1 //===- LoadValueNumbering.cpp - Load Value #'ing Implementation -*- C++ -*-===//
3 // This file implements a value numbering pass that value #'s load instructions.
4 // To do this, it finds lexically identical load instructions, and uses alias
5 // analysis to determine which loads are guaranteed to produce the same value.
7 // This pass builds off of another value numbering pass to implement value
8 // numbering for non-load instructions. It uses Alias Analysis so that it can
9 // disambiguate the load instructions. The more powerful these base analyses
10 // are, the more powerful the resultant analysis will be.
12 //===----------------------------------------------------------------------===//
14 #include "llvm/Analysis/LoadValueNumbering.h"
15 #include "llvm/Analysis/ValueNumbering.h"
16 #include "llvm/Analysis/AliasAnalysis.h"
17 #include "llvm/Analysis/Dominators.h"
18 #include "llvm/Pass.h"
19 #include "llvm/iMemory.h"
20 #include "llvm/BasicBlock.h"
21 #include "llvm/Support/CFG.h"
26 // FIXME: This should not be a functionpass.
27 struct LoadVN : public FunctionPass, public ValueNumbering {
29 /// Pass Implementation stuff. This doesn't do any analysis.
31 bool runOnFunction(Function &) { return false; }
33 /// getAnalysisUsage - Does not modify anything. It uses Value Numbering
34 /// and Alias Analysis.
36 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
38 /// getEqualNumberNodes - Return nodes with the same value number as the
39 /// specified Value. This fills in the argument vector with any equal
42 virtual void getEqualNumberNodes(Value *V1,
43 std::vector<Value*> &RetVals) const;
45 /// haveEqualValueNumber - Given two load instructions, determine if they
46 /// both produce the same value on every execution of the program, assuming
47 /// that their source operands always give the same value. This uses the
48 /// AliasAnalysis implementation to invalidate loads when stores or function
49 /// calls occur that could modify the value produced by the load.
51 bool haveEqualValueNumber(LoadInst *LI, LoadInst *LI2, AliasAnalysis &AA,
52 DominatorSet &DomSetInfo) const;
55 // Register this pass...
56 RegisterOpt<LoadVN> X("load-vn", "Load Value Numbering");
58 // Declare that we implement the ValueNumbering interface
59 RegisterAnalysisGroup<ValueNumbering, LoadVN> Y;
64 Pass *createLoadValueNumberingPass() { return new LoadVN(); }
67 /// getAnalysisUsage - Does not modify anything. It uses Value Numbering and
70 void LoadVN::getAnalysisUsage(AnalysisUsage &AU) const {
72 AU.addRequired<AliasAnalysis>();
73 AU.addRequired<ValueNumbering>();
74 AU.addRequired<DominatorSet>();
77 // getEqualNumberNodes - Return nodes with the same value number as the
78 // specified Value. This fills in the argument vector with any equal values.
80 void LoadVN::getEqualNumberNodes(Value *V,
81 std::vector<Value*> &RetVals) const {
83 if (LoadInst *LI = dyn_cast<LoadInst>(V)) {
84 // If we have a load instruction find all of the load instructions that use
85 // the same source operand. We implement this recursively, because there
86 // could be a load of a load of a load that are all identical. We are
87 // guaranteed that this cannot be an infinite recursion because load
88 // instructions would have to pass through a PHI node in order for there to
89 // be a cycle. The PHI node would be handled by the else case here,
90 // breaking the infinite recursion.
92 std::vector<Value*> PointerSources;
93 getEqualNumberNodes(LI->getOperand(0), PointerSources);
94 PointerSources.push_back(LI->getOperand(0));
96 Function *F = LI->getParent()->getParent();
98 // Now that we know the set of equivalent source pointers for the load
99 // instruction, look to see if there are any load candiates that are
102 std::vector<LoadInst*> CandidateLoads;
103 while (!PointerSources.empty()) {
104 Value *Source = PointerSources.back();
105 PointerSources.pop_back(); // Get a source pointer...
107 for (Value::use_iterator UI = Source->use_begin(), UE = Source->use_end();
109 if (LoadInst *Cand = dyn_cast<LoadInst>(*UI)) // Is a load of source?
110 if (Cand->getParent()->getParent() == F && // In the same function?
111 Cand != LI) // Not LI itself?
112 CandidateLoads.push_back(Cand); // Got one...
115 // Remove duplicates from the CandidateLoads list because alias analysis
116 // processing may be somewhat expensive and we don't want to do more work
119 std::sort(CandidateLoads.begin(), CandidateLoads.end());
120 CandidateLoads.erase(std::unique(CandidateLoads.begin(),
121 CandidateLoads.end()),
122 CandidateLoads.end());
124 // Get Alias Analysis...
125 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
126 DominatorSet &DomSetInfo = getAnalysis<DominatorSet>();
128 // Loop over all of the candindate loads. If they are not invalidated by
129 // stores or calls between execution of them and LI, then add them to
131 for (unsigned i = 0, e = CandidateLoads.size(); i != e; ++i)
132 if (haveEqualValueNumber(LI, CandidateLoads[i], AA, DomSetInfo))
133 RetVals.push_back(CandidateLoads[i]);
136 // Make sure passmanager doesn't try to fulfill our request with ourself!
137 assert(&getAnalysis<ValueNumbering>() != (ValueNumbering*)this &&
138 "getAnalysis() returned this!");
140 // Not a load instruction? Just chain to the base value numbering
141 // implementation to satisfy the request...
142 return getAnalysis<ValueNumbering>().getEqualNumberNodes(V, RetVals);
146 // CheckForInvalidatingInst - Return true if BB or any of the predecessors of BB
147 // (until DestBB) contain an instruction that might invalidate Ptr.
149 static bool CheckForInvalidatingInst(BasicBlock *BB, BasicBlock *DestBB,
150 Value *Ptr, AliasAnalysis &AA,
151 std::set<BasicBlock*> &VisitedSet) {
152 // Found the termination point!
153 if (BB == DestBB || VisitedSet.count(BB)) return false;
155 // Avoid infinite recursion!
156 VisitedSet.insert(BB);
158 // Can this basic block modify Ptr?
159 if (AA.canBasicBlockModify(*BB, Ptr))
162 // Check all of our predecessor blocks...
163 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
164 if (CheckForInvalidatingInst(*PI, DestBB, Ptr, AA, VisitedSet))
167 // None of our predecessor blocks contain an invalidating instruction, and we
173 /// haveEqualValueNumber - Given two load instructions, determine if they both
174 /// produce the same value on every execution of the program, assuming that
175 /// their source operands always give the same value. This uses the
176 /// AliasAnalysis implementation to invalidate loads when stores or function
177 /// calls occur that could modify the value produced by the load.
179 bool LoadVN::haveEqualValueNumber(LoadInst *L1, LoadInst *L2,
181 DominatorSet &DomSetInfo) const {
182 // Figure out which load dominates the other one. If neither dominates the
183 // other we cannot eliminate them.
185 // FIXME: This could be enhanced to some cases with a shared dominator!
187 if (DomSetInfo.dominates(L2, L1))
188 std::swap(L1, L2); // Make L1 dominate L2
189 else if (!DomSetInfo.dominates(L1, L2))
190 return false; // Neither instruction dominates the other one...
192 BasicBlock *BB1 = L1->getParent(), *BB2 = L2->getParent();
193 Value *LoadAddress = L1->getOperand(0);
195 // L1 now dominates L2. Check to see if the intervening instructions between
196 // the two loads include a store or call...
198 if (BB1 == BB2) { // In same basic block?
199 // In this degenerate case, no checking of global basic blocks has to occur
200 // just check the instructions BETWEEN L1 & L2...
202 if (AA.canInstructionRangeModify(*L1, *L2, LoadAddress))
203 return false; // Cannot eliminate load
205 // No instructions invalidate the loads, they produce the same value!
208 // Make sure that there are no store instructions between L1 and the end of
209 // it's basic block...
211 if (AA.canInstructionRangeModify(*L1, *BB1->getTerminator(), LoadAddress))
212 return false; // Cannot eliminate load
214 // Make sure that there are no store instructions between the start of BB2
215 // and the second load instruction...
217 if (AA.canInstructionRangeModify(BB2->front(), *L2, LoadAddress))
218 return false; // Cannot eliminate load
220 // Do a depth first traversal of the inverse CFG starting at L2's block,
221 // looking for L1's block. The inverse CFG is made up of the predecessor
222 // nodes of a block... so all of the edges in the graph are "backward".
224 std::set<BasicBlock*> VisitedSet;
225 for (pred_iterator PI = pred_begin(BB2), PE = pred_end(BB2); PI != PE; ++PI)
226 if (CheckForInvalidatingInst(*PI, BB1, LoadAddress, AA, VisitedSet))
229 // If we passed all of these checks then we are sure that the two loads
230 // produce the same value.