lib/Transforms/Utils/UnifyFunctionExitNodes.cpp

   1 //===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file was developed by the LLVM research group and is distributed under
   6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This pass is used to ensure that functions have at most one return
  11 // instruction in them.  Additionally, it keeps track of which node is the new
  12 // exit node of the CFG.  If there are no exit nodes in the CFG, the getExitNode
  13 // method will return a null pointer.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
  18 #include "llvm/Transforms/Scalar.h"
  19 #include "llvm/BasicBlock.h"
  20 #include "llvm/Function.h"
  21 #include "llvm/Instructions.h"
  22 #include "llvm/Type.h"
  23 using namespace llvm;
  24
  25 static RegisterOpt<UnifyFunctionExitNodes>
  26 X("mergereturn", "Unify function exit nodes");
  27
  28 int UnifyFunctionExitNodes::stub;
  29
  30 Pass *llvm::createUnifyFunctionExitNodesPass() {
  31   return new UnifyFunctionExitNodes();
  32 }
  33
  34 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
  35   // We preserve the non-critical-edgeness property
  36   AU.addPreservedID(BreakCriticalEdgesID);
  37   // This is a cluster of orthogonal Transforms
  38   AU.addPreservedID(PromoteMemoryToRegisterID);
  39   AU.addPreservedID(LowerSelectID);
  40   AU.addPreservedID(LowerSwitchID);
  41 }
  42
  43 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
  44 // BasicBlock, and converting all returns to unconditional branches to this
  45 // new basic block.  The singular exit node is returned.
  46 //
  47 // If there are no return stmts in the Function, a null pointer is returned.
  48 //
  49 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
  50   // Loop over all of the blocks in a function, tracking all of the blocks that
  51   // return.
  52   //
  53   std::vector<BasicBlock*> ReturningBlocks;
  54   std::vector<BasicBlock*> UnwindingBlocks;
  55   std::vector<BasicBlock*> UnreachableBlocks;
  56   for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
  57     if (isa<ReturnInst>(I->getTerminator()))
  58       ReturningBlocks.push_back(I);
  59     else if (isa<UnwindInst>(I->getTerminator()))
  60       UnwindingBlocks.push_back(I);
  61     else if (isa<UnreachableInst>(I->getTerminator()))
  62       UnreachableBlocks.push_back(I);
  63
  64   // Handle unwinding blocks first.
  65   if (UnwindingBlocks.empty()) {
  66     UnwindBlock = 0;
  67   } else if (UnwindingBlocks.size() == 1) {
  68     UnwindBlock = UnwindingBlocks.front();
  69   } else {
  70     UnwindBlock = new BasicBlock("UnifiedUnwindBlock", &F);
  71     new UnwindInst(UnwindBlock);
  72
  73     for (std::vector<BasicBlock*>::iterator I = UnwindingBlocks.begin(),
  74            E = UnwindingBlocks.end(); I != E; ++I) {
  75       BasicBlock *BB = *I;
  76       BB->getInstList().pop_back();  // Remove the unwind insn
  77       new BranchInst(UnwindBlock, BB);
  78     }
  79   }
  80
  81   // Then unreachable blocks.
  82   if (UnreachableBlocks.empty()) {
  83     UnreachableBlock = 0;
  84   } else if (UnreachableBlocks.size() == 1) {
  85     UnreachableBlock = UnreachableBlocks.front();
  86   } else {
  87     UnreachableBlock = new BasicBlock("UnifiedUnreachableBlock", &F);
  88     new UnreachableInst(UnreachableBlock);
  89
  90     for (std::vector<BasicBlock*>::iterator I = UnreachableBlocks.begin(),
  91            E = UnreachableBlocks.end(); I != E; ++I) {
  92       BasicBlock *BB = *I;
  93       BB->getInstList().pop_back();  // Remove the unreachable inst.
  94       new BranchInst(UnreachableBlock, BB);
  95     }
  96   }
  97
  98   // Now handle return blocks.
  99   if (ReturningBlocks.empty()) {
 100     ReturnBlock = 0;
 101     return false;                          // No blocks return
 102   } else if (ReturningBlocks.size() == 1) {
 103     ReturnBlock = ReturningBlocks.front(); // Already has a single return block
 104     return false;
 105   }
 106
 107   // Otherwise, we need to insert a new basic block into the function, add a PHI
 108   // node (if the function returns a value), and convert all of the return
 109   // instructions into unconditional branches.
 110   //
 111   BasicBlock *NewRetBlock = new BasicBlock("UnifiedReturnBlock", &F);
 112
 113   PHINode *PN = 0;
 114   if (F.getReturnType() != Type::VoidTy) {
 115     // If the function doesn't return void... add a PHI node to the block...
 116     PN = new PHINode(F.getReturnType(), "UnifiedRetVal");
 117     NewRetBlock->getInstList().push_back(PN);
 118   }
 119   new ReturnInst(PN, NewRetBlock);
 120
 121   // Loop over all of the blocks, replacing the return instruction with an
 122   // unconditional branch.
 123   //
 124   for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
 125          E = ReturningBlocks.end(); I != E; ++I) {
 126     BasicBlock *BB = *I;
 127
 128     // Add an incoming element to the PHI node for every return instruction that
 129     // is merging into this new block...
 130     if (PN) PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
 131
 132     BB->getInstList().pop_back();  // Remove the return insn
 133     new BranchInst(NewRetBlock, BB);
 134   }
 135   ReturnBlock = NewRetBlock;
 136   return true;
 137 }