| //===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This pass transforms loops by placing phi nodes at the end of the loops for |
| // all values that are live across the loop boundary. For example, it turns |
| // the left into the right code: |
| // |
| // for (...) for (...) |
| // if (c) if (c) |
| // X1 = ... X1 = ... |
| // else else |
| // X2 = ... X2 = ... |
| // X3 = phi(X1, X2) X3 = phi(X1, X2) |
| // ... = X3 + 4 X4 = phi(X3) |
| // ... = X4 + 4 |
| // |
| // This is still valid LLVM; the extra phi nodes are purely redundant, and will |
| // be trivially eliminated by InstCombine. The major benefit of this |
| // transformation is that it makes many other loop optimizations, such as |
| // LoopUnswitching, simpler. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Transforms/Scalar.h" |
| #include "llvm/ADT/STLExtras.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Analysis/LoopPass.h" |
| #include "llvm/Analysis/ScalarEvolution.h" |
| #include "llvm/IR/Constants.h" |
| #include "llvm/IR/Dominators.h" |
| #include "llvm/IR/Function.h" |
| #include "llvm/IR/Instructions.h" |
| #include "llvm/IR/PredIteratorCache.h" |
| #include "llvm/Pass.h" |
| #include "llvm/Transforms/Utils/LoopUtils.h" |
| #include "llvm/Transforms/Utils/SSAUpdater.h" |
| using namespace llvm; |
| |
| #define DEBUG_TYPE "lcssa" |
| |
| STATISTIC(NumLCSSA, "Number of live out of a loop variables"); |
| |
| /// Return true if the specified block is in the list. |
| static bool isExitBlock(BasicBlock *BB, |
| const SmallVectorImpl<BasicBlock *> &ExitBlocks) { |
| for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) |
| if (ExitBlocks[i] == BB) |
| return true; |
| return false; |
| } |
| |
| /// Given an instruction in the loop, check to see if it has any uses that are |
| /// outside the current loop. If so, insert LCSSA PHI nodes and rewrite the |
| /// uses. |
| static bool processInstruction(Loop &L, Instruction &Inst, DominatorTree &DT, |
| const SmallVectorImpl<BasicBlock *> &ExitBlocks, |
| PredIteratorCache &PredCache, LoopInfo *LI) { |
| SmallVector<Use *, 16> UsesToRewrite; |
| |
| BasicBlock *InstBB = Inst.getParent(); |
| |
| for (Use &U : Inst.uses()) { |
| Instruction *User = cast<Instruction>(U.getUser()); |
| BasicBlock *UserBB = User->getParent(); |
| if (PHINode *PN = dyn_cast<PHINode>(User)) |
| UserBB = PN->getIncomingBlock(U); |
| |
| if (InstBB != UserBB && !L.contains(UserBB)) |
| UsesToRewrite.push_back(&U); |
| } |
| |
| // If there are no uses outside the loop, exit with no change. |
| if (UsesToRewrite.empty()) |
| return false; |
| |
| ++NumLCSSA; // We are applying the transformation |
| |
| // Invoke instructions are special in that their result value is not available |
| // along their unwind edge. The code below tests to see whether DomBB |
| // dominates |
| // the value, so adjust DomBB to the normal destination block, which is |
| // effectively where the value is first usable. |
| BasicBlock *DomBB = Inst.getParent(); |
| if (InvokeInst *Inv = dyn_cast<InvokeInst>(&Inst)) |
| DomBB = Inv->getNormalDest(); |
| |
| DomTreeNode *DomNode = DT.getNode(DomBB); |
| |
| SmallVector<PHINode *, 16> AddedPHIs; |
| SmallVector<PHINode *, 8> PostProcessPHIs; |
| |
| SSAUpdater SSAUpdate; |
| SSAUpdate.Initialize(Inst.getType(), Inst.getName()); |
| |
| // Insert the LCSSA phi's into all of the exit blocks dominated by the |
| // value, and add them to the Phi's map. |
| for (SmallVectorImpl<BasicBlock *>::const_iterator BBI = ExitBlocks.begin(), |
| BBE = ExitBlocks.end(); |
| BBI != BBE; ++BBI) { |
| BasicBlock *ExitBB = *BBI; |
| if (!DT.dominates(DomNode, DT.getNode(ExitBB))) |
| continue; |
| |
| // If we already inserted something for this BB, don't reprocess it. |
| if (SSAUpdate.HasValueForBlock(ExitBB)) |
| continue; |
| |
| PHINode *PN = PHINode::Create(Inst.getType(), PredCache.size(ExitBB), |
| Inst.getName() + ".lcssa", ExitBB->begin()); |
| |
| // Add inputs from inside the loop for this PHI. |
| for (BasicBlock *Pred : PredCache.get(ExitBB)) { |
| PN->addIncoming(&Inst, Pred); |
| |
| // If the exit block has a predecessor not within the loop, arrange for |
| // the incoming value use corresponding to that predecessor to be |
| // rewritten in terms of a different LCSSA PHI. |
| if (!L.contains(Pred)) |
| UsesToRewrite.push_back( |
| &PN->getOperandUse(PN->getOperandNumForIncomingValue( |
| PN->getNumIncomingValues() - 1))); |
| } |
| |
| AddedPHIs.push_back(PN); |
| |
| // Remember that this phi makes the value alive in this block. |
| SSAUpdate.AddAvailableValue(ExitBB, PN); |
| |
| // LoopSimplify might fail to simplify some loops (e.g. when indirect |
| // branches are involved). In such situations, it might happen that an exit |
| // for Loop L1 is the header of a disjoint Loop L2. Thus, when we create |
| // PHIs in such an exit block, we are also inserting PHIs into L2's header. |
| // This could break LCSSA form for L2 because these inserted PHIs can also |
| // have uses outside of L2. Remember all PHIs in such situation as to |
| // revisit than later on. FIXME: Remove this if indirectbr support into |
| // LoopSimplify gets improved. |
| if (auto *OtherLoop = LI->getLoopFor(ExitBB)) |
| if (!L.contains(OtherLoop)) |
| PostProcessPHIs.push_back(PN); |
| } |
| |
| // Rewrite all uses outside the loop in terms of the new PHIs we just |
| // inserted. |
| for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) { |
| // If this use is in an exit block, rewrite to use the newly inserted PHI. |
| // This is required for correctness because SSAUpdate doesn't handle uses in |
| // the same block. It assumes the PHI we inserted is at the end of the |
| // block. |
| Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser()); |
| BasicBlock *UserBB = User->getParent(); |
| if (PHINode *PN = dyn_cast<PHINode>(User)) |
| UserBB = PN->getIncomingBlock(*UsesToRewrite[i]); |
| |
| if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) { |
| // Tell the VHs that the uses changed. This updates SCEV's caches. |
| if (UsesToRewrite[i]->get()->hasValueHandle()) |
| ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin()); |
| UsesToRewrite[i]->set(UserBB->begin()); |
| continue; |
| } |
| |
| // Otherwise, do full PHI insertion. |
| SSAUpdate.RewriteUse(*UsesToRewrite[i]); |
| } |
| |
| // Post process PHI instructions that were inserted into another disjoint loop |
| // and update their exits properly. |
| for (auto *I : PostProcessPHIs) { |
| if (I->use_empty()) |
| continue; |
| |
| BasicBlock *PHIBB = I->getParent(); |
| Loop *OtherLoop = LI->getLoopFor(PHIBB); |
| SmallVector<BasicBlock *, 8> EBs; |
| OtherLoop->getExitBlocks(EBs); |
| if (EBs.empty()) |
| continue; |
| |
| // Recurse and re-process each PHI instruction. FIXME: we should really |
| // convert this entire thing to a worklist approach where we process a |
| // vector of instructions... |
| processInstruction(*OtherLoop, *I, DT, EBs, PredCache, LI); |
| } |
| |
| // Remove PHI nodes that did not have any uses rewritten. |
| for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) { |
| if (AddedPHIs[i]->use_empty()) |
| AddedPHIs[i]->eraseFromParent(); |
| } |
| |
| return true; |
| } |
| |
| /// Return true if the specified block dominates at least |
| /// one of the blocks in the specified list. |
| static bool |
| blockDominatesAnExit(BasicBlock *BB, |
| DominatorTree &DT, |
| const SmallVectorImpl<BasicBlock *> &ExitBlocks) { |
| DomTreeNode *DomNode = DT.getNode(BB); |
| for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) |
| if (DT.dominates(DomNode, DT.getNode(ExitBlocks[i]))) |
| return true; |
| |
| return false; |
| } |
| |
| bool llvm::formLCSSA(Loop &L, DominatorTree &DT, LoopInfo *LI, |
| ScalarEvolution *SE) { |
| bool Changed = false; |
| |
| // Get the set of exiting blocks. |
| SmallVector<BasicBlock *, 8> ExitBlocks; |
| L.getExitBlocks(ExitBlocks); |
| |
| if (ExitBlocks.empty()) |
| return false; |
| |
| PredIteratorCache PredCache; |
| |
| // Look at all the instructions in the loop, checking to see if they have uses |
| // outside the loop. If so, rewrite those uses. |
| for (Loop::block_iterator BBI = L.block_begin(), BBE = L.block_end(); |
| BBI != BBE; ++BBI) { |
| BasicBlock *BB = *BBI; |
| |
| // For large loops, avoid use-scanning by using dominance information: In |
| // particular, if a block does not dominate any of the loop exits, then none |
| // of the values defined in the block could be used outside the loop. |
| if (!blockDominatesAnExit(BB, DT, ExitBlocks)) |
| continue; |
| |
| for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { |
| // Reject two common cases fast: instructions with no uses (like stores) |
| // and instructions with one use that is in the same block as this. |
| if (I->use_empty() || |
| (I->hasOneUse() && I->user_back()->getParent() == BB && |
| !isa<PHINode>(I->user_back()))) |
| continue; |
| |
| Changed |= processInstruction(L, *I, DT, ExitBlocks, PredCache, LI); |
| } |
| } |
| |
| // If we modified the code, remove any caches about the loop from SCEV to |
| // avoid dangling entries. |
| // FIXME: This is a big hammer, can we clear the cache more selectively? |
| if (SE && Changed) |
| SE->forgetLoop(&L); |
| |
| assert(L.isLCSSAForm(DT)); |
| |
| return Changed; |
| } |
| |
| /// Process a loop nest depth first. |
| bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT, LoopInfo *LI, |
| ScalarEvolution *SE) { |
| bool Changed = false; |
| |
| // Recurse depth-first through inner loops. |
| for (Loop::iterator I = L.begin(), E = L.end(); I != E; ++I) |
| Changed |= formLCSSARecursively(**I, DT, LI, SE); |
| |
| Changed |= formLCSSA(L, DT, LI, SE); |
| return Changed; |
| } |
| |
| namespace { |
| struct LCSSA : public FunctionPass { |
| static char ID; // Pass identification, replacement for typeid |
| LCSSA() : FunctionPass(ID) { |
| initializeLCSSAPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| // Cached analysis information for the current function. |
| DominatorTree *DT; |
| LoopInfo *LI; |
| ScalarEvolution *SE; |
| |
| bool runOnFunction(Function &F) override; |
| |
| /// This transformation requires natural loop information & requires that |
| /// loop preheaders be inserted into the CFG. It maintains both of these, |
| /// as well as the CFG. It also requires dominator information. |
| void getAnalysisUsage(AnalysisUsage &AU) const override { |
| AU.setPreservesCFG(); |
| |
| AU.addRequired<DominatorTreeWrapperPass>(); |
| AU.addRequired<LoopInfoWrapperPass>(); |
| AU.addPreservedID(LoopSimplifyID); |
| AU.addPreserved<AliasAnalysis>(); |
| AU.addPreserved<ScalarEvolution>(); |
| } |
| }; |
| } |
| |
| char LCSSA::ID = 0; |
| INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false) |
| INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) |
| INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) |
| INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false) |
| |
| Pass *llvm::createLCSSAPass() { return new LCSSA(); } |
| char &llvm::LCSSAID = LCSSA::ID; |
| |
| |
| /// Process all loops in the function, inner-most out. |
| bool LCSSA::runOnFunction(Function &F) { |
| bool Changed = false; |
| LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); |
| DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); |
| SE = getAnalysisIfAvailable<ScalarEvolution>(); |
| |
| // Simplify each loop nest in the function. |
| for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) |
| Changed |= formLCSSARecursively(**I, *DT, LI, SE); |
| |
| return Changed; |
| } |
| |