LLVM API Documentation

UnifyFunctionExitNodes.cpp

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00001 //===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
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 is used to ensure that functions have at most one return
00011 // instruction in them.  Additionally, it keeps track of which node is the new
00012 // exit node of the CFG.  If there are no exit nodes in the CFG, the getExitNode
00013 // method will return a null pointer.
00014 //
00015 //===----------------------------------------------------------------------===//
00016 
00017 #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
00018 #include "llvm/Transforms/Scalar.h"
00019 #include "llvm/BasicBlock.h"
00020 #include "llvm/Function.h"
00021 #include "llvm/Instructions.h"
00022 #include "llvm/Type.h"
00023 using namespace llvm;
00024 
00025 static RegisterOpt<UnifyFunctionExitNodes>
00026 X("mergereturn", "Unify function exit nodes");
00027 
00028 int UnifyFunctionExitNodes::stub;
00029 
00030 Pass *llvm::createUnifyFunctionExitNodesPass() {
00031   return new UnifyFunctionExitNodes();
00032 }
00033 
00034 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
00035   // We preserve the non-critical-edgeness property
00036   AU.addPreservedID(BreakCriticalEdgesID);
00037   // This is a cluster of orthogonal Transforms
00038   AU.addPreservedID(PromoteMemoryToRegisterID);
00039   AU.addPreservedID(LowerSelectID);
00040   AU.addPreservedID(LowerSwitchID);
00041 }
00042 
00043 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
00044 // BasicBlock, and converting all returns to unconditional branches to this
00045 // new basic block.  The singular exit node is returned.
00046 //
00047 // If there are no return stmts in the Function, a null pointer is returned.
00048 //
00049 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
00050   // Loop over all of the blocks in a function, tracking all of the blocks that
00051   // return.
00052   //
00053   std::vector<BasicBlock*> ReturningBlocks;
00054   std::vector<BasicBlock*> UnwindingBlocks;
00055   std::vector<BasicBlock*> UnreachableBlocks;
00056   for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
00057     if (isa<ReturnInst>(I->getTerminator()))
00058       ReturningBlocks.push_back(I);
00059     else if (isa<UnwindInst>(I->getTerminator()))
00060       UnwindingBlocks.push_back(I);
00061     else if (isa<UnreachableInst>(I->getTerminator()))
00062       UnreachableBlocks.push_back(I);
00063 
00064   // Handle unwinding blocks first.
00065   if (UnwindingBlocks.empty()) {
00066     UnwindBlock = 0;
00067   } else if (UnwindingBlocks.size() == 1) {
00068     UnwindBlock = UnwindingBlocks.front();
00069   } else {
00070     UnwindBlock = new BasicBlock("UnifiedUnwindBlock", &F);
00071     new UnwindInst(UnwindBlock);
00072 
00073     for (std::vector<BasicBlock*>::iterator I = UnwindingBlocks.begin(),
00074            E = UnwindingBlocks.end(); I != E; ++I) {
00075       BasicBlock *BB = *I;
00076       BB->getInstList().pop_back();  // Remove the unwind insn
00077       new BranchInst(UnwindBlock, BB);
00078     }
00079   }
00080 
00081   // Then unreachable blocks.
00082   if (UnreachableBlocks.empty()) {
00083     UnreachableBlock = 0;
00084   } else if (UnreachableBlocks.size() == 1) {
00085     UnreachableBlock = UnreachableBlocks.front();
00086   } else {
00087     UnreachableBlock = new BasicBlock("UnifiedUnreachableBlock", &F);
00088     new UnreachableInst(UnreachableBlock);
00089 
00090     for (std::vector<BasicBlock*>::iterator I = UnreachableBlocks.begin(),
00091            E = UnreachableBlocks.end(); I != E; ++I) {
00092       BasicBlock *BB = *I;
00093       BB->getInstList().pop_back();  // Remove the unreachable inst.
00094       new BranchInst(UnreachableBlock, BB);
00095     }
00096   }
00097 
00098   // Now handle return blocks.
00099   if (ReturningBlocks.empty()) {
00100     ReturnBlock = 0;
00101     return false;                          // No blocks return
00102   } else if (ReturningBlocks.size() == 1) {
00103     ReturnBlock = ReturningBlocks.front(); // Already has a single return block
00104     return false;
00105   }
00106 
00107   // Otherwise, we need to insert a new basic block into the function, add a PHI
00108   // node (if the function returns a value), and convert all of the return
00109   // instructions into unconditional branches.
00110   //
00111   BasicBlock *NewRetBlock = new BasicBlock("UnifiedReturnBlock", &F);
00112 
00113   PHINode *PN = 0;
00114   if (F.getReturnType() != Type::VoidTy) {
00115     // If the function doesn't return void... add a PHI node to the block...
00116     PN = new PHINode(F.getReturnType(), "UnifiedRetVal");
00117     NewRetBlock->getInstList().push_back(PN);
00118   }
00119   new ReturnInst(PN, NewRetBlock);
00120 
00121   // Loop over all of the blocks, replacing the return instruction with an
00122   // unconditional branch.
00123   //
00124   for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
00125          E = ReturningBlocks.end(); I != E; ++I) {
00126     BasicBlock *BB = *I;
00127 
00128     // Add an incoming element to the PHI node for every return instruction that
00129     // is merging into this new block...
00130     if (PN) PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
00131 
00132     BB->getInstList().pop_back();  // Remove the return insn
00133     new BranchInst(NewRetBlock, BB);
00134   }
00135   ReturnBlock = NewRetBlock;
00136   return true;
00137 }