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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/ConstantFolding.h"
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#include "llvm/Analysis/InlineCost.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/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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// The specific value of 50 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(50), cl::Hidden);
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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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SmallPtrSet<Value *,8> UnswitchedVals;
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DominanceFrontier *DF;
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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), DF(NULL), DT(NULL), loopHeader(NULL),
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loopPreheader(NULL) {}
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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<DominanceFrontier>();
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virtual void releaseMemory() {
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UnswitchedVals.clear();
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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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char LoopUnswitch::ID = 0;
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static RegisterPass<LoopUnswitch> X("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 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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DF = getAnalysisIfAvailable<DominanceFrontier>();
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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());
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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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DF->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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LLVMContext &Context = currentLoop->getHeader()->getContext();
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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();
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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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if (LoopCond && SI->getNumCases() > 1) {
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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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Constant *UnswitchVal = SI->getCaseValue(1);
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// Do not process same value again and again.
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if (!UnswitchedVals.insert(UnswitchVal))
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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 either:
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/// 1. Exit the loop with no side effects.
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/// 2. Branch to the latch block with no side-effects.
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/// If these conditions are 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 and Ok, end of recursion.
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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 are ok.
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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 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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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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// this. Note that we can't trivially unswitch on the default case.
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for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
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if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
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SI->getSuccessor(i)))) {
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// Okay, we found a trivial case, remember the value that is trivial.
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if (Val) *Val = SI->getCaseValue(i);
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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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// If LoopSimplify was unable to form a preheader, don't do any unswitching.
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Function *F = loopHeader->getParent();
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// If the condition is trivial, always unswitch. There is no code growth for
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if (!IsTrivialUnswitchCondition(LoopCond)) {
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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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// 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 = currentLoop->block_begin(),
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E = currentLoop->block_end();
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Metrics.analyzeBasicBlock(*I);
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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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if (Metrics.NumInsts > Threshold ||
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Metrics.NumBlocks * 5 > Threshold ||
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Metrics.NeverInline) {
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DEBUG(dbgs() << "NOT unswitching loop %"
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<< currentLoop->getHeader()->getName() << ", cost too high: "
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<< currentLoop->getBlocks().size() << "\n");
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BasicBlock *ExitBlock;
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if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) {
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UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock);
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UnswitchNontrivialCondition(LoopCond, Val, currentLoop);
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// RemapInstruction - Convert the instruction operands from referencing the
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// current values into those specified by ValueMap.
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static inline void RemapInstruction(Instruction *I,
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DenseMap<const Value *, Value*> &ValueMap) {
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for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
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Value *Op = I->getOperand(op);
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DenseMap<const Value *, Value*>::iterator It = ValueMap.find(Op);
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if (It != ValueMap.end()) Op = It->second;
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I->setOperand(op, Op);
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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, DenseMap<const Value*, Value*> &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, "tmp");
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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
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// form for enclosing loops.
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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
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/// condition in it (a cond branch from its header block to its latch block,
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/// where the path through the loop that doesn't execute its body has no
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/// side-effects), unswitch it. This doesn't involve any code duplication, just
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/// 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) {
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DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
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<< loopHeader->getName() << " [" << L->getBlocks().size()
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<< " 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
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// conditional branch on Cond.
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BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, this);
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// Now that we have a place to insert the conditional branch, create a place
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// to branch to: this is the exit block out of the loop that we should
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// Split this block now, so that the loop maintains its exit block, and so
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// that the jump from the preheader can execute the contents of the exit block
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// without actually branching to it (the exit block should be dominated by the
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// loop header, not the preheader).
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assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
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BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), this);
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// Okay, now we have a position to branch from and a position to branch to,
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// insert the new conditional branch.
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EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
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loopPreheader->getTerminator());
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LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L);
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loopPreheader->getTerminator()->eraseFromParent();
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// We need to reprocess this loop, it could be unswitched again.
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// Now that we know that the loop is never entered when this condition is a
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// particular value, rewrite the loop with this info. We know that this will
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// at least eliminate the old branch.
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RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
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/// SplitExitEdges - Split all of the edges from inside the loop to their exit
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/// blocks. Update the appropriate Phi nodes as we do so.
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void LoopUnswitch::SplitExitEdges(Loop *L,
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const SmallVector<BasicBlock *, 8> &ExitBlocks)
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
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BasicBlock *ExitBlock = ExitBlocks[i];
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SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
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pred_end(ExitBlock));
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SplitBlockPredecessors(ExitBlock, Preds.data(), Preds.size(),
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/// UnswitchNontrivialCondition - We determined that the loop is profitable
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/// to unswitch when LIC equal Val. Split it into loop versions and test the
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/// condition outside of either loop. Return the loops created as Out1/Out2.
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void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
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Function *F = loopHeader->getParent();
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DEBUG(dbgs() << "loop-unswitch: Unswitching loop %"
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<< loopHeader->getName() << " [" << L->getBlocks().size()
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<< " blocks] in Function " << F->getName()
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<< " when '" << *Val << "' == " << *LIC << "\n");
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// First step, split the preheader and exit blocks, and add these blocks to
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// the LoopBlocks list.
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BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, this);
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LoopBlocks.push_back(NewPreheader);
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// We want the loop to come after the preheader, but before the exit blocks.
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LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
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SmallVector<BasicBlock*, 8> ExitBlocks;
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L->getUniqueExitBlocks(ExitBlocks);
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// Split all of the edges from inside the loop to their exit blocks. Update
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// the appropriate Phi nodes as we do so.
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SplitExitEdges(L, ExitBlocks);
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// The exit blocks may have been changed due to edge splitting, recompute.
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L->getUniqueExitBlocks(ExitBlocks);
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// Add exit blocks to the loop blocks.
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LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
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// Next step, clone all of the basic blocks that make up the loop (including
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// the loop preheader and exit blocks), keeping track of the mapping between
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// the instructions and blocks.
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NewBlocks.reserve(LoopBlocks.size());
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DenseMap<const Value*, Value*> ValueMap;
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for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
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BasicBlock *New = CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F);
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NewBlocks.push_back(New);
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ValueMap[LoopBlocks[i]] = New; // Keep the BB mapping.
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LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], New, L);
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// Splice the newly inserted blocks into the function right before the
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// original preheader.
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F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
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NewBlocks[0], F->end());
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// Now we create the new Loop object for the versioned loop.
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Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI, LPM);
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Loop *ParentLoop = L->getParentLoop();
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// Make sure to add the cloned preheader and exit blocks to the parent loop
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ParentLoop->addBasicBlockToLoop(NewBlocks[0], LI->getBase());
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
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BasicBlock *NewExit = cast<BasicBlock>(ValueMap[ExitBlocks[i]]);
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// The new exit block should be in the same loop as the old one.
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if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
648
ExitBBLoop->addBasicBlockToLoop(NewExit, LI->getBase());
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assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
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"Exit block should have been split to have one successor!");
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BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
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// If the successor of the exit block had PHI nodes, add an entry for
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for (BasicBlock::iterator I = ExitSucc->begin();
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(PN = dyn_cast<PHINode>(I)); ++I) {
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Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
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DenseMap<const Value *, Value*>::iterator It = ValueMap.find(V);
661
if (It != ValueMap.end()) V = It->second;
662
PN->addIncoming(V, NewExit);
666
// Rewrite the code to refer to itself.
667
for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
668
for (BasicBlock::iterator I = NewBlocks[i]->begin(),
669
E = NewBlocks[i]->end(); I != E; ++I)
670
RemapInstruction(I, ValueMap);
672
// Rewrite the original preheader to select between versions of the loop.
673
BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator());
674
assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
675
"Preheader splitting did not work correctly!");
677
// Emit the new branch that selects between the two versions of this loop.
678
EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
679
LPM->deleteSimpleAnalysisValue(OldBR, L);
680
OldBR->eraseFromParent();
682
LoopProcessWorklist.push_back(NewLoop);
685
// Now we rewrite the original code to know that the condition is true and the
686
// new code to know that the condition is false.
687
RewriteLoopBodyWithConditionConstant(L , LIC, Val, false);
689
// It's possible that simplifying one loop could cause the other to be
690
// deleted. If so, don't simplify it.
691
if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop)
692
RewriteLoopBodyWithConditionConstant(NewLoop, LIC, Val, true);
696
/// RemoveFromWorklist - Remove all instances of I from the worklist vector
698
static void RemoveFromWorklist(Instruction *I,
699
std::vector<Instruction*> &Worklist) {
700
std::vector<Instruction*>::iterator WI = std::find(Worklist.begin(),
702
while (WI != Worklist.end()) {
703
unsigned Offset = WI-Worklist.begin();
705
WI = std::find(Worklist.begin()+Offset, Worklist.end(), I);
709
/// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
710
/// program, replacing all uses with V and update the worklist.
711
static void ReplaceUsesOfWith(Instruction *I, Value *V,
712
std::vector<Instruction*> &Worklist,
713
Loop *L, LPPassManager *LPM) {
714
DEBUG(dbgs() << "Replace with '" << *V << "': " << *I);
716
// Add uses to the worklist, which may be dead now.
717
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
718
if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
719
Worklist.push_back(Use);
721
// Add users to the worklist which may be simplified now.
722
for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
724
Worklist.push_back(cast<Instruction>(*UI));
725
LPM->deleteSimpleAnalysisValue(I, L);
726
RemoveFromWorklist(I, Worklist);
727
I->replaceAllUsesWith(V);
728
I->eraseFromParent();
732
/// RemoveBlockIfDead - If the specified block is dead, remove it, update loop
733
/// information, and remove any dead successors it has.
735
void LoopUnswitch::RemoveBlockIfDead(BasicBlock *BB,
736
std::vector<Instruction*> &Worklist,
738
if (pred_begin(BB) != pred_end(BB)) {
739
// This block isn't dead, since an edge to BB was just removed, see if there
740
// are any easy simplifications we can do now.
741
if (BasicBlock *Pred = BB->getSinglePredecessor()) {
742
// If it has one pred, fold phi nodes in BB.
743
while (isa<PHINode>(BB->begin()))
744
ReplaceUsesOfWith(BB->begin(),
745
cast<PHINode>(BB->begin())->getIncomingValue(0),
748
// If this is the header of a loop and the only pred is the latch, we now
749
// have an unreachable loop.
750
if (Loop *L = LI->getLoopFor(BB))
751
if (loopHeader == BB && L->contains(Pred)) {
752
// Remove the branch from the latch to the header block, this makes
753
// the header dead, which will make the latch dead (because the header
754
// dominates the latch).
755
LPM->deleteSimpleAnalysisValue(Pred->getTerminator(), L);
756
Pred->getTerminator()->eraseFromParent();
757
new UnreachableInst(BB->getContext(), Pred);
759
// The loop is now broken, remove it from LI.
760
RemoveLoopFromHierarchy(L);
762
// Reprocess the header, which now IS dead.
763
RemoveBlockIfDead(BB, Worklist, L);
767
// If pred ends in a uncond branch, add uncond branch to worklist so that
768
// the two blocks will get merged.
769
if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator()))
770
if (BI->isUnconditional())
771
Worklist.push_back(BI);
776
DEBUG(dbgs() << "Nuking dead block: " << *BB);
778
// Remove the instructions in the basic block from the worklist.
779
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
780
RemoveFromWorklist(I, Worklist);
782
// Anything that uses the instructions in this basic block should have their
783
// uses replaced with undefs.
784
// If I is not void type then replaceAllUsesWith undef.
785
// This allows ValueHandlers and custom metadata to adjust itself.
786
if (!I->getType()->isVoidTy())
787
I->replaceAllUsesWith(UndefValue::get(I->getType()));
790
// If this is the edge to the header block for a loop, remove the loop and
791
// promote all subloops.
792
if (Loop *BBLoop = LI->getLoopFor(BB)) {
793
if (BBLoop->getLoopLatch() == BB)
794
RemoveLoopFromHierarchy(BBLoop);
797
// Remove the block from the loop info, which removes it from any loops it
802
// Remove phi node entries in successors for this block.
803
TerminatorInst *TI = BB->getTerminator();
804
SmallVector<BasicBlock*, 4> Succs;
805
for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
806
Succs.push_back(TI->getSuccessor(i));
807
TI->getSuccessor(i)->removePredecessor(BB);
810
// Unique the successors, remove anything with multiple uses.
811
array_pod_sort(Succs.begin(), Succs.end());
812
Succs.erase(std::unique(Succs.begin(), Succs.end()), Succs.end());
814
// Remove the basic block, including all of the instructions contained in it.
815
LPM->deleteSimpleAnalysisValue(BB, L);
816
BB->eraseFromParent();
817
// Remove successor blocks here that are not dead, so that we know we only
818
// have dead blocks in this list. Nondead blocks have a way of becoming dead,
819
// then getting removed before we revisit them, which is badness.
821
for (unsigned i = 0; i != Succs.size(); ++i)
822
if (pred_begin(Succs[i]) != pred_end(Succs[i])) {
823
// One exception is loop headers. If this block was the preheader for a
824
// loop, then we DO want to visit the loop so the loop gets deleted.
825
// We know that if the successor is a loop header, that this loop had to
826
// be the preheader: the case where this was the latch block was handled
827
// above and headers can only have two predecessors.
828
if (!LI->isLoopHeader(Succs[i])) {
829
Succs.erase(Succs.begin()+i);
834
for (unsigned i = 0, e = Succs.size(); i != e; ++i)
835
RemoveBlockIfDead(Succs[i], Worklist, L);
838
/// RemoveLoopFromHierarchy - We have discovered that the specified loop has
839
/// become unwrapped, either because the backedge was deleted, or because the
840
/// edge into the header was removed. If the edge into the header from the
841
/// latch block was removed, the loop is unwrapped but subloops are still alive,
842
/// so they just reparent loops. If the loops are actually dead, they will be
844
void LoopUnswitch::RemoveLoopFromHierarchy(Loop *L) {
845
LPM->deleteLoopFromQueue(L);
846
RemoveLoopFromWorklist(L);
849
// RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
850
// the value specified by Val in the specified loop, or we know it does NOT have
851
// that value. Rewrite any uses of LIC or of properties correlated to it.
852
void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
855
assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
857
// FIXME: Support correlated properties, like:
864
// FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
865
// selects, switches.
866
std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
867
std::vector<Instruction*> Worklist;
868
LLVMContext &Context = Val->getContext();
871
// If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
872
// in the loop with the appropriate one directly.
873
if (IsEqual || (isa<ConstantInt>(Val) &&
874
Val->getType()->isIntegerTy(1))) {
879
Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()),
880
!cast<ConstantInt>(Val)->getZExtValue());
882
for (unsigned i = 0, e = Users.size(); i != e; ++i)
883
if (Instruction *U = cast<Instruction>(Users[i])) {
886
U->replaceUsesOfWith(LIC, Replacement);
887
Worklist.push_back(U);
890
// Otherwise, we don't know the precise value of LIC, but we do know that it
891
// is certainly NOT "Val". As such, simplify any uses in the loop that we
892
// can. This case occurs when we unswitch switch statements.
893
for (unsigned i = 0, e = Users.size(); i != e; ++i)
894
if (Instruction *U = cast<Instruction>(Users[i])) {
898
Worklist.push_back(U);
900
// If we know that LIC is not Val, use this info to simplify code.
901
if (SwitchInst *SI = dyn_cast<SwitchInst>(U)) {
902
for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) {
903
if (SI->getCaseValue(i) == Val) {
904
// Found a dead case value. Don't remove PHI nodes in the
905
// successor if they become single-entry, those PHI nodes may
906
// be in the Users list.
908
// FIXME: This is a hack. We need to keep the successor around
909
// and hooked up so as to preserve the loop structure, because
910
// trying to update it is complicated. So instead we preserve the
911
// loop structure and put the block on a dead code path.
912
BasicBlock *Switch = SI->getParent();
913
SplitEdge(Switch, SI->getSuccessor(i), this);
914
// Compute the successors instead of relying on the return value
915
// of SplitEdge, since it may have split the switch successor
917
BasicBlock *NewSISucc = SI->getSuccessor(i);
918
BasicBlock *OldSISucc = *succ_begin(NewSISucc);
919
// Create an "unreachable" destination.
920
BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable",
923
new UnreachableInst(Context, Abort);
924
// Force the new case destination to branch to the "unreachable"
925
// block while maintaining a (dead) CFG edge to the old block.
926
NewSISucc->getTerminator()->eraseFromParent();
927
BranchInst::Create(Abort, OldSISucc,
928
ConstantInt::getTrue(Context), NewSISucc);
929
// Release the PHI operands for this edge.
930
for (BasicBlock::iterator II = NewSISucc->begin();
931
PHINode *PN = dyn_cast<PHINode>(II); ++II)
932
PN->setIncomingValue(PN->getBasicBlockIndex(Switch),
933
UndefValue::get(PN->getType()));
934
// Tell the domtree about the new block. We don't fully update the
935
// domtree here -- instead we force it to do a full recomputation
936
// after the pass is complete -- but we do need to inform it of
939
DT->addNewBlock(Abort, NewSISucc);
945
// TODO: We could do other simplifications, for example, turning
946
// LIC == Val -> false.
950
SimplifyCode(Worklist, L);
953
/// SimplifyCode - Okay, now that we have simplified some instructions in the
954
/// loop, walk over it and constant prop, dce, and fold control flow where
955
/// possible. Note that this is effectively a very simple loop-structure-aware
956
/// optimizer. During processing of this loop, L could very well be deleted, so
957
/// it must not be used.
959
/// FIXME: When the loop optimizer is more mature, separate this out to a new
962
void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
963
while (!Worklist.empty()) {
964
Instruction *I = Worklist.back();
967
// Simple constant folding.
968
if (Constant *C = ConstantFoldInstruction(I)) {
969
ReplaceUsesOfWith(I, C, Worklist, L, LPM);
974
if (isInstructionTriviallyDead(I)) {
975
DEBUG(dbgs() << "Remove dead instruction '" << *I);
977
// Add uses to the worklist, which may be dead now.
978
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
979
if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
980
Worklist.push_back(Use);
981
LPM->deleteSimpleAnalysisValue(I, L);
982
RemoveFromWorklist(I, Worklist);
983
I->eraseFromParent();
988
// Special case hacks that appear commonly in unswitched code.
989
switch (I->getOpcode()) {
990
case Instruction::Select:
991
if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(0))) {
992
ReplaceUsesOfWith(I, I->getOperand(!CB->getZExtValue()+1), Worklist, L,
997
case Instruction::And:
998
if (isa<ConstantInt>(I->getOperand(0)) &&
1000
I->getOperand(0)->getType()->isIntegerTy(1))
1001
cast<BinaryOperator>(I)->swapOperands();
1002
if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1)))
1003
if (CB->getType()->isIntegerTy(1)) {
1004
if (CB->isOne()) // X & 1 -> X
1005
ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM);
1007
ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM);
1011
case Instruction::Or:
1012
if (isa<ConstantInt>(I->getOperand(0)) &&
1014
I->getOperand(0)->getType()->isIntegerTy(1))
1015
cast<BinaryOperator>(I)->swapOperands();
1016
if (ConstantInt *CB = dyn_cast<ConstantInt>(I->getOperand(1)))
1017
if (CB->getType()->isIntegerTy(1)) {
1018
if (CB->isOne()) // X | 1 -> 1
1019
ReplaceUsesOfWith(I, I->getOperand(1), Worklist, L, LPM);
1021
ReplaceUsesOfWith(I, I->getOperand(0), Worklist, L, LPM);
1025
case Instruction::Br: {
1026
BranchInst *BI = cast<BranchInst>(I);
1027
if (BI->isUnconditional()) {
1028
// If BI's parent is the only pred of the successor, fold the two blocks
1030
BasicBlock *Pred = BI->getParent();
1031
BasicBlock *Succ = BI->getSuccessor(0);
1032
BasicBlock *SinglePred = Succ->getSinglePredecessor();
1033
if (!SinglePred) continue; // Nothing to do.
1034
assert(SinglePred == Pred && "CFG broken");
1036
DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- "
1037
<< Succ->getName() << "\n");
1039
// Resolve any single entry PHI nodes in Succ.
1040
while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
1041
ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM);
1043
// Move all of the successor contents from Succ to Pred.
1044
Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
1046
LPM->deleteSimpleAnalysisValue(BI, L);
1047
BI->eraseFromParent();
1048
RemoveFromWorklist(BI, Worklist);
1050
// If Succ has any successors with PHI nodes, update them to have
1051
// entries coming from Pred instead of Succ.
1052
Succ->replaceAllUsesWith(Pred);
1054
// Remove Succ from the loop tree.
1055
LI->removeBlock(Succ);
1056
LPM->deleteSimpleAnalysisValue(Succ, L);
1057
Succ->eraseFromParent();
1059
} else if (ConstantInt *CB = dyn_cast<ConstantInt>(BI->getCondition())){
1060
// Conditional branch. Turn it into an unconditional branch, then
1061
// remove dead blocks.
1062
break; // FIXME: Enable.
1064
DEBUG(dbgs() << "Folded branch: " << *BI);
1065
BasicBlock *DeadSucc = BI->getSuccessor(CB->getZExtValue());
1066
BasicBlock *LiveSucc = BI->getSuccessor(!CB->getZExtValue());
1067
DeadSucc->removePredecessor(BI->getParent(), true);
1068
Worklist.push_back(BranchInst::Create(LiveSucc, BI));
1069
LPM->deleteSimpleAnalysisValue(BI, L);
1070
BI->eraseFromParent();
1071
RemoveFromWorklist(BI, Worklist);
1074
RemoveBlockIfDead(DeadSucc, Worklist, L);