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
12 /// into 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 /// TODO: The transformation implemented here handles all irreducible control
21 /// flow, without exponential code-size expansion, though it does so by creating
22 /// inefficient code in many cases. Ideally, we should add other
23 /// transformations, including code-duplicating cases, which can be more
24 /// efficient in common cases, and they can fall back to this conservative
25 /// implementation as needed.
27 //===----------------------------------------------------------------------===//
29 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
30 #include "WebAssembly.h"
31 #include "WebAssemblyMachineFunctionInfo.h"
32 #include "WebAssemblySubtarget.h"
33 #include "llvm/ADT/PriorityQueue.h"
34 #include "llvm/ADT/SCCIterator.h"
35 #include "llvm/ADT/SetVector.h"
36 #include "llvm/CodeGen/MachineDominators.h"
37 #include "llvm/CodeGen/MachineFunction.h"
38 #include "llvm/CodeGen/MachineInstrBuilder.h"
39 #include "llvm/CodeGen/MachineLoopInfo.h"
40 #include "llvm/CodeGen/MachineRegisterInfo.h"
41 #include "llvm/CodeGen/Passes.h"
42 #include "llvm/Support/Debug.h"
43 #include "llvm/Support/raw_ostream.h"
46 #define DEBUG_TYPE "wasm-fix-irreducible-control-flow"
49 class WebAssemblyFixIrreducibleControlFlow final : public MachineFunctionPass {
50 StringRef getPassName() const override {
51 return "WebAssembly Fix Irreducible Control Flow";
54 void getAnalysisUsage(AnalysisUsage &AU) const override {
56 AU.addRequired<MachineDominatorTree>();
57 AU.addPreserved<MachineDominatorTree>();
58 AU.addRequired<MachineLoopInfo>();
59 AU.addPreserved<MachineLoopInfo>();
60 MachineFunctionPass::getAnalysisUsage(AU);
63 bool runOnMachineFunction(MachineFunction &MF) override;
65 bool VisitLoop(MachineFunction &MF, MachineLoopInfo &MLI, MachineLoop *Loop);
68 static char ID; // Pass identification, replacement for typeid
69 WebAssemblyFixIrreducibleControlFlow() : MachineFunctionPass(ID) {}
71 } // end anonymous namespace
73 char WebAssemblyFixIrreducibleControlFlow::ID = 0;
74 INITIALIZE_PASS(WebAssemblyFixIrreducibleControlFlow, DEBUG_TYPE,
75 "Removes irreducible control flow", false, false)
77 FunctionPass *llvm::createWebAssemblyFixIrreducibleControlFlow() {
78 return new WebAssemblyFixIrreducibleControlFlow();
83 /// A utility for walking the blocks of a loop, handling a nested inner
84 /// loop as a monolithic conceptual block.
86 MachineBasicBlock *Block;
87 SmallVector<MachineBasicBlock *, 2> Preds;
88 SmallVector<MachineBasicBlock *, 2> Succs;
91 explicit MetaBlock(MachineBasicBlock *MBB)
92 : Block(MBB), Preds(MBB->pred_begin(), MBB->pred_end()),
93 Succs(MBB->succ_begin(), MBB->succ_end()) {}
95 explicit MetaBlock(MachineLoop *Loop) : Block(Loop->getHeader()) {
96 Loop->getExitBlocks(Succs);
97 for (MachineBasicBlock *Pred : Block->predecessors())
98 if (!Loop->contains(Pred))
99 Preds.push_back(Pred);
102 MachineBasicBlock *getBlock() const { return Block; }
104 const SmallVectorImpl<MachineBasicBlock *> &predecessors() const {
107 const SmallVectorImpl<MachineBasicBlock *> &successors() const {
111 bool operator==(const MetaBlock &MBB) { return Block == MBB.Block; }
112 bool operator!=(const MetaBlock &MBB) { return Block != MBB.Block; }
115 class SuccessorList final : public MetaBlock {
120 explicit SuccessorList(MachineBasicBlock *MBB)
121 : MetaBlock(MBB), Index(0), Num(successors().size()) {}
123 explicit SuccessorList(MachineLoop *Loop)
124 : MetaBlock(Loop), Index(0), Num(successors().size()) {}
126 bool HasNext() const { return Index != Num; }
128 MachineBasicBlock *Next() {
130 return successors()[Index++];
134 } // end anonymous namespace
136 bool WebAssemblyFixIrreducibleControlFlow::VisitLoop(MachineFunction &MF,
137 MachineLoopInfo &MLI,
139 MachineBasicBlock *Header = Loop ? Loop->getHeader() : &*MF.begin();
140 SetVector<MachineBasicBlock *> RewriteSuccs;
142 // DFS through Loop's body, looking for irreducible control flow. Loop is
143 // natural, and we stay in its body, and we treat any nested loops
144 // monolithically, so any cycles we encounter indicate irreducibility.
145 SmallPtrSet<MachineBasicBlock *, 8> OnStack;
146 SmallPtrSet<MachineBasicBlock *, 8> Visited;
147 SmallVector<SuccessorList, 4> LoopWorklist;
148 LoopWorklist.push_back(SuccessorList(Header));
149 OnStack.insert(Header);
150 Visited.insert(Header);
151 while (!LoopWorklist.empty()) {
152 SuccessorList &Top = LoopWorklist.back();
154 MachineBasicBlock *Next = Top.Next();
155 if (Next == Header || (Loop && !Loop->contains(Next)))
157 if (LLVM_LIKELY(OnStack.insert(Next).second)) {
158 if (!Visited.insert(Next).second) {
162 MachineLoop *InnerLoop = MLI.getLoopFor(Next);
163 if (InnerLoop != Loop)
164 LoopWorklist.push_back(SuccessorList(InnerLoop));
166 LoopWorklist.push_back(SuccessorList(Next));
168 RewriteSuccs.insert(Top.getBlock());
172 OnStack.erase(Top.getBlock());
173 LoopWorklist.pop_back();
176 // Most likely, we didn't find any irreducible control flow.
177 if (LLVM_LIKELY(RewriteSuccs.empty()))
180 LLVM_DEBUG(dbgs() << "Irreducible control flow detected!\n");
182 // Ok. We have irreducible control flow! Create a dispatch block which will
183 // contains a jump table to any block in the problematic set of blocks.
184 MachineBasicBlock *Dispatch = MF.CreateMachineBasicBlock();
185 MF.insert(MF.end(), Dispatch);
186 MLI.changeLoopFor(Dispatch, Loop);
188 // Add the jump table.
189 const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
190 MachineInstrBuilder MIB = BuildMI(*Dispatch, Dispatch->end(), DebugLoc(),
191 TII.get(WebAssembly::BR_TABLE_I32));
193 // Add the register which will be used to tell the jump table which block to
195 MachineRegisterInfo &MRI = MF.getRegInfo();
196 unsigned Reg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
199 // Collect all the blocks which need to have their successors rewritten,
200 // add the successors to the jump table, and remember their index.
201 DenseMap<MachineBasicBlock *, unsigned> Indices;
202 SmallVector<MachineBasicBlock *, 4> SuccWorklist(RewriteSuccs.begin(),
204 while (!SuccWorklist.empty()) {
205 MachineBasicBlock *MBB = SuccWorklist.pop_back_val();
206 auto Pair = Indices.insert(std::make_pair(MBB, 0));
210 unsigned Index = MIB.getInstr()->getNumExplicitOperands() - 1;
211 LLVM_DEBUG(dbgs() << printMBBReference(*MBB) << " has index " << Index
214 Pair.first->second = Index;
215 for (auto Pred : MBB->predecessors())
216 RewriteSuccs.insert(Pred);
219 Dispatch->addSuccessor(MBB);
222 for (auto *Succ : Meta.successors())
223 if (Succ != Header && (!Loop || Loop->contains(Succ)))
224 SuccWorklist.push_back(Succ);
227 // Rewrite the problematic successors for every block in RewriteSuccs.
228 // For simplicity, we just introduce a new block for every edge we need to
229 // rewrite. Fancier things are possible.
230 for (MachineBasicBlock *MBB : RewriteSuccs) {
231 DenseMap<MachineBasicBlock *, MachineBasicBlock *> Map;
232 for (auto *Succ : MBB->successors()) {
233 if (!Indices.count(Succ))
236 MachineBasicBlock *Split = MF.CreateMachineBasicBlock();
237 MF.insert(MBB->isLayoutSuccessor(Succ) ? MachineFunction::iterator(Succ)
240 MLI.changeLoopFor(Split, Loop);
242 // Set the jump table's register of the index of the block we wish to
243 // jump to, and jump to the jump table.
244 BuildMI(*Split, Split->end(), DebugLoc(), TII.get(WebAssembly::CONST_I32),
246 .addImm(Indices[Succ]);
247 BuildMI(*Split, Split->end(), DebugLoc(), TII.get(WebAssembly::BR))
249 Split->addSuccessor(Dispatch);
252 // Remap the terminator operands and the successor list.
253 for (MachineInstr &Term : MBB->terminators())
254 for (auto &Op : Term.explicit_uses())
255 if (Op.isMBB() && Indices.count(Op.getMBB()))
256 Op.setMBB(Map[Op.getMBB()]);
257 for (auto Rewrite : Map)
258 MBB->replaceSuccessor(Rewrite.first, Rewrite.second);
261 // Create a fake default label, because br_table requires one.
262 MIB.addMBB(MIB.getInstr()
263 ->getOperand(MIB.getInstr()->getNumExplicitOperands() - 1)
269 bool WebAssemblyFixIrreducibleControlFlow::runOnMachineFunction(
270 MachineFunction &MF) {
271 LLVM_DEBUG(dbgs() << "********** Fixing Irreducible Control Flow **********\n"
272 "********** Function: "
273 << MF.getName() << '\n');
275 bool Changed = false;
276 auto &MLI = getAnalysis<MachineLoopInfo>();
278 // Visit the function body, which is identified as a null loop.
279 Changed |= VisitLoop(MF, MLI, nullptr);
281 // Visit all the loops.
282 SmallVector<MachineLoop *, 8> Worklist(MLI.begin(), MLI.end());
283 while (!Worklist.empty()) {
284 MachineLoop *CurLoop = Worklist.pop_back_val();
285 Worklist.append(CurLoop->begin(), CurLoop->end());
286 Changed |= VisitLoop(MF, MLI, CurLoop);
289 // If we made any changes, completely recompute everything.
290 if (LLVM_UNLIKELY(Changed)) {
291 LLVM_DEBUG(dbgs() << "Recomputing dominators and loops.\n");
292 MF.getRegInfo().invalidateLiveness();
294 getAnalysis<MachineDominatorTree>().runOnMachineFunction(MF);
295 MLI.runOnMachineFunction(MF);