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