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LoopUnswitch.cpp

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00001 //===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
00002 // 
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file was developed by the LLVM research group and is distributed under
00006 // the University of Illinois Open Source License. See LICENSE.TXT for details.
00007 // 
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This pass transforms loops that contain branches on loop-invariant conditions
00011 // to have multiple loops.  For example, it turns the left into the right code:
00012 //
00013 //  for (...)                  if (lic)
00014 //    A                          for (...)
00015 //    if (lic)                     A; B; C
00016 //      B                      else
00017 //    C                          for (...)
00018 //                                 A; C
00019 //
00020 // This can increase the size of the code exponentially (doubling it every time
00021 // a loop is unswitched) so we only unswitch if the resultant code will be
00022 // smaller than a threshold.
00023 //
00024 // This pass expects LICM to be run before it to hoist invariant conditions out
00025 // of the loop, to make the unswitching opportunity obvious.
00026 //
00027 //===----------------------------------------------------------------------===//
00028 
00029 #define DEBUG_TYPE "loop-unswitch"
00030 #include "llvm/Transforms/Scalar.h"
00031 #include "llvm/Constants.h"
00032 #include "llvm/Function.h"
00033 #include "llvm/Instructions.h"
00034 #include "llvm/Analysis/Dominators.h"
00035 #include "llvm/Analysis/LoopInfo.h"
00036 #include "llvm/Transforms/Utils/Cloning.h"
00037 #include "llvm/Transforms/Utils/Local.h"
00038 #include "llvm/Support/Debug.h"
00039 #include "llvm/ADT/Statistic.h"
00040 #include <algorithm>
00041 using namespace llvm;
00042 
00043 namespace {
00044   Statistic<> NumUnswitched("loop-unswitch", "Number of loops unswitched");
00045 
00046   class LoopUnswitch : public FunctionPass {
00047     LoopInfo *LI;  // Loop information
00048     DominatorSet *DS;
00049   public:
00050     virtual bool runOnFunction(Function &F);
00051     bool visitLoop(Loop *L);
00052 
00053     /// This transformation requires natural loop information & requires that
00054     /// loop preheaders be inserted into the CFG...
00055     ///
00056     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
00057       AU.addRequiredID(LoopSimplifyID);
00058       //AU.addRequired<DominatorSet>();
00059       AU.addRequired<LoopInfo>();
00060       AU.addPreserved<LoopInfo>();
00061     }
00062 
00063   private:
00064     void VersionLoop(Value *LIC, Loop *L);
00065     BasicBlock *SplitBlock(BasicBlock *BB, bool SplitAtTop);
00066     void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, bool Val);
00067   };
00068   RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
00069 }
00070 
00071 FunctionPass *createLoopUnswitchPass() { return new LoopUnswitch(); }
00072 
00073 bool LoopUnswitch::runOnFunction(Function &F) {
00074   bool Changed = false;
00075   LI = &getAnalysis<LoopInfo>();
00076   DS = 0; //&getAnalysis<DominatorSet>();
00077 
00078   // Transform all the top-level loops.  Copy the loop list so that the child
00079   // can update the loop tree if it needs to delete the loop.
00080   std::vector<Loop*> SubLoops(LI->begin(), LI->end());
00081   for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
00082     Changed |= visitLoop(SubLoops[i]);
00083 
00084   return Changed;
00085 }
00086 
00087 bool LoopUnswitch::visitLoop(Loop *L) {
00088   bool Changed = false;
00089 
00090   // Recurse through all subloops before we process this loop.  Copy the loop
00091   // list so that the child can update the loop tree if it needs to delete the
00092   // loop.
00093   std::vector<Loop*> SubLoops(L->begin(), L->end());
00094   for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
00095     Changed |= visitLoop(SubLoops[i]);
00096 
00097   // Loop over all of the basic blocks in the loop.  If we find an interior
00098   // block that is branching on a loop-invariant condition, we can unswitch this
00099   // loop.
00100   for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
00101        I != E; ++I) {
00102     TerminatorInst *TI = (*I)->getTerminator();
00103     if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
00104       if (!isa<Constant>(SI) && L->isLoopInvariant(SI->getCondition()))
00105         DEBUG(std::cerr << "Can't unswitching 'switch' loop %"
00106               << L->getHeader()->getName() << ", cost = "
00107               << L->getBlocks().size() << "\n" << **I);
00108     } else if (BranchInst *BI = dyn_cast<BranchInst>(TI))
00109       if (BI->isConditional() && !isa<Constant>(BI->getCondition()) &&
00110           L->isLoopInvariant(BI->getCondition())) {
00111         // Check to see if it would be profitable to unswitch this loop.
00112         if (L->getBlocks().size() > 10) {
00113           DEBUG(std::cerr << "NOT unswitching loop %"
00114                 << L->getHeader()->getName() << ", cost too high: "
00115                 << L->getBlocks().size() << "\n");
00116         } else {
00117           // FIXME: check for profitability.
00118           //std::cerr << "BEFORE:\n"; LI->dump();
00119           
00120           VersionLoop(BI->getCondition(), L);
00121           
00122           //std::cerr << "AFTER:\n"; LI->dump();
00123           return true;
00124         }
00125       }
00126   }
00127   
00128   return Changed;
00129 }
00130 
00131 /// SplitBlock - Split the specified basic block into two pieces.  If SplitAtTop
00132 /// is false, this splits the block so the second half only has an unconditional
00133 /// branch.  If SplitAtTop is true, it makes it so the first half of the block
00134 /// only has an unconditional branch in it.
00135 ///
00136 /// This method updates the LoopInfo for this function to correctly reflect the
00137 /// CFG changes made.
00138 BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *BB, bool SplitAtTop) {
00139   BasicBlock::iterator SplitPoint;
00140   if (!SplitAtTop)
00141     SplitPoint = BB->getTerminator();
00142   else {
00143     SplitPoint = BB->begin();
00144     while (isa<PHINode>(SplitPoint)) ++SplitPoint;
00145   }
00146 
00147   BasicBlock *New = BB->splitBasicBlock(SplitPoint, BB->getName()+".tail");
00148   // New now lives in whichever loop that BB used to.
00149   if (Loop *L = LI->getLoopFor(BB))
00150     L->addBasicBlockToLoop(New, *LI);
00151   return SplitAtTop ? BB : New;
00152 }
00153 
00154 
00155 // RemapInstruction - Convert the instruction operands from referencing the 
00156 // current values into those specified by ValueMap.
00157 //
00158 static inline void RemapInstruction(Instruction *I, 
00159                                     std::map<const Value *, Value*> &ValueMap) {
00160   for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
00161     Value *Op = I->getOperand(op);
00162     std::map<const Value *, Value*>::iterator It = ValueMap.find(Op);
00163     if (It != ValueMap.end()) Op = It->second;
00164     I->setOperand(op, Op);
00165   }
00166 }
00167 
00168 /// CloneLoop - Recursively clone the specified loop and all of its children,
00169 /// mapping the blocks with the specified map.
00170 static Loop *CloneLoop(Loop *L, Loop *PL, std::map<const Value*, Value*> &VM,
00171                        LoopInfo *LI) {
00172   Loop *New = new Loop();
00173 
00174   if (PL)
00175     PL->addChildLoop(New);
00176   else
00177     LI->addTopLevelLoop(New);
00178 
00179   // Add all of the blocks in L to the new loop.
00180   for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
00181        I != E; ++I)
00182     if (LI->getLoopFor(*I) == L)
00183       New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
00184 
00185   // Add all of the subloops to the new loop.
00186   for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
00187     CloneLoop(*I, New, VM, LI);
00188   
00189   return New;
00190 }
00191 
00192 
00193 /// InsertPHINodesForUsesOutsideLoop - If this instruction is used outside of
00194 /// the specified loop, insert a PHI node in the appropriate exit block to merge
00195 /// the values in the two different loop versions.
00196 ///
00197 /// Most values are not used outside of the loop they are defined in, so be
00198 /// efficient for this case.
00199 ///
00200 static AllocaInst *
00201 InsertPHINodesForUsesOutsideLoop(Instruction *OI, Instruction *NI,
00202                                  DominatorSet &DS, Loop *OL, Loop *NL,
00203                                  std::vector<BasicBlock*> &OldExitBlocks,
00204                                  std::map<const Value*, Value*> &ValueMap) {
00205   assert(OI->getType() == NI->getType() && OI->getOpcode() == NI->getOpcode() &&
00206          "Hrm, should be mapping between identical instructions!");
00207   for (Value::use_iterator UI = OI->use_begin(), E = OI->use_end(); UI != E;
00208        ++UI)
00209     if (!OL->contains(cast<Instruction>(*UI)->getParent()) &&
00210         !NL->contains(cast<Instruction>(*UI)->getParent()))
00211       goto UsedOutsideOfLoop;
00212   return 0;
00213   
00214 UsedOutsideOfLoop:
00215   // Okay, this instruction is used outside of the current loop.  Insert a PHI
00216   // nodes for the instruction merging the values together.
00217 
00218   // FIXME: For now we just spill the object to the stack, assuming that a
00219   // subsequent mem2reg pass will clean up after us.  This should be improved in
00220   // two ways:
00221   //  1. If there is only one exit block, trivially insert the PHI nodes
00222   //  2. Once we update domfrontier, we should do the promotion after everything
00223   //     is stable again.
00224   AllocaInst *Result = DemoteRegToStack(*OI);
00225 
00226   // Store to the stack location right after the new instruction.
00227   BasicBlock::iterator InsertPoint = NI;
00228   if (InvokeInst *II = dyn_cast<InvokeInst>(NI))
00229     InsertPoint = II->getNormalDest()->begin();
00230   else
00231     ++InsertPoint;
00232   while (isa<PHINode>(InsertPoint)) ++InsertPoint;
00233   new StoreInst(NI, Result, InsertPoint);
00234   return Result;
00235 }
00236 
00237 
00238 
00239 /// VersionLoop - We determined that the loop is profitable to unswitch and
00240 /// contains a branch on a loop invariant condition.  Split it into loop
00241 /// versions and test the condition outside of either loop.
00242 void LoopUnswitch::VersionLoop(Value *LIC, Loop *L) {
00243   Function *F = L->getHeader()->getParent();
00244 
00245   DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
00246         << L->getHeader()->getName() << " [" << L->getBlocks().size()
00247         << " blocks] in Function " << F->getName()
00248         << " on cond:" << *LIC << "\n");
00249 
00250   std::vector<BasicBlock*> LoopBlocks;
00251 
00252   // First step, split the preheader and exit blocks, and add these blocks to
00253   // the LoopBlocks list.
00254   BasicBlock *OrigPreheader = L->getLoopPreheader();
00255   LoopBlocks.push_back(SplitBlock(OrigPreheader, false));
00256 
00257   // We want the loop to come after the preheader, but before the exit blocks.
00258   LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
00259 
00260   std::vector<BasicBlock*> ExitBlocks;
00261   L->getExitBlocks(ExitBlocks);
00262   std::sort(ExitBlocks.begin(), ExitBlocks.end());
00263   ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
00264                    ExitBlocks.end());
00265   for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
00266     LoopBlocks.push_back(ExitBlocks[i] = SplitBlock(ExitBlocks[i], true));
00267 
00268   // Next step, clone all of the basic blocks that make up the loop (including
00269   // the loop preheader and exit blocks), keeping track of the mapping between
00270   // the instructions and blocks.
00271   std::vector<BasicBlock*> NewBlocks;
00272   NewBlocks.reserve(LoopBlocks.size());
00273   std::map<const Value*, Value*> ValueMap;
00274   for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
00275     NewBlocks.push_back(CloneBasicBlock(LoopBlocks[i], ValueMap, ".us", F));
00276     ValueMap[LoopBlocks[i]] = NewBlocks.back();  // Keep the BB mapping.
00277   }
00278 
00279   // Splice the newly inserted blocks into the function right before the
00280   // original preheader.
00281   F->getBasicBlockList().splice(LoopBlocks[0], F->getBasicBlockList(),
00282                                 NewBlocks[0], F->end());
00283 
00284   // Now we create the new Loop object for the versioned loop.
00285   Loop *NewLoop = CloneLoop(L, L->getParentLoop(), ValueMap, LI);
00286   if (Loop *Parent = L->getParentLoop()) {
00287     // Make sure to add the cloned preheader and exit blocks to the parent loop
00288     // as well.
00289     Parent->addBasicBlockToLoop(NewBlocks[0], *LI);
00290     for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
00291       Parent->addBasicBlockToLoop(cast<BasicBlock>(ValueMap[ExitBlocks[i]]),
00292                                   *LI);
00293   }
00294 
00295   // Rewrite the code to refer to itself.
00296   for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
00297     for (BasicBlock::iterator I = NewBlocks[i]->begin(),
00298            E = NewBlocks[i]->end(); I != E; ++I)
00299       RemapInstruction(I, ValueMap);
00300 
00301   // If the instructions are used outside of the loop, insert a PHI node in any
00302   // exit blocks dominated by the instruction.
00303   for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
00304     for (BasicBlock::iterator OI = LoopBlocks[i]->begin(),
00305            E = LoopBlocks[i]->end(); OI != E; ++OI)
00306       if (!OI->use_empty()) {
00307         std::map<const Value*,Value*>::iterator OII = ValueMap.find(OI);
00308         // The PHINode rewriting stuff can insert stores that are not in the
00309         // mapping.  Don't mess around with them.
00310         if (OII != ValueMap.end()) {
00311           Instruction *NI = cast<Instruction>(OII->second);
00312           InsertPHINodesForUsesOutsideLoop(OI, NI, *DS, L, NewLoop,
00313                                            ExitBlocks, ValueMap);
00314         }
00315       }
00316 
00317   // Rewrite the original preheader to select between versions of the loop.
00318   assert(isa<BranchInst>(OrigPreheader->getTerminator()) &&
00319          cast<BranchInst>(OrigPreheader->getTerminator())->isUnconditional() &&
00320          OrigPreheader->getTerminator()->getSuccessor(0) == LoopBlocks[0] &&
00321          "Preheader splitting did not work correctly!");
00322   // Remove the unconditional branch to LoopBlocks[0].
00323   OrigPreheader->getInstList().pop_back();
00324 
00325   // Insert a conditional branch on LIC to the two preheaders.  The original
00326   // code is the true version and the new code is the false version.
00327   new BranchInst(LoopBlocks[0], NewBlocks[0], LIC, OrigPreheader);
00328 
00329   // Now we rewrite the original code to know that the condition is true and the
00330   // new code to know that the condition is false.
00331   RewriteLoopBodyWithConditionConstant(L, LIC, true);
00332   RewriteLoopBodyWithConditionConstant(NewLoop, LIC, false);
00333   ++NumUnswitched;
00334 
00335   // Try to unswitch each of our new loops now!
00336   visitLoop(L);
00337   visitLoop(NewLoop);
00338 }
00339 
00340 // RewriteLoopBodyWithConditionConstant - We know that the boolean value LIC has
00341 // the value specified by Val in the specified loop.  Rewrite any uses of LIC or
00342 // of properties correlated to it.
00343 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
00344                                                         bool Val) {
00345   // FIXME: Support correlated properties, like:
00346   //  for (...)
00347   //    if (li1 < li2)
00348   //      ...
00349   //    if (li1 > li2)
00350   //      ...
00351   ConstantBool *BoolVal = ConstantBool::get(Val);
00352 
00353   std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
00354   for (unsigned i = 0, e = Users.size(); i != e; ++i)
00355     if (Instruction *U = dyn_cast<Instruction>(Users[i]))
00356       if (L->contains(U->getParent()))
00357         U->replaceUsesOfWith(LIC, BoolVal);
00358 }