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+//===-- 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/Utils/LCSSA.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/BasicAliasAnalysis.h"
+#include "llvm/Analysis/GlobalsModRef.h"
+#include "llvm/Analysis/LoopPass.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.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/Scalar.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");
+
+#ifdef EXPENSIVE_CHECKS
+static bool VerifyLoopLCSSA = true;
+#else
+static bool VerifyLoopLCSSA = false;
+#endif
+static cl::opt<bool,true>
+VerifyLoopLCSSAFlag("verify-loop-lcssa", cl::location(VerifyLoopLCSSA),
+ cl::desc("Verify loop lcssa form (time consuming)"));
+
+/// Return true if the specified block is in the list.
+static bool isExitBlock(BasicBlock *BB,
+ const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
+ return is_contained(ExitBlocks, BB);
+}
+
+/// For every instruction from the worklist, check to see if it has any uses
+/// that are outside the current loop. If so, insert LCSSA PHI nodes and
+/// rewrite the uses.
+bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
+ DominatorTree &DT, LoopInfo &LI) {
+ SmallVector<Use *, 16> UsesToRewrite;
+ SmallSetVector<PHINode *, 16> PHIsToRemove;
+ PredIteratorCache PredCache;
+ bool Changed = false;
+
+ // Cache the Loop ExitBlocks across this loop. We expect to get a lot of
+ // instructions within the same loops, computing the exit blocks is
+ // expensive, and we're not mutating the loop structure.
+ SmallDenseMap<Loop*, SmallVector<BasicBlock *,1>> LoopExitBlocks;
+
+ while (!Worklist.empty()) {
+ UsesToRewrite.clear();
+
+ Instruction *I = Worklist.pop_back_val();
+ BasicBlock *InstBB = I->getParent();
+ Loop *L = LI.getLoopFor(InstBB);
+ if (!LoopExitBlocks.count(L))
+ L->getExitBlocks(LoopExitBlocks[L]);
+ assert(LoopExitBlocks.count(L));
+ const SmallVectorImpl<BasicBlock *> &ExitBlocks = LoopExitBlocks[L];
+
+ if (ExitBlocks.empty())
+ continue;
+
+ // Tokens cannot be used in PHI nodes, so we skip over them.
+ // We can run into tokens which are live out of a loop with catchswitch
+ // instructions in Windows EH if the catchswitch has one catchpad which
+ // is inside the loop and another which is not.
+ if (I->getType()->isTokenTy())
+ continue;
+
+ for (Use &U : I->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())
+ continue;
+
+ ++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 = InstBB;
+ if (InvokeInst *Inv = dyn_cast<InvokeInst>(I))
+ DomBB = Inv->getNormalDest();
+
+ DomTreeNode *DomNode = DT.getNode(DomBB);
+
+ SmallVector<PHINode *, 16> AddedPHIs;
+ SmallVector<PHINode *, 8> PostProcessPHIs;
+
+ SmallVector<PHINode *, 4> InsertedPHIs;
+ SSAUpdater SSAUpdate(&InsertedPHIs);
+ SSAUpdate.Initialize(I->getType(), I->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 (BasicBlock *ExitBB : ExitBlocks) {
+ 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(I->getType(), PredCache.size(ExitBB),
+ I->getName() + ".lcssa", &ExitBB->front());
+
+ // Add inputs from inside the loop for this PHI.
+ for (BasicBlock *Pred : PredCache.get(ExitBB)) {
+ PN->addIncoming(I, 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 (Use *UseToRewrite : UsesToRewrite) {
+ // 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>(UseToRewrite->getUser());
+ BasicBlock *UserBB = User->getParent();
+ if (PHINode *PN = dyn_cast<PHINode>(User))
+ UserBB = PN->getIncomingBlock(*UseToRewrite);
+
+ if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
+ // Tell the VHs that the uses changed. This updates SCEV's caches.
+ if (UseToRewrite->get()->hasValueHandle())
+ ValueHandleBase::ValueIsRAUWd(*UseToRewrite, &UserBB->front());
+ UseToRewrite->set(&UserBB->front());
+ continue;
+ }
+
+ // Otherwise, do full PHI insertion.
+ SSAUpdate.RewriteUse(*UseToRewrite);
+ }
+
+ // SSAUpdater might have inserted phi-nodes inside other loops. We'll need
+ // to post-process them to keep LCSSA form.
+ for (PHINode *InsertedPN : InsertedPHIs) {
+ if (auto *OtherLoop = LI.getLoopFor(InsertedPN->getParent()))
+ if (!L->contains(OtherLoop))
+ PostProcessPHIs.push_back(InsertedPN);
+ }
+
+ // Post process PHI instructions that were inserted into another disjoint
+ // loop and update their exits properly.
+ for (auto *PostProcessPN : PostProcessPHIs) {
+ if (PostProcessPN->use_empty())
+ continue;
+
+ // Reprocess each PHI instruction.
+ Worklist.push_back(PostProcessPN);
+ }
+
+ // Keep track of PHI nodes that we want to remove because they did not have
+ // any uses rewritten.
+ for (PHINode *PN : AddedPHIs)
+ if (PN->use_empty())
+ PHIsToRemove.insert(PN);
+
+ Changed = true;
+ }
+ // Remove PHI nodes that did not have any uses rewritten.
+ for (PHINode *PN : PHIsToRemove) {
+ assert (PN->use_empty() && "Trying to remove a phi with uses.");
+ PN->eraseFromParent();
+ }
+ return Changed;
+}
+
+/// 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);
+ return any_of(ExitBlocks, [&](BasicBlock *EB) {
+ return DT.dominates(DomNode, DT.getNode(EB));
+ });
+}
+
+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;
+
+ SmallVector<Instruction *, 8> Worklist;
+
+ // Look at all the instructions in the loop, checking to see if they have uses
+ // outside the loop. If so, put them into the worklist to rewrite those uses.
+ for (BasicBlock *BB : L.blocks()) {
+ // 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 (Instruction &I : *BB) {
+ // 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;
+
+ Worklist.push_back(&I);
+ }
+ }
+ Changed = formLCSSAForInstructions(Worklist, DT, *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 *SubLoop : L.getSubLoops())
+ Changed |= formLCSSARecursively(*SubLoop, DT, LI, SE);
+
+ Changed |= formLCSSA(L, DT, LI, SE);
+ return Changed;
+}
+
+/// Process all loops in the function, inner-most out.
+static bool formLCSSAOnAllLoops(LoopInfo *LI, DominatorTree &DT,
+ ScalarEvolution *SE) {
+ bool Changed = false;
+ for (auto &L : *LI)
+ Changed |= formLCSSARecursively(*L, DT, LI, SE);
+ return Changed;
+}
+
+namespace {
+struct LCSSAWrapperPass : public FunctionPass {
+ static char ID; // Pass identification, replacement for typeid
+ LCSSAWrapperPass() : FunctionPass(ID) {
+ initializeLCSSAWrapperPassPass(*PassRegistry::getPassRegistry());
+ }
+
+ // Cached analysis information for the current function.
+ DominatorTree *DT;
+ LoopInfo *LI;
+ ScalarEvolution *SE;
+
+ bool runOnFunction(Function &F) override;
+ void verifyAnalysis() const override {
+ // This check is very expensive. On the loop intensive compiles it may cause
+ // up to 10x slowdown. Currently it's disabled by default. LPPassManager
+ // always does limited form of the LCSSA verification. Similar reasoning
+ // was used for the LoopInfo verifier.
+ if (VerifyLoopLCSSA) {
+ assert(all_of(*LI,
+ [&](Loop *L) {
+ return L->isRecursivelyLCSSAForm(*DT, *LI);
+ }) &&
+ "LCSSA form is broken!");
+ }
+ };
+
+ /// 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<AAResultsWrapperPass>();
+ AU.addPreserved<BasicAAWrapperPass>();
+ AU.addPreserved<GlobalsAAWrapperPass>();
+ AU.addPreserved<ScalarEvolutionWrapperPass>();
+ AU.addPreserved<SCEVAAWrapperPass>();
+
+ // This is needed to perform LCSSA verification inside LPPassManager
+ AU.addRequired<LCSSAVerificationPass>();
+ AU.addPreserved<LCSSAVerificationPass>();
+ }
+};
+}
+
+char LCSSAWrapperPass::ID = 0;
+INITIALIZE_PASS_BEGIN(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
+ false, false)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(LCSSAVerificationPass)
+INITIALIZE_PASS_END(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
+ false, false)
+
+Pass *llvm::createLCSSAPass() { return new LCSSAWrapperPass(); }
+char &llvm::LCSSAID = LCSSAWrapperPass::ID;
+
+/// Transform \p F into loop-closed SSA form.
+bool LCSSAWrapperPass::runOnFunction(Function &F) {
+ LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+ DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+ auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
+ SE = SEWP ? &SEWP->getSE() : nullptr;
+
+ return formLCSSAOnAllLoops(LI, *DT, SE);
+}
+
+PreservedAnalyses LCSSAPass::run(Function &F, FunctionAnalysisManager &AM) {
+ auto &LI = AM.getResult<LoopAnalysis>(F);
+ auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
+ auto *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);
+ if (!formLCSSAOnAllLoops(&LI, DT, SE))
+ return PreservedAnalyses::all();
+
+ // FIXME: This should also 'preserve the CFG'.
+ PreservedAnalyses PA;
+ PA.preserve<BasicAA>();
+ PA.preserve<GlobalsAA>();
+ PA.preserve<SCEVAA>();
+ PA.preserve<ScalarEvolutionAnalysis>();
+ return PA;
+}
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