1 //=- WebAssemblyFixIrreducibleControlFlow.cpp - Fix irreducible control flow -//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
11 /// This file implements a pass that transforms irreducible control flow into
12 /// reducible control flow. Irreducible control flow means multiple-entry
13 /// loops; they appear as CFG cycles that are not recorded in MachineLoopInfo
14 /// due to being unnatural.
16 /// Note that LLVM has a generic pass that lowers irreducible control flow, but
17 /// it linearizes control flow, turning diamonds into two triangles, which is
18 /// both unnecessary and undesirable for WebAssembly.
20 /// The big picture: Ignoring natural loops (seeing them monolithically), we
21 /// find all the blocks which can return to themselves ("loopers"). Loopers
22 /// reachable from the non-loopers are loop entries: if there are 2 or more,
23 /// then we have irreducible control flow. We fix that as follows: a new block
24 /// is created that can dispatch to each of the loop entries, based on the
25 /// value of a label "helper" variable, and we replace direct branches to the
26 /// entries with assignments to the label variable and a branch to the dispatch
27 /// block. Then the dispatch block is the single entry in a new natural loop.
29 /// This is similar to what the Relooper [1] does, both identify looping code
30 /// that requires multiple entries, and resolve it in a similar way. In
31 /// Relooper terminology, we implement a Multiple shape in a Loop shape. Note
32 /// also that like the Relooper, we implement a "minimal" intervention: we only
33 /// use the "label" helper for the blocks we absolutely must and no others. We
34 /// also prioritize code size and do not perform node splitting (i.e. we don't
35 /// duplicate code in order to resolve irreducibility).
37 /// The difference between this code and the Relooper is that the Relooper also
38 /// generates ifs and loops and works in a recursive manner, knowing at each
39 /// point what the entries are, and recursively breaks down the problem. Here
40 /// we just want to resolve irreducible control flow, and we also want to use
41 /// as much LLVM infrastructure as possible. So we use the MachineLoopInfo to
42 /// identify natural loops, etc., and we start with the whole CFG and must
43 /// identify both the looping code and its entries.
45 /// [1] Alon Zakai. 2011. Emscripten: an LLVM-to-JavaScript compiler. In
46 /// Proceedings of the ACM international conference companion on Object oriented
47 /// programming systems languages and applications companion (SPLASH '11). ACM,
48 /// New York, NY, USA, 301-312. DOI=10.1145/2048147.2048224
49 /// http://doi.acm.org/10.1145/2048147.2048224
51 //===----------------------------------------------------------------------===//
53 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
54 #include "WebAssembly.h"
55 #include "WebAssemblyMachineFunctionInfo.h"
56 #include "WebAssemblySubtarget.h"
57 #include "llvm/ADT/PriorityQueue.h"
58 #include "llvm/ADT/SCCIterator.h"
59 #include "llvm/ADT/SetVector.h"
60 #include "llvm/CodeGen/MachineDominators.h"
61 #include "llvm/CodeGen/MachineFunction.h"
62 #include "llvm/CodeGen/MachineInstrBuilder.h"
63 #include "llvm/CodeGen/MachineLoopInfo.h"
64 #include "llvm/CodeGen/MachineRegisterInfo.h"
65 #include "llvm/CodeGen/Passes.h"
66 #include "llvm/Support/Debug.h"
67 #include "llvm/Support/raw_ostream.h"
70 #define DEBUG_TYPE "wasm-fix-irreducible-control-flow"
76 LoopFixer(MachineFunction &MF, MachineLoopInfo &MLI, MachineLoop *Loop)
77 : MF(MF), MLI(MLI), Loop(Loop) {}
79 // Run the fixer on the given inputs. Returns whether changes were made.
87 MachineBasicBlock *Header;
88 SmallPtrSet<MachineBasicBlock *, 4> LoopBlocks;
90 using BlockSet = SmallPtrSet<MachineBasicBlock *, 4>;
91 DenseMap<MachineBasicBlock *, BlockSet> Reachable;
93 // The worklist contains pairs of recent additions, (a, b), where we just
94 // added a link a => b.
95 using BlockPair = std::pair<MachineBasicBlock *, MachineBasicBlock *>;
96 SmallVector<BlockPair, 4> WorkList;
98 // Get a canonical block to represent a block or a loop: the block, or if in
99 // an inner loop, the loop header, of it in an outer loop scope, we can
100 // ignore it. We need to call this on all blocks we work on.
101 MachineBasicBlock *canonicalize(MachineBasicBlock *MBB) {
102 MachineLoop *InnerLoop = MLI.getLoopFor(MBB);
103 if (InnerLoop == Loop) {
106 // This is either in an outer or an inner loop, and not in ours.
107 if (!LoopBlocks.count(MBB)) {
108 // It's in outer code, ignore it.
112 // It's in an inner loop, canonicalize it to the header of that loop.
113 return InnerLoop->getHeader();
117 // For a successor we can additionally ignore it if it's a branch back to a
118 // natural loop top, as when we are in the scope of a loop, we just care
119 // about internal irreducibility, and can ignore the loop we are in. We need
120 // to call this on all blocks in a context where they are a successor.
121 MachineBasicBlock *canonicalizeSuccessor(MachineBasicBlock *MBB) {
122 if (Loop && MBB == Loop->getHeader()) {
123 // Ignore branches going to the loop's natural header.
126 return canonicalize(MBB);
129 // Potentially insert a new reachable edge, and if so, note it as further
131 void maybeInsert(MachineBasicBlock *MBB, MachineBasicBlock *Succ) {
132 assert(MBB == canonicalize(MBB));
134 // Succ may not be interesting as a sucessor.
135 Succ = canonicalizeSuccessor(Succ);
138 if (Reachable[MBB].insert(Succ).second) {
139 // For there to be further work, it means that we have
141 // for some other X, and in that case X => Succ would be a new edge for
142 // us to discover later. However, if we don't care about MBB as a
143 // successor, then we don't care about that anyhow.
144 if (canonicalizeSuccessor(MBB)) {
145 WorkList.emplace_back(MBB, Succ);
151 bool LoopFixer::run() {
152 Header = Loop ? Loop->getHeader() : &*MF.begin();
154 // Identify all the blocks in this loop scope.
156 for (auto *MBB : Loop->getBlocks()) {
157 LoopBlocks.insert(MBB);
160 for (auto &MBB : MF) {
161 LoopBlocks.insert(&MBB);
165 // Compute which (canonicalized) blocks each block can reach.
167 // Add all the initial work.
168 for (auto *MBB : LoopBlocks) {
169 MachineLoop *InnerLoop = MLI.getLoopFor(MBB);
171 if (InnerLoop == Loop) {
172 for (auto *Succ : MBB->successors()) {
173 maybeInsert(MBB, Succ);
176 // It can't be in an outer loop - we loop on LoopBlocks - and so it must
179 // Check if we are the canonical block for this loop.
180 if (canonicalize(MBB) != MBB) {
183 // The successors are those of the loop.
184 SmallVector<MachineBasicBlock *, 2> ExitBlocks;
185 InnerLoop->getExitBlocks(ExitBlocks);
186 for (auto *Succ : ExitBlocks) {
187 maybeInsert(MBB, Succ);
192 // Do work until we are all done.
193 while (!WorkList.empty()) {
194 MachineBasicBlock *MBB;
195 MachineBasicBlock *Succ;
196 std::tie(MBB, Succ) = WorkList.pop_back_val();
197 // The worklist item is an edge we just added, so it must have valid blocks
198 // (and not something canonicalized to nullptr).
201 // The successor in that pair must also be a valid successor.
202 assert(MBB == canonicalizeSuccessor(MBB));
203 // We recently added MBB => Succ, and that means we may have enabled
204 // Pred => MBB => Succ. Check all the predecessors. Note that our loop here
205 // is correct for both a block and a block representing a loop, as the loop
206 // is natural and so the predecessors are all predecessors of the loop
207 // header, which is the block we have here.
208 for (auto *Pred : MBB->predecessors()) {
209 // Canonicalize, make sure it's relevant, and check it's not the same
210 // block (an update to the block itself doesn't help compute that same
212 Pred = canonicalize(Pred);
213 if (Pred && Pred != MBB) {
214 maybeInsert(Pred, Succ);
219 // It's now trivial to identify the loopers.
220 SmallPtrSet<MachineBasicBlock *, 4> Loopers;
221 for (auto MBB : LoopBlocks) {
222 if (Reachable[MBB].count(MBB)) {
226 // The header cannot be a looper. At the toplevel, LLVM does not allow the
227 // entry to be in a loop, and in a natural loop we should ignore the header.
228 assert(Loopers.count(Header) == 0);
230 // Find the entries, loopers reachable from non-loopers.
231 SmallPtrSet<MachineBasicBlock *, 4> Entries;
232 SmallVector<MachineBasicBlock *, 4> SortedEntries;
233 for (auto *Looper : Loopers) {
234 for (auto *Pred : Looper->predecessors()) {
235 Pred = canonicalize(Pred);
236 if (Pred && !Loopers.count(Pred)) {
237 Entries.insert(Looper);
238 SortedEntries.push_back(Looper);
244 // Check if we found irreducible control flow.
245 if (LLVM_LIKELY(Entries.size() <= 1))
248 // Sort the entries to ensure a deterministic build.
249 llvm::sort(SortedEntries,
250 [&](const MachineBasicBlock *A, const MachineBasicBlock *B) {
251 auto ANum = A->getNumber();
252 auto BNum = B->getNumber();
257 for (auto Block : SortedEntries)
258 assert(Block->getNumber() != -1);
259 if (SortedEntries.size() > 1) {
260 for (auto I = SortedEntries.begin(), E = SortedEntries.end() - 1;
262 auto ANum = (*I)->getNumber();
263 auto BNum = (*(std::next(I)))->getNumber();
264 assert(ANum != BNum);
269 // Create a dispatch block which will contain a jump table to the entries.
270 MachineBasicBlock *Dispatch = MF.CreateMachineBasicBlock();
271 MF.insert(MF.end(), Dispatch);
272 MLI.changeLoopFor(Dispatch, Loop);
274 // Add the jump table.
275 const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
276 MachineInstrBuilder MIB = BuildMI(*Dispatch, Dispatch->end(), DebugLoc(),
277 TII.get(WebAssembly::BR_TABLE_I32));
279 // Add the register which will be used to tell the jump table which block to
281 MachineRegisterInfo &MRI = MF.getRegInfo();
282 unsigned Reg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
285 // Compute the indices in the superheader, one for each bad block, and
286 // add them as successors.
287 DenseMap<MachineBasicBlock *, unsigned> Indices;
288 for (auto *MBB : SortedEntries) {
289 auto Pair = Indices.insert(std::make_pair(MBB, 0));
294 unsigned Index = MIB.getInstr()->getNumExplicitOperands() - 1;
295 Pair.first->second = Index;
298 Dispatch->addSuccessor(MBB);
301 // Rewrite the problematic successors for every block that wants to reach the
302 // bad blocks. For simplicity, we just introduce a new block for every edge
303 // we need to rewrite. (Fancier things are possible.)
305 SmallVector<MachineBasicBlock *, 4> AllPreds;
306 for (auto *MBB : SortedEntries) {
307 for (auto *Pred : MBB->predecessors()) {
308 if (Pred != Dispatch) {
309 AllPreds.push_back(Pred);
314 for (MachineBasicBlock *MBB : AllPreds) {
315 DenseMap<MachineBasicBlock *, MachineBasicBlock *> Map;
316 for (auto *Succ : MBB->successors()) {
317 if (!Entries.count(Succ)) {
321 // This is a successor we need to rewrite.
322 MachineBasicBlock *Split = MF.CreateMachineBasicBlock();
323 MF.insert(MBB->isLayoutSuccessor(Succ) ? MachineFunction::iterator(Succ)
326 MLI.changeLoopFor(Split, Loop);
328 // Set the jump table's register of the index of the block we wish to
329 // jump to, and jump to the jump table.
330 BuildMI(*Split, Split->end(), DebugLoc(), TII.get(WebAssembly::CONST_I32),
332 .addImm(Indices[Succ]);
333 BuildMI(*Split, Split->end(), DebugLoc(), TII.get(WebAssembly::BR))
335 Split->addSuccessor(Dispatch);
338 // Remap the terminator operands and the successor list.
339 for (MachineInstr &Term : MBB->terminators())
340 for (auto &Op : Term.explicit_uses())
341 if (Op.isMBB() && Indices.count(Op.getMBB()))
342 Op.setMBB(Map[Op.getMBB()]);
343 for (auto Rewrite : Map)
344 MBB->replaceSuccessor(Rewrite.first, Rewrite.second);
347 // Create a fake default label, because br_table requires one.
348 MIB.addMBB(MIB.getInstr()
349 ->getOperand(MIB.getInstr()->getNumExplicitOperands() - 1)
355 class WebAssemblyFixIrreducibleControlFlow final : public MachineFunctionPass {
356 StringRef getPassName() const override {
357 return "WebAssembly Fix Irreducible Control Flow";
360 void getAnalysisUsage(AnalysisUsage &AU) const override {
361 AU.setPreservesCFG();
362 AU.addRequired<MachineDominatorTree>();
363 AU.addPreserved<MachineDominatorTree>();
364 AU.addRequired<MachineLoopInfo>();
365 AU.addPreserved<MachineLoopInfo>();
366 MachineFunctionPass::getAnalysisUsage(AU);
369 bool runOnMachineFunction(MachineFunction &MF) override;
371 bool runIteration(MachineFunction &MF, MachineLoopInfo &MLI) {
372 // Visit the function body, which is identified as a null loop.
373 if (LoopFixer(MF, MLI, nullptr).run()) {
377 // Visit all the loops.
378 SmallVector<MachineLoop *, 8> Worklist(MLI.begin(), MLI.end());
379 while (!Worklist.empty()) {
380 MachineLoop *Loop = Worklist.pop_back_val();
381 Worklist.append(Loop->begin(), Loop->end());
382 if (LoopFixer(MF, MLI, Loop).run()) {
391 static char ID; // Pass identification, replacement for typeid
392 WebAssemblyFixIrreducibleControlFlow() : MachineFunctionPass(ID) {}
394 } // end anonymous namespace
396 char WebAssemblyFixIrreducibleControlFlow::ID = 0;
397 INITIALIZE_PASS(WebAssemblyFixIrreducibleControlFlow, DEBUG_TYPE,
398 "Removes irreducible control flow", false, false)
400 FunctionPass *llvm::createWebAssemblyFixIrreducibleControlFlow() {
401 return new WebAssemblyFixIrreducibleControlFlow();
404 bool WebAssemblyFixIrreducibleControlFlow::runOnMachineFunction(
405 MachineFunction &MF) {
406 LLVM_DEBUG(dbgs() << "********** Fixing Irreducible Control Flow **********\n"
407 "********** Function: "
408 << MF.getName() << '\n');
410 bool Changed = false;
411 auto &MLI = getAnalysis<MachineLoopInfo>();
413 // When we modify something, bail out and recompute MLI, then start again, as
414 // we create a new natural loop when we resolve irreducible control flow, and
415 // other loops may become nested in it, etc. In practice this is not an issue
416 // because irreducible control flow is rare, only very few cycles are needed
418 while (LLVM_UNLIKELY(runIteration(MF, MLI))) {
419 // We rewrote part of the function; recompute MLI and start again.
420 LLVM_DEBUG(dbgs() << "Recomputing loops.\n");
421 MF.getRegInfo().invalidateLiveness();
423 getAnalysis<MachineDominatorTree>().runOnMachineFunction(MF);
424 MLI.runOnMachineFunction(MF);