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| //===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// 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/IR/BasicBlock.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Type.h"
#include "llvm/Transforms/Utils.h"
using namespace llvm;
char UnifyFunctionExitNodes::ID = 0;
INITIALIZE_PASS(UnifyFunctionExitNodes, "mergereturn",
"Unify function exit nodes", false, false)
Pass *llvm::createUnifyFunctionExitNodesPass() {
return new UnifyFunctionExitNodes();
}
void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
// We preserve the non-critical-edgeness property
AU.addPreservedID(BreakCriticalEdgesID);
// This is a cluster of orthogonal Transforms
AU.addPreservedID(LowerSwitchID);
}
// 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.
//
std::vector<BasicBlock*> ReturningBlocks;
std::vector<BasicBlock*> UnreachableBlocks;
for (BasicBlock &I : F)
if (isa<ReturnInst>(I.getTerminator()))
ReturningBlocks.push_back(&I);
else if (isa<UnreachableInst>(I.getTerminator()))
UnreachableBlocks.push_back(&I);
// Then unreachable blocks.
if (UnreachableBlocks.empty()) {
UnreachableBlock = nullptr;
} else if (UnreachableBlocks.size() == 1) {
UnreachableBlock = UnreachableBlocks.front();
} else {
UnreachableBlock = BasicBlock::Create(F.getContext(),
"UnifiedUnreachableBlock", &F);
new UnreachableInst(F.getContext(), UnreachableBlock);
for (BasicBlock *BB : UnreachableBlocks) {
BB->getInstList().pop_back(); // Remove the unreachable inst.
BranchInst::Create(UnreachableBlock, BB);
}
}
// Now handle return blocks.
if (ReturningBlocks.empty()) {
ReturnBlock = nullptr;
return false; // No blocks return
} else if (ReturningBlocks.size() == 1) {
ReturnBlock = 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
// nodes (if the function returns values), and convert all of the return
// instructions into unconditional branches.
//
BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(),
"UnifiedReturnBlock", &F);
PHINode *PN = nullptr;
if (F.getReturnType()->isVoidTy()) {
ReturnInst::Create(F.getContext(), nullptr, NewRetBlock);
} else {
// If the function doesn't return void... add a PHI node to the block...
PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
"UnifiedRetVal");
NewRetBlock->getInstList().push_back(PN);
ReturnInst::Create(F.getContext(), PN, NewRetBlock);
}
// Loop over all of the blocks, replacing the return instruction with an
// unconditional branch.
//
for (BasicBlock *BB : ReturningBlocks) {
// Add an incoming element to the PHI node for every return instruction that
// is merging into this new block...
if (PN)
PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
BB->getInstList().pop_back(); // Remove the return insn
BranchInst::Create(NewRetBlock, BB);
}
ReturnBlock = NewRetBlock;
return true;
}
|