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//===- LiveValues.cpp - Liveness information for LLVM IR Values. ----------===//
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// The LLVM Compiler Infrastructure
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//===----------------------------------------------------------------------===//
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// This file defines the implementation for the LLVM IR Value liveness
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/LiveValues.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/LoopInfo.h"
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FunctionPass *createLiveValuesPass() { return new LiveValues(); }
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char LiveValues::ID = 0;
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static RegisterPass<LiveValues>
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X("live-values", "Value Liveness Analysis", false, true);
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LiveValues::LiveValues() : FunctionPass(&ID) {}
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void LiveValues::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<DominatorTree>();
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AU.addRequired<LoopInfo>();
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bool LiveValues::runOnFunction(Function &F) {
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DT = &getAnalysis<DominatorTree>();
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LI = &getAnalysis<LoopInfo>();
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// This pass' values are computed lazily, so there's nothing to do here.
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void LiveValues::releaseMemory() {
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/// isUsedInBlock - Test if the given value is used in the given block.
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bool LiveValues::isUsedInBlock(const Value *V, const BasicBlock *BB) {
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return M.Used.count(BB);
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/// isLiveThroughBlock - Test if the given value is known to be
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/// live-through the given block, meaning that the block is properly
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/// dominated by the value's definition, and there exists a block
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/// reachable from it that contains a use. This uses a conservative
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/// approximation that errs on the side of returning false.
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bool LiveValues::isLiveThroughBlock(const Value *V,
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const BasicBlock *BB) {
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return M.LiveThrough.count(BB);
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/// isKilledInBlock - Test if the given value is known to be killed in
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/// the given block, meaning that the block contains a use of the value,
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/// and no blocks reachable from the block contain a use. This uses a
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/// conservative approximation that errs on the side of returning false.
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bool LiveValues::isKilledInBlock(const Value *V, const BasicBlock *BB) {
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return M.Killed.count(BB);
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/// getMemo - Retrieve an existing Memo for the given value if one
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/// is available, otherwise compute a new one.
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LiveValues::Memo &LiveValues::getMemo(const Value *V) {
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DenseMap<const Value *, Memo>::iterator I = Memos.find(V);
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/// getImmediateDominator - A handy utility for the specific DominatorTree
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/// query that we need here.
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static const BasicBlock *getImmediateDominator(const BasicBlock *BB,
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const DominatorTree *DT) {
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DomTreeNode *Node = DT->getNode(const_cast<BasicBlock *>(BB))->getIDom();
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return Node ? Node->getBlock() : 0;
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/// compute - Compute a new Memo for the given value.
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LiveValues::Memo &LiveValues::compute(const Value *V) {
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// Determine the block containing the definition.
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const BasicBlock *DefBB;
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// Instructions define values with meaningful live ranges.
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if (const Instruction *I = dyn_cast<Instruction>(V))
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DefBB = I->getParent();
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// Arguments can be analyzed as values defined in the entry block.
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else if (const Argument *A = dyn_cast<Argument>(V))
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DefBB = &A->getParent()->getEntryBlock();
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// Constants and other things aren't meaningful here, so just
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// return having computed an empty Memo so that we don't come
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// here again. The assumption here is that client code won't
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// be asking about such values very often.
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// Determine if the value is defined inside a loop. This is used
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// to track whether the value is ever used outside the loop, so
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// it'll be set to null if the value is either not defined in a
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// loop or used outside the loop in which it is defined.
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const Loop *L = LI->getLoopFor(DefBB);
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// Track whether the value is used anywhere outside of the block
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// in which it is defined.
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bool LiveOutOfDefBB = false;
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// Examine each use of the value.
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for (Value::use_const_iterator I = V->use_begin(), E = V->use_end();
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const BasicBlock *UseBB = cast<Instruction>(U)->getParent();
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// Note the block in which this use occurs.
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M.Used.insert(UseBB);
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// If the use block doesn't have successors, the value can be
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// considered killed.
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if (succ_begin(UseBB) == succ_end(UseBB))
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M.Killed.insert(UseBB);
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// Observe whether the value is used outside of the loop in which
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// it is defined. Switch to an enclosing loop if necessary.
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for (; L; L = L->getParentLoop())
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if (L->contains(UseBB))
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// Search for live-through blocks.
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const BasicBlock *BB;
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if (const PHINode *PHI = dyn_cast<PHINode>(U)) {
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// For PHI nodes, start the search at the incoming block paired with the
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// incoming value, which must be dominated by the definition.
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unsigned Num = PHI->getIncomingValueNumForOperand(I.getOperandNo());
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BB = PHI->getIncomingBlock(Num);
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// A PHI-node use means the value is live-out of it's defining block
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// even if that block also contains the only use.
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LiveOutOfDefBB = true;
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// Otherwise just start the search at the use.
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// Note if the use is outside the defining block.
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LiveOutOfDefBB |= UseBB != DefBB;
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// Climb the immediate dominator tree from the use to the definition
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// and mark all intermediate blocks as live-through.
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for (; BB != DefBB; BB = getImmediateDominator(BB, DT)) {
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if (BB != UseBB && !M.LiveThrough.insert(BB))
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// If the value is defined inside a loop and is not live outside
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// the loop, then each exit block of the loop in which the value
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// is used is a kill block.
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SmallVector<BasicBlock *, 4> ExitingBlocks;
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L->getExitingBlocks(ExitingBlocks);
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for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
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const BasicBlock *ExitingBlock = ExitingBlocks[i];
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if (M.Used.count(ExitingBlock))
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M.Killed.insert(ExitingBlock);
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// If the value was never used outside the block in which it was
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// defined, it's killed in that block.
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M.Killed.insert(DefBB);