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//===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
//
// This pass is used to ensure that functions have at most one return
// instruction in them. Additionally, it keeps track of which node is the new
// exit node of the CFG. If there are no exit nodes in the CFG, the getExitNode
// method will return a null pointer.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
#include "llvm/BasicBlock.h"
#include "llvm/Function.h"
#include "llvm/iTerminators.h"
#include "llvm/iPHINode.h"
#include "llvm/Type.h"
using std::vector;
static RegisterOpt<UnifyFunctionExitNodes>
X("mergereturn", "Unify function exit nodes");
// UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
// BasicBlock, and converting all returns to unconditional branches to this
// new basic block. The singular exit node is returned.
//
// If there are no return stmts in the Function, a null pointer is returned.
//
bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
// Loop over all of the blocks in a function, tracking all of the blocks that
// return.
//
vector<BasicBlock*> ReturningBlocks;
for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
if (isa<ReturnInst>(I->getTerminator()))
ReturningBlocks.push_back(I);
if (ReturningBlocks.empty()) {
ExitNode = 0;
return false; // No blocks return
} else if (ReturningBlocks.size() == 1) {
ExitNode = ReturningBlocks.front(); // Already has a single return block
return false;
}
// Otherwise, we need to insert a new basic block into the function, add a PHI
// node (if the function returns a value), and convert all of the return
// instructions into unconditional branches.
//
BasicBlock *NewRetBlock = new BasicBlock("UnifiedExitNode", &F);
if (F.getReturnType() != Type::VoidTy) {
// If the function doesn't return void... add a PHI node to the block...
PHINode *PN = new PHINode(F.getReturnType(), "UnifiedRetVal",
NewRetBlock->end());
// Add an incoming element to the PHI node for every return instruction that
// is merging into this new block...
for (vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
E = ReturningBlocks.end(); I != E; ++I)
PN->addIncoming((*I)->getTerminator()->getOperand(0), *I);
// Add a return instruction to return the result of the PHI node...
new ReturnInst(PN, NewRetBlock->end());
} else {
// If it returns void, just add a return void instruction to the block
new ReturnInst(0, NewRetBlock->end());
}
// Loop over all of the blocks, replacing the return instruction with an
// unconditional branch.
//
for (vector<BasicBlock*>::iterator I = ReturningBlocks.begin(),
E = ReturningBlocks.end(); I != E; ++I) {
(*I)->getInstList().pop_back(); // Remove the return insn
new BranchInst(NewRetBlock, (*I)->end());
}
ExitNode = NewRetBlock;
return true;
}
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