1 //===- LoadValueNumbering.cpp - Load Value #'ing Implementation -*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements a value numbering pass that value #'s load instructions.
11 // To do this, it finds lexically identical load instructions, and uses alias
12 // analysis to determine which loads are guaranteed to produce the same value.
14 // This pass builds off of another value numbering pass to implement value
15 // numbering for non-load instructions. It uses Alias Analysis so that it can
16 // disambiguate the load instructions. The more powerful these base analyses
17 // are, the more powerful the resultant analysis will be.
19 //===----------------------------------------------------------------------===//
21 #include "llvm/Analysis/LoadValueNumbering.h"
22 #include "llvm/Analysis/ValueNumbering.h"
23 #include "llvm/Analysis/AliasAnalysis.h"
24 #include "llvm/Analysis/Dominators.h"
25 #include "llvm/Target/TargetData.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Type.h"
28 #include "llvm/iMemory.h"
29 #include "llvm/BasicBlock.h"
30 #include "llvm/Support/CFG.h"
36 // FIXME: This should not be a FunctionPass.
37 struct LoadVN : public FunctionPass, public ValueNumbering {
39 /// Pass Implementation stuff. This doesn't do any analysis.
41 bool runOnFunction(Function &) { return false; }
43 /// getAnalysisUsage - Does not modify anything. It uses Value Numbering
44 /// and Alias Analysis.
46 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
48 /// getEqualNumberNodes - Return nodes with the same value number as the
49 /// specified Value. This fills in the argument vector with any equal
52 virtual void getEqualNumberNodes(Value *V1,
53 std::vector<Value*> &RetVals) const;
55 /// haveEqualValueNumber - Given two load instructions, determine if they
56 /// both produce the same value on every execution of the program, assuming
57 /// that their source operands always give the same value. This uses the
58 /// AliasAnalysis implementation to invalidate loads when stores or function
59 /// calls occur that could modify the value produced by the load.
61 bool haveEqualValueNumber(LoadInst *LI, LoadInst *LI2, AliasAnalysis &AA,
62 DominatorSet &DomSetInfo) const;
63 bool haveEqualValueNumber(LoadInst *LI, StoreInst *SI, AliasAnalysis &AA,
64 DominatorSet &DomSetInfo) const;
67 // Register this pass...
68 RegisterOpt<LoadVN> X("load-vn", "Load Value Numbering");
70 // Declare that we implement the ValueNumbering interface
71 RegisterAnalysisGroup<ValueNumbering, LoadVN> Y;
74 Pass *llvm::createLoadValueNumberingPass() { return new LoadVN(); }
77 /// getAnalysisUsage - Does not modify anything. It uses Value Numbering and
80 void LoadVN::getAnalysisUsage(AnalysisUsage &AU) const {
82 AU.addRequired<AliasAnalysis>();
83 AU.addRequired<ValueNumbering>();
84 AU.addRequired<DominatorSet>();
85 AU.addRequired<TargetData>();
88 // getEqualNumberNodes - Return nodes with the same value number as the
89 // specified Value. This fills in the argument vector with any equal values.
91 void LoadVN::getEqualNumberNodes(Value *V,
92 std::vector<Value*> &RetVals) const {
93 // If the alias analysis has any must alias information to share with us, we
94 // can definitely use it.
95 if (isa<PointerType>(V->getType()))
96 getAnalysis<AliasAnalysis>().getMustAliases(V, RetVals);
98 if (LoadInst *LI = dyn_cast<LoadInst>(V)) {
99 // Volatile loads cannot be replaced with the value of other loads.
100 if (LI->isVolatile())
101 return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
103 // If we have a load instruction, find all of the load and store
104 // instructions that use the same source operand. We implement this
105 // recursively, because there could be a load of a load of a load that are
106 // all identical. We are guaranteed that this cannot be an infinite
107 // recursion because load instructions would have to pass through a PHI node
108 // in order for there to be a cycle. The PHI node would be handled by the
109 // else case here, breaking the infinite recursion.
111 std::vector<Value*> PointerSources;
112 getEqualNumberNodes(LI->getOperand(0), PointerSources);
113 PointerSources.push_back(LI->getOperand(0));
115 Function *F = LI->getParent()->getParent();
117 // Now that we know the set of equivalent source pointers for the load
118 // instruction, look to see if there are any load or store candidates that
121 std::vector<LoadInst*> CandidateLoads;
122 std::vector<StoreInst*> CandidateStores;
124 while (!PointerSources.empty()) {
125 Value *Source = PointerSources.back();
126 PointerSources.pop_back(); // Get a source pointer...
128 for (Value::use_iterator UI = Source->use_begin(), UE = Source->use_end();
130 if (LoadInst *Cand = dyn_cast<LoadInst>(*UI)) {// Is a load of source?
131 if (Cand->getParent()->getParent() == F && // In the same function?
132 Cand != LI && !Cand->isVolatile()) // Not LI itself?
133 CandidateLoads.push_back(Cand); // Got one...
134 } else if (StoreInst *Cand = dyn_cast<StoreInst>(*UI)) {
135 if (Cand->getParent()->getParent() == F && !Cand->isVolatile() &&
136 Cand->getOperand(1) == Source) // It's a store THROUGH the ptr...
137 CandidateStores.push_back(Cand);
141 // Remove duplicates from the CandidateLoads list because alias analysis
142 // processing may be somewhat expensive and we don't want to do more work
145 unsigned OldSize = CandidateLoads.size();
146 std::sort(CandidateLoads.begin(), CandidateLoads.end());
147 CandidateLoads.erase(std::unique(CandidateLoads.begin(),
148 CandidateLoads.end()),
149 CandidateLoads.end());
150 // FIXME: REMOVE THIS SORTING AND UNIQUING IF IT CAN'T HAPPEN
151 assert(CandidateLoads.size() == OldSize && "Shrunk the candloads list?");
153 // Get Alias Analysis...
154 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
155 DominatorSet &DomSetInfo = getAnalysis<DominatorSet>();
157 // Loop over all of the candidate loads. If they are not invalidated by
158 // stores or calls between execution of them and LI, then add them to
160 for (unsigned i = 0, e = CandidateLoads.size(); i != e; ++i)
161 if (haveEqualValueNumber(LI, CandidateLoads[i], AA, DomSetInfo))
162 RetVals.push_back(CandidateLoads[i]);
163 for (unsigned i = 0, e = CandidateStores.size(); i != e; ++i)
164 if (haveEqualValueNumber(LI, CandidateStores[i], AA, DomSetInfo))
165 RetVals.push_back(CandidateStores[i]->getOperand(0));
168 assert(&getAnalysis<ValueNumbering>() != (ValueNumbering*)this &&
169 "getAnalysis() returned this!");
171 // Not a load instruction? Just chain to the base value numbering
172 // implementation to satisfy the request...
173 return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
177 // CheckForInvalidatingInst - Return true if BB or any of the predecessors of BB
178 // (until DestBB) contain an instruction that might invalidate Ptr.
180 static bool CheckForInvalidatingInst(BasicBlock *BB, BasicBlock *DestBB,
181 Value *Ptr, unsigned Size,
183 std::set<BasicBlock*> &VisitedSet) {
184 // Found the termination point!
185 if (BB == DestBB || VisitedSet.count(BB)) return false;
187 // Avoid infinite recursion!
188 VisitedSet.insert(BB);
190 // Can this basic block modify Ptr?
191 if (AA.canBasicBlockModify(*BB, Ptr, Size))
194 // Check all of our predecessor blocks...
195 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
196 if (CheckForInvalidatingInst(*PI, DestBB, Ptr, Size, AA, VisitedSet))
199 // None of our predecessor blocks contain an invalidating instruction, and we
205 /// haveEqualValueNumber - Given two load instructions, determine if they both
206 /// produce the same value on every execution of the program, assuming that
207 /// their source operands always give the same value. This uses the
208 /// AliasAnalysis implementation to invalidate loads when stores or function
209 /// calls occur that could modify the value produced by the load.
211 bool LoadVN::haveEqualValueNumber(LoadInst *L1, LoadInst *L2,
213 DominatorSet &DomSetInfo) const {
214 assert(L1 != L2 && "haveEqualValueNumber assumes differing loads!");
215 assert(L1->getType() == L2->getType() &&
216 "How could the same source pointer return different types?");
217 Value *LoadAddress = L1->getOperand(0);
219 // Find out how many bytes of memory are loaded by the load instruction...
220 unsigned LoadSize = getAnalysis<TargetData>().getTypeSize(L1->getType());
222 // If the two loads are in the same basic block, just do a local analysis.
223 if (L1->getParent() == L2->getParent()) {
224 // It can be _very_ expensive to determine which instruction occurs first in
225 // the basic block if the block is large (see PR209). For this reason,
226 // instead of figuring out which block is first, then scanning all of the
227 // instructions, we scan the instructions both ways from L1 until we find
228 // L2. Along the way if we find a potentially modifying instruction, we
229 // kill the search. This helps in cases where we have large blocks the have
230 // potentially modifying instructions in them which stop the search.
232 BasicBlock *BB = L1->getParent();
233 BasicBlock::iterator UpIt = L1, DownIt = L1; ++DownIt;
234 bool NoModifiesUp = true, NoModifiesDown = true;
236 // Scan up and down looking for L2, a modifying instruction, or the end of a
238 while (UpIt != BB->begin() && DownIt != BB->end()) {
242 return NoModifiesUp; // No instructions invalidate the loads!
245 !(AA.getModRefInfo(UpIt, LoadAddress, LoadSize) & AliasAnalysis::Mod);
248 return NoModifiesDown;
251 !(AA.getModRefInfo(DownIt, LoadAddress, LoadSize)
252 & AliasAnalysis::Mod);
256 // If we got here, we ran into one end of the basic block or the other.
257 if (UpIt != BB->begin()) {
258 // If we know that the upward scan found a modifier, return false.
259 if (!NoModifiesUp) return false;
261 // Otherwise, continue the scan looking for a modifier or L2.
262 for (--UpIt; &*UpIt != L2; --UpIt)
263 if (AA.getModRefInfo(UpIt, LoadAddress, LoadSize) & AliasAnalysis::Mod)
267 // If we know that the downward scan found a modifier, return false.
268 assert(DownIt != BB->end() && "Didn't find instructions??");
269 if (!NoModifiesDown) return false;
271 // Otherwise, continue the scan looking for a modifier or L2.
272 for (; &*DownIt != L2; ++DownIt) {
273 if (AA.getModRefInfo(DownIt, LoadAddress, LoadSize) &AliasAnalysis::Mod)
279 // Figure out which load dominates the other one. If neither dominates the
280 // other we cannot eliminate them.
282 // FIXME: This could be enhanced greatly!
284 if (DomSetInfo.dominates(L2, L1))
285 std::swap(L1, L2); // Make L1 dominate L2
286 else if (!DomSetInfo.dominates(L1, L2))
287 return false; // Neither instruction dominates the other one...
289 BasicBlock *BB1 = L1->getParent(), *BB2 = L2->getParent();
291 // L1 now dominates L2. Check to see if the intervening instructions
292 // between the two loads might modify the loaded location.
294 // Make sure that there are no modifying instructions between L1 and the end
295 // of its basic block.
297 if (AA.canInstructionRangeModify(*L1, *BB1->getTerminator(), LoadAddress,
299 return false; // Cannot eliminate load
301 // Make sure that there are no modifying instructions between the start of
302 // BB2 and the second load instruction.
304 if (AA.canInstructionRangeModify(BB2->front(), *L2, LoadAddress, LoadSize))
305 return false; // Cannot eliminate load
307 // Do a depth first traversal of the inverse CFG starting at L2's block,
308 // looking for L1's block. The inverse CFG is made up of the predecessor
309 // nodes of a block... so all of the edges in the graph are "backward".
311 std::set<BasicBlock*> VisitedSet;
312 for (pred_iterator PI = pred_begin(BB2), PE = pred_end(BB2); PI != PE; ++PI)
313 if (CheckForInvalidatingInst(*PI, BB1, LoadAddress, LoadSize, AA,
317 // If we passed all of these checks then we are sure that the two loads
318 // produce the same value.
324 /// haveEqualValueNumber - Given a load instruction and a store instruction,
325 /// determine if the stored value reaches the loaded value unambiguously on
326 /// every execution of the program. This uses the AliasAnalysis implementation
327 /// to invalidate the stored value when stores or function calls occur that
328 /// could modify the value produced by the load.
330 bool LoadVN::haveEqualValueNumber(LoadInst *Load, StoreInst *Store,
332 DominatorSet &DomSetInfo) const {
333 // If the store does not dominate the load, we cannot do anything...
334 if (!DomSetInfo.dominates(Store, Load))
337 BasicBlock *BB1 = Store->getParent(), *BB2 = Load->getParent();
338 Value *LoadAddress = Load->getOperand(0);
340 assert(LoadAddress->getType() == Store->getOperand(1)->getType() &&
341 "How could the same source pointer return different types?");
343 // Find out how many bytes of memory are loaded by the load instruction...
344 unsigned LoadSize = getAnalysis<TargetData>().getTypeSize(Load->getType());
346 // Compute a basic block iterator pointing to the instruction after the store.
347 BasicBlock::iterator StoreIt = Store; ++StoreIt;
349 // Check to see if the intervening instructions between the two store and load
350 // include a store or call...
352 if (BB1 == BB2) { // In same basic block?
353 // In this degenerate case, no checking of global basic blocks has to occur
354 // just check the instructions BETWEEN Store & Load...
356 if (AA.canInstructionRangeModify(*StoreIt, *Load, LoadAddress, LoadSize))
357 return false; // Cannot eliminate load
359 // No instructions invalidate the stored value, they produce the same value!
362 // Make sure that there are no store instructions between the Store and the
363 // end of its basic block...
365 if (AA.canInstructionRangeModify(*StoreIt, *BB1->getTerminator(),
366 LoadAddress, LoadSize))
367 return false; // Cannot eliminate load
369 // Make sure that there are no store instructions between the start of BB2
370 // and the second load instruction...
372 if (AA.canInstructionRangeModify(BB2->front(), *Load, LoadAddress,LoadSize))
373 return false; // Cannot eliminate load
375 // Do a depth first traversal of the inverse CFG starting at L2's block,
376 // looking for L1's block. The inverse CFG is made up of the predecessor
377 // nodes of a block... so all of the edges in the graph are "backward".
379 std::set<BasicBlock*> VisitedSet;
380 for (pred_iterator PI = pred_begin(BB2), PE = pred_end(BB2); PI != PE; ++PI)
381 if (CheckForInvalidatingInst(*PI, BB1, LoadAddress, LoadSize, AA,
385 // If we passed all of these checks then we are sure that the two loads
386 // produce the same value.