AssumptionCacheTracker>();
189 AU.addRequiredID(LoopSimplifyID);
190 AU.addPreservedID(LoopSimplifyID);
191 AU.addRequired<LoopInfoWrapperPass>();
192 AU.addPreserved<LoopInfoWrapperPass>();
193 AU.addRequiredID(LCSSAID);
194 AU.addPreservedID(LCSSAID);
195 AU.addPreserved<DominatorTreeWrapperPass>();
196 AU.addPreserved<ScalarEvolutionWrapperPass>();
197 AU.addRequired<TargetTransformInfoWrapperPass>();
198 }
199
200 private:
201
202 void releaseMemory() override {
203 BranchesInfo.forgetLoop(currentLoop);
204 }
205
206 void initLoopData() {
207 loopHeader = currentLoop->getHeader();
208 loopPreheader = currentLoop->getLoopPreheader();
209 }
210
211 /// Split all of the edges from inside the loop to their exit blocks.
212 /// Update the appropriate Phi nodes as we do so.
213 void SplitExitEdges(Loop *L,
214 const SmallVectorImpl<BasicBlock *> &ExitBlocks);
215
216 bool TryTrivialLoopUnswitch(bool &Changed);
217
218 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,
219 TerminatorInst *TI = nullptr);
220 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
221 BasicBlock *ExitBlock, TerminatorInst *TI);
222 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L,
223 TerminatorInst *TI);
224
225 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
226 Constant *Val, bool isEqual);
227
228 void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
229 BasicBlock *TrueDest,
230 BasicBlock *FalseDest,
231 Instruction *InsertPt,
232 TerminatorInst *TI);
233
234 void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L);
235 };
236}
237
238// Analyze loop. Check its size, calculate is it possible to unswitch
239// it. Returns true if we can unswitch this loop.
240bool LUAnalysisCache::countLoop(const Loop *L, const TargetTransformInfo &TTI,
241 AssumptionCache *AC) {
242
243 LoopPropsMapIt PropsIt;
244 bool Inserted;
245 std::tie(PropsIt, Inserted) =
246 LoopsProperties.insert(std::make_pair(L, LoopProperties()));
247
248 LoopProperties &Props = PropsIt->second;
249
250 if (Inserted) {
251 // New loop.
252
253 // Limit the number of instructions to avoid causing significant code
254 // expansion, and the number of basic blocks, to avoid loops with
255 // large numbers of branches which cause loop unswitching to go crazy.
256 // This is a very ad-hoc heuristic.
257
258 SmallPtrSet<const Value *, 32> EphValues;
259 CodeMetrics::collectEphemeralValues(L, AC, EphValues);
260
261 // FIXME: This is overly conservative because it does not take into
262 // consideration code simplification opportunities and code that can
263 // be shared by the resultant unswitched loops.
264 CodeMetrics Metrics;
265 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); I != E;
266 ++I)
267 Metrics.analyzeBasicBlock(*I, TTI, EphValues);
268
269 Props.SizeEstimation = Metrics.NumInsts;
270 Props.CanBeUnswitchedCount = MaxSize / (Props.SizeEstimation);
271 Props.WasUnswitchedCount = 0;
272 MaxSize -= Props.SizeEstimation * Props.CanBeUnswitchedCount;
273
274 if (Metrics.notDuplicatable) {
275 DEBUG(dbgs() << "NOT unswitching loop %"do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "NOT unswitching loop %"
<< L->getHeader()->getName() << ", contents cannot be "
<< "duplicated!\n"; } } while (0)
276 << L->getHeader()->getName() << ", contents cannot be "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "NOT unswitching loop %"
<< L->getHeader()->getName() << ", contents cannot be "
<< "duplicated!\n"; } } while (0)
277 << "duplicated!\n")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "NOT unswitching loop %"
<< L->getHeader()->getName() << ", contents cannot be "
<< "duplicated!\n"; } } while (0);
278 return false;
279 }
280 }
281
282 // Be careful. This links are good only before new loop addition.
283 CurrentLoopProperties = &Props;
284 CurLoopInstructions = &Props.UnswitchedVals;
285
286 return true;
287}
288
289// Clean all data related to given loop.
290void LUAnalysisCache::forgetLoop(const Loop *L) {
291
292 LoopPropsMapIt LIt = LoopsProperties.find(L);
293
294 if (LIt != LoopsProperties.end()) {
295 LoopProperties &Props = LIt->second;
296 MaxSize += (Props.CanBeUnswitchedCount + Props.WasUnswitchedCount) *
297 Props.SizeEstimation;
298 LoopsProperties.erase(LIt);
299 }
300
301 CurrentLoopProperties = nullptr;
302 CurLoopInstructions = nullptr;
303}
304
305// Mark case value as unswitched.
306// Since SI instruction can be partly unswitched, in order to avoid
307// extra unswitching in cloned loops keep track all unswitched values.
308void LUAnalysisCache::setUnswitched(const SwitchInst *SI, const Value *V) {
309 (*CurLoopInstructions)[SI].insert(V);
310}
311
312// Check was this case value unswitched before or not.
313bool LUAnalysisCache::isUnswitched(const SwitchInst *SI, const Value *V) {
314 return (*CurLoopInstructions)[SI].count(V);
315}
316
317bool LUAnalysisCache::CostAllowsUnswitching() {
318 return CurrentLoopProperties->CanBeUnswitchedCount > 0;
319}
320
321// Clone all loop-unswitch related loop properties.
322// Redistribute unswitching quotas.
323// Note, that new loop data is stored inside the VMap.
324void LUAnalysisCache::cloneData(const Loop *NewLoop, const Loop *OldLoop,
325 const ValueToValueMapTy &VMap) {
326
327 LoopProperties &NewLoopProps = LoopsProperties[NewLoop];
328 LoopProperties &OldLoopProps = *CurrentLoopProperties;
329 UnswitchedValsMap &Insts = OldLoopProps.UnswitchedVals;
330
331 // Reallocate "can-be-unswitched quota"
332
333 --OldLoopProps.CanBeUnswitchedCount;
334 ++OldLoopProps.WasUnswitchedCount;
335 NewLoopProps.WasUnswitchedCount = 0;
336 unsigned Quota = OldLoopProps.CanBeUnswitchedCount;
337 NewLoopProps.CanBeUnswitchedCount = Quota / 2;
338 OldLoopProps.CanBeUnswitchedCount = Quota - Quota / 2;
339
340 NewLoopProps.SizeEstimation = OldLoopProps.SizeEstimation;
341
342 // Clone unswitched values info:
343 // for new loop switches we clone info about values that was
344 // already unswitched and has redundant successors.
345 for (UnswitchedValsIt I = Insts.begin(); I != Insts.end(); ++I) {
346 const SwitchInst *OldInst = I->first;
347 Value *NewI = VMap.lookup(OldInst);
348 const SwitchInst *NewInst = cast_or_null<SwitchInst>(NewI);
349 assert(NewInst && "All instructions that are in SrcBB must be in VMap.")((NewInst && "All instructions that are in SrcBB must be in VMap."
) ? static_cast<void> (0) : __assert_fail ("NewInst && \"All instructions that are in SrcBB must be in VMap.\""
, "/tmp/buildd/llvm-toolchain-snapshot-3.8~svn246424/lib/Transforms/Scalar/LoopUnswitch.cpp"
, 349, __PRETTY_FUNCTION__));
350
351 NewLoopProps.UnswitchedVals[NewInst] = OldLoopProps.UnswitchedVals[OldInst];
352 }
353}
354
355char LoopUnswitch::ID = 0;
356INITIALIZE_PASS_BEGIN(LoopUnswitch, "loop-unswitch", "Unswitch loops",static void* initializeLoopUnswitchPassOnce(PassRegistry &
Registry) {
357 false, false)static void* initializeLoopUnswitchPassOnce(PassRegistry &
Registry) {
358INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)initializeTargetTransformInfoWrapperPassPass(Registry);
359INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)initializeAssumptionCacheTrackerPass(Registry);
360INITIALIZE_PASS_DEPENDENCY(LoopSimplify)initializeLoopSimplifyPass(Registry);
361INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)initializeLoopInfoWrapperPassPass(Registry);
362INITIALIZE_PASS_DEPENDENCY(LCSSA)initializeLCSSAPass(Registry);
363INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops",PassInfo *PI = new PassInfo("Unswitch loops", "loop-unswitch"
, & LoopUnswitch ::ID, PassInfo::NormalCtor_t(callDefaultCtor
< LoopUnswitch >), false, false); Registry.registerPass
(*PI, true); return PI; } void llvm::initializeLoopUnswitchPass
(PassRegistry &Registry) { static volatile sys::cas_flag initialized
= 0; sys::cas_flag old_val = sys::CompareAndSwap(&initialized
, 1, 0); if (old_val == 0) { initializeLoopUnswitchPassOnce(Registry
); sys::MemoryFence(); ; ; initialized = 2; ; } else { sys::cas_flag
tmp = initialized; sys::MemoryFence(); while (tmp != 2) { tmp
= initialized; sys::MemoryFence(); } } ; }
364 false, false)PassInfo *PI = new PassInfo("Unswitch loops", "loop-unswitch"
, & LoopUnswitch ::ID, PassInfo::NormalCtor_t(callDefaultCtor
< LoopUnswitch >), false, false); Registry.registerPass
(*PI, true); return PI; } void llvm::initializeLoopUnswitchPass
(PassRegistry &Registry) { static volatile sys::cas_flag initialized
= 0; sys::cas_flag old_val = sys::CompareAndSwap(&initialized
, 1, 0); if (old_val == 0) { initializeLoopUnswitchPassOnce(Registry
); sys::MemoryFence(); ; ; initialized = 2; ; } else { sys::cas_flag
tmp = initialized; sys::MemoryFence(); while (tmp != 2) { tmp
= initialized; sys::MemoryFence(); } } ; }
365
366Pass *llvm::createLoopUnswitchPass(bool Os) {
367 return new LoopUnswitch(Os);
368}
369
370/// Cond is a condition that occurs in L. If it is invariant in the loop, or has
371/// an invariant piece, return the invariant. Otherwise, return null.
372static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
373
374 // We started analyze new instruction, increment scanned instructions counter.
375 ++TotalInsts;
376
377 // We can never unswitch on vector conditions.
378 if (Cond->getType()->isVectorTy())
379 return nullptr;
380
381 // Constants should be folded, not unswitched on!
382 if (isa<Constant>(Cond)) return nullptr;
383
384 // TODO: Handle: br (VARIANT|INVARIANT).
385
386 // Hoist simple values out.
387 if (L->makeLoopInvariant(Cond, Changed))
388 return Cond;
389
390 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
391 if (BO->getOpcode() == Instruction::And ||
392 BO->getOpcode() == Instruction::Or) {
393 // If either the left or right side is invariant, we can unswitch on this,
394 // which will cause the branch to go away in one loop and the condition to
395 // simplify in the other one.
396 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
397 return LHS;
398 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
399 return RHS;
400 }
401
402 return nullptr;
403}
404
405bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
406 if (skipOptnoneFunction(L))
1
Taking false branch
→
407 return false;
408
409 AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
410 *L->getHeader()->getParent());
411 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
412 LPM = &LPM_Ref;
413 DominatorTreeWrapperPass *DTWP =
414 getAnalysisIfAvailable<DominatorTreeWrapperPass>();
415 DT = DTWP ? &DTWP->getDomTree() : nullptr;
2
←
Assuming 'DTWP' is null
→
3
←
'?' condition is false
→
4
←
Null pointer value stored to field 'DT'
→
416 currentLoop = L;
417 Function *F = currentLoop->getHeader()->getParent();
418 bool Changed = false;
419 do {
420 assert(currentLoop->isLCSSAForm(*DT))((currentLoop->isLCSSAForm(*DT)) ? static_cast<void>
(0) : __assert_fail ("currentLoop->isLCSSAForm(*DT)", "/tmp/buildd/llvm-toolchain-snapshot-3.8~svn246424/lib/Transforms/Scalar/LoopUnswitch.cpp"
, 420, __PRETTY_FUNCTION__));
5
←
Within the expansion of the macro 'assert':
a
Forming reference to null pointer
421 redoLoop = false;
422 Changed |= processCurrentLoop();
423 } while(redoLoop);
424
425 if (Changed) {
426 // FIXME: Reconstruct dom info, because it is not preserved properly.
427 if (DT)
428 DT->recalculate(*F);
429 }
430 return Changed;
431}
432
433/// Do actual work and unswitch loop if possible and profitable.
434bool LoopUnswitch::processCurrentLoop() {
435 bool Changed = false;
436
437 initLoopData();
438
439 // If LoopSimplify was unable to form a preheader, don't do any unswitching.
440 if (!loopPreheader)
441 return false;
442
443 // Loops with indirectbr cannot be cloned.
444 if (!currentLoop->isSafeToClone())
445 return false;
446
447 // Without dedicated exits, splitting the exit edge may fail.
448 if (!currentLoop->hasDedicatedExits())
449 return false;
450
451 LLVMContext &Context = loopHeader->getContext();
452
453 // Probably we reach the quota of branches for this loop. If so
454 // stop unswitching.
455 if (!BranchesInfo.countLoop(
456 currentLoop, getAnalysis<TargetTransformInfoWrapperPass>().getTTI(
457 *currentLoop->getHeader()->getParent()),
458 AC))
459 return false;
460
461 // Try trivial unswitch first before loop over other basic blocks in the loop.
462 if (TryTrivialLoopUnswitch(Changed)) {
463 return true;
464 }
465
466 // Do not do non-trivial unswitch while optimizing for size.
467 // FIXME: Use Function::optForSize().
468 if (OptimizeForSize ||
469 loopHeader->getParent()->hasFnAttribute(Attribute::OptimizeForSize))
470 return false;
471
472 // Loop over all of the basic blocks in the loop. If we find an interior
473 // block that is branching on a loop-invariant condition, we can unswitch this
474 // loop.
475 for (Loop::block_iterator I = currentLoop->block_begin(),
476 E = currentLoop->block_end(); I != E; ++I) {
477 TerminatorInst *TI = (*I)->getTerminator();
478 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
479 // If this isn't branching on an invariant condition, we can't unswitch
480 // it.
481 if (BI->isConditional()) {
482 // See if this, or some part of it, is loop invariant. If so, we can
483 // unswitch on it if we desire.
484 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
485 currentLoop, Changed);
486 if (LoopCond &&
487 UnswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context), TI)) {
488 ++NumBranches;
489 return true;
490 }
491 }
492 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
493 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
494 currentLoop, Changed);
495 unsigned NumCases = SI->getNumCases();
496 if (LoopCond && NumCases) {
497 // Find a value to unswitch on:
498 // FIXME: this should chose the most expensive case!
499 // FIXME: scan for a case with a non-critical edge?
500 Constant *UnswitchVal = nullptr;
501
502 // Do not process same value again and again.
503 // At this point we have some cases already unswitched and
504 // some not yet unswitched. Let's find the first not yet unswitched one.
505 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
506 i != e; ++i) {
507 Constant *UnswitchValCandidate = i.getCaseValue();
508 if (!BranchesInfo.isUnswitched(SI, UnswitchValCandidate)) {
509 UnswitchVal = UnswitchValCandidate;
510 break;
511 }
512 }
513
514 if (!UnswitchVal)
515 continue;
516
517 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) {
518 ++NumSwitches;
519 return true;
520 }
521 }
522 }
523
524 // Scan the instructions to check for unswitchable values.
525 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
526 BBI != E; ++BBI)
527 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
528 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
529 currentLoop, Changed);
530 if (LoopCond && UnswitchIfProfitable(LoopCond,
531 ConstantInt::getTrue(Context))) {
532 ++NumSelects;
533 return true;
534 }
535 }
536 }
537 return Changed;
538}
539
540/// Check to see if all paths from BB exit the loop with no side effects
541/// (including infinite loops).
542///
543/// If true, we return true and set ExitBB to the block we
544/// exit through.
545///
546static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
547 BasicBlock *&ExitBB,
548 std::set<BasicBlock*> &Visited) {
549 if (!Visited.insert(BB).second) {
550 // Already visited. Without more analysis, this could indicate an infinite
551 // loop.
552 return false;
553 }
554 if (!L->contains(BB)) {
555 // Otherwise, this is a loop exit, this is fine so long as this is the
556 // first exit.
557 if (ExitBB) return false;
558 ExitBB = BB;
559 return true;
560 }
561
562 // Otherwise, this is an unvisited intra-loop node. Check all successors.
563 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
564 // Check to see if the successor is a trivial loop exit.
565 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
566 return false;
567 }
568
569 // Okay, everything after this looks good, check to make sure that this block
570 // doesn't include any side effects.
571 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
572 if (I->mayHaveSideEffects())
573 return false;
574
575 return true;
576}
577
578/// Return true if the specified block unconditionally leads to an exit from
579/// the specified loop, and has no side-effects in the process. If so, return
580/// the block that is exited to, otherwise return null.
581static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
582 std::set<BasicBlock*> Visited;
583 Visited.insert(L->getHeader()); // Branches to header make infinite loops.
584 BasicBlock *ExitBB = nullptr;
585 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
586 return ExitBB;
587 return nullptr;
588}
589
590/// We have found that we can unswitch currentLoop when LoopCond == Val to
591/// simplify the loop. If we decide that this is profitable,
592/// unswitch the loop, reprocess the pieces, then return true.
593bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,
594 TerminatorInst *TI) {
595 // Check to see if it would be profitable to unswitch current loop.
596 if (!BranchesInfo.CostAllowsUnswitching()) {
597 DEBUG(dbgs() << "NOT unswitching loop %"do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "NOT unswitching loop %"
<< currentLoop->getHeader()->getName() << " at non-trivial condition '"
<< *Val << "' == " << *LoopCond << "\n"
<< ". Cost too high.\n"; } } while (0)
598 << currentLoop->getHeader()->getName()do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "NOT unswitching loop %"
<< currentLoop->getHeader()->getName() << " at non-trivial condition '"
<< *Val << "' == " << *LoopCond << "\n"
<< ". Cost too high.\n"; } } while (0)
599 << " at non-trivial condition '" << *Valdo { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "NOT unswitching loop %"
<< currentLoop->getHeader()->getName() << " at non-trivial condition '"
<< *Val << "' == " << *LoopCond << "\n"
<< ". Cost too high.\n"; } } while (0)
600 << "' == " << *LoopCond << "\n"do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "NOT unswitching loop %"
<< currentLoop->getHeader()->getName() << " at non-trivial condition '"
<< *Val << "' == " << *LoopCond << "\n"
<< ". Cost too high.\n"; } } while (0)
601 << ". Cost too high.\n")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "NOT unswitching loop %"
<< currentLoop->getHeader()->getName() << " at non-trivial condition '"
<< *Val << "' == " << *LoopCond << "\n"
<< ". Cost too high.\n"; } } while (0);
602 return false;
603 }
604
605 UnswitchNontrivialCondition(LoopCond, Val, currentLoop, TI);
606 return true;
607}
608
609/// Recursively clone the specified loop and all of its children,
610/// mapping the blocks with the specified map.
611static Loop *CloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM,
612 LoopInfo *LI, LPPassManager *LPM) {
613 Loop *New = new Loop();
614 LPM->insertLoop(New, PL);
615
616 // Add all of the blocks in L to the new loop.
617 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
618 I != E; ++I)
619 if (LI->getLoopFor(*I) == L)
620 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
621
622 // Add all of the subloops to the new loop.
623 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
624 CloneLoop(*I, New, VM, LI, LPM);
625
626 return New;
627}
628
629static void copyMetadata(Instruction *DstInst, const Instruction *SrcInst,
630 bool Swapped) {
631 if (!SrcInst || !SrcInst->hasMetadata())
632 return;
633
634 SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
635 SrcInst->getAllMetadata(MDs);
636 for (auto &MD : MDs) {
637 switch (MD.first) {
638 default:
639 break;
640 case LLVMContext::MD_prof:
641 if (Swapped && MD.second->getNumOperands() == 3 &&
642 isa<MDString>(MD.second->getOperand(0))) {
643 MDString *MDName = cast<MDString>(MD.second->getOperand(0));
644 if (MDName->getString() == "branch_weights") {
645 auto *ValT = cast_or_null<ConstantAsMetadata>(
646 MD.second->getOperand(1))->getValue();
647 auto *ValF = cast_or_null<ConstantAsMetadata>(
648 MD.second->getOperand(2))->getValue();
649 assert(ValT && ValF && "Invalid Operands of branch_weights")((ValT && ValF && "Invalid Operands of branch_weights"
) ? static_cast<void> (0) : __assert_fail ("ValT && ValF && \"Invalid Operands of branch_weights\""
, "/tmp/buildd/llvm-toolchain-snapshot-3.8~svn246424/lib/Transforms/Scalar/LoopUnswitch.cpp"
, 649, __PRETTY_FUNCTION__));
650 auto NewMD =
651 MDBuilder(DstInst->getParent()->getContext())
652 .createBranchWeights(cast<ConstantInt>(ValF)->getZExtValue(),
653 cast<ConstantInt>(ValT)->getZExtValue());
654 MD.second = NewMD;
655 }
656 }
657 // fallthrough.
658 case LLVMContext::MD_make_implicit:
659 case LLVMContext::MD_dbg:
660 DstInst->setMetadata(MD.first, MD.second);
661 }
662 }
663}
664
665/// Emit a conditional branch on two values if LIC == Val, branch to TrueDst,
666/// otherwise branch to FalseDest. Insert the code immediately before InsertPt.
667void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
668 BasicBlock *TrueDest,
669 BasicBlock *FalseDest,
670 Instruction *InsertPt,
671 TerminatorInst *TI) {
672 // Insert a conditional branch on LIC to the two preheaders. The original
673 // code is the true version and the new code is the false version.
674 Value *BranchVal = LIC;
675 bool Swapped = false;
676 if (!isa<ConstantInt>(Val) ||
677 Val->getType() != Type::getInt1Ty(LIC->getContext()))
678 BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val);
679 else if (Val != ConstantInt::getTrue(Val->getContext())) {
680 // We want to enter the new loop when the condition is true.
681 std::swap(TrueDest, FalseDest);
682 Swapped = true;
683 }
684
685 // Insert the new branch.
686 BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt);
687 copyMetadata(BI, TI, Swapped);
688
689 // If either edge is critical, split it. This helps preserve LoopSimplify
690 // form for enclosing loops.
691 auto Options = CriticalEdgeSplittingOptions(DT, LI).setPreserveLCSSA();
692 SplitCriticalEdge(BI, 0, Options);
693 SplitCriticalEdge(BI, 1, Options);
694}
695
696/// Given a loop that has a trivial unswitchable condition in it (a cond branch
697/// from its header block to its latch block, where the path through the loop
698/// that doesn't execute its body has no side-effects), unswitch it. This
699/// doesn't involve any code duplication, just moving the conditional branch
700/// outside of the loop and updating loop info.
701void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
702 BasicBlock *ExitBlock,
703 TerminatorInst *TI) {
704 DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %"do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
<< loopHeader->getName() << " [" << L->
getBlocks().size() << " blocks] in Function " << L
->getHeader()->getParent()->getName() << " on cond: "
<< *Val << " == " << *Cond << "\n"; }
} while (0)
705 << loopHeader->getName() << " [" << L->getBlocks().size()do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
<< loopHeader->getName() << " [" << L->
getBlocks().size() << " blocks] in Function " << L
->getHeader()->getParent()->getName() << " on cond: "
<< *Val << " == " << *Cond << "\n"; }
} while (0)
706 << " blocks] in Function "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
<< loopHeader->getName() << " [" << L->
getBlocks().size() << " blocks] in Function " << L
->getHeader()->getParent()->getName() << " on cond: "
<< *Val << " == " << *Cond << "\n"; }
} while (0)
707 << L->getHeader()->getParent()->getName() << " on cond: " << *Valdo { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
<< loopHeader->getName() << " [" << L->
getBlocks().size() << " blocks] in Function " << L
->getHeader()->getParent()->getName() << " on cond: "
<< *Val << " == " << *Cond << "\n"; }
} while (0)
708 << " == " << *Cond << "\n")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
<< loopHeader->getName() << " [" << L->
getBlocks().size() << " blocks] in Function " << L
->getHeader()->getParent()->getName() << " on cond: "
<< *Val << " == " << *Cond << "\n"; }
} while (0);
709
710 // First step, split the preheader, so that we know that there is a safe place
711 // to insert the conditional branch. We will change loopPreheader to have a
712 // conditional branch on Cond.
713 BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, DT, LI);
714
715 // Now that we have a place to insert the conditional branch, create a place
716 // to branch to: this is the exit block out of the loop that we should
717 // short-circuit to.
718
719 // Split this block now, so that the loop maintains its exit block, and so
720 // that the jump from the preheader can execute the contents of the exit block
721 // without actually branching to it (the exit block should be dominated by the
722 // loop header, not the preheader).
723 assert(!L->contains(ExitBlock) && "Exit block is in the loop?")((!L->contains(ExitBlock) && "Exit block is in the loop?"
) ? static_cast<void> (0) : __assert_fail ("!L->contains(ExitBlock) && \"Exit block is in the loop?\""
, "/tmp/buildd/llvm-toolchain-snapshot-3.8~svn246424/lib/Transforms/Scalar/LoopUnswitch.cpp"
, 723, __PRETTY_FUNCTION__));
724 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), DT, LI);
725
726 // Okay, now we have a position to branch from and a position to branch to,
727 // insert the new conditional branch.
728 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
729 loopPreheader->getTerminator(), TI);
730 LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L);
731 loopPreheader->getTerminator()->eraseFromParent();
732
733 // We need to reprocess this loop, it could be unswitched again.
734 redoLoop = true;
735
736 // Now that we know that the loop is never entered when this condition is a
737 // particular value, rewrite the loop with this info. We know that this will
738 // at least eliminate the old branch.
739 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
740 ++NumTrivial;
741}
742
743/// Check if the first non-constant condition starting from the loop header is
744/// a trivial unswitch condition: that is, a condition controls whether or not
745/// the loop does anything at all. If it is a trivial condition, unswitching
746/// produces no code duplications (equivalently, it produces a simpler loop and
747/// a new empty loop, which gets deleted). Therefore always unswitch trivial
748/// condition.
749bool LoopUnswitch::TryTrivialLoopUnswitch(bool &Changed) {
750 BasicBlock *CurrentBB = currentLoop->getHeader();
751 TerminatorInst *CurrentTerm = CurrentBB->getTerminator();
752 LLVMContext &Context = CurrentBB->getContext();
753
754 // If loop header has only one reachable successor (currently via an
755 // unconditional branch or constant foldable conditional branch, but
756 // should also consider adding constant foldable switch instruction in
757 // future), we should keep looking for trivial condition candidates in
758 // the successor as well. An alternative is to constant fold conditions
759 // and merge successors into loop header (then we only need to check header's
760 // terminator). The reason for not doing this in LoopUnswitch pass is that
761 // it could potentially break LoopPassManager's invariants. Folding dead
762 // branches could either eliminate the current loop or make other loops
763 // unreachable. LCSSA form might also not be preserved after deleting
764 // branches. The following code keeps traversing loop header's successors
765 // until it finds the trivial condition candidate (condition that is not a
766 // constant). Since unswitching generates branches with constant conditions,
767 // this scenario could be very common in practice.
768 SmallSet<BasicBlock*, 8> Visited;
769
770 while (true) {
771 // If we exit loop or reach a previous visited block, then
772 // we can not reach any trivial condition candidates (unfoldable
773 // branch instructions or switch instructions) and no unswitch
774 // can happen. Exit and return false.
775 if (!currentLoop->contains(CurrentBB) || !Visited.insert(CurrentBB).second)
776 return false;
777
778 // Check if this loop will execute any side-effecting instructions (e.g.
779 // stores, calls, volatile loads) in the part of the loop that the code
780 // *would* execute. Check the header first.
781 for (BasicBlock::iterator I : *CurrentBB)
782 if (I->mayHaveSideEffects())
783 return false;
784
785 // FIXME: add check for constant foldable switch instructions.
786 if (BranchInst *BI = dyn_cast<BranchInst>(CurrentTerm)) {
787 if (BI->isUnconditional()) {
788 CurrentBB = BI->getSuccessor(0);
789 } else if (BI->getCondition() == ConstantInt::getTrue(Context)) {
790 CurrentBB = BI->getSuccessor(0);
791 } else if (BI->getCondition() == ConstantInt::getFalse(Context)) {
792 CurrentBB = BI->getSuccessor(1);
793 } else {
794 // Found a trivial condition candidate: non-foldable conditional branch.
795 break;
796 }
797 } else {
798 break;
799 }
800
801 CurrentTerm = CurrentBB->getTerminator();
802 }
803
804 // CondVal is the condition that controls the trivial condition.
805 // LoopExitBB is the BasicBlock that loop exits when meets trivial condition.
806 Constant *CondVal = nullptr;
807 BasicBlock *LoopExitBB = nullptr;
808
809 if (BranchInst *BI = dyn_cast<BranchInst>(CurrentTerm)) {
810 // If this isn't branching on an invariant condition, we can't unswitch it.
811 if (!BI->isConditional())
812 return false;
813
814 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
815 currentLoop, Changed);
816
817 // Unswitch only if the trivial condition itself is an LIV (not
818 // partial LIV which could occur in and/or)
819 if (!LoopCond || LoopCond != BI->getCondition())
820 return false;
821
822 // Check to see if a successor of the branch is guaranteed to
823 // exit through a unique exit block without having any
824 // side-effects. If so, determine the value of Cond that causes
825 // it to do this.
826 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
827 BI->getSuccessor(0)))) {
828 CondVal = ConstantInt::getTrue(Context);
829 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
830 BI->getSuccessor(1)))) {
831 CondVal = ConstantInt::getFalse(Context);
832 }
833
834 // If we didn't find a single unique LoopExit block, or if the loop exit
835 // block contains phi nodes, this isn't trivial.
836 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
837 return false; // Can't handle this.
838
839 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, LoopExitBB,
840 CurrentTerm);
841 ++NumBranches;
842 return true;
843 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(CurrentTerm)) {
844 // If this isn't switching on an invariant condition, we can't unswitch it.
845 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
846 currentLoop, Changed);
847
848 // Unswitch only if the trivial condition itself is an LIV (not
849 // partial LIV which could occur in and/or)
850 if (!LoopCond || LoopCond != SI->getCondition())
851 return false;
852
853 // Check to see if a successor of the switch is guaranteed to go to the
854 // latch block or exit through a one exit block without having any
855 // side-effects. If so, determine the value of Cond that causes it to do
856 // this.
857 // Note that we can't trivially unswitch on the default case or
858 // on already unswitched cases.
859 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
860 i != e; ++i) {
861 BasicBlock *LoopExitCandidate;
862 if ((LoopExitCandidate = isTrivialLoopExitBlock(currentLoop,
863 i.getCaseSuccessor()))) {
864 // Okay, we found a trivial case, remember the value that is trivial.
865 ConstantInt *CaseVal = i.getCaseValue();
866
867 // Check that it was not unswitched before, since already unswitched
868 // trivial vals are looks trivial too.
869 if (BranchesInfo.isUnswitched(SI, CaseVal))
870 continue;
871 LoopExitBB = LoopExitCandidate;
872 CondVal = CaseVal;
873 break;
874 }
875 }
876
877 // If we didn't find a single unique LoopExit block, or if the loop exit
878 // block contains phi nodes, this isn't trivial.
879 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
880 return false; // Can't handle this.
881
882 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, LoopExitBB,
883 nullptr);
884 ++NumSwitches;
885 return true;
886 }
887 return false;
888}
889
890/// Split all of the edges from inside the loop to their exit blocks.
891/// Update the appropriate Phi nodes as we do so.
892void LoopUnswitch::SplitExitEdges(Loop *L,
893 const SmallVectorImpl<BasicBlock *> &ExitBlocks){
894
895 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
896 BasicBlock *ExitBlock = ExitBlocks[i];
897 SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
898 pred_end(ExitBlock));
899
900 // Although SplitBlockPredecessors doesn't preserve loop-simplify in
901 // general, if we call it on all predecessors of all exits then it does.
902 SplitBlockPredecessors(ExitBlock, Preds, ".us-lcssa", DT, LI,
903 /*PreserveLCSSA*/ true);
904 }
905}
906
907/// We determined that the loop is profitable to unswitch when LIC equal Val.
908/// Split it into loop versions and test the condition outside of either loop.
909/// Return the loops created as Out1/Out2.
910void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
911 Loop *L, TerminatorInst *TI) {
912 Function *F = loopHeader->getParent();
913 DEBUG(dbgs() << "loop-unswitch: Unswitching loop %"do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "loop-unswitch: Unswitching loop %"
<< loopHeader->getName() << " [" << L->
getBlocks().size() << " blocks] in Function " << F
->getName() << " when '" << *Val << "' == "
<< *LIC << "\n"; } } while (0)
914 << loopHeader->getName() << " [" << L->getBlocks().size()do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "loop-unswitch: Unswitching loop %"
<< loopHeader->getName() << " [" << L->
getBlocks().size() << " blocks] in Function " << F
->getName() << " when '" << *Val << "' == "
<< *LIC << "\n"; } } while (0)
915 << " blocks] in Function " << F->getName()do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "loop-unswitch: Unswitching loop %"
<< loopHeader->getName() << " [" << L->
getBlocks().size() << " blocks] in Function " << F
->getName() << " when '" << *Val << "' == "
<< *LIC << "\n"; } } while (0)
916 << " when '" << *Val << "' == " << *LIC << "\n")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "loop-unswitch: Unswitching loop %"
<< loopHeader->getName() << " [" << L->
getBlocks().size() << " blocks] in Function " << F
->getName() << " when '" << *Val << "' == "
<< *LIC << "\n"; } } while (0);
917
918 if (auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>())
919 SEWP->getSE().forgetLoop(L);
920
921 LoopBlocks.clear();
922 NewBlocks.clear();
923
924 // First step, split the preheader and exit blocks, and add these blocks to
925 // the LoopBlocks list.
926 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, DT, LI);
927 LoopBlocks.push_back(NewPreheader);
928
929 // We want the loop to come after the preheader, but before the exit blocks.
930 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
931
932 SmallVector<BasicBlock*, 8> ExitBlocks;
933 L->getUniqueExitBlocks(ExitBlocks);
934
935 // Split all of the edges from inside the loop to their exit blocks. Update
936 // the appropriate Phi nodes as we do so.
937 SplitExitEdges(L, ExitBlocks);
938
939 // The exit blocks may have been changed due to edge splitting, recompute.
940 ExitBlocks.clear();
941 L->getUniqueExitBlocks(ExitBlocks);
942
943 // Add exit blocks to the loop blocks.
944 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
945
946 // Next step, clone all of the basic blocks that make up the loop (including
947 // the loop preheader and exit blocks), keeping track of the mapping between
948 // the instructions and blocks.
949 NewBlocks.reserve(LoopBlocks.size());
950 ValueToValueMapTy VMap;
951 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
952 BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[i], VMap, ".us", F);
953
954 NewBlocks.push_back(NewBB);
955 VMap[LoopBlocks[i]] = NewBB; // Keep the BB mapping.
956 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L);
957 }
958
959 // Splice the newly inserted blocks into the function right before the
960 // original preheader.
961 F->getBasicBlockList().splice(NewPreheader, F->getBasicBlockList(),
962 NewBlocks[0], F->end());
963
964 // FIXME: We could register any cloned assumptions instead of clearing the
965 // whole function's cache.
966 AC->clear();
967
968 // Now we create the new Loop object for the versioned loop.
969 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), VMap, LI, LPM);
970
971 // Recalculate unswitching quota, inherit simplified switches info for NewBB,
972 // Probably clone more loop-unswitch related loop properties.
973 BranchesInfo.cloneData(NewLoop, L, VMap);
974
975 Loop *ParentLoop = L->getParentLoop();
976 if (ParentLoop) {
977 // Make sure to add the cloned preheader and exit blocks to the parent loop
978 // as well.
979 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI);
980 }
981
982 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
983 BasicBlock *NewExit = cast<BasicBlock>(VMap[ExitBlocks[i]]);
984 // The new exit block should be in the same loop as the old one.
985 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
986 ExitBBLoop->addBasicBlockToLoop(NewExit, *LI);
987
988 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&((NewExit->getTerminator()->getNumSuccessors() == 1 &&
"Exit block should have been split to have one successor!") ?
static_cast<void> (0) : __assert_fail ("NewExit->getTerminator()->getNumSuccessors() == 1 && \"Exit block should have been split to have one successor!\""
, "/tmp/buildd/llvm-toolchain-snapshot-3.8~svn246424/lib/Transforms/Scalar/LoopUnswitch.cpp"
, 989, __PRETTY_FUNCTION__))
989 "Exit block should have been split to have one successor!")((NewExit->getTerminator()->getNumSuccessors() == 1 &&
"Exit block should have been split to have one successor!") ?
static_cast<void> (0) : __assert_fail ("NewExit->getTerminator()->getNumSuccessors() == 1 && \"Exit block should have been split to have one successor!\""
, "/tmp/buildd/llvm-toolchain-snapshot-3.8~svn246424/lib/Transforms/Scalar/LoopUnswitch.cpp"
, 989, __PRETTY_FUNCTION__));
990 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
991
992 // If the successor of the exit block had PHI nodes, add an entry for
993 // NewExit.
994 for (BasicBlock::iterator I = ExitSucc->begin();
995 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
996 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
997 ValueToValueMapTy::iterator It = VMap.find(V);
998 if (It != VMap.end()) V = It->second;
999 PN->addIncoming(V, NewExit);
1000 }
1001
1002 if (LandingPadInst *LPad = NewExit->getLandingPadInst()) {
1003 PHINode *PN = PHINode::Create(LPad->getType(), 0, "",
1004 ExitSucc->getFirstInsertionPt());
1005
1006 for (pred_iterator I = pred_begin(ExitSucc), E = pred_end(ExitSucc);
1007 I != E; ++I) {
1008 BasicBlock *BB = *I;
1009 LandingPadInst *LPI = BB->getLandingPadInst();
1010 LPI->replaceAllUsesWith(PN);
1011 PN->addIncoming(LPI, BB);
1012 }
1013 }
1014 }
1015
1016 // Rewrite the code to refer to itself.
1017 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
1018 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
1019 E = NewBlocks[i]->end(); I != E; ++I)
1020 RemapInstruction(I, VMap,RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
1021
1022 // Rewrite the original preheader to select between versions of the loop.
1023 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator());
1024 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&((OldBR->isUnconditional() && OldBR->getSuccessor
(0) == LoopBlocks[0] && "Preheader splitting did not work correctly!"
) ? static_cast<void> (0) : __assert_fail ("OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && \"Preheader splitting did not work correctly!\""
, "/tmp/buildd/llvm-toolchain-snapshot-3.8~svn246424/lib/Transforms/Scalar/LoopUnswitch.cpp"
, 1025, __PRETTY_FUNCTION__))
1025 "Preheader splitting did not work correctly!")((OldBR->isUnconditional() && OldBR->getSuccessor
(0) == LoopBlocks[0] && "Preheader splitting did not work correctly!"
) ? static_cast<void> (0) : __assert_fail ("OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] && \"Preheader splitting did not work correctly!\""
, "/tmp/buildd/llvm-toolchain-snapshot-3.8~svn246424/lib/Transforms/Scalar/LoopUnswitch.cpp"
, 1025, __PRETTY_FUNCTION__));
1026
1027 // Emit the new branch that selects between the two versions of this loop.
1028 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR,
1029 TI);
1030 LPM->deleteSimpleAnalysisValue(OldBR, L);
1031 OldBR->eraseFromParent();
1032
1033 LoopProcessWorklist.push_back(NewLoop);
1034 redoLoop = true;
1035
1036 // Keep a WeakVH holding onto LIC. If the first call to RewriteLoopBody
1037 // deletes the instruction (for example by simplifying a PHI that feeds into
1038 // the condition that we're unswitching on), we don't rewrite the second
1039 // iteration.
1040 WeakVH LICHandle(LIC);
1041
1042 // Now we rewrite the original code to know that the condition is true and the
1043 // new code to know that the condition is false.
1044 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
1045
1046 // It's possible that simplifying one loop could cause the other to be
1047 // changed to another value or a constant. If its a constant, don't simplify
1048 // it.
1049 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop &&
1050 LICHandle && !isa<Constant>(LICHandle))
1051 RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true);
1052}
1053
1054/// Remove all instances of I from the worklist vector specified.
1055static void RemoveFromWorklist(Instruction *I,
1056 std::vector<Instruction*> &Worklist) {
1057
1058 Worklist.erase(std::remove(Worklist.begin(), Worklist.end(), I),
1059 Worklist.end());
1060}
1061
1062/// When we find that I really equals V, remove I from the
1063/// program, replacing all uses with V and update the worklist.
1064static void ReplaceUsesOfWith(Instruction *I, Value *V,
1065 std::vector<Instruction*> &Worklist,
1066 Loop *L, LPPassManager *LPM) {
1067 DEBUG(dbgs() << "Replace with '" << *V << "': " << *I)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "Replace with '" <<
*V << "': " << *I; } } while (0);
1068
1069 // Add uses to the worklist, which may be dead now.
1070 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
1071 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
1072 Worklist.push_back(Use);
1073
1074 // Add users to the worklist which may be simplified now.
1075 for (User *U : I->users())
1076 Worklist.push_back(cast<Instruction>(U));
1077 LPM->deleteSimpleAnalysisValue(I, L);
1078 RemoveFromWorklist(I, Worklist);
1079 I->replaceAllUsesWith(V);
1080 I->eraseFromParent();
1081 ++NumSimplify;
1082}
1083
1084/// We know either that the value LIC has the value specified by Val in the
1085/// specified loop, or we know it does NOT have that value.
1086/// Rewrite any uses of LIC or of properties correlated to it.
1087void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
1088 Constant *Val,
1089 bool IsEqual) {
1090 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?")((!isa<Constant>(LIC) && "Why are we unswitching on a constant?"
) ? static_cast<void> (0) : __assert_fail ("!isa<Constant>(LIC) && \"Why are we unswitching on a constant?\""
, "/tmp/buildd/llvm-toolchain-snapshot-3.8~svn246424/lib/Transforms/Scalar/LoopUnswitch.cpp"
, 1090, __PRETTY_FUNCTION__));
1091
1092 // FIXME: Support correlated properties, like:
1093 // for (...)
1094 // if (li1 < li2)
1095 // ...
1096 // if (li1 > li2)
1097 // ...
1098
1099 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
1100 // selects, switches.
1101 std::vector<Instruction*> Worklist;
1102 LLVMContext &Context = Val->getContext();
1103
1104 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
1105 // in the loop with the appropriate one directly.
1106 if (IsEqual || (isa<ConstantInt>(Val) &&
1107 Val->getType()->isIntegerTy(1))) {
1108 Value *Replacement;
1109 if (IsEqual)
1110 Replacement = Val;
1111 else
1112 Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()),
1113 !cast<ConstantInt>(Val)->getZExtValue());
1114
1115 for (User *U : LIC->users()) {
1116 Instruction *UI = dyn_cast<Instruction>(U);
1117 if (!UI || !L->contains(UI))
1118 continue;
1119 Worklist.push_back(UI);
1120 }
1121
1122 for (std::vector<Instruction*>::iterator UI = Worklist.begin(),
1123 UE = Worklist.end(); UI != UE; ++UI)
1124 (*UI)->replaceUsesOfWith(LIC, Replacement);
1125
1126 SimplifyCode(Worklist, L);
1127 return;
1128 }
1129
1130 // Otherwise, we don't know the precise value of LIC, but we do know that it
1131 // is certainly NOT "Val". As such, simplify any uses in the loop that we
1132 // can. This case occurs when we unswitch switch statements.
1133 for (User *U : LIC->users()) {
1134 Instruction *UI = dyn_cast<Instruction>(U);
1135 if (!UI || !L->contains(UI))
1136 continue;
1137
1138 Worklist.push_back(UI);
1139
1140 // TODO: We could do other simplifications, for example, turning
1141 // 'icmp eq LIC, Val' -> false.
1142
1143 // If we know that LIC is not Val, use this info to simplify code.
1144 SwitchInst *SI = dyn_cast<SwitchInst>(UI);
1145 if (!SI || !isa<ConstantInt>(Val)) continue;
1146
1147 SwitchInst::CaseIt DeadCase = SI->findCaseValue(cast<ConstantInt>(Val));
1148 // Default case is live for multiple values.
1149 if (DeadCase == SI->case_default()) continue;
1150
1151 // Found a dead case value. Don't remove PHI nodes in the
1152 // successor if they become single-entry, those PHI nodes may
1153 // be in the Users list.
1154
1155 BasicBlock *Switch = SI->getParent();
1156 BasicBlock *SISucc = DeadCase.getCaseSuccessor();
1157 BasicBlock *Latch = L->getLoopLatch();
1158
1159 BranchesInfo.setUnswitched(SI, Val);
1160
1161 if (!SI->findCaseDest(SISucc)) continue; // Edge is critical.
1162 // If the DeadCase successor dominates the loop latch, then the
1163 // transformation isn't safe since it will delete the sole predecessor edge
1164 // to the latch.
1165 if (Latch && DT->dominates(SISucc, Latch))
1166 continue;
1167
1168 // FIXME: This is a hack. We need to keep the successor around
1169 // and hooked up so as to preserve the loop structure, because
1170 // trying to update it is complicated. So instead we preserve the
1171 // loop structure and put the block on a dead code path.
1172 SplitEdge(Switch, SISucc, DT, LI);
1173 // Compute the successors instead of relying on the return value
1174 // of SplitEdge, since it may have split the switch successor
1175 // after PHI nodes.
1176 BasicBlock *NewSISucc = DeadCase.getCaseSuccessor();
1177 BasicBlock *OldSISucc = *succ_begin(NewSISucc);
1178 // Create an "unreachable" destination.
1179 BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable",
1180 Switch->getParent(),
1181 OldSISucc);
1182 new UnreachableInst(Context, Abort);
1183 // Force the new case destination to branch to the "unreachable"
1184 // block while maintaining a (dead) CFG edge to the old block.
1185 NewSISucc->getTerminator()->eraseFromParent();
1186 BranchInst::Create(Abort, OldSISucc,
1187 ConstantInt::getTrue(Context), NewSISucc);
1188 // Release the PHI operands for this edge.
1189 for (BasicBlock::iterator II = NewSISucc->begin();
1190 PHINode *PN = dyn_cast<PHINode>(II); ++II)
1191 PN->setIncomingValue(PN->getBasicBlockIndex(Switch),
1192 UndefValue::get(PN->getType()));
1193 // Tell the domtree about the new block. We don't fully update the
1194 // domtree here -- instead we force it to do a full recomputation
1195 // after the pass is complete -- but we do need to inform it of
1196 // new blocks.
1197 if (DT)
1198 DT->addNewBlock(Abort, NewSISucc);
1199 }
1200
1201 SimplifyCode(Worklist, L);
1202}
1203
1204/// Now that we have simplified some instructions in the loop, walk over it and
1205/// constant prop, dce, and fold control flow where possible. Note that this is
1206/// effectively a very simple loop-structure-aware optimizer. During processing
1207/// of this loop, L could very well be deleted, so it must not be used.
1208///
1209/// FIXME: When the loop optimizer is more mature, separate this out to a new
1210/// pass.
1211///
1212void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
1213 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
1214 while (!Worklist.empty()) {
1215 Instruction *I = Worklist.back();
1216 Worklist.pop_back();
1217
1218 // Simple DCE.
1219 if (isInstructionTriviallyDead(I)) {
1220 DEBUG(dbgs() << "Remove dead instruction '" << *I)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "Remove dead instruction '"
<< *I; } } while (0);
1221
1222 // Add uses to the worklist, which may be dead now.
1223 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
1224 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
1225 Worklist.push_back(Use);
1226 LPM->deleteSimpleAnalysisValue(I, L);
1227 RemoveFromWorklist(I, Worklist);
1228 I->eraseFromParent();
1229 ++NumSimplify;
1230 continue;
1231 }
1232
1233 // See if instruction simplification can hack this up. This is common for
1234 // things like "select false, X, Y" after unswitching made the condition be
1235 // 'false'. TODO: update the domtree properly so we can pass it here.
1236 if (Value *V = SimplifyInstruction(I, DL))
1237 if (LI->replacementPreservesLCSSAForm(I, V)) {
1238 ReplaceUsesOfWith(I, V, Worklist, L, LPM);
1239 continue;
1240 }
1241
1242 // Special case hacks that appear commonly in unswitched code.
1243 if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
1244 if (BI->isUnconditional()) {
1245 // If BI's parent is the only pred of the successor, fold the two blocks
1246 // together.
1247 BasicBlock *Pred = BI->getParent();
1248 BasicBlock *Succ = BI->getSuccessor(0);
1249 BasicBlock *SinglePred = Succ->getSinglePredecessor();
1250 if (!SinglePred) continue; // Nothing to do.
1251 assert(SinglePred == Pred && "CFG broken")((SinglePred == Pred && "CFG broken") ? static_cast<
void> (0) : __assert_fail ("SinglePred == Pred && \"CFG broken\""
, "/tmp/buildd/llvm-toolchain-snapshot-3.8~svn246424/lib/Transforms/Scalar/LoopUnswitch.cpp"
, 1251, __PRETTY_FUNCTION__));
1252
1253 DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "Merging blocks: " <<
Pred->getName() << " <- " << Succ->getName
() << "\n"; } } while (0)
1254 << Succ->getName() << "\n")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("loop-unswitch")) { dbgs() << "Merging blocks: " <<
Pred->getName() << " <- " << Succ->getName
() << "\n"; } } while (0);
1255
1256 // Resolve any single entry PHI nodes in Succ.
1257 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
1258 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM);
1259
1260 // If Succ has any successors with PHI nodes, update them to have
1261 // entries coming from Pred instead of Succ.
1262 Succ->replaceAllUsesWith(Pred);
1263
1264 // Move all of the successor contents from Succ to Pred.
1265 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
1266 Succ->end());
1267 LPM->deleteSimpleAnalysisValue(BI, L);
1268 BI->eraseFromParent();
1269 RemoveFromWorklist(BI, Worklist);
1270
1271 // Remove Succ from the loop tree.
1272 LI->removeBlock(Succ);
1273 LPM->deleteSimpleAnalysisValue(Succ, L);
1274 Succ->eraseFromParent();
1275 ++NumSimplify;
1276 continue;
1277 }
1278
1279 continue;
1280 }
1281 }
1282}