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//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
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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 pass transforms loops by placing phi nodes at the end of the loops for
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// all values that are live across the loop boundary. For example, it turns
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// the left into the right code:
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// for (...) for (...)
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// X3 = phi(X1, X2) X3 = phi(X1, X2)
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// ... = X3 + 4 X4 = phi(X3)
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// This is still valid LLVM; the extra phi nodes are purely redundant, and will
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// be trivially eliminated by InstCombine. The major benefit of this
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// transformation is that it makes many other loop optimizations, such as
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// LoopUnswitching, simpler.
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "lcssa"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Constants.h"
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#include "llvm/Pass.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Transforms/Utils/SSAUpdater.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/Support/PredIteratorCache.h"
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STATISTIC(NumLCSSA, "Number of live out of a loop variables");
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struct LCSSA : public LoopPass {
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static char ID; // Pass identification, replacement for typeid
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LCSSA() : LoopPass(&ID) {}
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// Cached analysis information for the current function.
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std::vector<BasicBlock*> LoopBlocks;
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PredIteratorCache PredCache;
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virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
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/// This transformation requires natural loop information & requires that
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/// loop preheaders be inserted into the CFG. It maintains both of these,
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/// as well as the CFG. It also requires dominator information.
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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// LCSSA doesn't actually require LoopSimplify, but the PassManager
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// doesn't know how to schedule LoopSimplify by itself.
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AU.addRequiredID(LoopSimplifyID);
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AU.addPreservedID(LoopSimplifyID);
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AU.addRequiredTransitive<LoopInfo>();
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AU.addPreserved<LoopInfo>();
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AU.addRequiredTransitive<DominatorTree>();
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AU.addPreserved<ScalarEvolution>();
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AU.addPreserved<DominatorTree>();
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// Request DominanceFrontier now, even though LCSSA does
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// not use it. This allows Pass Manager to schedule Dominance
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// Frontier early enough such that one LPPassManager can handle
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// multiple loop transformation passes.
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AU.addRequired<DominanceFrontier>();
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AU.addPreserved<DominanceFrontier>();
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bool ProcessInstruction(Instruction *Inst,
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const SmallVectorImpl<BasicBlock*> &ExitBlocks);
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/// verifyAnalysis() - Verify loop nest.
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virtual void verifyAnalysis() const {
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// Check the special guarantees that LCSSA makes.
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assert(L->isLCSSAForm() && "LCSSA form not preserved!");
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/// inLoop - returns true if the given block is within the current loop
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bool inLoop(BasicBlock *B) const {
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return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
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static RegisterPass<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
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Pass *llvm::createLCSSAPass() { return new LCSSA(); }
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const PassInfo *const llvm::LCSSAID = &X;
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/// BlockDominatesAnExit - Return true if the specified block dominates at least
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/// one of the blocks in the specified list.
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static bool BlockDominatesAnExit(BasicBlock *BB,
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const SmallVectorImpl<BasicBlock*> &ExitBlocks,
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DomTreeNode *DomNode = DT->getNode(BB);
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
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if (DT->dominates(DomNode, DT->getNode(ExitBlocks[i])))
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/// runOnFunction - Process all loops in the function, inner-most out.
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bool LCSSA::runOnLoop(Loop *TheLoop, LPPassManager &LPM) {
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DT = &getAnalysis<DominatorTree>();
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// Get the set of exiting blocks.
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SmallVector<BasicBlock*, 8> ExitBlocks;
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L->getExitBlocks(ExitBlocks);
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if (ExitBlocks.empty())
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// Speed up queries by creating a sorted vector of blocks.
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LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
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array_pod_sort(LoopBlocks.begin(), LoopBlocks.end());
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// Look at all the instructions in the loop, checking to see if they have uses
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// outside the loop. If so, rewrite those uses.
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bool MadeChange = false;
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for (Loop::block_iterator BBI = L->block_begin(), E = L->block_end();
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BasicBlock *BB = *BBI;
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// For large loops, avoid use-scanning by using dominance information: In
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// particular, if a block does not dominate any of the loop exits, then none
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// of the values defined in the block could be used outside the loop.
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if (!BlockDominatesAnExit(BB, ExitBlocks, DT))
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for (BasicBlock::iterator I = BB->begin(), E = BB->end();
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// Reject two common cases fast: instructions with no uses (like stores)
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// and instructions with one use that is in the same block as this.
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if (I->use_empty() ||
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(I->hasOneUse() && I->use_back()->getParent() == BB &&
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!isa<PHINode>(I->use_back())))
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MadeChange |= ProcessInstruction(I, ExitBlocks);
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assert(L->isLCSSAForm());
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/// isExitBlock - Return true if the specified block is in the list.
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static bool isExitBlock(BasicBlock *BB,
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const SmallVectorImpl<BasicBlock*> &ExitBlocks) {
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
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if (ExitBlocks[i] == BB)
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/// ProcessInstruction - Given an instruction in the loop, check to see if it
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/// has any uses that are outside the current loop. If so, insert LCSSA PHI
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/// nodes and rewrite the uses.
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bool LCSSA::ProcessInstruction(Instruction *Inst,
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const SmallVectorImpl<BasicBlock*> &ExitBlocks) {
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SmallVector<Use*, 16> UsesToRewrite;
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BasicBlock *InstBB = Inst->getParent();
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for (Value::use_iterator UI = Inst->use_begin(), E = Inst->use_end();
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BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
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if (PHINode *PN = dyn_cast<PHINode>(*UI))
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UserBB = PN->getIncomingBlock(UI);
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if (InstBB != UserBB && !inLoop(UserBB))
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UsesToRewrite.push_back(&UI.getUse());
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// If there are no uses outside the loop, exit with no change.
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if (UsesToRewrite.empty()) return false;
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++NumLCSSA; // We are applying the transformation
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// Invoke instructions are special in that their result value is not available
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// along their unwind edge. The code below tests to see whether DomBB dominates
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// the value, so adjust DomBB to the normal destination block, which is
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// effectively where the value is first usable.
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BasicBlock *DomBB = Inst->getParent();
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if (InvokeInst *Inv = dyn_cast<InvokeInst>(Inst))
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DomBB = Inv->getNormalDest();
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DomTreeNode *DomNode = DT->getNode(DomBB);
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SSAUpdater SSAUpdate;
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SSAUpdate.Initialize(Inst);
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// Insert the LCSSA phi's into all of the exit blocks dominated by the
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// value, and add them to the Phi's map.
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for (SmallVectorImpl<BasicBlock*>::const_iterator BBI = ExitBlocks.begin(),
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BBE = ExitBlocks.end(); BBI != BBE; ++BBI) {
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BasicBlock *ExitBB = *BBI;
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if (!DT->dominates(DomNode, DT->getNode(ExitBB))) continue;
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// If we already inserted something for this BB, don't reprocess it.
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if (SSAUpdate.HasValueForBlock(ExitBB)) continue;
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PHINode *PN = PHINode::Create(Inst->getType(), Inst->getName()+".lcssa",
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PN->reserveOperandSpace(PredCache.GetNumPreds(ExitBB));
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// Add inputs from inside the loop for this PHI.
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for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) {
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PN->addIncoming(Inst, *PI);
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// If the exit block has a predecessor not within the loop, arrange for
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// the incoming value use corresponding to that predecessor to be
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// rewritten in terms of a different LCSSA PHI.
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UsesToRewrite.push_back(
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PN->getOperandNumForIncomingValue(PN->getNumIncomingValues()-1)));
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// Remember that this phi makes the value alive in this block.
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SSAUpdate.AddAvailableValue(ExitBB, PN);
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// Rewrite all uses outside the loop in terms of the new PHIs we just
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for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
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// If this use is in an exit block, rewrite to use the newly inserted PHI.
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// This is required for correctness because SSAUpdate doesn't handle uses in
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// the same block. It assumes the PHI we inserted is at the end of the
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Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser());
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BasicBlock *UserBB = User->getParent();
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if (PHINode *PN = dyn_cast<PHINode>(User))
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UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);
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if (isa<PHINode>(UserBB->begin()) &&
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isExitBlock(UserBB, ExitBlocks)) {
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UsesToRewrite[i]->set(UserBB->begin());
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// Otherwise, do full PHI insertion.
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SSAUpdate.RewriteUse(*UsesToRewrite[i]);