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//===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
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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 that contain branches on loop-invariant conditions
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// to have multiple loops. For example, it turns the left into the right code:
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// This can increase the size of the code exponentially (doubling it every time
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// a loop is unswitched) so we only unswitch if the resultant code will be
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// smaller than a threshold.
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// This pass expects LICM to be run before it to hoist invariant conditions out
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// of the loop, to make the unswitching opportunity obvious.
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "loop-unswitch"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Constants.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Function.h"
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#include "llvm/Instructions.h"
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#include "llvm/Analysis/CodeMetrics.h"
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#include "llvm/Analysis/InstructionSimplify.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Transforms/Utils/Cloning.h"
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/STLExtras.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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STATISTIC(NumBranches, "Number of branches unswitched");
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STATISTIC(NumSwitches, "Number of switches unswitched");
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STATISTIC(NumSelects , "Number of selects unswitched");
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STATISTIC(NumTrivial , "Number of unswitches that are trivial");
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STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
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STATISTIC(TotalInsts, "Total number of instructions analyzed");
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// The specific value of 100 here was chosen based only on intuition and a
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// few specific examples.
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static cl::opt<unsigned>
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Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
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cl::init(100), cl::Hidden);
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class LUAnalysisCache {
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typedef DenseMap<const SwitchInst*, SmallPtrSet<const Value *, 8> >
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typedef UnswitchedValsMap::iterator UnswitchedValsIt;
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struct LoopProperties {
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unsigned CanBeUnswitchedCount;
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unsigned SizeEstimation;
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UnswitchedValsMap UnswitchedVals;
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// Here we use std::map instead of DenseMap, since we need to keep valid
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// LoopProperties pointer for current loop for better performance.
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typedef std::map<const Loop*, LoopProperties> LoopPropsMap;
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typedef LoopPropsMap::iterator LoopPropsMapIt;
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LoopPropsMap LoopsProperties;
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UnswitchedValsMap* CurLoopInstructions;
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LoopProperties* CurrentLoopProperties;
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// Max size of code we can produce on remained iterations.
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CurLoopInstructions(NULL), CurrentLoopProperties(NULL),
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// Analyze loop. Check its size, calculate is it possible to unswitch
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// it. Returns true if we can unswitch this loop.
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bool countLoop(const Loop* L);
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// Clean all data related to given loop.
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void forgetLoop(const Loop* L);
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// Mark case value as unswitched.
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// Since SI instruction can be partly unswitched, in order to avoid
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// extra unswitching in cloned loops keep track all unswitched values.
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void setUnswitched(const SwitchInst* SI, const Value* V);
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// Check was this case value unswitched before or not.
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bool isUnswitched(const SwitchInst* SI, const Value* V);
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// Clone all loop-unswitch related loop properties.
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// Redistribute unswitching quotas.
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// Note, that new loop data is stored inside the VMap.
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void cloneData(const Loop* NewLoop, const Loop* OldLoop,
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const ValueToValueMapTy& VMap);
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class LoopUnswitch : public LoopPass {
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LoopInfo *LI; // Loop information
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// LoopProcessWorklist - Used to check if second loop needs processing
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// after RewriteLoopBodyWithConditionConstant rewrites first loop.
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std::vector<Loop*> LoopProcessWorklist;
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LUAnalysisCache BranchesInfo;
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bool OptimizeForSize;
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BasicBlock *loopHeader;
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BasicBlock *loopPreheader;
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// LoopBlocks contains all of the basic blocks of the loop, including the
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// preheader of the loop, the body of the loop, and the exit blocks of the
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// loop, in that order.
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std::vector<BasicBlock*> LoopBlocks;
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// NewBlocks contained cloned copy of basic blocks from LoopBlocks.
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std::vector<BasicBlock*> NewBlocks;
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static char ID; // Pass ID, replacement for typeid
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explicit LoopUnswitch(bool Os = false) :
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LoopPass(ID), OptimizeForSize(Os), redoLoop(false),
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currentLoop(NULL), DT(NULL), loopHeader(NULL),
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loopPreheader(NULL) {
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initializeLoopUnswitchPass(*PassRegistry::getPassRegistry());
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bool runOnLoop(Loop *L, LPPassManager &LPM);
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bool processCurrentLoop();
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/// This transformation requires natural loop information & requires that
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/// loop preheaders be inserted into the CFG.
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequiredID(LoopSimplifyID);
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AU.addPreservedID(LoopSimplifyID);
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AU.addRequired<LoopInfo>();
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AU.addPreserved<LoopInfo>();
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AU.addRequiredID(LCSSAID);
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AU.addPreservedID(LCSSAID);
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AU.addPreserved<DominatorTree>();
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AU.addPreserved<ScalarEvolution>();
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virtual void releaseMemory() {
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BranchesInfo.forgetLoop(currentLoop);
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/// RemoveLoopFromWorklist - If the specified loop is on the loop worklist,
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void RemoveLoopFromWorklist(Loop *L) {
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std::vector<Loop*>::iterator I = std::find(LoopProcessWorklist.begin(),
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LoopProcessWorklist.end(), L);
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if (I != LoopProcessWorklist.end())
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LoopProcessWorklist.erase(I);
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void initLoopData() {
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loopHeader = currentLoop->getHeader();
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loopPreheader = currentLoop->getLoopPreheader();
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/// Split all of the edges from inside the loop to their exit blocks.
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/// Update the appropriate Phi nodes as we do so.
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void SplitExitEdges(Loop *L, const SmallVector<BasicBlock *, 8> &ExitBlocks);
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bool UnswitchIfProfitable(Value *LoopCond, Constant *Val);
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void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
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BasicBlock *ExitBlock);
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void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
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void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
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Constant *Val, bool isEqual);
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void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
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BasicBlock *TrueDest,
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BasicBlock *FalseDest,
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Instruction *InsertPt);
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void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L);
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void RemoveBlockIfDead(BasicBlock *BB,
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std::vector<Instruction*> &Worklist, Loop *l);
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void RemoveLoopFromHierarchy(Loop *L);
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bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = 0,
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BasicBlock **LoopExit = 0);
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// Analyze loop. Check its size, calculate is it possible to unswitch
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// it. Returns true if we can unswitch this loop.
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bool LUAnalysisCache::countLoop(const Loop* L) {
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std::pair<LoopPropsMapIt, bool> InsertRes =
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LoopsProperties.insert(std::make_pair(L, LoopProperties()));
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LoopProperties& Props = InsertRes.first->second;
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if (InsertRes.second) {
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// Limit the number of instructions to avoid causing significant code
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// expansion, and the number of basic blocks, to avoid loops with
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// large numbers of branches which cause loop unswitching to go crazy.
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// This is a very ad-hoc heuristic.
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// FIXME: This is overly conservative because it does not take into
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// consideration code simplification opportunities and code that can
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// be shared by the resultant unswitched loops.
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for (Loop::block_iterator I = L->block_begin(),
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Metrics.analyzeBasicBlock(*I);
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Props.SizeEstimation = std::min(Metrics.NumInsts, Metrics.NumBlocks * 5);
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Props.CanBeUnswitchedCount = MaxSize / (Props.SizeEstimation);
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MaxSize -= Props.SizeEstimation * Props.CanBeUnswitchedCount;
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if (!Props.CanBeUnswitchedCount) {
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DEBUG(dbgs() << "NOT unswitching loop %"
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<< L->getHeader()->getName() << ", cost too high: "
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<< L->getBlocks().size() << "\n");
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// Be careful. This links are good only before new loop addition.
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CurrentLoopProperties = &Props;
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CurLoopInstructions = &Props.UnswitchedVals;
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// Clean all data related to given loop.
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void LUAnalysisCache::forgetLoop(const Loop* L) {
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LoopPropsMapIt LIt = LoopsProperties.find(L);
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if (LIt != LoopsProperties.end()) {
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LoopProperties& Props = LIt->second;
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MaxSize += Props.CanBeUnswitchedCount * Props.SizeEstimation;
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LoopsProperties.erase(LIt);
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CurrentLoopProperties = NULL;
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CurLoopInstructions = NULL;
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// Mark case value as unswitched.
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// Since SI instruction can be partly unswitched, in order to avoid
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// extra unswitching in cloned loops keep track all unswitched values.
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void LUAnalysisCache::setUnswitched(const SwitchInst* SI, const Value* V) {
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(*CurLoopInstructions)[SI].insert(V);
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// Check was this case value unswitched before or not.
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bool LUAnalysisCache::isUnswitched(const SwitchInst* SI, const Value* V) {
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return (*CurLoopInstructions)[SI].count(V);
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// Clone all loop-unswitch related loop properties.
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// Redistribute unswitching quotas.
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// Note, that new loop data is stored inside the VMap.
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void LUAnalysisCache::cloneData(const Loop* NewLoop, const Loop* OldLoop,
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const ValueToValueMapTy& VMap) {
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LoopProperties& NewLoopProps = LoopsProperties[NewLoop];
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LoopProperties& OldLoopProps = *CurrentLoopProperties;
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UnswitchedValsMap& Insts = OldLoopProps.UnswitchedVals;
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// Reallocate "can-be-unswitched quota"
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--OldLoopProps.CanBeUnswitchedCount;
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unsigned Quota = OldLoopProps.CanBeUnswitchedCount;
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NewLoopProps.CanBeUnswitchedCount = Quota / 2;
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OldLoopProps.CanBeUnswitchedCount = Quota - Quota / 2;
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NewLoopProps.SizeEstimation = OldLoopProps.SizeEstimation;
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// Clone unswitched values info:
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// for new loop switches we clone info about values that was
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// already unswitched and has redundant successors.
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for (UnswitchedValsIt I = Insts.begin(); I != Insts.end(); ++I) {
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const SwitchInst* OldInst = I->first;
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Value* NewI = VMap.lookup(OldInst);
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const SwitchInst* NewInst = cast_or_null<SwitchInst>(NewI);
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assert(NewInst && "All instructions that are in SrcBB must be in VMap.");
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NewLoopProps.UnswitchedVals[NewInst] = OldLoopProps.UnswitchedVals[OldInst];
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char LoopUnswitch::ID = 0;
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INITIALIZE_PASS_BEGIN(LoopUnswitch, "loop-unswitch", "Unswitch loops",
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INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
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INITIALIZE_PASS_DEPENDENCY(LoopInfo)
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INITIALIZE_PASS_DEPENDENCY(LCSSA)
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INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops",
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Pass *llvm::createLoopUnswitchPass(bool Os) {
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return new LoopUnswitch(Os);
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/// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
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/// invariant in the loop, or has an invariant piece, return the invariant.
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/// Otherwise, return null.
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static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
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// We started analyze new instruction, increment scanned instructions counter.
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// We can never unswitch on vector conditions.
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if (Cond->getType()->isVectorTy())
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// Constants should be folded, not unswitched on!
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if (isa<Constant>(Cond)) return 0;
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// TODO: Handle: br (VARIANT|INVARIANT).
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// Hoist simple values out.
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if (L->makeLoopInvariant(Cond, Changed))
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if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
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if (BO->getOpcode() == Instruction::And ||
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BO->getOpcode() == Instruction::Or) {
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// If either the left or right side is invariant, we can unswitch on this,
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// which will cause the branch to go away in one loop and the condition to
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// simplify in the other one.
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if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
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if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
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bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
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LI = &getAnalysis<LoopInfo>();
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DT = getAnalysisIfAvailable<DominatorTree>();
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Function *F = currentLoop->getHeader()->getParent();
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bool Changed = false;
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assert(currentLoop->isLCSSAForm(*DT));
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Changed |= processCurrentLoop();
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// FIXME: Reconstruct dom info, because it is not preserved properly.
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DT->runOnFunction(*F);
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/// processCurrentLoop - Do actual work and unswitch loop if possible
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bool LoopUnswitch::processCurrentLoop() {
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bool Changed = false;
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// If LoopSimplify was unable to form a preheader, don't do any unswitching.
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LLVMContext &Context = loopHeader->getContext();
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// Probably we reach the quota of branches for this loop. If so
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if (!BranchesInfo.countLoop(currentLoop))
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// Loop over all of the basic blocks in the loop. If we find an interior
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// block that is branching on a loop-invariant condition, we can unswitch this
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for (Loop::block_iterator I = currentLoop->block_begin(),
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E = currentLoop->block_end(); I != E; ++I) {
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TerminatorInst *TI = (*I)->getTerminator();
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if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
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// If this isn't branching on an invariant condition, we can't unswitch
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if (BI->isConditional()) {
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// See if this, or some part of it, is loop invariant. If so, we can
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// unswitch on it if we desire.
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Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
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currentLoop, Changed);
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if (LoopCond && UnswitchIfProfitable(LoopCond,
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ConstantInt::getTrue(Context))) {
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} else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
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Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
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currentLoop, Changed);
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unsigned NumCases = SI->getNumCases();
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if (LoopCond && NumCases) {
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// Find a value to unswitch on:
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// FIXME: this should chose the most expensive case!
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// FIXME: scan for a case with a non-critical edge?
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Constant *UnswitchVal = NULL;
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// Do not process same value again and again.
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// At this point we have some cases already unswitched and
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// some not yet unswitched. Let's find the first not yet unswitched one.
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for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
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Constant* UnswitchValCandidate = i.getCaseValue();
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if (!BranchesInfo.isUnswitched(SI, UnswitchValCandidate)) {
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UnswitchVal = UnswitchValCandidate;
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if (UnswitchIfProfitable(LoopCond, UnswitchVal)) {
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// Scan the instructions to check for unswitchable values.
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for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
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if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
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Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
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currentLoop, Changed);
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if (LoopCond && UnswitchIfProfitable(LoopCond,
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ConstantInt::getTrue(Context))) {
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/// isTrivialLoopExitBlock - Check to see if all paths from BB exit the
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/// loop with no side effects (including infinite loops).
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/// If true, we return true and set ExitBB to the block we
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static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
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std::set<BasicBlock*> &Visited) {
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if (!Visited.insert(BB).second) {
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// Already visited. Without more analysis, this could indicate an infinite
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} else if (!L->contains(BB)) {
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// Otherwise, this is a loop exit, this is fine so long as this is the
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if (ExitBB != 0) return false;
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// Otherwise, this is an unvisited intra-loop node. Check all successors.
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for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
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// Check to see if the successor is a trivial loop exit.
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if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
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// Okay, everything after this looks good, check to make sure that this block
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// doesn't include any side effects.
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
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if (I->mayHaveSideEffects())
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/// isTrivialLoopExitBlock - Return true if the specified block unconditionally
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/// leads to an exit from the specified loop, and has no side-effects in the
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/// process. If so, return the block that is exited to, otherwise return null.
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static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
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std::set<BasicBlock*> Visited;
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Visited.insert(L->getHeader()); // Branches to header make infinite loops.
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BasicBlock *ExitBB = 0;
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if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
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/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
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/// trivial: that is, that the condition controls whether or not the loop does
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/// anything at all. If this is a trivial condition, unswitching produces no
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/// code duplications (equivalently, it produces a simpler loop and a new empty
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/// loop, which gets deleted).
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/// If this is a trivial condition, return true, otherwise return false. When
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/// returning true, this sets Cond and Val to the condition that controls the
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/// trivial condition: when Cond dynamically equals Val, the loop is known to
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/// exit. Finally, this sets LoopExit to the BB that the loop exits to when
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bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
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BasicBlock **LoopExit) {
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BasicBlock *Header = currentLoop->getHeader();
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TerminatorInst *HeaderTerm = Header->getTerminator();
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LLVMContext &Context = Header->getContext();
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BasicBlock *LoopExitBB = 0;
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if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
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// If the header block doesn't end with a conditional branch on Cond, we
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if (!BI->isConditional() || BI->getCondition() != Cond)
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// Check to see if a successor of the branch is guaranteed to
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// exit through a unique exit block without having any
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// side-effects. If so, determine the value of Cond that causes it to do
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if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
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BI->getSuccessor(0)))) {
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if (Val) *Val = ConstantInt::getTrue(Context);
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} else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
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BI->getSuccessor(1)))) {
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if (Val) *Val = ConstantInt::getFalse(Context);
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} else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
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// If this isn't a switch on Cond, we can't handle it.
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if (SI->getCondition() != Cond) return false;
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// Check to see if a successor of the switch is guaranteed to go to the
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// latch block or exit through a one exit block without having any
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// side-effects. If so, determine the value of Cond that causes it to do
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// Note that we can't trivially unswitch on the default case or
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// on already unswitched cases.
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for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
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BasicBlock* LoopExitCandidate;
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if ((LoopExitCandidate = isTrivialLoopExitBlock(currentLoop,
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i.getCaseSuccessor()))) {
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// Okay, we found a trivial case, remember the value that is trivial.
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ConstantInt* CaseVal = i.getCaseValue();
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// Check that it was not unswitched before, since already unswitched
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// trivial vals are looks trivial too.
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if (BranchesInfo.isUnswitched(SI, CaseVal))
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LoopExitBB = LoopExitCandidate;
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if (Val) *Val = CaseVal;
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// If we didn't find a single unique LoopExit block, or if the loop exit block
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// contains phi nodes, this isn't trivial.
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if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
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return false; // Can't handle this.
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if (LoopExit) *LoopExit = LoopExitBB;
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// We already know that nothing uses any scalar values defined inside of this
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// loop. As such, we just have to check to see if this loop will execute any
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// side-effecting instructions (e.g. stores, calls, volatile loads) in the
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// part of the loop that the code *would* execute. We already checked the
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// tail, check the header now.
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for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
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if (I->mayHaveSideEffects())
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/// UnswitchIfProfitable - We have found that we can unswitch currentLoop when
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/// LoopCond == Val to simplify the loop. If we decide that this is profitable,
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/// unswitch the loop, reprocess the pieces, then return true.
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bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val) {
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Function *F = loopHeader->getParent();
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Constant *CondVal = 0;
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BasicBlock *ExitBlock = 0;
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if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) {
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// If the condition is trivial, always unswitch. There is no code growth
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UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock);
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// Check to see if it would be profitable to unswitch current loop.
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// Do not do non-trivial unswitch while optimizing for size.
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if (OptimizeForSize || F->hasFnAttr(Attribute::OptimizeForSize))
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UnswitchNontrivialCondition(LoopCond, Val, currentLoop);
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/// CloneLoop - Recursively clone the specified loop and all of its children,
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/// mapping the blocks with the specified map.
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static Loop *CloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM,
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LoopInfo *LI, LPPassManager *LPM) {
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Loop *New = new Loop();
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LPM->insertLoop(New, PL);
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// Add all of the blocks in L to the new loop.
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for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
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if (LI->getLoopFor(*I) == L)
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New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), LI->getBase());
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// Add all of the subloops to the new loop.
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for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
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CloneLoop(*I, New, VM, LI, LPM);
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/// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
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/// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
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/// code immediately before InsertPt.
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void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
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BasicBlock *TrueDest,
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BasicBlock *FalseDest,
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Instruction *InsertPt) {
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// Insert a conditional branch on LIC to the two preheaders. The original
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// code is the true version and the new code is the false version.
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Value *BranchVal = LIC;
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if (!isa<ConstantInt>(Val) ||
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Val->getType() != Type::getInt1Ty(LIC->getContext()))
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BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val);
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else if (Val != ConstantInt::getTrue(Val->getContext()))
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// We want to enter the new loop when the condition is true.
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std::swap(TrueDest, FalseDest);
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// Insert the new branch.
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BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt);
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// If either edge is critical, split it. This helps preserve LoopSimplify
682
// form for enclosing loops.
683
SplitCriticalEdge(BI, 0, this);
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SplitCriticalEdge(BI, 1, this);
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/// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
688
/// condition in it (a cond branch from its header block to its latch block,
689
/// where the path through the loop that doesn't execute its body has no
690
/// side-effects), unswitch it. This doesn't involve any code duplication, just
691
/// moving the conditional branch outside of the loop and updating loop info.
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void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
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BasicBlock *ExitBlock) {
695
DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
696
<< loopHeader->getName() << " [" << L->getBlocks().size()
697
<< " blocks] in Function " << L->getHeader()->getParent()->getName()
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<< " on cond: " << *Val << " == " << *Cond << "\n");
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// First step, split the preheader, so that we know that there is a safe place
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// to insert the conditional branch. We will change loopPreheader to have a
702
// conditional branch on Cond.
703
BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this);
705
// Now that we have a place to insert the conditional branch, create a place
706
// to branch to: this is the exit block out of the loop that we should
709
// Split this block now, so that the loop maintains its exit block, and so
710
// that the jump from the preheader can execute the contents of the exit block
711
// without actually branching to it (the exit block should be dominated by the
712
// loop header, not the preheader).
713
assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
714
BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this);
716
// Okay, now we have a position to branch from and a position to branch to,
717
// insert the new conditional branch.
718
EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
719
loopPreheader->getTerminator());
720
LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L);
721
loopPreheader->getTerminator()->eraseFromParent();
723
// We need to reprocess this loop, it could be unswitched again.
726
// Now that we know that the loop is never entered when this condition is a
727
// particular value, rewrite the loop with this info. We know that this will
728
// at least eliminate the old branch.
729
RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
733
/// SplitExitEdges - Split all of the edges from inside the loop to their exit
734
/// blocks. Update the appropriate Phi nodes as we do so.
735
void LoopUnswitch::SplitExitEdges(Loop *L,
736
const SmallVector<BasicBlock *, 8> &ExitBlocks){
738
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
739
BasicBlock *ExitBlock = ExitBlocks[i];
740
SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
741
pred_end(ExitBlock));
743
// Although SplitBlockPredecessors doesn't preserve loop-simplify in
744
// general, if we call it on all predecessors of all exits then it does.
745
if (!ExitBlock->isLandingPad()) {
746
SplitBlockPredecessors(ExitBlock, Preds, ".us-lcssa", this);
748
SmallVector<BasicBlock*, 2> NewBBs;
749
SplitLandingPadPredecessors(ExitBlock, Preds, ".us-lcssa", ".us-lcssa",
755
/// UnswitchNontrivialCondition - We determined that the loop is profitable
756
/// to unswitch when LIC equal Val. Split it into loop versions and test the
757
/// condition outside of either loop. Return the loops created as Out1/Out2.
758
void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
760
Function *F = loopHeader->getParent();
761
DEBUG(dbgs() << "loop-unswitch: Unswitching loop %"
762
<< loopHeader->getName() << " [" << L->getBlocks().size()
763
<< " blocks] in Function " << F->getName()
764
<< " when '" << *Val << "' == " << *LIC << "\n");
766
if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>())
772
// First step, split the preheader and exit blocks, and add these blocks to
773
// the LoopBlocks list.
774
BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this);
775
LoopBlocks.push_back(NewPreheader);
777
// We want the loop to come after the preheader, but before the exit blocks.
778
LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
780
SmallVector<BasicBlock*, 8> ExitBlocks;
781
L->getUniqueExitBlocks(ExitBlocks);
783
// Split all of the edges from inside the loop to their exit blocks. Update
784
// the appropriate Phi nodes as we do so.
785
SplitExitEdges(L, ExitBlocks);
787
// The exit blocks may have been changed due to edge splitting, recompute.
789
L->getUniqueExitBlocks(ExitBlocks);
791
// Add exit blocks to the loop blocks.
792
LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
794
// Next step, clone all of the basic blocks that make up the loop (including
795
// the loop preheader and exit blocks), keeping track of the mapping between
796
// the instructions and blocks.
797
NewBlocks.reserve(LoopBlocks.size());
798
ValueToValueMapTy VMap;
799
for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
800
BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[i], VMap, ".us", F);
802
NewBlocks.push_back(NewBB);
803
VMap[LoopBlocks[i]] = NewBB; // Keep the BB mapping.
804
LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L);
807
// Splice the newly inserted blocks into the function right before the
808
// original preheader.
809
F->getBasicBlockList().splice(NewPreheader, F->getBasicBlockList(),
810
NewBlocks[0], F->end());
812
// Now we create the new Loop object for the versioned loop.
813
Loop *NewLoop = CloneLoop(L, L->getParentLoop(), VMap, LI, LPM);
815
// Recalculate unswitching quota, inherit simplified switches info for NewBB,
816
// Probably clone more loop-unswitch related loop properties.
817
BranchesInfo.cloneData(NewLoop, L, VMap);
819
Loop *ParentLoop = L->getParentLoop();
821
// Make sure to add the cloned preheader and exit blocks to the parent loop
823
ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase());
826
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
827
BasicBlock *NewExit = cast<BasicBlock>(VMap[ExitBlocks[i]]);
828
// The new exit block should be in the same loop as the old one.
829
if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
830
ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase());
832
assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
833
"Exit block should have been split to have one successor!");
834
BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
836
// If the successor of the exit block had PHI nodes, add an entry for
839
for (BasicBlock::iterator I = ExitSucc->begin(); isa<PHINode>(I); ++I) {
840
PN = cast<PHINode>(I);
841
Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
842
ValueToValueMapTy::iterator It = VMap.find(V);
843
if (It != VMap.end()) V = It->second;
844
PN->addIncoming(V, NewExit);
847
if (LandingPadInst *LPad = NewExit->getLandingPadInst()) {
848
PN = PHINode::Create(LPad->getType(), 0, "",
849
ExitSucc->getFirstInsertionPt());
851
for (pred_iterator I = pred_begin(ExitSucc), E = pred_end(ExitSucc);
854
LandingPadInst *LPI = BB->getLandingPadInst();
855
LPI->replaceAllUsesWith(PN);
856
PN->addIncoming(LPI, BB);
861
// Rewrite the code to refer to itself.
862
for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
863
for (BasicBlock::iterator I = NewBlocks[i]->begin(),
864
E = NewBlocks[i]->end(); I != E; ++I)
865
RemapInstruction(I, VMap,RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
867
// Rewrite the original preheader to select between versions of the loop.
868
BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator());
869
assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
870
"Preheader splitting did not work correctly!");
872
// Emit the new branch that selects between the two versions of this loop.
873
EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
874
LPM->deleteSimpleAnalysisValue(OldBR, L);
875
OldBR->eraseFromParent();
877
LoopProcessWorklist.push_back(NewLoop);
880
// Keep a WeakVH holding onto LIC. If the first call to RewriteLoopBody
881
// deletes the instruction (for example by simplifying a PHI that feeds into
882
// the condition that we're unswitching on), we don't rewrite the second
884
WeakVH LICHandle(LIC);
886
// Now we rewrite the original code to know that the condition is true and the
887
// new code to know that the condition is false.
888
RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
890
// It's possible that simplifying one loop could cause the other to be
891
// changed to another value or a constant. If its a constant, don't simplify
893
if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop &&
894
LICHandle && !isa<Constant>(LICHandle))
895
RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true);
898
/// RemoveFromWorklist - Remove all instances of I from the worklist vector
900
static void RemoveFromWorklist(Instruction *I,
901
std::vector<Instruction*> &Worklist) {
902
std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
904
while (WI != Worklist.end()) {
905
unsigned Offset = WI-Worklist.begin();
907
WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
911
/// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
912
/// program, replacing all uses with V and update the worklist.
913
static void ReplaceUsesOfWith(Instruction *I, Value *V,
914
std::vector<Instruction*> &Worklist,
915
Loop *L, LPPassManager *LPM) {
916
DEBUG(dbgs() << "Replace with '" << *V << "': " << *I);
918
// Add uses to the worklist, which may be dead now.
919
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
920
if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
921
Worklist.push_back(Use);
923
// Add users to the worklist which may be simplified now.
924
for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
926
Worklist.push_back(cast<Instruction>(*UI));
927
LPM->deleteSimpleAnalysisValue(I, L);
928
RemoveFromWorklist(I, Worklist);
929
I->replaceAllUsesWith(V);
930
I->eraseFromParent();
934
/// RemoveBlockIfDead - If the specified block is dead, remove it, update loop
935
/// information, and remove any dead successors it has.
937
void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
938
std::vector<Instruction*> &Worklist,
940
if (pred_begin(BB) != pred_end(BB)) {
941
// This block isn't dead, since an edge to BB was just removed, see if there
942
// are any easy simplifications we can do now.
943
if (BasicBlock *Pred = BB->getSinglePredecessor()) {
944
// If it has one pred, fold phi nodes in BB.
945
while (isa<PHINode>(BB->begin()))
946
ReplaceUsesOfWith(BB->begin(),
947
cast<PHINode>(BB->begin())->getIncomingValue(0),
950
// If this is the header of a loop and the only pred is the latch, we now
951
// have an unreachable loop.
952
if (Loop *L = LI->getLoopFor(BB))
953
if (loopHeader == BB && L->contains(Pred)) {
954
// Remove the branch from the latch to the header block, this makes
955
// the header dead, which will make the latch dead (because the header
956
// dominates the latch).
957
LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L);
958
Pred->getTerminator()->eraseFromParent();
959
new UnreachableInst(BB->getContext(), Pred);
961
// The loop is now broken, remove it from LI.
962
RemoveLoopFromHierarchy(L);
964
// Reprocess the header, which now IS dead.
965
RemoveBlockIfDead(BB, Worklist, L);
969
// If pred ends in a uncond branch, add uncond branch to worklist so that
970
// the two blocks will get merged.
971
if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
972
if (BI->isUnconditional())
973
Worklist.push_back(BI);
978
DEBUG(dbgs() << "Nuking dead block: " << *BB);
980
// Remove the instructions in the basic block from the worklist.
981
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
982
RemoveFromWorklist(I, Worklist);
984
// Anything that uses the instructions in this basic block should have their
985
// uses replaced with undefs.
986
// If I is not void type then replaceAllUsesWith undef.
987
// This allows ValueHandlers and custom metadata to adjust itself.
988
if (!I->getType()->isVoidTy())
989
I->replaceAllUsesWith(UndefValue::get(I->getType()));
992
// If this is the edge to the header block for a loop, remove the loop and
993
// promote all subloops.
994
if (Loop *BBLoop = LI->getLoopFor(BB)) {
995
if (BBLoop->getLoopLatch() == BB) {
996
RemoveLoopFromHierarchy(BBLoop);
997
if (currentLoop == BBLoop) {
1004
// Remove the block from the loop info, which removes it from any loops it
1006
LI->removeBlock(BB);
1009
// Remove phi node entries in successors for this block.
1010
TerminatorInst *TI = BB->getTerminator();
1011
SmallVector<BasicBlock*, 4> Succs;
1012
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
1013
Succs.push_back(TI->getSuccessor(i));
1014
TI->getSuccessor(i)->removePredecessor(BB);
1017
// Unique the successors, remove anything with multiple uses.
1018
array_pod_sort(Succs.begin(), Succs.end());
1019
Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end());
1021
// Remove the basic block, including all of the instructions contained in it.
1022
LPM->deleteSimpleAnalysisValue(BB, L);
1023
BB->eraseFromParent();
1024
// Remove successor blocks here that are not dead, so that we know we only
1025
// have dead blocks in this list. Nondead blocks have a way of becoming dead,
1026
// then getting removed before we revisit them, which is badness.
1028
for (unsigned i = 0; i != Succs.size(); ++i)
1029
if (pred_begin(Succs[i]) != pred_end(Succs[i])) {
1030
// One exception is loop headers. If this block was the preheader for a
1031
// loop, then we DO want to visit the loop so the loop gets deleted.
1032
// We know that if the successor is a loop header, that this loop had to
1033
// be the preheader: the case where this was the latch block was handled
1034
// above and headers can only have two predecessors.
1035
if (!LI->isLoopHeader(Succs[i])) {
1036
Succs.erase(Succs.begin()+i);
1041
for (unsigned i = 0, e = Succs.size(); i != e; ++i)
1042
RemoveBlockIfDead(Succs[i], Worklist, L);
1045
/// RemoveLoopFromHierarchy - We have discovered that the specified loop has
1046
/// become unwrapped, either because the backedge was deleted, or because the
1047
/// edge into the header was removed. If the edge into the header from the
1048
/// latch block was removed, the loop is unwrapped but subloops are still alive,
1049
/// so they just reparent loops. If the loops are actually dead, they will be
1051
void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) {
1052
LPM->deleteLoopFromQueue(L);
1053
RemoveLoopFromWorklist(L);
1056
// RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
1057
// the value specified by Val in the specified loop, or we know it does NOT have
1058
// that value. Rewrite any uses of LIC or of properties correlated to it.
1059
void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
1062
assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
1064
// FIXME: Support correlated properties, like:
1071
// FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
1072
// selects, switches.
1073
std::vector<Instruction*> Worklist;
1074
LLVMContext &Context = Val->getContext();
1077
// If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
1078
// in the loop with the appropriate one directly.
1079
if (IsEqual || (isa<ConstantInt>(Val) &&
1080
Val->getType()->isIntegerTy(1))) {
1085
Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()),
1086
!cast<ConstantInt>(Val)->getZExtValue());
1088
for (Value::use_iterator UI = LIC->use_begin(), E = LIC->use_end();
1090
Instruction *U = dyn_cast<Instruction>(*UI);
1091
if (!U || !L->contains(U))
1093
Worklist.push_back(U);
1096
for (std::vector<Instruction*>::iterator UI = Worklist.begin();
1097
UI != Worklist.end(); ++UI)
1098
(*UI)->replaceUsesOfWith(LIC, Replacement);
1100
SimplifyCode(Worklist, L);
1104
// Otherwise, we don't know the precise value of LIC, but we do know that it
1105
// is certainly NOT "Val". As such, simplify any uses in the loop that we
1106
// can. This case occurs when we unswitch switch statements.
1107
for (Value::use_iterator UI = LIC->use_begin(), E = LIC->use_end();
1109
Instruction *U = dyn_cast<Instruction>(*UI);
1110
if (!U || !L->contains(U))
1113
Worklist.push_back(U);
1115
// TODO: We could do other simplifications, for example, turning
1116
// 'icmp eq LIC, Val' -> false.
1118
// If we know that LIC is not Val, use this info to simplify code.
1119
SwitchInst *SI = dyn_cast<SwitchInst>(U);
1120
if (SI == 0 || !isa<ConstantInt>(Val)) continue;
1122
SwitchInst::CaseIt DeadCase = SI->findCaseValue(cast<ConstantInt>(Val));
1123
// Default case is live for multiple values.
1124
if (DeadCase == SI->case_default()) continue;
1126
// Found a dead case value. Don't remove PHI nodes in the
1127
// successor if they become single-entry, those PHI nodes may
1128
// be in the Users list.
1130
BasicBlock *Switch = SI->getParent();
1131
BasicBlock *SISucc = DeadCase.getCaseSuccessor();
1132
BasicBlock *Latch = L->getLoopLatch();
1134
BranchesInfo.setUnswitched(SI, Val);
1136
if (!SI->findCaseDest(SISucc)) continue; // Edge is critical.
1137
// If the DeadCase successor dominates the loop latch, then the
1138
// transformation isn't safe since it will delete the sole predecessor edge
1140
if (Latch && DT->dominates(SISucc, Latch))
1143
// FIXME: This is a hack. We need to keep the successor around
1144
// and hooked up so as to preserve the loop structure, because
1145
// trying to update it is complicated. So instead we preserve the
1146
// loop structure and put the block on a dead code path.
1147
SplitEdge(Switch, SISucc, this);
1148
// Compute the successors instead of relying on the return value
1149
// of SplitEdge, since it may have split the switch successor
1151
BasicBlock *NewSISucc = DeadCase.getCaseSuccessor();
1152
BasicBlock *OldSISucc = *succ_begin(NewSISucc);
1153
// Create an "unreachable" destination.
1154
BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable",
1155
Switch->getParent(),
1157
new UnreachableInst(Context, Abort);
1158
// Force the new case destination to branch to the "unreachable"
1159
// block while maintaining a (dead) CFG edge to the old block.
1160
NewSISucc->getTerminator()->eraseFromParent();
1161
BranchInst::Create(Abort, OldSISucc,
1162
ConstantInt::getTrue(Context), NewSISucc);
1163
// Release the PHI operands for this edge.
1164
for (BasicBlock::iterator II = NewSISucc->begin();
1165
PHINode *PN = dyn_cast<PHINode>(II); ++II)
1166
PN->setIncomingValue(PN->getBasicBlockIndex(Switch),
1167
UndefValue::get(PN->getType()));
1168
// Tell the domtree about the new block. We don't fully update the
1169
// domtree here -- instead we force it to do a full recomputation
1170
// after the pass is complete -- but we do need to inform it of
1173
DT->addNewBlock(Abort, NewSISucc);
1176
SimplifyCode(Worklist, L);
1179
/// SimplifyCode - Okay, now that we have simplified some instructions in the
1180
/// loop, walk over it and constant prop, dce, and fold control flow where
1181
/// possible. Note that this is effectively a very simple loop-structure-aware
1182
/// optimizer. During processing of this loop, L could very well be deleted, so
1183
/// it must not be used.
1185
/// FIXME: When the loop optimizer is more mature, separate this out to a new
1188
void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
1189
while (!Worklist.empty()) {
1190
Instruction *I = Worklist.back();
1191
Worklist.pop_back();
1194
if (isInstructionTriviallyDead(I)) {
1195
DEBUG(dbgs() << "Remove dead instruction '" << *I);
1197
// Add uses to the worklist, which may be dead now.
1198
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
1199
if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
1200
Worklist.push_back(Use);
1201
LPM->deleteSimpleAnalysisValue(I, L);
1202
RemoveFromWorklist(I, Worklist);
1203
I->eraseFromParent();
1208
// See if instruction simplification can hack this up. This is common for
1209
// things like "select false, X, Y" after unswitching made the condition be
1211
if (Value *V = SimplifyInstruction(I, 0, 0, DT))
1212
if (LI->replacementPreservesLCSSAForm(I, V)) {
1213
ReplaceUsesOfWith(I, V, Worklist, L, LPM);
1217
// Special case hacks that appear commonly in unswitched code.
1218
if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
1219
if (BI->isUnconditional()) {
1220
// If BI's parent is the only pred of the successor, fold the two blocks
1222
BasicBlock *Pred = BI->getParent();
1223
BasicBlock *Succ = BI->getSuccessor(0);
1224
BasicBlock *SinglePred = Succ->getSinglePredecessor();
1225
if (!SinglePred) continue; // Nothing to do.
1226
assert(SinglePred == Pred && "CFG broken");
1228
DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- "
1229
<< Succ->getName() << "\n");
1231
// Resolve any single entry PHI nodes in Succ.
1232
while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
1233
ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM);
1235
// If Succ has any successors with PHI nodes, update them to have
1236
// entries coming from Pred instead of Succ.
1237
Succ->replaceAllUsesWith(Pred);
1239
// Move all of the successor contents from Succ to Pred.
1240
Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
1242
LPM->deleteSimpleAnalysisValue(BI, L);
1243
BI->eraseFromParent();
1244
RemoveFromWorklist(BI, Worklist);
1246
// Remove Succ from the loop tree.
1247
LI->removeBlock(Succ);
1248
LPM->deleteSimpleAnalysisValue(Succ, L);
1249
Succ->eraseFromParent();
1254
if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){
1255
// Conditional branch. Turn it into an unconditional branch, then
1256
// remove dead blocks.
1257
continue; // FIXME: Enable.
1259
DEBUG(dbgs() << "Folded branch: " << *BI);
1260
BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue());
1261
BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue());
1262
DeadSucc->removePredecessor(BI->getParent(), true);
1263
Worklist.push_back(BranchInst::Create(LiveSucc, BI));
1264
LPM->deleteSimpleAnalysisValue(BI, L);
1265
BI->eraseFromParent();
1266
RemoveFromWorklist(BI, Worklist);
1269
RemoveBlockIfDead(DeadSucc, Worklist, L);
added
removed
lib/Transforms/Scalar/LoopUnswitch.cpp
