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 INITIALIZE_PASS_BEGIN(LiveValues, "live-values",
  26                 "Value Liveness Analysis", false, true)
  27 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
  28 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  29 INITIALIZE_PASS_END(LiveValues, "live-values",
  30                 "Value Liveness Analysis", false, true)
  31
  32 LiveValues::LiveValues() : FunctionPass(ID) {
  33   initializeLiveValuesPass(*PassRegistry::getPassRegistry());
  34 }
  35
  36 void LiveValues::getAnalysisUsage(AnalysisUsage &AU) const {
  37   AU.addRequired<DominatorTree>();
  38   AU.addRequired<LoopInfo>();
  39   AU.setPreservesAll();
  40 }
  41
  42 bool LiveValues::runOnFunction(Function &F) {
  43   DT = &getAnalysis<DominatorTree>();
  44   LI = &getAnalysis<LoopInfo>();
  45
  46   // This pass' values are computed lazily, so there's nothing to do here.
  47
  48   return false;
  49 }
  50
  51 void LiveValues::releaseMemory() {
  52   Memos.clear();
  53 }
  54
  55 /// isUsedInBlock - Test if the given value is used in the given block.
  56 ///
  57 bool LiveValues::isUsedInBlock(const Value *V, const BasicBlock *BB) {
  58   Memo &M = getMemo(V);
  59   return M.Used.count(BB);
  60 }
  61
  62 /// isLiveThroughBlock - Test if the given value is known to be
  63 /// live-through the given block, meaning that the block is properly
  64 /// dominated by the value's definition, and there exists a block
  65 /// reachable from it that contains a use. This uses a conservative
  66 /// approximation that errs on the side of returning false.
  67 ///
  68 bool LiveValues::isLiveThroughBlock(const Value *V,
  69                                     const BasicBlock *BB) {
  70   Memo &M = getMemo(V);
  71   return M.LiveThrough.count(BB);
  72 }
  73
  74 /// isKilledInBlock - Test if the given value is known to be killed in
  75 /// the given block, meaning that the block contains a use of the value,
  76 /// and no blocks reachable from the block contain a use. This uses a
  77 /// conservative approximation that errs on the side of returning false.
  78 ///
  79 bool LiveValues::isKilledInBlock(const Value *V, const BasicBlock *BB) {
  80   Memo &M = getMemo(V);
  81   return M.Killed.count(BB);
  82 }
  83
  84 /// getMemo - Retrieve an existing Memo for the given value if one
  85 /// is available, otherwise compute a new one.
  86 ///
  87 LiveValues::Memo &LiveValues::getMemo(const Value *V) {
  88   DenseMap<const Value *, Memo>::iterator I = Memos.find(V);
  89   if (I != Memos.end())
  90     return I->second;
  91   return compute(V);
  92 }
  93
  94 /// getImmediateDominator - A handy utility for the specific DominatorTree
  95 /// query that we need here.
  96 ///
  97 static const BasicBlock *getImmediateDominator(const BasicBlock *BB,
  98                                                const DominatorTree *DT) {
  99   DomTreeNode *Node = DT->getNode(const_cast<BasicBlock *>(BB))->getIDom();
 100   return Node ? Node->getBlock() : 0;
 101 }
 102
 103 /// compute - Compute a new Memo for the given value.
 104 ///
 105 LiveValues::Memo &LiveValues::compute(const Value *V) {
 106   Memo &M = Memos[V];
 107
 108   // Determine the block containing the definition.
 109   const BasicBlock *DefBB;
 110   // Instructions define values with meaningful live ranges.
 111   if (const Instruction *I = dyn_cast<Instruction>(V))
 112     DefBB = I->getParent();
 113   // Arguments can be analyzed as values defined in the entry block.
 114   else if (const Argument *A = dyn_cast<Argument>(V))
 115     DefBB = &A->getParent()->getEntryBlock();
 116   // Constants and other things aren't meaningful here, so just
 117   // return having computed an empty Memo so that we don't come
 118   // here again. The assumption here is that client code won't
 119   // be asking about such values very often.
 120   else
 121     return M;
 122
 123   // Determine if the value is defined inside a loop. This is used
 124   // to track whether the value is ever used outside the loop, so
 125   // it'll be set to null if the value is either not defined in a
 126   // loop or used outside the loop in which it is defined.
 127   const Loop *L = LI->getLoopFor(DefBB);
 128
 129   // Track whether the value is used anywhere outside of the block
 130   // in which it is defined.
 131   bool LiveOutOfDefBB = false;
 132
 133   // Examine each use of the value.
 134   for (Value::const_use_iterator I = V->use_begin(), E = V->use_end();
 135        I != E; ++I) {
 136     const User *U = *I;
 137     const BasicBlock *UseBB = cast<Instruction>(U)->getParent();
 138
 139     // Note the block in which this use occurs.
 140     M.Used.insert(UseBB);
 141
 142     // If the use block doesn't have successors, the value can be
 143     // considered killed.
 144     if (succ_begin(UseBB) == succ_end(UseBB))
 145       M.Killed.insert(UseBB);
 146
 147     // Observe whether the value is used outside of the loop in which
 148     // it is defined. Switch to an enclosing loop if necessary.
 149     for (; L; L = L->getParentLoop())
 150       if (L->contains(UseBB))
 151         break;
 152
 153     // Search for live-through blocks.
 154     const BasicBlock *BB;
 155     if (const PHINode *PHI = dyn_cast<PHINode>(U)) {
 156       // For PHI nodes, start the search at the incoming block paired with the
 157       // incoming value, which must be dominated by the definition.
 158       unsigned Num = PHI->getIncomingValueNumForOperand(I.getOperandNo());
 159       BB = PHI->getIncomingBlock(Num);
 160
 161       // A PHI-node use means the value is live-out of it's defining block
 162       // even if that block also contains the only use.
 163       LiveOutOfDefBB = true;
 164     } else {
 165       // Otherwise just start the search at the use.
 166       BB = UseBB;
 167
 168       // Note if the use is outside the defining block.
 169       LiveOutOfDefBB |= UseBB != DefBB;
 170     }
 171
 172     // Climb the immediate dominator tree from the use to the definition
 173     // and mark all intermediate blocks as live-through.
 174     for (; BB != DefBB; BB = getImmediateDominator(BB, DT)) {
 175       if (BB != UseBB && !M.LiveThrough.insert(BB))
 176         break;
 177     }
 178   }
 179
 180   // If the value is defined inside a loop and is not live outside
 181   // the loop, then each exit block of the loop in which the value
 182   // is used is a kill block.
 183   if (L) {
 184     SmallVector<BasicBlock *, 4> ExitingBlocks;
 185     L->getExitingBlocks(ExitingBlocks);
 186     for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
 187       const BasicBlock *ExitingBlock = ExitingBlocks[i];
 188       if (M.Used.count(ExitingBlock))
 189         M.Killed.insert(ExitingBlock);
 190     }
 191   }
 192
 193   // If the value was never used outside the block in which it was
 194   // defined, it's killed in that block.
 195   if (!LiveOutOfDefBB)
 196     M.Killed.insert(DefBB);
 197
 198   return M;
 199 }