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Diffstat (limited to 'llvm/examples/IRTransforms/SimplifyCFG.cpp')
| -rw-r--r-- | llvm/examples/IRTransforms/SimplifyCFG.cpp | 414 |
1 files changed, 0 insertions, 414 deletions
diff --git a/llvm/examples/IRTransforms/SimplifyCFG.cpp b/llvm/examples/IRTransforms/SimplifyCFG.cpp deleted file mode 100644 index 10658c9f095..00000000000 --- a/llvm/examples/IRTransforms/SimplifyCFG.cpp +++ /dev/null @@ -1,414 +0,0 @@ -//===- SimplifyCFG.cpp ----------------------------------------------------===// -// -// -// 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 file implements the control flow graph (CFG) simplifications -// presented as part of the 'Getting Started With LLVM: Basics' tutorial at the -// US LLVM Developers Meeting 2019. It also contains additional material. -// -// The current file contains three different CFG simplifications. There are -// multiple versions of each implementation (e.g. _v1 and _v2), which implement -// additional functionality (e.g. preserving analysis like the DominatorTree) or -// use additional utilities to simplify the code (e.g. LLVM's PatternMatch.h). -// The available simplifications are: -// 1. Trivially Dead block Removal (removeDeadBlocks_v[1,2]). -// This simplifications removes all blocks without predecessors in the CFG -// from a function. -// 2. Conditional Branch Elimination (eliminateCondBranches_v[1,2,3]) -// This simplification replaces conditional branches with constant integer -// conditions with unconditional branches. -// 3. Single Predecessor Block Merging (mergeIntoSinglePredecessor_v[1,2]) -// This simplification merges blocks with a single predecessor into the -// predecessor, if that block has a single successor. -// -// TODOs -// * Hook up pass to the new pass manager. -// * Preserve LoopInfo. -// * Add fixed point iteration to delete all dead blocks -// * Add implementation using reachability to discover dead blocks. -//===----------------------------------------------------------------------===// - -#include "SimplifyCFG.h" -#include "InitializePasses.h" -#include "llvm/Analysis/DomTreeUpdater.h" -#include "llvm/IR/Dominators.h" -#include "llvm/IR/Function.h" -#include "llvm/IR/PassManager.h" -#include "llvm/IR/PatternMatch.h" -#include "llvm/InitializePasses.h" -#include "llvm/Support/CommandLine.h" - -using namespace llvm; -using namespace PatternMatch; - -enum TutorialVersion { V1, V2, V3 }; -static cl::opt<TutorialVersion> - Version("tut-simplifycfg-version", cl::desc("Select tutorial version"), - cl::Hidden, cl::ValueOptional, cl::init(V1), - cl::values(clEnumValN(V1, "v1", "version 1"), - clEnumValN(V2, "v2", "version 2"), - clEnumValN(V3, "v3", "version 3"), - // Sentinel value for unspecified option. - clEnumValN(V3, "", ""))); - -#define DEBUG_TYPE "tut-simplifycfg" - -// Remove trivially dead blocks. First version, not preserving the -// DominatorTree. -static bool removeDeadBlocks_v1(Function &F) { - bool Changed = false; - - // Remove trivially dead blocks. - for (BasicBlock &BB : make_early_inc_range(F)) { - // Skip blocks we know to not be trivially dead. We know a block is - // guaranteed to be dead, iff it is neither the entry block nor - // has any predecessors. - if (&F.getEntryBlock() == &BB || !pred_empty(&BB)) - continue; - - // Notify successors of BB that BB is going to be removed. This removes - // incoming values from BB from PHIs in the successors. Note that this will - // not actually remove BB from the predecessor lists of its successors. - for (BasicBlock *Succ : successors(&BB)) - Succ->removePredecessor(&BB); - // TODO: Find a better place to put such small variations. - // Alternatively, we can update the PHI nodes manually: - // for (PHINode &PN : make_early_inc_range(Succ->phis())) - // PN.removeIncomingValue(&BB); - - // Replace all instructions in BB with an undef constant. The block is - // unreachable, so the results of the instructions should never get used. - while (!BB.empty()) { - Instruction &I = BB.back(); - I.replaceAllUsesWith(UndefValue::get(I.getType())); - I.eraseFromParent(); - } - - // Finally remove the basic block. - BB.eraseFromParent(); - Changed = true; - } - - return Changed; -} - -// Remove trivially dead blocks. This is the second version and preserves the -// dominator tree. -static bool removeDeadBlocks_v2(Function &F, DominatorTree &DT) { - bool Changed = false; - DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy); - SmallVector<DominatorTree::UpdateType, 8> DTUpdates; - - // Remove trivially dead blocks. - for (BasicBlock &BB : make_early_inc_range(F)) { - // Skip blocks we know to not be trivially dead. We know a block is - // guaranteed to be dead, iff it is neither the entry block nor - // has any predecessors. - if (&F.getEntryBlock() == &BB || !pred_empty(&BB)) - continue; - - // Notify successors of BB that BB is going to be removed. This removes - // incoming values from BB from PHIs in the successors. Note that this will - // not actually remove BB from the predecessor lists of its successors. - for (BasicBlock *Succ : successors(&BB)) { - Succ->removePredecessor(&BB); - - // Collect updates that need to be applied to the dominator tree. - DTUpdates.push_back({DominatorTree::Delete, &BB, Succ}); - } - - // Remove BB via the DomTreeUpdater. DomTreeUpdater::deleteBB conveniently - // removes the instructions in BB as well. - DTU.deleteBB(&BB); - Changed = true; - } - - // Apply updates permissively, to remove duplicates. - DTU.applyUpdatesPermissive(DTUpdates); - - return Changed; -} - -// Eliminate branches with constant conditionals. This is the first version, -// which *does not* preserve the dominator tree. -static bool eliminateCondBranches_v1(Function &F) { - bool Changed = false; - - // Eliminate branches with constant conditionals. - for (BasicBlock &BB : F) { - // Skip blocks without conditional branches as terminators. - BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator()); - if (!BI || !BI->isConditional()) - continue; - - // Skip blocks with conditional branches without ConstantInt conditions. - ConstantInt *CI = dyn_cast<ConstantInt>(BI->getCondition()); - if (!CI) - continue; - - // We use the branch condition (CI), to select the successor we remove: - // if CI == 1 (true), we remove the second successor, otherwise the first. - BasicBlock *RemovedSucc = BI->getSuccessor(CI->isOne()); - // Tell RemovedSucc we will remove BB from its predecessors. - RemovedSucc->removePredecessor(&BB); - - // Replace the conditional branch with an unconditional one, by creating - // a new unconditional branch to the selected successor and removing the - // conditional one. - BranchInst::Create(BI->getSuccessor(CI->isZero()), BI); - BI->eraseFromParent(); - Changed = true; - } - - return Changed; -} - -// Eliminate branches with constant conditionals. This is the second -// version, which *does* preserve the dominator tree. -static bool eliminateCondBranches_v2(Function &F, DominatorTree &DT) { - bool Changed = false; - - DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy); - SmallVector<DominatorTree::UpdateType, 8> DTUpdates; - // Eliminate branches with constant conditionals. - for (BasicBlock &BB : F) { - // Skip blocks without conditional branches as terminators. - BranchInst *BI = dyn_cast<BranchInst>(BB.getTerminator()); - if (!BI || !BI->isConditional()) - continue; - - // Skip blocks with conditional branches without ConstantInt conditions. - ConstantInt *CI = dyn_cast<ConstantInt>(BI->getCondition()); - if (!CI) - continue; - - // We use the branch condition (CI), to select the successor we remove: - // if CI == 1 (true), we remove the second successor, otherwise the first. - BasicBlock *RemovedSucc = BI->getSuccessor(CI->isOne()); - // Tell RemovedSucc we will remove BB from its predecessors. - RemovedSucc->removePredecessor(&BB); - - // Replace the conditional branch with an unconditional one, by creating - // a new unconditional branch to the selected successor and removing the - // conditional one. - BranchInst *NewBranch = - BranchInst::Create(BI->getSuccessor(CI->isZero()), BI); - BI->eraseFromParent(); - - // Delete the edge between BB and RemovedSucc in the DominatorTree, iff - // the conditional branch did not use RemovedSucc as both the true and false - // branches. - if (NewBranch->getSuccessor(0) != RemovedSucc) - DTUpdates.push_back({DominatorTree::Delete, &BB, RemovedSucc}); - Changed = true; - } - - // Apply updates permissively, to remove duplicates. - DTU.applyUpdatesPermissive(DTUpdates); - - return Changed; -} - -// Eliminate branches with constant conditionals. This is the third -// version, which uses PatternMatch.h. -static bool eliminateCondBranches_v3(Function &F, DominatorTree &DT) { - bool Changed = false; - DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy); - SmallVector<DominatorTree::UpdateType, 8> DTUpdates; - - // Eliminate branches with constant conditionals. - for (BasicBlock &BB : F) { - ConstantInt *CI = nullptr; - BasicBlock *TakenSucc, *RemovedSucc; - // Check if the terminator is a conditional branch, with constant integer - // condition and also capture the successor blocks as TakenSucc and - // RemovedSucc. - if (!match(BB.getTerminator(), - m_Br(m_ConstantInt(CI), m_BasicBlock(TakenSucc), - m_BasicBlock(RemovedSucc)))) - continue; - - // If the condition is false, swap TakenSucc and RemovedSucc. - if (CI->isZero()) - std::swap(TakenSucc, RemovedSucc); - - // Tell RemovedSucc we will remove BB from its predecessors. - RemovedSucc->removePredecessor(&BB); - - // Replace the conditional branch with an unconditional one, by creating - // a new unconditional branch to the selected successor and removing the - // conditional one. - - BranchInst *NewBranch = BranchInst::Create(TakenSucc, BB.getTerminator()); - BB.getTerminator()->eraseFromParent(); - - // Delete the edge between BB and RemovedSucc in the DominatorTree, iff - // the conditional branch did not use RemovedSucc as both the true and false - // branches. - if (NewBranch->getSuccessor(0) != RemovedSucc) - DTUpdates.push_back({DominatorTree::Delete, &BB, RemovedSucc}); - Changed = true; - } - - // Apply updates permissively, to remove duplicates. - DTU.applyUpdatesPermissive(DTUpdates); - return Changed; -} - -// Merge basic blocks into their single predecessor, if their predecessor has a -// single successor. This is the first version and does not preserve the -// DominatorTree. -static bool mergeIntoSinglePredecessor_v1(Function &F) { - bool Changed = false; - - // Merge blocks with single predecessors. - for (BasicBlock &BB : make_early_inc_range(F)) { - BasicBlock *Pred = BB.getSinglePredecessor(); - // Make sure BB has a single predecessor Pred and BB is the single - // successor of Pred. - if (!Pred || Pred->getSingleSuccessor() != &BB) - continue; - - // Do not try to merge self loops. That can happen in dead blocks. - if (Pred == &BB) - continue; - - // Need to replace it before nuking the branch. - BB.replaceAllUsesWith(Pred); - // PHI nodes in BB can only have a single incoming value. Remove them. - for (PHINode &PN : make_early_inc_range(BB.phis())) { - PN.replaceAllUsesWith(PN.getIncomingValue(0)); - PN.eraseFromParent(); - } - // Move all instructions from BB to Pred. - for (Instruction &I : make_early_inc_range(BB)) - I.moveBefore(Pred->getTerminator()); - - // Remove the Pred's terminator (which jumped to BB). BB's terminator - // will become Pred's terminator. - Pred->getTerminator()->eraseFromParent(); - BB.eraseFromParent(); - - Changed = true; - } - - return Changed; -} - -// Merge basic blocks into their single predecessor, if their predecessor has a -// single successor. This is the second version and does preserve the -// DominatorTree. -static bool mergeIntoSinglePredecessor_v2(Function &F, DominatorTree &DT) { - bool Changed = false; - DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy); - SmallVector<DominatorTree::UpdateType, 8> DTUpdates; - - // Merge blocks with single predecessors. - for (BasicBlock &BB : make_early_inc_range(F)) { - BasicBlock *Pred = BB.getSinglePredecessor(); - // Make sure BB has a single predecessor Pred and BB is the single - // successor of Pred. - if (!Pred || Pred->getSingleSuccessor() != &BB) - continue; - - // Do not try to merge self loops. That can happen in dead blocks. - if (Pred == &BB) - continue; - - // Tell DTU about the changes to the CFG: All edges from BB to its - // successors get removed and we add edges between Pred and BB's successors. - for (BasicBlock *Succ : successors(&BB)) { - DTUpdates.push_back({DominatorTree::Delete, &BB, Succ}); - DTUpdates.push_back({DominatorTree::Insert, Pred, Succ}); - } - // Also remove the edge between Pred and BB. - DTUpdates.push_back({DominatorTree::Delete, Pred, &BB}); - - // Need to replace it before nuking the branch. - BB.replaceAllUsesWith(Pred); - // PHI nodes in BB can only have a single incoming value. Remove them. - for (PHINode &PN : make_early_inc_range(BB.phis())) { - PN.replaceAllUsesWith(PN.getIncomingValue(0)); - PN.eraseFromParent(); - } - // Move all instructions from BB to Pred. - for (Instruction &I : make_early_inc_range(BB)) - I.moveBefore(Pred->getTerminator()); - - // Remove the Pred's terminator (which jumped to BB). BB's terminator - // will become Pred's terminator. - Pred->getTerminator()->eraseFromParent(); - DTU.deleteBB(&BB); - - Changed = true; - } - - // Apply updates permissively, to remove duplicates. - DTU.applyUpdatesPermissive(DTUpdates); - return Changed; -} - -static bool doSimplify_v1(Function &F) { - return eliminateCondBranches_v1(F) & mergeIntoSinglePredecessor_v1(F) & - removeDeadBlocks_v1(F); -} - -static bool doSimplify_v2(Function &F, DominatorTree &DT) { - return eliminateCondBranches_v2(F, DT) & - mergeIntoSinglePredecessor_v2(F, DT) & removeDeadBlocks_v2(F, DT); -} - -static bool doSimplify_v3(Function &F, DominatorTree &DT) { - return eliminateCondBranches_v3(F, DT) & - mergeIntoSinglePredecessor_v2(F, DT) & removeDeadBlocks_v2(F, DT); -} - -namespace { -struct SimplifyCFGLegacyPass : public FunctionPass { - static char ID; - SimplifyCFGLegacyPass() : FunctionPass(ID) { - initializeSimplifyCFGLegacyPassPass(*PassRegistry::getPassRegistry()); - } - - void getAnalysisUsage(AnalysisUsage &AU) const override { - AU.addRequired<DominatorTreeWrapperPass>(); - // Version 1 of the implementation does not preserve the dominator tree. - if (Version != V1) - AU.addPreserved<DominatorTreeWrapperPass>(); - - FunctionPass::getAnalysisUsage(AU); - } - - bool runOnFunction(Function &F) override { - if (skipFunction(F)) - return false; - - switch (Version) { - case V1: - return doSimplify_v1(F); - case V2: { - auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree(); - return doSimplify_v2(F, DT); - } - case V3: { - auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree(); - return doSimplify_v3(F, DT); - } - } - - llvm_unreachable("Unsupported version"); - } -}; -} // namespace - -char SimplifyCFGLegacyPass::ID = 0; -INITIALIZE_PASS_BEGIN(SimplifyCFGLegacyPass, DEBUG_TYPE, - "Tutorial CFG simplification", false, false) -INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) -INITIALIZE_PASS_END(SimplifyCFGLegacyPass, DEBUG_TYPE, - "Tutorial CFG simplifications", false, false) |
