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 is distributed under the University of Illinois Open Source
   6 // 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 #include "llvm/ADT/StringExtras.h"
  24 using namespace llvm;
  25
  26 char UnifyFunctionExitNodes::ID = 0;
  27 static RegisterPass<UnifyFunctionExitNodes>
  28 X("mergereturn", "Unify function exit nodes");
  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(LowerSwitchID);
  40 }
  41
  42 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
  43 // BasicBlock, and converting all returns to unconditional branches to this
  44 // new basic block.  The singular exit node is returned.
  45 //
  46 // If there are no return stmts in the Function, a null pointer is returned.
  47 //
  48 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
  49   // Loop over all of the blocks in a function, tracking all of the blocks that
  50   // return.
  51   //
  52   std::vector<BasicBlock*> ReturningBlocks;
  53   std::vector<BasicBlock*> UnwindingBlocks;
  54   std::vector<BasicBlock*> UnreachableBlocks;
  55   for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
  56     if (isa<ReturnInst>(I->getTerminator()))
  57       ReturningBlocks.push_back(I);
  58     else if (isa<UnwindInst>(I->getTerminator()))
  59       UnwindingBlocks.push_back(I);
  60     else if (isa<UnreachableInst>(I->getTerminator()))
  61       UnreachableBlocks.push_back(I);
  62
  63   // Handle unwinding blocks first.
  64   if (UnwindingBlocks.empty()) {
  65     UnwindBlock = 0;
  66   } else if (UnwindingBlocks.size() == 1) {
  67     UnwindBlock = UnwindingBlocks.front();
  68   } else {
  69     UnwindBlock = BasicBlock::Create("UnifiedUnwindBlock", &F);
  70     new UnwindInst(UnwindBlock);
  71
  72     for (std::vector<BasicBlock*>::iterator I = UnwindingBlocks.begin(),
  73            E = UnwindingBlocks.end(); I != E; ++I) {
  74       BasicBlock *BB = *I;
  75       BB->getInstList().pop_back();  // Remove the unwind insn
  76       BranchInst::Create(UnwindBlock, BB);
  77     }
  78   }
  79
  80   // Then unreachable blocks.
  81   if (UnreachableBlocks.empty()) {
  82     UnreachableBlock = 0;
  83   } else if (UnreachableBlocks.size() == 1) {
  84     UnreachableBlock = UnreachableBlocks.front();
  85   } else {
  86     UnreachableBlock = BasicBlock::Create("UnifiedUnreachableBlock", &F);
  87     new UnreachableInst(UnreachableBlock);
  88
  89     for (std::vector<BasicBlock*>::iterator I = UnreachableBlocks.begin(),
  90            E = UnreachableBlocks.end(); I != E; ++I) {
  91       BasicBlock *BB = *I;
  92       BB->getInstList().pop_back();  // Remove the unreachable inst.
  93       BranchInst::Create(UnreachableBlock, BB);
  94     }
  95   }
  96
  97   // Now handle return blocks.
  98   if (ReturningBlocks.empty()) {
  99     ReturnBlock = 0;
 100     return false;                          // No blocks return
 101   } else if (ReturningBlocks.size() == 1) {
 102     ReturnBlock = ReturningBlocks.front(); // Already has a single return block
 103     return false;
 104   }
 105
 106   // Otherwise, we need to insert a new basic block into the function, add a PHI
 107   // nodes (if the function returns values), and convert all of the return
 108   // instructions into unconditional branches.
 109   //
 110   BasicBlock *NewRetBlock = BasicBlock::Create("UnifiedReturnBlock", &F);
 111
 112   PHINode *PN = 0;
 113   if (F.getReturnType() == Type::VoidTy) {
 114     ReturnInst::Create(NULL, NewRetBlock);
 115   } else {
 116     // If the function doesn't return void... add a PHI node to the block...
 117     PN = PHINode::Create(F.getReturnType(), "UnifiedRetVal");
 118     NewRetBlock->getInstList().push_back(PN);
 119     ReturnInst::Create(PN, NewRetBlock);
 120   }
 121
 122   // Loop over all of the blocks, replacing the return instruction with an
 123   // unconditional branch.
 124   //
 125   for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
 126          E = ReturningBlocks.end(); I != E; ++I) {
 127     BasicBlock *BB = *I;
 128
 129     // Add an incoming element to the PHI node for every return instruction that
 130     // is merging into this new block...
 131     if (PN)
 132       PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
 133
 134     BB->getInstList().pop_back();  // Remove the return insn
 135     BranchInst::Create(NewRetBlock, BB);
 136   }
 137   ReturnBlock = NewRetBlock;
 138   return true;
 139 }