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      1 //===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
      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 file implements dead code elimination and basic block merging, along
     11 // with a collection of other peephole control flow optimizations.  For example:
     12 //
     13 //   * Removes basic blocks with no predecessors.
     14 //   * Merges a basic block into its predecessor if there is only one and the
     15 //     predecessor only has one successor.
     16 //   * Eliminates PHI nodes for basic blocks with a single predecessor.
     17 //   * Eliminates a basic block that only contains an unconditional branch.
     18 //   * Changes invoke instructions to nounwind functions to be calls.
     19 //   * Change things like "if (x) if (y)" into "if (x&y)".
     20 //   * etc..
     21 //
     22 //===----------------------------------------------------------------------===//
     23 
     24 #include "llvm/Transforms/Scalar.h"
     25 #include "llvm/ADT/SmallPtrSet.h"
     26 #include "llvm/ADT/SmallVector.h"
     27 #include "llvm/ADT/Statistic.h"
     28 #include "llvm/Analysis/TargetTransformInfo.h"
     29 #include "llvm/IR/Attributes.h"
     30 #include "llvm/IR/CFG.h"
     31 #include "llvm/IR/Constants.h"
     32 #include "llvm/IR/DataLayout.h"
     33 #include "llvm/IR/Instructions.h"
     34 #include "llvm/IR/IntrinsicInst.h"
     35 #include "llvm/IR/Module.h"
     36 #include "llvm/Pass.h"
     37 #include "llvm/Transforms/Utils/Local.h"
     38 using namespace llvm;
     39 
     40 #define DEBUG_TYPE "simplifycfg"
     41 
     42 STATISTIC(NumSimpl, "Number of blocks simplified");
     43 
     44 namespace {
     45 struct CFGSimplifyPass : public FunctionPass {
     46   static char ID; // Pass identification, replacement for typeid
     47   CFGSimplifyPass() : FunctionPass(ID) {
     48     initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
     49   }
     50   bool runOnFunction(Function &F) override;
     51 
     52   void getAnalysisUsage(AnalysisUsage &AU) const override {
     53     AU.addRequired<TargetTransformInfo>();
     54   }
     55 };
     56 }
     57 
     58 char CFGSimplifyPass::ID = 0;
     59 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
     60                       false)
     61 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
     62 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
     63                     false)
     64 
     65 // Public interface to the CFGSimplification pass
     66 FunctionPass *llvm::createCFGSimplificationPass() {
     67   return new CFGSimplifyPass();
     68 }
     69 
     70 /// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
     71 /// node) return blocks, merge them together to promote recursive block merging.
     72 static bool mergeEmptyReturnBlocks(Function &F) {
     73   bool Changed = false;
     74 
     75   BasicBlock *RetBlock = nullptr;
     76 
     77   // Scan all the blocks in the function, looking for empty return blocks.
     78   for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
     79     BasicBlock &BB = *BBI++;
     80 
     81     // Only look at return blocks.
     82     ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
     83     if (!Ret) continue;
     84 
     85     // Only look at the block if it is empty or the only other thing in it is a
     86     // single PHI node that is the operand to the return.
     87     if (Ret != &BB.front()) {
     88       // Check for something else in the block.
     89       BasicBlock::iterator I = Ret;
     90       --I;
     91       // Skip over debug info.
     92       while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
     93         --I;
     94       if (!isa<DbgInfoIntrinsic>(I) &&
     95           (!isa<PHINode>(I) || I != BB.begin() ||
     96            Ret->getNumOperands() == 0 ||
     97            Ret->getOperand(0) != I))
     98         continue;
     99     }
    100 
    101     // If this is the first returning block, remember it and keep going.
    102     if (!RetBlock) {
    103       RetBlock = &BB;
    104       continue;
    105     }
    106 
    107     // Otherwise, we found a duplicate return block.  Merge the two.
    108     Changed = true;
    109 
    110     // Case when there is no input to the return or when the returned values
    111     // agree is trivial.  Note that they can't agree if there are phis in the
    112     // blocks.
    113     if (Ret->getNumOperands() == 0 ||
    114         Ret->getOperand(0) ==
    115           cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
    116       BB.replaceAllUsesWith(RetBlock);
    117       BB.eraseFromParent();
    118       continue;
    119     }
    120 
    121     // If the canonical return block has no PHI node, create one now.
    122     PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
    123     if (!RetBlockPHI) {
    124       Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
    125       pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
    126       RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
    127                                     std::distance(PB, PE), "merge",
    128                                     &RetBlock->front());
    129 
    130       for (pred_iterator PI = PB; PI != PE; ++PI)
    131         RetBlockPHI->addIncoming(InVal, *PI);
    132       RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
    133     }
    134 
    135     // Turn BB into a block that just unconditionally branches to the return
    136     // block.  This handles the case when the two return blocks have a common
    137     // predecessor but that return different things.
    138     RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
    139     BB.getTerminator()->eraseFromParent();
    140     BranchInst::Create(RetBlock, &BB);
    141   }
    142 
    143   return Changed;
    144 }
    145 
    146 /// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
    147 /// iterating until no more changes are made.
    148 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
    149                                    const DataLayout *DL) {
    150   bool Changed = false;
    151   bool LocalChange = true;
    152   while (LocalChange) {
    153     LocalChange = false;
    154 
    155     // Loop over all of the basic blocks and remove them if they are unneeded...
    156     //
    157     for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
    158       if (SimplifyCFG(BBIt++, TTI, DL)) {
    159         LocalChange = true;
    160         ++NumSimpl;
    161       }
    162     }
    163     Changed |= LocalChange;
    164   }
    165   return Changed;
    166 }
    167 
    168 // It is possible that we may require multiple passes over the code to fully
    169 // simplify the CFG.
    170 //
    171 bool CFGSimplifyPass::runOnFunction(Function &F) {
    172   if (skipOptnoneFunction(F))
    173     return false;
    174 
    175   const TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
    176   DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
    177   const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr;
    178   bool EverChanged = removeUnreachableBlocks(F);
    179   EverChanged |= mergeEmptyReturnBlocks(F);
    180   EverChanged |= iterativelySimplifyCFG(F, TTI, DL);
    181 
    182   // If neither pass changed anything, we're done.
    183   if (!EverChanged) return false;
    184 
    185   // iterativelySimplifyCFG can (rarely) make some loops dead.  If this happens,
    186   // removeUnreachableBlocks is needed to nuke them, which means we should
    187   // iterate between the two optimizations.  We structure the code like this to
    188   // avoid reruning iterativelySimplifyCFG if the second pass of
    189   // removeUnreachableBlocks doesn't do anything.
    190   if (!removeUnreachableBlocks(F))
    191     return true;
    192 
    193   do {
    194     EverChanged = iterativelySimplifyCFG(F, TTI, DL);
    195     EverChanged |= removeUnreachableBlocks(F);
    196   } while (EverChanged);
    197 
    198   return true;
    199 }
    200