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      1 //===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This pass transforms loops by placing phi nodes at the end of the loops for
     11 // all values that are live across the loop boundary.  For example, it turns
     12 // the left into the right code:
     13 //
     14 // for (...)                for (...)
     15 //   if (c)                   if (c)
     16 //     X1 = ...                 X1 = ...
     17 //   else                     else
     18 //     X2 = ...                 X2 = ...
     19 //   X3 = phi(X1, X2)         X3 = phi(X1, X2)
     20 // ... = X3 + 4             X4 = phi(X3)
     21 //                          ... = X4 + 4
     22 //
     23 // This is still valid LLVM; the extra phi nodes are purely redundant, and will
     24 // be trivially eliminated by InstCombine.  The major benefit of this
     25 // transformation is that it makes many other loop optimizations, such as
     26 // LoopUnswitching, simpler.
     27 //
     28 //===----------------------------------------------------------------------===//
     29 
     30 #include "llvm/Transforms/Scalar.h"
     31 #include "llvm/ADT/STLExtras.h"
     32 #include "llvm/ADT/Statistic.h"
     33 #include "llvm/Analysis/AliasAnalysis.h"
     34 #include "llvm/Analysis/LoopPass.h"
     35 #include "llvm/Analysis/ScalarEvolution.h"
     36 #include "llvm/IR/Constants.h"
     37 #include "llvm/IR/Dominators.h"
     38 #include "llvm/IR/Function.h"
     39 #include "llvm/IR/Instructions.h"
     40 #include "llvm/IR/PredIteratorCache.h"
     41 #include "llvm/Pass.h"
     42 #include "llvm/Transforms/Utils/LoopUtils.h"
     43 #include "llvm/Transforms/Utils/SSAUpdater.h"
     44 using namespace llvm;
     45 
     46 #define DEBUG_TYPE "lcssa"
     47 
     48 STATISTIC(NumLCSSA, "Number of live out of a loop variables");
     49 
     50 /// Return true if the specified block is in the list.
     51 static bool isExitBlock(BasicBlock *BB,
     52                         const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
     53   for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
     54     if (ExitBlocks[i] == BB)
     55       return true;
     56   return false;
     57 }
     58 
     59 /// Given an instruction in the loop, check to see if it has any uses that are
     60 /// outside the current loop.  If so, insert LCSSA PHI nodes and rewrite the
     61 /// uses.
     62 static bool processInstruction(Loop &L, Instruction &Inst, DominatorTree &DT,
     63                                const SmallVectorImpl<BasicBlock *> &ExitBlocks,
     64                                PredIteratorCache &PredCache) {
     65   SmallVector<Use *, 16> UsesToRewrite;
     66 
     67   BasicBlock *InstBB = Inst.getParent();
     68 
     69   for (Use &U : Inst.uses()) {
     70     Instruction *User = cast<Instruction>(U.getUser());
     71     BasicBlock *UserBB = User->getParent();
     72     if (PHINode *PN = dyn_cast<PHINode>(User))
     73       UserBB = PN->getIncomingBlock(U);
     74 
     75     if (InstBB != UserBB && !L.contains(UserBB))
     76       UsesToRewrite.push_back(&U);
     77   }
     78 
     79   // If there are no uses outside the loop, exit with no change.
     80   if (UsesToRewrite.empty())
     81     return false;
     82 
     83   ++NumLCSSA; // We are applying the transformation
     84 
     85   // Invoke instructions are special in that their result value is not available
     86   // along their unwind edge. The code below tests to see whether DomBB
     87   // dominates
     88   // the value, so adjust DomBB to the normal destination block, which is
     89   // effectively where the value is first usable.
     90   BasicBlock *DomBB = Inst.getParent();
     91   if (InvokeInst *Inv = dyn_cast<InvokeInst>(&Inst))
     92     DomBB = Inv->getNormalDest();
     93 
     94   DomTreeNode *DomNode = DT.getNode(DomBB);
     95 
     96   SmallVector<PHINode *, 16> AddedPHIs;
     97 
     98   SSAUpdater SSAUpdate;
     99   SSAUpdate.Initialize(Inst.getType(), Inst.getName());
    100 
    101   // Insert the LCSSA phi's into all of the exit blocks dominated by the
    102   // value, and add them to the Phi's map.
    103   for (SmallVectorImpl<BasicBlock *>::const_iterator BBI = ExitBlocks.begin(),
    104                                                      BBE = ExitBlocks.end();
    105        BBI != BBE; ++BBI) {
    106     BasicBlock *ExitBB = *BBI;
    107     if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
    108       continue;
    109 
    110     // If we already inserted something for this BB, don't reprocess it.
    111     if (SSAUpdate.HasValueForBlock(ExitBB))
    112       continue;
    113 
    114     PHINode *PN = PHINode::Create(Inst.getType(), PredCache.GetNumPreds(ExitBB),
    115                                   Inst.getName() + ".lcssa", ExitBB->begin());
    116 
    117     // Add inputs from inside the loop for this PHI.
    118     for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) {
    119       PN->addIncoming(&Inst, *PI);
    120 
    121       // If the exit block has a predecessor not within the loop, arrange for
    122       // the incoming value use corresponding to that predecessor to be
    123       // rewritten in terms of a different LCSSA PHI.
    124       if (!L.contains(*PI))
    125         UsesToRewrite.push_back(
    126             &PN->getOperandUse(PN->getOperandNumForIncomingValue(
    127                  PN->getNumIncomingValues() - 1)));
    128     }
    129 
    130     AddedPHIs.push_back(PN);
    131 
    132     // Remember that this phi makes the value alive in this block.
    133     SSAUpdate.AddAvailableValue(ExitBB, PN);
    134   }
    135 
    136   // Rewrite all uses outside the loop in terms of the new PHIs we just
    137   // inserted.
    138   for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
    139     // If this use is in an exit block, rewrite to use the newly inserted PHI.
    140     // This is required for correctness because SSAUpdate doesn't handle uses in
    141     // the same block.  It assumes the PHI we inserted is at the end of the
    142     // block.
    143     Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser());
    144     BasicBlock *UserBB = User->getParent();
    145     if (PHINode *PN = dyn_cast<PHINode>(User))
    146       UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);
    147 
    148     if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
    149       // Tell the VHs that the uses changed. This updates SCEV's caches.
    150       if (UsesToRewrite[i]->get()->hasValueHandle())
    151         ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin());
    152       UsesToRewrite[i]->set(UserBB->begin());
    153       continue;
    154     }
    155 
    156     // Otherwise, do full PHI insertion.
    157     SSAUpdate.RewriteUse(*UsesToRewrite[i]);
    158   }
    159 
    160   // Remove PHI nodes that did not have any uses rewritten.
    161   for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) {
    162     if (AddedPHIs[i]->use_empty())
    163       AddedPHIs[i]->eraseFromParent();
    164   }
    165 
    166   return true;
    167 }
    168 
    169 /// Return true if the specified block dominates at least
    170 /// one of the blocks in the specified list.
    171 static bool
    172 blockDominatesAnExit(BasicBlock *BB,
    173                      DominatorTree &DT,
    174                      const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
    175   DomTreeNode *DomNode = DT.getNode(BB);
    176   for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
    177     if (DT.dominates(DomNode, DT.getNode(ExitBlocks[i])))
    178       return true;
    179 
    180   return false;
    181 }
    182 
    183 bool llvm::formLCSSA(Loop &L, DominatorTree &DT, ScalarEvolution *SE) {
    184   bool Changed = false;
    185 
    186   // Get the set of exiting blocks.
    187   SmallVector<BasicBlock *, 8> ExitBlocks;
    188   L.getExitBlocks(ExitBlocks);
    189 
    190   if (ExitBlocks.empty())
    191     return false;
    192 
    193   PredIteratorCache PredCache;
    194 
    195   // Look at all the instructions in the loop, checking to see if they have uses
    196   // outside the loop.  If so, rewrite those uses.
    197   for (Loop::block_iterator BBI = L.block_begin(), BBE = L.block_end();
    198        BBI != BBE; ++BBI) {
    199     BasicBlock *BB = *BBI;
    200 
    201     // For large loops, avoid use-scanning by using dominance information:  In
    202     // particular, if a block does not dominate any of the loop exits, then none
    203     // of the values defined in the block could be used outside the loop.
    204     if (!blockDominatesAnExit(BB, DT, ExitBlocks))
    205       continue;
    206 
    207     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
    208       // Reject two common cases fast: instructions with no uses (like stores)
    209       // and instructions with one use that is in the same block as this.
    210       if (I->use_empty() ||
    211           (I->hasOneUse() && I->user_back()->getParent() == BB &&
    212            !isa<PHINode>(I->user_back())))
    213         continue;
    214 
    215       Changed |= processInstruction(L, *I, DT, ExitBlocks, PredCache);
    216     }
    217   }
    218 
    219   // If we modified the code, remove any caches about the loop from SCEV to
    220   // avoid dangling entries.
    221   // FIXME: This is a big hammer, can we clear the cache more selectively?
    222   if (SE && Changed)
    223     SE->forgetLoop(&L);
    224 
    225   assert(L.isLCSSAForm(DT));
    226 
    227   return Changed;
    228 }
    229 
    230 /// Process a loop nest depth first.
    231 bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT,
    232                                 ScalarEvolution *SE) {
    233   bool Changed = false;
    234 
    235   // Recurse depth-first through inner loops.
    236   for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
    237     Changed |= formLCSSARecursively(**LI, DT, SE);
    238 
    239   Changed |= formLCSSA(L, DT, SE);
    240   return Changed;
    241 }
    242 
    243 namespace {
    244 struct LCSSA : public FunctionPass {
    245   static char ID; // Pass identification, replacement for typeid
    246   LCSSA() : FunctionPass(ID) {
    247     initializeLCSSAPass(*PassRegistry::getPassRegistry());
    248   }
    249 
    250   // Cached analysis information for the current function.
    251   DominatorTree *DT;
    252   LoopInfo *LI;
    253   ScalarEvolution *SE;
    254 
    255   bool runOnFunction(Function &F) override;
    256 
    257   /// This transformation requires natural loop information & requires that
    258   /// loop preheaders be inserted into the CFG.  It maintains both of these,
    259   /// as well as the CFG.  It also requires dominator information.
    260   void getAnalysisUsage(AnalysisUsage &AU) const override {
    261     AU.setPreservesCFG();
    262 
    263     AU.addRequired<DominatorTreeWrapperPass>();
    264     AU.addRequired<LoopInfo>();
    265     AU.addPreservedID(LoopSimplifyID);
    266     AU.addPreserved<AliasAnalysis>();
    267     AU.addPreserved<ScalarEvolution>();
    268   }
    269 
    270 private:
    271   void verifyAnalysis() const override;
    272 };
    273 }
    274 
    275 char LCSSA::ID = 0;
    276 INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
    277 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
    278 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
    279 INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
    280 
    281 Pass *llvm::createLCSSAPass() { return new LCSSA(); }
    282 char &llvm::LCSSAID = LCSSA::ID;
    283 
    284 
    285 /// Process all loops in the function, inner-most out.
    286 bool LCSSA::runOnFunction(Function &F) {
    287   bool Changed = false;
    288   LI = &getAnalysis<LoopInfo>();
    289   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
    290   SE = getAnalysisIfAvailable<ScalarEvolution>();
    291 
    292   // Simplify each loop nest in the function.
    293   for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
    294     Changed |= formLCSSARecursively(**I, *DT, SE);
    295 
    296   return Changed;
    297 }
    298 
    299 static void verifyLoop(Loop &L, DominatorTree &DT) {
    300   // Recurse depth-first through inner loops.
    301   for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
    302     verifyLoop(**LI, DT);
    303 
    304   // Check the special guarantees that LCSSA makes.
    305   //assert(L.isLCSSAForm(DT) && "LCSSA form not preserved!");
    306 }
    307 
    308 void LCSSA::verifyAnalysis() const {
    309   // Verify each loop nest in the function, assuming LI still points at that
    310   // function's loop info.
    311   for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
    312     verifyLoop(**I, *DT);
    313 }
    314