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