lib/Transforms/Utils/UnifyFunctionExitNodes.cpp

   1 //===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This pass is used to ensure that functions have at most one return
  10 // instruction in them.  Additionally, it keeps track of which node is the new
  11 // exit node of the CFG.  If there are no exit nodes in the CFG, the getExitNode
  12 // method will return a null pointer.
  13 //
  14 //===----------------------------------------------------------------------===//
  15
  16 #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
  17 #include "llvm/IR/BasicBlock.h"
  18 #include "llvm/IR/Function.h"
  19 #include "llvm/IR/Instructions.h"
  20 #include "llvm/IR/Type.h"
  21 #include "llvm/Transforms/Utils.h"
  22 using namespace llvm;
  23
  24 char UnifyFunctionExitNodes::ID = 0;
  25 INITIALIZE_PASS(UnifyFunctionExitNodes, "mergereturn",
  26                 "Unify function exit nodes", false, false)
  27
  28 Pass *llvm::createUnifyFunctionExitNodesPass() {
  29   return new UnifyFunctionExitNodes();
  30 }
  31
  32 void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
  33   // We preserve the non-critical-edgeness property
  34   AU.addPreservedID(BreakCriticalEdgesID);
  35   // This is a cluster of orthogonal Transforms
  36   AU.addPreservedID(LowerSwitchID);
  37 }
  38
  39 // UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
  40 // BasicBlock, and converting all returns to unconditional branches to this
  41 // new basic block.  The singular exit node is returned.
  42 //
  43 // If there are no return stmts in the Function, a null pointer is returned.
  44 //
  45 bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
  46   // Loop over all of the blocks in a function, tracking all of the blocks that
  47   // return.
  48   //
  49   std::vector<BasicBlock*> ReturningBlocks;
  50   std::vector<BasicBlock*> UnreachableBlocks;
  51   for (BasicBlock &I : F)
  52     if (isa<ReturnInst>(I.getTerminator()))
  53       ReturningBlocks.push_back(&I);
  54     else if (isa<UnreachableInst>(I.getTerminator()))
  55       UnreachableBlocks.push_back(&I);
  56
  57   // Then unreachable blocks.
  58   if (UnreachableBlocks.empty()) {
  59     UnreachableBlock = nullptr;
  60   } else if (UnreachableBlocks.size() == 1) {
  61     UnreachableBlock = UnreachableBlocks.front();
  62   } else {
  63     UnreachableBlock = BasicBlock::Create(F.getContext(),
  64                                           "UnifiedUnreachableBlock", &F);
  65     new UnreachableInst(F.getContext(), UnreachableBlock);
  66
  67     for (BasicBlock *BB : UnreachableBlocks) {
  68       BB->getInstList().pop_back();  // Remove the unreachable inst.
  69       BranchInst::Create(UnreachableBlock, BB);
  70     }
  71   }
  72
  73   // Now handle return blocks.
  74   if (ReturningBlocks.empty()) {
  75     ReturnBlock = nullptr;
  76     return false;                          // No blocks return
  77   } else if (ReturningBlocks.size() == 1) {
  78     ReturnBlock = ReturningBlocks.front(); // Already has a single return block
  79     return false;
  80   }
  81
  82   // Otherwise, we need to insert a new basic block into the function, add a PHI
  83   // nodes (if the function returns values), and convert all of the return
  84   // instructions into unconditional branches.
  85   //
  86   BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(),
  87                                                "UnifiedReturnBlock", &F);
  88
  89   PHINode *PN = nullptr;
  90   if (F.getReturnType()->isVoidTy()) {
  91     ReturnInst::Create(F.getContext(), nullptr, NewRetBlock);
  92   } else {
  93     // If the function doesn't return void... add a PHI node to the block...
  94     PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
  95                          "UnifiedRetVal");
  96     NewRetBlock->getInstList().push_back(PN);
  97     ReturnInst::Create(F.getContext(), PN, NewRetBlock);
  98   }
  99
 100   // Loop over all of the blocks, replacing the return instruction with an
 101   // unconditional branch.
 102   //
 103   for (BasicBlock *BB : ReturningBlocks) {
 104     // Add an incoming element to the PHI node for every return instruction that
 105     // is merging into this new block...
 106     if (PN)
 107       PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
 108
 109     BB->getInstList().pop_back();  // Remove the return insn
 110     BranchInst::Create(NewRetBlock, BB);
 111   }
 112   ReturnBlock = NewRetBlock;
 113   return true;
 114 }