1 //===- FixIrreducible.cpp - Convert irreducible control-flow into loops ---===//
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //===----------------------------------------------------------------------===//
9 // An irreducible SCC is one which has multiple "header" blocks, i.e., blocks
10 // with control-flow edges incident from outside the SCC. This pass converts a
11 // irreducible SCC into a natural loop by applying the following transformation:
13 // 1. Collect the set of headers H of the SCC.
14 // 2. Collect the set of predecessors P of these headers. These may be inside as
15 // well as outside the SCC.
16 // 3. Create block N and redirect every edge from set P to set H through N.
18 // This converts the SCC into a natural loop with N as the header: N is the only
19 // block with edges incident from outside the SCC, and all backedges in the SCC
20 // are incident on N, i.e., for every backedge, the head now dominates the tail.
22 // INPUT CFG: The blocks A and B form an irreducible loop with two headers.
33 // OUTPUT CFG: Edges incident on A and B are now redirected through a
34 // new block N, forming a natural loop consisting of N, A and B.
48 // The transformation is applied to every maximal SCC that is not already
49 // recognized as a loop. The pass operates on all maximal SCCs found in the
50 // function body outside of any loop, as well as those found inside each loop,
51 // including inside any newly created loops. This ensures that any SCC hidden
52 // inside a maximal SCC is also transformed.
54 // The actual transformation is handled by function CreateControlFlowHub, which
55 // takes a set of incoming blocks (the predecessors) and outgoing blocks (the
56 // headers). The function also moves every PHINode in an outgoing block to the
57 // hub. Since the hub dominates all the outgoing blocks, each such PHINode
58 // continues to dominate its uses. Since every header in an SCC has at least two
59 // predecessors, every value used in the header (or later) but defined in a
60 // predecessor (or earlier) is represented by a PHINode in a header. Hence the
61 // above handling of PHINodes is sufficient and no further processing is
62 // required to restore SSA.
64 // Limitation: The pass cannot handle switch statements and indirect
65 // branches. Both must be lowered to plain branches first.
67 //===----------------------------------------------------------------------===//
69 #include "llvm/ADT/SCCIterator.h"
70 #include "llvm/Analysis/LoopIterator.h"
71 #include "llvm/InitializePasses.h"
72 #include "llvm/Pass.h"
73 #include "llvm/Transforms/Utils.h"
74 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
76 #define DEBUG_TYPE "fix-irreducible"
81 struct FixIrreducible : public FunctionPass {
83 FixIrreducible() : FunctionPass(ID) {
84 initializeFixIrreduciblePass(*PassRegistry::getPassRegistry());
87 void getAnalysisUsage(AnalysisUsage &AU) const override {
88 AU.addRequiredID(LowerSwitchID);
89 AU.addRequired<DominatorTreeWrapperPass>();
90 AU.addRequired<LoopInfoWrapperPass>();
91 AU.addPreservedID(LowerSwitchID);
92 AU.addPreserved<DominatorTreeWrapperPass>();
93 AU.addPreserved<LoopInfoWrapperPass>();
96 bool runOnFunction(Function &F) override;
100 char FixIrreducible::ID = 0;
102 FunctionPass *llvm::createFixIrreduciblePass() { return new FixIrreducible(); }
104 INITIALIZE_PASS_BEGIN(FixIrreducible, "fix-irreducible",
105 "Convert irreducible control-flow into natural loops",
106 false /* Only looks at CFG */, false /* Analysis Pass */)
107 INITIALIZE_PASS_DEPENDENCY(LowerSwitch)
108 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
109 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
110 INITIALIZE_PASS_END(FixIrreducible, "fix-irreducible",
111 "Convert irreducible control-flow into natural loops",
112 false /* Only looks at CFG */, false /* Analysis Pass */)
114 // When a new loop is created, existing children of the parent loop may now be
115 // fully inside the new loop. Reconnect these as children of the new loop.
116 static void reconnectChildLoops(LoopInfo &LI, Loop *ParentLoop, Loop *NewLoop,
117 SetVector<BasicBlock *> &Blocks,
118 SetVector<BasicBlock *> &Headers) {
119 auto &CandidateLoops = ParentLoop ? ParentLoop->getSubLoopsVector()
120 : LI.getTopLevelLoopsVector();
121 // The new loop cannot be its own child, and any candidate is a
122 // child iff its header is owned by the new loop. Move all the
123 // children to a new vector.
124 auto FirstChild = std::partition(
125 CandidateLoops.begin(), CandidateLoops.end(), [&](Loop *L) {
126 return L == NewLoop || Blocks.count(L->getHeader()) == 0;
128 SmallVector<Loop *, 8> ChildLoops(FirstChild, CandidateLoops.end());
129 CandidateLoops.erase(FirstChild, CandidateLoops.end());
131 for (auto II = ChildLoops.begin(), IE = ChildLoops.end(); II != IE; ++II) {
133 LLVM_DEBUG(dbgs() << "child loop: " << Child->getHeader()->getName()
135 // TODO: A child loop whose header is also a header in the current
136 // SCC gets destroyed since its backedges are removed. That may
137 // not be necessary if we can retain such backedges.
138 if (Headers.count(Child->getHeader())) {
139 for (auto BB : Child->blocks()) {
140 LI.changeLoopFor(BB, NewLoop);
141 LLVM_DEBUG(dbgs() << "moved block from child: " << BB->getName()
145 LLVM_DEBUG(dbgs() << "subsumed child loop (common header)\n");
149 Child->setParentLoop(nullptr);
150 NewLoop->addChildLoop(Child);
151 LLVM_DEBUG(dbgs() << "added child loop to new loop\n");
155 // Given a set of blocks and headers in an irreducible SCC, convert it into a
156 // natural loop. Also insert this new loop at its appropriate place in the
157 // hierarchy of loops.
158 static void createNaturalLoopInternal(LoopInfo &LI, DominatorTree &DT,
160 SetVector<BasicBlock *> &Blocks,
161 SetVector<BasicBlock *> &Headers) {
163 // All headers are part of the SCC
164 for (auto H : Headers) {
165 assert(Blocks.count(H));
169 SetVector<BasicBlock *> Predecessors;
170 for (auto H : Headers) {
171 for (auto P : predecessors(H)) {
172 Predecessors.insert(P);
177 dbgs() << "Found predecessors:";
178 for (auto P : Predecessors) {
179 dbgs() << " " << P->getName();
183 // Redirect all the backedges through a "hub" consisting of a series
184 // of guard blocks that manage the flow of control from the
185 // predecessors to the headers.
186 SmallVector<BasicBlock *, 8> GuardBlocks;
187 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
188 CreateControlFlowHub(&DTU, GuardBlocks, Predecessors, Headers, "irr");
189 #if defined(EXPENSIVE_CHECKS)
190 assert(DT.verify(DominatorTree::VerificationLevel::Full));
192 assert(DT.verify(DominatorTree::VerificationLevel::Fast));
195 // Create a new loop from the now-transformed cycle
196 auto NewLoop = LI.AllocateLoop();
198 ParentLoop->addChildLoop(NewLoop);
200 LI.addTopLevelLoop(NewLoop);
203 // Add the guard blocks to the new loop. The first guard block is
204 // the head of all the backedges, and it is the first to be inserted
205 // in the loop. This ensures that it is recognized as the
206 // header. Since the new loop is already in LoopInfo, the new blocks
207 // are also propagated up the chain of parent loops.
208 for (auto G : GuardBlocks) {
209 LLVM_DEBUG(dbgs() << "added guard block: " << G->getName() << "\n");
210 NewLoop->addBasicBlockToLoop(G, LI);
213 // Add the SCC blocks to the new loop.
214 for (auto BB : Blocks) {
215 NewLoop->addBlockEntry(BB);
216 if (LI.getLoopFor(BB) == ParentLoop) {
217 LLVM_DEBUG(dbgs() << "moved block from parent: " << BB->getName()
219 LI.changeLoopFor(BB, NewLoop);
221 LLVM_DEBUG(dbgs() << "added block from child: " << BB->getName() << "\n");
224 LLVM_DEBUG(dbgs() << "header for new loop: "
225 << NewLoop->getHeader()->getName() << "\n");
227 reconnectChildLoops(LI, ParentLoop, NewLoop, Blocks, Headers);
229 NewLoop->verifyLoop();
231 ParentLoop->verifyLoop();
233 #if defined(EXPENSIVE_CHECKS)
235 #endif // EXPENSIVE_CHECKS
239 // Enable the graph traits required for traversing a Loop body.
240 template <> struct GraphTraits<Loop> : LoopBodyTraits {};
243 // Overloaded wrappers to go with the function template below.
244 static BasicBlock *unwrapBlock(BasicBlock *B) { return B; }
245 static BasicBlock *unwrapBlock(LoopBodyTraits::NodeRef &N) { return N.second; }
247 static void createNaturalLoop(LoopInfo &LI, DominatorTree &DT, Function *F,
248 SetVector<BasicBlock *> &Blocks,
249 SetVector<BasicBlock *> &Headers) {
250 createNaturalLoopInternal(LI, DT, nullptr, Blocks, Headers);
253 static void createNaturalLoop(LoopInfo &LI, DominatorTree &DT, Loop &L,
254 SetVector<BasicBlock *> &Blocks,
255 SetVector<BasicBlock *> &Headers) {
256 createNaturalLoopInternal(LI, DT, &L, Blocks, Headers);
259 // Convert irreducible SCCs; Graph G may be a Function* or a Loop&.
260 template <class Graph>
261 static bool makeReducible(LoopInfo &LI, DominatorTree &DT, Graph &&G) {
262 bool Changed = false;
263 for (auto Scc = scc_begin(G); !Scc.isAtEnd(); ++Scc) {
266 SetVector<BasicBlock *> Blocks;
267 LLVM_DEBUG(dbgs() << "Found SCC:");
268 for (auto N : *Scc) {
269 auto BB = unwrapBlock(N);
270 LLVM_DEBUG(dbgs() << " " << BB->getName());
273 LLVM_DEBUG(dbgs() << "\n");
275 // Minor optimization: The SCC blocks are usually discovered in an order
276 // that is the opposite of the order in which these blocks appear as branch
277 // targets. This results in a lot of condition inversions in the control
278 // flow out of the new ControlFlowHub, which can be mitigated if the orders
279 // match. So we discover the headers using the reverse of the block order.
280 SetVector<BasicBlock *> Headers;
281 LLVM_DEBUG(dbgs() << "Found headers:");
282 for (auto BB : reverse(Blocks)) {
283 for (const auto P : predecessors(BB)) {
284 // Skip unreachable predecessors.
285 if (!DT.isReachableFromEntry(P))
287 if (!Blocks.count(P)) {
288 LLVM_DEBUG(dbgs() << " " << BB->getName());
294 LLVM_DEBUG(dbgs() << "\n");
296 if (Headers.size() == 1) {
297 assert(LI.isLoopHeader(Headers.front()));
298 LLVM_DEBUG(dbgs() << "Natural loop with a single header: skipped\n");
301 createNaturalLoop(LI, DT, G, Blocks, Headers);
307 bool FixIrreducible::runOnFunction(Function &F) {
308 LLVM_DEBUG(dbgs() << "===== Fix irreducible control-flow in function: "
309 << F.getName() << "\n");
310 auto &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
311 auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
313 bool Changed = false;
314 SmallVector<Loop *, 8> WorkList;
316 LLVM_DEBUG(dbgs() << "visiting top-level\n");
317 Changed |= makeReducible(LI, DT, &F);
319 // Any SCCs reduced are now already in the list of top-level loops, so simply
320 // add them all to the worklist.
322 WorkList.push_back(L);
325 while (!WorkList.empty()) {
326 auto L = WorkList.back();
328 LLVM_DEBUG(dbgs() << "visiting loop with header "
329 << L->getHeader()->getName() << "\n");
330 Changed |= makeReducible(LI, DT, *L);
331 // Any SCCs reduced are now already in the list of child loops, so simply
332 // add them all to the worklist.
333 WorkList.append(L->begin(), L->end());