+++ /dev/null
-//===- 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)
+++ /dev/null
-; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt -tut-simplifycfg -tut-simplifycfg-version=v1 < %s -S -verify-dom-info | FileCheck %s
-; RUN: opt -tut-simplifycfg -tut-simplifycfg-version=v2 < %s -S -verify-dom-info | FileCheck %s
-; RUN: opt -tut-simplifycfg -tut-simplifycfg-version=v3 < %s -S -verify-dom-info | FileCheck %s
-
-define void @test() {
-; CHECK-LABEL: @test(
-; CHECK-NEXT: entry:
-; CHECK-NEXT: switch i32 undef, label [[SW_DEFAULT23:%.*]] [
-; CHECK-NEXT: i32 129, label [[SW_BB:%.*]]
-; CHECK-NEXT: i32 215, label [[SW_BB1:%.*]]
-; CHECK-NEXT: i32 117, label [[SW_BB1]]
-; CHECK-NEXT: i32 207, label [[SW_BB1]]
-; CHECK-NEXT: i32 158, label [[SW_BB1]]
-; CHECK-NEXT: i32 94, label [[SW_BB1]]
-; CHECK-NEXT: i32 219, label [[SW_BB1]]
-; CHECK-NEXT: i32 88, label [[SW_BB1]]
-; CHECK-NEXT: i32 168, label [[SW_BB1]]
-; CHECK-NEXT: i32 295, label [[SW_BB1]]
-; CHECK-NEXT: i32 294, label [[SW_BB1]]
-; CHECK-NEXT: i32 296, label [[SW_BB1]]
-; CHECK-NEXT: i32 67, label [[SW_BB1]]
-; CHECK-NEXT: i32 293, label [[SW_BB1]]
-; CHECK-NEXT: i32 382, label [[SW_BB1]]
-; CHECK-NEXT: i32 335, label [[SW_BB1]]
-; CHECK-NEXT: i32 393, label [[SW_BB1]]
-; CHECK-NEXT: i32 415, label [[SW_BB1]]
-; CHECK-NEXT: i32 400, label [[SW_BB1]]
-; CHECK-NEXT: i32 383, label [[SW_BB1]]
-; CHECK-NEXT: i32 421, label [[SW_BB1]]
-; CHECK-NEXT: i32 422, label [[SW_BB1]]
-; CHECK-NEXT: i32 302, label [[SW_BB1]]
-; CHECK-NEXT: i32 303, label [[SW_BB1]]
-; CHECK-NEXT: i32 304, label [[SW_BB1]]
-; CHECK-NEXT: i32 420, label [[SW_BB1]]
-; CHECK-NEXT: i32 401, label [[SW_EPILOG24:%.*]]
-; CHECK-NEXT: i32 53, label [[SW_BB12:%.*]]
-; CHECK-NEXT: i32 44, label [[SW_BB12]]
-; CHECK-NEXT: ]
-; CHECK: sw.bb:
-; CHECK-NEXT: unreachable
-; CHECK: sw.bb1:
-; CHECK-NEXT: br label [[SW_EPILOG24]]
-; CHECK: sw.bb12:
-; CHECK-NEXT: switch i32 undef, label [[SW_DEFAULT:%.*]] [
-; CHECK-NEXT: i32 47, label [[SW_BB13:%.*]]
-; CHECK-NEXT: i32 8, label [[SW_BB13]]
-; CHECK-NEXT: ]
-; CHECK: sw.bb13:
-; CHECK-NEXT: unreachable
-; CHECK: sw.default:
-; CHECK-NEXT: unreachable
-; CHECK: sw.default23:
-; CHECK-NEXT: unreachable
-; CHECK: sw.epilog24:
-; CHECK-NEXT: [[PREVIOUS_3:%.*]] = phi i32 [ undef, [[SW_BB1]] ], [ 401, [[ENTRY:%.*]] ]
-; CHECK-NEXT: unreachable
-;
-entry:
- br label %while.body
-
-while.body: ; preds = %entry
- switch i32 undef, label %sw.default23 [
- i32 129, label %sw.bb
- i32 215, label %sw.bb1
- i32 117, label %sw.bb1
- i32 207, label %sw.bb1
- i32 158, label %sw.bb1
- i32 94, label %sw.bb1
- i32 219, label %sw.bb1
- i32 88, label %sw.bb1
- i32 168, label %sw.bb1
- i32 295, label %sw.bb1
- i32 294, label %sw.bb1
- i32 296, label %sw.bb1
- i32 67, label %sw.bb1
- i32 293, label %sw.bb1
- i32 382, label %sw.bb1
- i32 335, label %sw.bb1
- i32 393, label %sw.bb1
- i32 415, label %sw.bb1
- i32 400, label %sw.bb1
- i32 383, label %sw.bb1
- i32 421, label %sw.bb1
- i32 422, label %sw.bb1
- i32 302, label %sw.bb1
- i32 303, label %sw.bb1
- i32 304, label %sw.bb1
- i32 420, label %sw.bb1
- i32 401, label %sw.epilog24
- i32 53, label %sw.bb12
- i32 44, label %sw.bb12
- ]
-
-sw.bb: ; preds = %while.body
- unreachable
-
-sw.bb1: ; preds = %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body, %while.body
- br i1 false, label %land.lhs.true, label %sw.epilog24
-
-land.lhs.true: ; preds = %sw.bb1
- br label %sw.epilog24
-
-sw.bb12: ; preds = %while.body, %while.body
- switch i32 undef, label %sw.default [
- i32 47, label %sw.bb13
- i32 8, label %sw.bb13
- ]
-
-sw.bb13: ; preds = %sw.bb12, %sw.bb12
- unreachable
-
-sw.default: ; preds = %sw.bb12
- unreachable
-
-sw.default23: ; preds = %while.body
- unreachable
-
-sw.epilog24: ; preds = %land.lhs.true, %sw.bb1, %while.body
- %Previous.3 = phi i32 [ undef, %land.lhs.true ], [ undef, %sw.bb1 ], [ 401, %while.body ]
- unreachable
-}