1 //===- MemoryDependenceAnalysis.cpp - Mem Deps Implementation --*- C++ -*-===//
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 file implements an analysis that determines, for a given memory
11 // operation, what preceding memory operations it depends on. It builds on
12 // alias analysis information, and tries to provide a lazy, caching interface to
13 // a common kind of alias information query.
15 //===----------------------------------------------------------------------===//
17 #define DEBUG_TYPE "memdep"
18 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
19 #include "llvm/Constants.h"
20 #include "llvm/Instructions.h"
21 #include "llvm/Function.h"
22 #include "llvm/Analysis/AliasAnalysis.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/ADT/STLExtras.h"
25 #include "llvm/Support/CFG.h"
26 #include "llvm/Support/CommandLine.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Target/TargetData.h"
31 STATISTIC(NumCacheNonLocal, "Number of cached non-local responses");
32 STATISTIC(NumUncacheNonLocal, "Number of uncached non-local responses");
34 char MemoryDependenceAnalysis::ID = 0;
36 // Register this pass...
37 static RegisterPass<MemoryDependenceAnalysis> X("memdep",
38 "Memory Dependence Analysis", false, true);
40 /// getAnalysisUsage - Does not modify anything. It uses Alias Analysis.
42 void MemoryDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
44 AU.addRequiredTransitive<AliasAnalysis>();
45 AU.addRequiredTransitive<TargetData>();
48 /// getCallSiteDependency - Private helper for finding the local dependencies
50 MemoryDependenceAnalysis::DepResultTy MemoryDependenceAnalysis::
51 getCallSiteDependency(CallSite C, BasicBlock::iterator ScanIt,
53 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
54 TargetData &TD = getAnalysis<TargetData>();
56 // Walk backwards through the block, looking for dependencies
57 while (ScanIt != BB->begin()) {
58 Instruction *Inst = --ScanIt;
60 // If this inst is a memory op, get the pointer it accessed
62 uint64_t PointerSize = 0;
63 if (StoreInst *S = dyn_cast<StoreInst>(Inst)) {
64 Pointer = S->getPointerOperand();
65 PointerSize = TD.getTypeStoreSize(S->getOperand(0)->getType());
66 } else if (AllocationInst *AI = dyn_cast<AllocationInst>(Inst)) {
68 if (ConstantInt *C = dyn_cast<ConstantInt>(AI->getArraySize()))
69 // Use ABI size (size between elements), not store size (size of one
70 // element without padding).
71 PointerSize = C->getZExtValue() *
72 TD.getABITypeSize(AI->getAllocatedType());
75 } else if (VAArgInst *V = dyn_cast<VAArgInst>(Inst)) {
76 Pointer = V->getOperand(0);
77 PointerSize = TD.getTypeStoreSize(V->getType());
78 } else if (FreeInst *F = dyn_cast<FreeInst>(Inst)) {
79 Pointer = F->getPointerOperand();
81 // FreeInsts erase the entire structure
83 } else if (isa<CallInst>(Inst) || isa<InvokeInst>(Inst)) {
84 if (AA.getModRefBehavior(CallSite::get(Inst)) ==
85 AliasAnalysis::DoesNotAccessMemory)
87 return DepResultTy(Inst, Normal);
91 if (AA.getModRefInfo(C, Pointer, PointerSize) != AliasAnalysis::NoModRef)
92 return DepResultTy(Inst, Normal);
95 // No dependence found.
96 return DepResultTy(0, NonLocal);
99 /// getDependency - Return the instruction on which a memory operation
100 /// depends. The local parameter indicates if the query should only
101 /// evaluate dependencies within the same basic block.
102 MemoryDependenceAnalysis::DepResultTy MemoryDependenceAnalysis::
103 getDependencyFromInternal(Instruction *QueryInst, BasicBlock::iterator ScanIt,
105 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
106 TargetData &TD = getAnalysis<TargetData>();
108 // Get the pointer value for which dependence will be determined
110 uint64_t MemSize = 0;
111 bool MemVolatile = false;
113 if (StoreInst* S = dyn_cast<StoreInst>(QueryInst)) {
114 MemPtr = S->getPointerOperand();
115 MemSize = TD.getTypeStoreSize(S->getOperand(0)->getType());
116 MemVolatile = S->isVolatile();
117 } else if (LoadInst* L = dyn_cast<LoadInst>(QueryInst)) {
118 MemPtr = L->getPointerOperand();
119 MemSize = TD.getTypeStoreSize(L->getType());
120 MemVolatile = L->isVolatile();
121 } else if (VAArgInst* V = dyn_cast<VAArgInst>(QueryInst)) {
122 MemPtr = V->getOperand(0);
123 MemSize = TD.getTypeStoreSize(V->getType());
124 } else if (FreeInst* F = dyn_cast<FreeInst>(QueryInst)) {
125 MemPtr = F->getPointerOperand();
126 // FreeInsts erase the entire structure, not just a field.
128 } else if (isa<CallInst>(QueryInst) || isa<InvokeInst>(QueryInst))
129 return getCallSiteDependency(CallSite::get(QueryInst), ScanIt, BB);
130 else // Non-memory instructions depend on nothing.
131 return DepResultTy(0, None);
133 // Walk backwards through the basic block, looking for dependencies
134 while (ScanIt != BB->begin()) {
135 Instruction *Inst = --ScanIt;
137 // If the access is volatile and this is a volatile load/store, return a
140 ((isa<LoadInst>(Inst) && cast<LoadInst>(Inst)->isVolatile()) ||
141 (isa<StoreInst>(Inst) && cast<StoreInst>(Inst)->isVolatile())))
142 return DepResultTy(Inst, Normal);
144 // MemDep is broken w.r.t. loads: it says that two loads of the same pointer
145 // depend on each other. :(
146 if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
147 Value *Pointer = L->getPointerOperand();
148 uint64_t PointerSize = TD.getTypeStoreSize(L->getType());
150 // If we found a pointer, check if it could be the same as our pointer
151 AliasAnalysis::AliasResult R =
152 AA.alias(Pointer, PointerSize, MemPtr, MemSize);
154 if (R == AliasAnalysis::NoAlias)
157 // May-alias loads don't depend on each other without a dependence.
158 if (isa<LoadInst>(QueryInst) && R == AliasAnalysis::MayAlias)
160 return DepResultTy(Inst, Normal);
163 // If this is an allocation, and if we know that the accessed pointer is to
164 // the allocation, return None. This means that there is no dependence and
165 // the access can be optimized based on that. For example, a load could
167 if (AllocationInst *AI = dyn_cast<AllocationInst>(Inst)) {
168 Value *AccessPtr = MemPtr->getUnderlyingObject();
170 if (AccessPtr == AI ||
171 AA.alias(AI, 1, AccessPtr, 1) == AliasAnalysis::MustAlias)
172 return DepResultTy(0, None);
176 // See if this instruction mod/ref's the pointer.
177 AliasAnalysis::ModRefResult MRR = AA.getModRefInfo(Inst, MemPtr, MemSize);
179 if (MRR == AliasAnalysis::NoModRef)
182 // Loads don't depend on read-only instructions.
183 if (isa<LoadInst>(QueryInst) && MRR == AliasAnalysis::Ref)
186 // Otherwise, there is a dependence.
187 return DepResultTy(Inst, Normal);
190 // If we found nothing, return the non-local flag.
191 return DepResultTy(0, NonLocal);
194 /// getDependency - Return the instruction on which a memory operation
196 MemDepResult MemoryDependenceAnalysis::getDependency(Instruction *QueryInst) {
197 Instruction *ScanPos = QueryInst;
199 // Check for a cached result
200 DepResultTy &LocalCache = LocalDeps[QueryInst];
202 // If the cached entry is non-dirty, just return it. Note that this depends
203 // on DepResultTy's default constructing to 'dirty'.
204 if (LocalCache.getInt() != Dirty)
205 return ConvToResult(LocalCache);
207 // Otherwise, if we have a dirty entry, we know we can start the scan at that
208 // instruction, which may save us some work.
209 if (Instruction *Inst = LocalCache.getPointer())
213 LocalCache = getDependencyFromInternal(QueryInst, ScanPos,
214 QueryInst->getParent());
216 // Remember the result!
217 if (Instruction *I = LocalCache.getPointer())
218 ReverseLocalDeps[I].insert(QueryInst);
220 return ConvToResult(LocalCache);
223 /// getNonLocalDependency - Perform a full dependency query for the
224 /// specified instruction, returning the set of blocks that the value is
225 /// potentially live across. The returned set of results will include a
226 /// "NonLocal" result for all blocks where the value is live across.
228 /// This method assumes the instruction returns a "nonlocal" dependency
229 /// within its own block.
231 void MemoryDependenceAnalysis::
232 getNonLocalDependency(Instruction *QueryInst,
233 SmallVectorImpl<std::pair<BasicBlock*,
234 MemDepResult> > &Result) {
235 assert(getDependency(QueryInst).isNonLocal() &&
236 "getNonLocalDependency should only be used on insts with non-local deps!");
237 DenseMap<BasicBlock*, DepResultTy> &Cache = NonLocalDeps[QueryInst];
239 /// DirtyBlocks - This is the set of blocks that need to be recomputed. In
240 /// the cached case, this can happen due to instructions being deleted etc. In
241 /// the uncached case, this starts out as the set of predecessors we care
243 SmallVector<BasicBlock*, 32> DirtyBlocks;
245 if (!Cache.empty()) {
246 // If we already have a partially computed set of results, scan them to
247 // determine what is dirty, seeding our initial DirtyBlocks worklist.
248 // FIXME: In the "don't need to be updated" case, this is expensive, why not
249 // have a per-"cache" flag saying it is undirty?
250 for (DenseMap<BasicBlock*, DepResultTy>::iterator I = Cache.begin(),
251 E = Cache.end(); I != E; ++I)
252 if (I->second.getInt() == Dirty)
253 DirtyBlocks.push_back(I->first);
257 //cerr << "CACHED CASE: " << DirtyBlocks.size() << " dirty: "
258 // << Cache.size() << " cached: " << *QueryInst;
260 // Seed DirtyBlocks with each of the preds of QueryInst's block.
261 BasicBlock *QueryBB = QueryInst->getParent();
262 DirtyBlocks.append(pred_begin(QueryBB), pred_end(QueryBB));
263 NumUncacheNonLocal++;
266 // Iterate while we still have blocks to update.
267 while (!DirtyBlocks.empty()) {
268 BasicBlock *DirtyBB = DirtyBlocks.back();
269 DirtyBlocks.pop_back();
271 // Get the entry for this block. Note that this relies on DepResultTy
272 // default initializing to Dirty.
273 DepResultTy &DirtyBBEntry = Cache[DirtyBB];
275 // If DirtyBBEntry isn't dirty, it ended up on the worklist multiple times.
276 if (DirtyBBEntry.getInt() != Dirty) continue;
278 // If the dirty entry has a pointer, start scanning from it so we don't have
279 // to rescan the entire block.
280 BasicBlock::iterator ScanPos = DirtyBB->end();
281 if (Instruction *Inst = DirtyBBEntry.getPointer())
284 // Find out if this block has a local dependency for QueryInst.
285 DirtyBBEntry = getDependencyFromInternal(QueryInst, ScanPos, DirtyBB);
287 // If the block has a dependency (i.e. it isn't completely transparent to
288 // the value), remember it!
289 if (DirtyBBEntry.getInt() != NonLocal) {
290 // Keep the ReverseNonLocalDeps map up to date so we can efficiently
291 // update this when we remove instructions.
292 if (Instruction *Inst = DirtyBBEntry.getPointer())
293 ReverseNonLocalDeps[Inst].insert(QueryInst);
297 // If the block *is* completely transparent to the load, we need to check
298 // the predecessors of this block. Add them to our worklist.
299 DirtyBlocks.append(pred_begin(DirtyBB), pred_end(DirtyBB));
303 // Copy the result into the output set.
304 for (DenseMap<BasicBlock*, DepResultTy>::iterator I = Cache.begin(),
305 E = Cache.end(); I != E; ++I)
306 Result.push_back(std::make_pair(I->first, ConvToResult(I->second)));
309 /// removeInstruction - Remove an instruction from the dependence analysis,
310 /// updating the dependence of instructions that previously depended on it.
311 /// This method attempts to keep the cache coherent using the reverse map.
312 void MemoryDependenceAnalysis::removeInstruction(Instruction *RemInst) {
313 // Walk through the Non-local dependencies, removing this one as the value
314 // for any cached queries.
315 for (DenseMap<BasicBlock*, DepResultTy>::iterator DI =
316 NonLocalDeps[RemInst].begin(), DE = NonLocalDeps[RemInst].end();
318 if (Instruction *Inst = DI->second.getPointer())
319 ReverseNonLocalDeps[Inst].erase(RemInst);
321 // If we have a cached local dependence query for this instruction, remove it.
323 LocalDepMapType::iterator LocalDepEntry = LocalDeps.find(RemInst);
324 if (LocalDepEntry != LocalDeps.end()) {
325 // Remove us from DepInst's reverse set now that the local dep info is gone.
326 if (Instruction *Inst = LocalDepEntry->second.getPointer()) {
327 SmallPtrSet<Instruction*, 4> &RLD = ReverseLocalDeps[Inst];
330 ReverseLocalDeps.erase(Inst);
333 // Remove this local dependency info.
334 LocalDeps.erase(LocalDepEntry);
337 // Loop over all of the things that depend on the instruction we're removing.
339 SmallVector<std::pair<Instruction*, Instruction*>, 8> ReverseDepsToAdd;
341 ReverseDepMapType::iterator ReverseDepIt = ReverseLocalDeps.find(RemInst);
342 if (ReverseDepIt != ReverseLocalDeps.end()) {
343 SmallPtrSet<Instruction*, 4> &ReverseDeps = ReverseDepIt->second;
344 // RemInst can't be the terminator if it has stuff depending on it.
345 assert(!ReverseDeps.empty() && !isa<TerminatorInst>(RemInst) &&
346 "Nothing can locally depend on a terminator");
348 // Anything that was locally dependent on RemInst is now going to be
349 // dependent on the instruction after RemInst. It will have the dirty flag
350 // set so it will rescan. This saves having to scan the entire block to get
352 Instruction *NewDepInst = next(BasicBlock::iterator(RemInst));
354 for (SmallPtrSet<Instruction*, 4>::iterator I = ReverseDeps.begin(),
355 E = ReverseDeps.end(); I != E; ++I) {
356 Instruction *InstDependingOnRemInst = *I;
358 // If we thought the instruction depended on itself (possible for
359 // unconfirmed dependencies) ignore the update.
360 if (InstDependingOnRemInst == RemInst) continue;
362 LocalDeps[InstDependingOnRemInst] = DepResultTy(NewDepInst, Dirty);
364 // Make sure to remember that new things depend on NewDepInst.
365 ReverseDepsToAdd.push_back(std::make_pair(NewDepInst,
366 InstDependingOnRemInst));
369 ReverseLocalDeps.erase(ReverseDepIt);
371 // Add new reverse deps after scanning the set, to avoid invalidating the
372 // 'ReverseDeps' reference.
373 while (!ReverseDepsToAdd.empty()) {
374 ReverseLocalDeps[ReverseDepsToAdd.back().first]
375 .insert(ReverseDepsToAdd.back().second);
376 ReverseDepsToAdd.pop_back();
380 ReverseDepIt = ReverseNonLocalDeps.find(RemInst);
381 if (ReverseDepIt != ReverseNonLocalDeps.end()) {
382 SmallPtrSet<Instruction*, 4>& set = ReverseDepIt->second;
383 for (SmallPtrSet<Instruction*, 4>::iterator I = set.begin(), E = set.end();
385 for (DenseMap<BasicBlock*, DepResultTy>::iterator
386 DI = NonLocalDeps[*I].begin(), DE = NonLocalDeps[*I].end();
388 if (DI->second.getPointer() == RemInst) {
389 // Convert to a dirty entry for the subsequent instruction.
390 DI->second.setInt(Dirty);
391 if (RemInst->isTerminator())
392 DI->second.setPointer(0);
394 Instruction *NextI = next(BasicBlock::iterator(RemInst));
395 DI->second.setPointer(NextI);
396 assert(NextI != RemInst);
397 ReverseDepsToAdd.push_back(std::make_pair(NextI, *I));
401 ReverseNonLocalDeps.erase(ReverseDepIt);
403 // Add new reverse deps after scanning the set, to avoid invalidating 'Set'
404 while (!ReverseDepsToAdd.empty()) {
405 ReverseNonLocalDeps[ReverseDepsToAdd.back().first]
406 .insert(ReverseDepsToAdd.back().second);
407 ReverseDepsToAdd.pop_back();
411 NonLocalDeps.erase(RemInst);
412 getAnalysis<AliasAnalysis>().deleteValue(RemInst);
413 DEBUG(verifyRemoved(RemInst));
416 /// verifyRemoved - Verify that the specified instruction does not occur
417 /// in our internal data structures.
418 void MemoryDependenceAnalysis::verifyRemoved(Instruction *D) const {
419 for (LocalDepMapType::const_iterator I = LocalDeps.begin(),
420 E = LocalDeps.end(); I != E; ++I) {
421 assert(I->first != D && "Inst occurs in data structures");
422 assert(I->second.getPointer() != D &&
423 "Inst occurs in data structures");
426 for (NonLocalDepMapType::const_iterator I = NonLocalDeps.begin(),
427 E = NonLocalDeps.end(); I != E; ++I) {
428 assert(I->first != D && "Inst occurs in data structures");
429 for (DenseMap<BasicBlock*, DepResultTy>::iterator II = I->second.begin(),
430 EE = I->second.end(); II != EE; ++II)
431 assert(II->second.getPointer() != D && "Inst occurs in data structures");
434 for (ReverseDepMapType::const_iterator I = ReverseLocalDeps.begin(),
435 E = ReverseLocalDeps.end(); I != E; ++I)
436 for (SmallPtrSet<Instruction*, 4>::const_iterator II = I->second.begin(),
437 EE = I->second.end(); II != EE; ++II)
438 assert(*II != D && "Inst occurs in data structures");
440 for (ReverseDepMapType::const_iterator I = ReverseNonLocalDeps.begin(),
441 E = ReverseNonLocalDeps.end();
443 for (SmallPtrSet<Instruction*, 4>::const_iterator II = I->second.begin(),
444 EE = I->second.end(); II != EE; ++II)
445 assert(*II != D && "Inst occurs in data structures");