lib/Analysis/LiveValues.cpp

   1 //===- LiveValues.cpp - Liveness information for LLVM IR Values. ----------===//
   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 file defines the implementation for the LLVM IR Value liveness
  11 // analysis pass.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #include "llvm/Analysis/LiveValues.h"
  16 #include "llvm/Analysis/Dominators.h"
  17 #include "llvm/Analysis/LoopInfo.h"
  18 using namespace llvm;
  19
  20 namespace llvm {
  21   FunctionPass *createLiveValuesPass() { return new LiveValues(); }
  22 }
  23
  24 char LiveValues::ID = 0;
  25 static RegisterPass<LiveValues>
  26 X("live-values", "Value Liveness Analysis", false, true);
  27
  28 LiveValues::LiveValues() : FunctionPass(&ID) {}
  29
  30 void LiveValues::getAnalysisUsage(AnalysisUsage &AU) const {
  31   AU.addRequired<DominatorTree>();
  32   AU.addRequired<LoopInfo>();
  33   AU.setPreservesAll();
  34 }
  35
  36 bool LiveValues::runOnFunction(Function &F) {
  37   DT = &getAnalysis<DominatorTree>();
  38   LI = &getAnalysis<LoopInfo>();
  39
  40   // This pass' values are computed lazily, so there's nothing to do here.
  41
  42   return false;
  43 }
  44
  45 void LiveValues::releaseMemory() {
  46   Memos.clear();
  47 }
  48
  49 /// isUsedInBlock - Test if the given value is used in the given block.
  50 ///
  51 bool LiveValues::isUsedInBlock(const Value *V, const BasicBlock *BB) {
  52   Memo &M = getMemo(V);
  53   return M.Used.count(BB);
  54 }
  55
  56 /// isLiveThroughBlock - Test if the given value is known to be
  57 /// live-through the given block, meaning that the block is properly
  58 /// dominated by the value's definition, and there exists a block
  59 /// reachable from it that contains a use. This uses a conservative
  60 /// approximation that errs on the side of returning false.
  61 ///
  62 bool LiveValues::isLiveThroughBlock(const Value *V,
  63                                     const BasicBlock *BB) {
  64   Memo &M = getMemo(V);
  65   return M.LiveThrough.count(BB);
  66 }
  67
  68 /// isKilledInBlock - Test if the given value is known to be killed in
  69 /// the given block, meaning that the block contains a use of the value,
  70 /// and no blocks reachable from the block contain a use. This uses a
  71 /// conservative approximation that errs on the side of returning false.
  72 ///
  73 bool LiveValues::isKilledInBlock(const Value *V, const BasicBlock *BB) {
  74   Memo &M = getMemo(V);
  75   return M.Killed.count(BB);
  76 }
  77
  78 /// getMemo - Retrieve an existing Memo for the given value if one
  79 /// is available, otherwise compute a new one.
  80 ///
  81 LiveValues::Memo &LiveValues::getMemo(const Value *V) {
  82   DenseMap<const Value *, Memo>::iterator I = Memos.find(V);
  83   if (I != Memos.end())
  84     return I->second;
  85   return compute(V);
  86 }
  87
  88 /// getImmediateDominator - A handy utility for the specific DominatorTree
  89 /// query that we need here.
  90 ///
  91 static const BasicBlock *getImmediateDominator(const BasicBlock *BB,
  92                                                const DominatorTree *DT) {
  93   DomTreeNode *Node = DT->getNode(const_cast<BasicBlock *>(BB))->getIDom();
  94   return Node ? Node->getBlock() : 0;
  95 }
  96
  97 /// compute - Compute a new Memo for the given value.
  98 ///
  99 LiveValues::Memo &LiveValues::compute(const Value *V) {
 100   Memo &M = Memos[V];
 101
 102   // Determine the block containing the definition.
 103   const BasicBlock *DefBB;
 104   // Instructions define values with meaningful live ranges.
 105   if (const Instruction *I = dyn_cast<Instruction>(V))
 106     DefBB = I->getParent();
 107   // Arguments can be analyzed as values defined in the entry block.
 108   else if (const Argument *A = dyn_cast<Argument>(V))
 109     DefBB = &A->getParent()->getEntryBlock();
 110   // Constants and other things aren't meaningful here, so just
 111   // return having computed an empty Memo so that we don't come
 112   // here again. The assumption here is that client code won't
 113   // be asking about such values very often.
 114   else
 115     return M;
 116
 117   // Determine if the value is defined inside a loop. This is used
 118   // to track whether the value is ever used outside the loop, so
 119   // it'll be set to null if the value is either not defined in a
 120   // loop or used outside the loop in which it is defined.
 121   const Loop *L = LI->getLoopFor(DefBB);
 122
 123   // Track whether the value is used anywhere outside of the block
 124   // in which it is defined.
 125   bool LiveOutOfDefBB = false;
 126
 127   // Examine each use of the value.
 128   for (Value::use_const_iterator I = V->use_begin(), E = V->use_end();
 129        I != E; ++I) {
 130     const User *U = *I;
 131     const BasicBlock *UseBB = cast<Instruction>(U)->getParent();
 132
 133     // Note the block in which this use occurs.
 134     M.Used.insert(UseBB);
 135
 136     // If the use block doesn't have successors, the value can be
 137     // considered killed.
 138     if (succ_begin(UseBB) == succ_end(UseBB))
 139       M.Killed.insert(UseBB);
 140
 141     // Observe whether the value is used outside of the loop in which
 142     // it is defined. Switch to an enclosing loop if necessary.
 143     for (; L; L = L->getParentLoop())
 144       if (L->contains(UseBB))
 145         break;
 146
 147     // Search for live-through blocks.
 148     const BasicBlock *BB;
 149     if (const PHINode *PHI = dyn_cast<PHINode>(U)) {
 150       // For PHI nodes, start the search at the incoming block paired with the
 151       // incoming value, which must be dominated by the definition.
 152       unsigned Num = PHI->getIncomingValueNumForOperand(I.getOperandNo());
 153       BB = PHI->getIncomingBlock(Num);
 154
 155       // A PHI-node use means the value is live-out of it's defining block
 156       // even if that block also contains the only use.
 157       LiveOutOfDefBB = true;
 158     } else {
 159       // Otherwise just start the search at the use.
 160       BB = UseBB;
 161
 162       // Note if the use is outside the defining block.
 163       LiveOutOfDefBB |= UseBB != DefBB;
 164     }
 165
 166     // Climb the immediate dominator tree from the use to the definition
 167     // and mark all intermediate blocks as live-through.
 168     for (; BB != DefBB; BB = getImmediateDominator(BB, DT)) {
 169       if (BB != UseBB && !M.LiveThrough.insert(BB))
 170         break;
 171     }
 172   }
 173
 174   // If the value is defined inside a loop and is not live outside
 175   // the loop, then each exit block of the loop in which the value
 176   // is used is a kill block.
 177   if (L) {
 178     SmallVector<BasicBlock *, 4> ExitingBlocks;
 179     L->getExitingBlocks(ExitingBlocks);
 180     for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
 181       const BasicBlock *ExitingBlock = ExitingBlocks[i];
 182       if (M.Used.count(ExitingBlock))
 183         M.Killed.insert(ExitingBlock);
 184     }
 185   }
 186
 187   // If the value was never used outside the the block in which it was
 188   // defined, it's killed in that block.
 189   if (!LiveOutOfDefBB)
 190     M.Killed.insert(DefBB);
 191
 192   return M;
 193 }