1 //===- MemoryDependenceAnalysis.cpp - Mem Deps Implementation --*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the Owen Anderson and is distributed under
6 // the University of Illinois Open Source 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 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
18 #include "llvm/Constants.h"
19 #include "llvm/Instructions.h"
20 #include "llvm/Function.h"
21 #include "llvm/Analysis/AliasAnalysis.h"
22 #include "llvm/Support/CFG.h"
23 #include "llvm/Target/TargetData.h"
27 char MemoryDependenceAnalysis::ID = 0;
29 Instruction* const MemoryDependenceAnalysis::NonLocal = (Instruction*)-3;
30 Instruction* const MemoryDependenceAnalysis::None = (Instruction*)-4;
32 // Register this pass...
33 static RegisterPass<MemoryDependenceAnalysis> X("memdep",
34 "Memory Dependence Analysis");
36 /// getAnalysisUsage - Does not modify anything. It uses Alias Analysis.
38 void MemoryDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
40 AU.addRequiredTransitive<AliasAnalysis>();
41 AU.addRequiredTransitive<TargetData>();
44 /// getCallSiteDependency - Private helper for finding the local dependencies
46 Instruction* MemoryDependenceAnalysis::getCallSiteDependency(CallSite C,
50 AliasAnalysis& AA = getAnalysis<AliasAnalysis>();
51 TargetData& TD = getAnalysis<TargetData>();
52 BasicBlock::iterator blockBegin = C.getInstruction()->getParent()->begin();
53 BasicBlock::iterator QI = C.getInstruction();
55 // If the starting point was specifiy, use it
58 blockBegin = start->getParent()->end();
59 // If the starting point wasn't specified, but the block was, use it
60 } else if (!start && block) {
62 blockBegin = block->end();
65 // Walk backwards through the block, looking for dependencies
66 while (QI != blockBegin) {
69 // If this inst is a memory op, get the pointer it accessed
71 uint64_t pointerSize = 0;
72 if (StoreInst* S = dyn_cast<StoreInst>(QI)) {
73 pointer = S->getPointerOperand();
74 pointerSize = TD.getTypeSize(S->getOperand(0)->getType());
75 } else if (LoadInst* L = dyn_cast<LoadInst>(QI)) {
76 pointer = L->getPointerOperand();
77 pointerSize = TD.getTypeSize(L->getType());
78 } else if (AllocationInst* AI = dyn_cast<AllocationInst>(QI)) {
80 if (ConstantInt* C = dyn_cast<ConstantInt>(AI->getArraySize()))
81 pointerSize = C->getZExtValue() * \
82 TD.getTypeSize(AI->getAllocatedType());
85 } else if (VAArgInst* V = dyn_cast<VAArgInst>(QI)) {
86 pointer = V->getOperand(0);
87 pointerSize = TD.getTypeSize(V->getType());
88 } else if (FreeInst* F = dyn_cast<FreeInst>(QI)) {
89 pointer = F->getPointerOperand();
91 // FreeInsts erase the entire structure
93 } else if (CallSite::get(QI).getInstruction() != 0) {
94 if (AA.getModRefInfo(C, CallSite::get(QI)) != AliasAnalysis::NoModRef) {
95 if (!start && !block) {
96 depGraphLocal.insert(std::make_pair(C.getInstruction(),
97 std::make_pair(QI, true)));
98 reverseDep[QI].insert(C.getInstruction());
107 if (AA.getModRefInfo(C, pointer, pointerSize) != AliasAnalysis::NoModRef) {
108 if (!start && !block) {
109 depGraphLocal.insert(std::make_pair(C.getInstruction(),
110 std::make_pair(QI, true)));
111 reverseDep[QI].insert(C.getInstruction());
117 // No dependence found
118 depGraphLocal.insert(std::make_pair(C.getInstruction(),
119 std::make_pair(NonLocal, true)));
120 reverseDep[NonLocal].insert(C.getInstruction());
124 /// nonLocalHelper - Private helper used to calculate non-local dependencies
125 /// by doing DFS on the predecessors of a block to find its dependencies
126 void MemoryDependenceAnalysis::nonLocalHelper(Instruction* query,
128 DenseMap<BasicBlock*, Value*>& resp) {
129 // Set of blocks that we've already visited in our DFS
130 SmallPtrSet<BasicBlock*, 4> visited;
131 // Current stack of the DFS
132 SmallVector<BasicBlock*, 4> stack;
133 stack.push_back(block);
136 while (!stack.empty()) {
137 BasicBlock* BB = stack.back();
139 // If we've already visited this block, no need to revist
140 if (visited.count(BB)) {
145 // If we find a new block with a local dependency for query,
146 // then we insert the new dependency and backtrack.
150 Instruction* localDep = getDependency(query, 0, BB);
151 if (localDep != NonLocal) {
152 resp.insert(std::make_pair(BB, localDep));
157 // If we re-encounter the starting block, we still need to search it
158 // because there might be a dependency in the starting block AFTER
159 // the position of the query. This is necessary to get loops right.
160 } else if (BB == block && stack.size() > 1) {
163 Instruction* localDep = getDependency(query, 0, BB);
164 if (localDep != query)
165 resp.insert(std::make_pair(BB, localDep));
172 // If we didn't find anything, recurse on the precessors of this block
173 bool predOnStack = false;
174 bool inserted = false;
175 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
177 if (!visited.count(*PI)) {
178 stack.push_back(*PI);
183 // If we inserted a new predecessor, then we'll come back to this block
186 // If we didn't insert because we have no predecessors, then this
187 // query has no dependency at all.
188 else if (!inserted && !predOnStack) {
189 resp.insert(std::make_pair(BB, None));
190 // If we didn't insert because our predecessors are already on the stack,
191 // then we might still have a dependency, but it will be discovered during
193 } else if (!inserted && predOnStack){
194 resp.insert(std::make_pair(BB, NonLocal));
201 /// getNonLocalDependency - Fills the passed-in map with the non-local
202 /// dependencies of the queries. The map will contain NonLocal for
203 /// blocks between the query and its dependencies.
204 void MemoryDependenceAnalysis::getNonLocalDependency(Instruction* query,
205 DenseMap<BasicBlock*, Value*>& resp) {
206 // First check that we don't actually have a local dependency.
207 Instruction* localDep = getDependency(query);
208 if (localDep != NonLocal) {
209 resp.insert(std::make_pair(query->getParent(),localDep));
213 // If not, go ahead and search for non-local ones.
214 nonLocalHelper(query, query->getParent(), resp);
217 /// getDependency - Return the instruction on which a memory operation
218 /// depends. The local paramter indicates if the query should only
219 /// evaluate dependencies within the same basic block.
220 Instruction* MemoryDependenceAnalysis::getDependency(Instruction* query,
223 // Start looking for dependencies with the queried inst
224 BasicBlock::iterator QI = query;
226 // Check for a cached result
227 std::pair<Instruction*, bool> cachedResult = depGraphLocal[query];
228 // If we have a _confirmed_ cached entry, return it
229 if (cachedResult.second)
230 return cachedResult.first;
231 else if (cachedResult.first && cachedResult.first != NonLocal)
232 // If we have an unconfirmed cached entry, we can start our search from there
233 QI = cachedResult.first;
237 else if (!start && block)
240 AliasAnalysis& AA = getAnalysis<AliasAnalysis>();
241 TargetData& TD = getAnalysis<TargetData>();
243 // Get the pointer value for which dependence will be determined
245 uint64_t dependeeSize = 0;
246 bool queryIsVolatile = false;
247 if (StoreInst* S = dyn_cast<StoreInst>(query)) {
248 dependee = S->getPointerOperand();
249 dependeeSize = TD.getTypeSize(S->getOperand(0)->getType());
250 queryIsVolatile = S->isVolatile();
251 } else if (LoadInst* L = dyn_cast<LoadInst>(query)) {
252 dependee = L->getPointerOperand();
253 dependeeSize = TD.getTypeSize(L->getType());
254 queryIsVolatile = L->isVolatile();
255 } else if (VAArgInst* V = dyn_cast<VAArgInst>(query)) {
256 dependee = V->getOperand(0);
257 dependeeSize = TD.getTypeSize(V->getType());
258 } else if (FreeInst* F = dyn_cast<FreeInst>(query)) {
259 dependee = F->getPointerOperand();
261 // FreeInsts erase the entire structure, not just a field
263 } else if (CallSite::get(query).getInstruction() != 0)
264 return getCallSiteDependency(CallSite::get(query), start, block);
265 else if (isa<AllocationInst>(query))
270 BasicBlock::iterator blockBegin = block ? block->begin()
271 : query->getParent()->begin();
273 // Walk backwards through the basic block, looking for dependencies
274 while (QI != blockBegin) {
277 // If this inst is a memory op, get the pointer it accessed
279 uint64_t pointerSize = 0;
280 if (StoreInst* S = dyn_cast<StoreInst>(QI)) {
281 // All volatile loads/stores depend on each other
282 if (queryIsVolatile && S->isVolatile()) {
283 if (!start && !block) {
284 depGraphLocal.insert(std::make_pair(query, std::make_pair(S, true)));
285 reverseDep[S].insert(query);
291 pointer = S->getPointerOperand();
292 pointerSize = TD.getTypeSize(S->getOperand(0)->getType());
293 } else if (LoadInst* L = dyn_cast<LoadInst>(QI)) {
294 // All volatile loads/stores depend on each other
295 if (queryIsVolatile && L->isVolatile()) {
296 if (!start && !block) {
297 depGraphLocal.insert(std::make_pair(query, std::make_pair(L, true)));
298 reverseDep[L].insert(query);
304 pointer = L->getPointerOperand();
305 pointerSize = TD.getTypeSize(L->getType());
306 } else if (AllocationInst* AI = dyn_cast<AllocationInst>(QI)) {
308 if (ConstantInt* C = dyn_cast<ConstantInt>(AI->getArraySize()))
309 pointerSize = C->getZExtValue() * \
310 TD.getTypeSize(AI->getAllocatedType());
313 } else if (VAArgInst* V = dyn_cast<VAArgInst>(QI)) {
314 pointer = V->getOperand(0);
315 pointerSize = TD.getTypeSize(V->getType());
316 } else if (FreeInst* F = dyn_cast<FreeInst>(QI)) {
317 pointer = F->getPointerOperand();
319 // FreeInsts erase the entire structure
321 } else if (CallSite::get(QI).getInstruction() != 0) {
322 // Call insts need special handling. Check if they can modify our pointer
323 AliasAnalysis::ModRefResult MR = AA.getModRefInfo(CallSite::get(QI),
324 dependee, dependeeSize);
326 if (MR != AliasAnalysis::NoModRef) {
327 // Loads don't depend on read-only calls
328 if (isa<LoadInst>(query) && MR == AliasAnalysis::Ref)
331 if (!start && !block) {
332 depGraphLocal.insert(std::make_pair(query,
333 std::make_pair(QI, true)));
334 reverseDep[QI].insert(query);
343 // If we found a pointer, check if it could be the same as our pointer
345 AliasAnalysis::AliasResult R = AA.alias(pointer, pointerSize,
346 dependee, dependeeSize);
348 if (R != AliasAnalysis::NoAlias) {
349 // May-alias loads don't depend on each other
350 if (isa<LoadInst>(query) && isa<LoadInst>(QI) &&
351 R == AliasAnalysis::MayAlias)
354 if (!start && !block) {
355 depGraphLocal.insert(std::make_pair(query,
356 std::make_pair(QI, true)));
357 reverseDep[QI].insert(query);
365 // If we found nothing, return the non-local flag
366 if (!start && !block) {
367 depGraphLocal.insert(std::make_pair(query,
368 std::make_pair(NonLocal, true)));
369 reverseDep[NonLocal].insert(query);
375 /// removeInstruction - Remove an instruction from the dependence analysis,
376 /// updating the dependence of instructions that previously depended on it.
377 /// This method attempts to keep the cache coherent using the reverse map.
378 void MemoryDependenceAnalysis::removeInstruction(Instruction* rem) {
379 // Figure out the new dep for things that currently depend on rem
380 Instruction* newDep = NonLocal;
382 depMapType::iterator depGraphEntry = depGraphLocal.find(rem);
383 // We assume here that it's not in the reverse map if it's not in
384 // the dep map. Checking it could be expensive, so don't do it.
386 if (depGraphEntry != depGraphLocal.end()) {
387 if (depGraphEntry->second.first != NonLocal &&
388 depGraphEntry->second.second) {
389 // If we have dep info for rem, set them to it
390 BasicBlock::iterator RI = depGraphEntry->second.first;
393 } else if (depGraphEntry->second.first == NonLocal &&
394 depGraphEntry->second.second ) {
395 // If we have a confirmed non-local flag, use it
398 // Otherwise, use the immediate successor of rem
399 // NOTE: This is because, when getDependence is called, it will first
400 // check the immediate predecessor of what is in the cache.
401 BasicBlock::iterator RI = rem;
406 SmallPtrSet<Instruction*, 4>& set = reverseDep[rem];
407 for (SmallPtrSet<Instruction*, 4>::iterator I = set.begin(), E = set.end();
409 // Insert the new dependencies
410 // Mark it as unconfirmed as long as it is not the non-local flag
411 depGraphLocal[*I] = std::make_pair(newDep, !newDep);
413 reverseDep.erase(rem);
416 getAnalysis<AliasAnalysis>().deleteValue(rem);