1//===- ControlFlowUtils.cpp - Control Flow Utilities -----------------------==//
2//
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
6//
7//===----------------------------------------------------------------------===//
8//
9// Utilities to manipulate the CFG and restore SSA for the new control flow.
10//
11//===----------------------------------------------------------------------===//
12
13#include "llvm/Transforms/Utils/ControlFlowUtils.h"
14#include "llvm/ADT/SetVector.h"
15#include "llvm/ADT/SmallSet.h"
16#include "llvm/Analysis/DomTreeUpdater.h"
17#include "llvm/IR/Constants.h"
18#include "llvm/IR/Instructions.h"
19#include "llvm/IR/ValueHandle.h"
20#include "llvm/Transforms/Utils/Local.h"
21
22#define DEBUG_TYPE "control-flow-hub"
23
24using namespace llvm;
25
26using BBPredicates = DenseMap<BasicBlock *, Instruction *>;
27using EdgeDescriptor = ControlFlowHub::BranchDescriptor;
28
29// Redirects the terminator of the incoming block to the first guard block in
30// the hub. Returns the branch condition from `BB` if it exits.
31// - If only one of Succ0 or Succ1 is not null, the corresponding branch
32// successor is redirected to the FirstGuardBlock.
33// - Else both are not null, and branch is replaced with an unconditional
34// branch to the FirstGuardBlock.
35static Value *redirectToHub(BasicBlock *BB, BasicBlock *Succ0,
36 BasicBlock *Succ1, BasicBlock *FirstGuardBlock) {
37 assert(isa<BranchInst>(BB->getTerminator()) &&
38 "Only support branch terminator.");
39 auto *Branch = cast<BranchInst>(Val: BB->getTerminator());
40 auto *Condition = Branch->isConditional() ? Branch->getCondition() : nullptr;
41
42 assert(Succ0 || Succ1);
43
44 if (Branch->isUnconditional()) {
45 assert(Succ0 == Branch->getSuccessor(0));
46 assert(!Succ1);
47 Branch->setSuccessor(idx: 0, NewSucc: FirstGuardBlock);
48 } else {
49 assert(!Succ1 || Succ1 == Branch->getSuccessor(1));
50 if (Succ0 && !Succ1) {
51 Branch->setSuccessor(idx: 0, NewSucc: FirstGuardBlock);
52 } else if (Succ1 && !Succ0) {
53 Branch->setSuccessor(idx: 1, NewSucc: FirstGuardBlock);
54 } else {
55 Branch->eraseFromParent();
56 BranchInst::Create(IfTrue: FirstGuardBlock, InsertBefore: BB);
57 }
58 }
59
60 return Condition;
61}
62
63// Setup the branch instructions for guard blocks.
64//
65// Each guard block terminates in a conditional branch that transfers
66// control to the corresponding outgoing block or the next guard
67// block. The last guard block has two outgoing blocks as successors.
68static void setupBranchForGuard(ArrayRef<BasicBlock *> GuardBlocks,
69 ArrayRef<BasicBlock *> Outgoing,
70 BBPredicates &GuardPredicates) {
71 assert(Outgoing.size() > 1);
72 assert(GuardBlocks.size() == Outgoing.size() - 1);
73 int I = 0;
74 for (int E = GuardBlocks.size() - 1; I != E; ++I) {
75 BasicBlock *Out = Outgoing[I];
76 BranchInst::Create(IfTrue: Out, IfFalse: GuardBlocks[I + 1], Cond: GuardPredicates[Out],
77 InsertBefore: GuardBlocks[I]);
78 }
79 BasicBlock *Out = Outgoing[I];
80 BranchInst::Create(IfTrue: Out, IfFalse: Outgoing[I + 1], Cond: GuardPredicates[Out],
81 InsertBefore: GuardBlocks[I]);
82}
83
84// Assign an index to each outgoing block. At the corresponding guard
85// block, compute the branch condition by comparing this index.
86static void calcPredicateUsingInteger(ArrayRef<EdgeDescriptor> Branches,
87 ArrayRef<BasicBlock *> Outgoing,
88 ArrayRef<BasicBlock *> GuardBlocks,
89 BBPredicates &GuardPredicates) {
90 LLVMContext &Context = GuardBlocks.front()->getContext();
91 BasicBlock *FirstGuardBlock = GuardBlocks.front();
92 Type *Int32Ty = Type::getInt32Ty(C&: Context);
93
94 auto *Phi = PHINode::Create(Ty: Int32Ty, NumReservedValues: Branches.size(), NameStr: "merged.bb.idx",
95 InsertBefore: FirstGuardBlock);
96
97 for (auto [BB, Succ0, Succ1] : Branches) {
98 Value *Condition = redirectToHub(BB, Succ0, Succ1, FirstGuardBlock);
99 Value *IncomingId = nullptr;
100 if (Succ0 && Succ1) {
101 auto Succ0Iter = find(Range&: Outgoing, Val: Succ0);
102 auto Succ1Iter = find(Range&: Outgoing, Val: Succ1);
103 Value *Id0 =
104 ConstantInt::get(Ty: Int32Ty, V: std::distance(first: Outgoing.begin(), last: Succ0Iter));
105 Value *Id1 =
106 ConstantInt::get(Ty: Int32Ty, V: std::distance(first: Outgoing.begin(), last: Succ1Iter));
107 IncomingId = SelectInst::Create(C: Condition, S1: Id0, S2: Id1, NameStr: "target.bb.idx",
108 InsertBefore: BB->getTerminator()->getIterator());
109 } else {
110 // Get the index of the non-null successor.
111 auto SuccIter = Succ0 ? find(Range&: Outgoing, Val: Succ0) : find(Range&: Outgoing, Val: Succ1);
112 IncomingId =
113 ConstantInt::get(Ty: Int32Ty, V: std::distance(first: Outgoing.begin(), last: SuccIter));
114 }
115 Phi->addIncoming(V: IncomingId, BB);
116 }
117
118 for (int I = 0, E = Outgoing.size() - 1; I != E; ++I) {
119 BasicBlock *Out = Outgoing[I];
120 LLVM_DEBUG(dbgs() << "Creating integer guard for " << Out->getName()
121 << "\n");
122 auto *Cmp = ICmpInst::Create(Op: Instruction::ICmp, Pred: ICmpInst::ICMP_EQ, S1: Phi,
123 S2: ConstantInt::get(Ty: Int32Ty, V: I),
124 Name: Out->getName() + ".predicate", InsertBefore: GuardBlocks[I]);
125 GuardPredicates[Out] = Cmp;
126 }
127}
128
129// Determine the branch condition to be used at each guard block from the
130// original boolean values.
131static void calcPredicateUsingBooleans(
132 ArrayRef<EdgeDescriptor> Branches, ArrayRef<BasicBlock *> Outgoing,
133 SmallVectorImpl<BasicBlock *> &GuardBlocks, BBPredicates &GuardPredicates,
134 SmallVectorImpl<WeakVH> &DeletionCandidates) {
135 LLVMContext &Context = GuardBlocks.front()->getContext();
136 auto *BoolTrue = ConstantInt::getTrue(Context);
137 auto *BoolFalse = ConstantInt::getFalse(Context);
138 BasicBlock *FirstGuardBlock = GuardBlocks.front();
139
140 // The predicate for the last outgoing is trivially true, and so we
141 // process only the first N-1 successors.
142 for (int I = 0, E = Outgoing.size() - 1; I != E; ++I) {
143 BasicBlock *Out = Outgoing[I];
144 LLVM_DEBUG(dbgs() << "Creating boolean guard for " << Out->getName()
145 << "\n");
146
147 auto *Phi =
148 PHINode::Create(Ty: Type::getInt1Ty(C&: Context), NumReservedValues: Branches.size(),
149 NameStr: StringRef("Guard.") + Out->getName(), InsertBefore: FirstGuardBlock);
150 GuardPredicates[Out] = Phi;
151 }
152
153 for (auto [BB, Succ0, Succ1] : Branches) {
154 Value *Condition = redirectToHub(BB, Succ0, Succ1, FirstGuardBlock);
155
156 // Optimization: Consider an incoming block A with both successors
157 // Succ0 and Succ1 in the set of outgoing blocks. The predicates
158 // for Succ0 and Succ1 complement each other. If Succ0 is visited
159 // first in the loop below, control will branch to Succ0 using the
160 // corresponding predicate. But if that branch is not taken, then
161 // control must reach Succ1, which means that the incoming value of
162 // the predicate from `BB` is true for Succ1.
163 bool OneSuccessorDone = false;
164 for (int I = 0, E = Outgoing.size() - 1; I != E; ++I) {
165 BasicBlock *Out = Outgoing[I];
166 PHINode *Phi = cast<PHINode>(Val: GuardPredicates[Out]);
167 if (Out != Succ0 && Out != Succ1) {
168 Phi->addIncoming(V: BoolFalse, BB);
169 } else if (!Succ0 || !Succ1 || OneSuccessorDone) {
170 // Optimization: When only one successor is an outgoing block,
171 // the incoming predicate from `BB` is always true.
172 Phi->addIncoming(V: BoolTrue, BB);
173 } else {
174 assert(Succ0 && Succ1);
175 if (Out == Succ0) {
176 Phi->addIncoming(V: Condition, BB);
177 } else {
178 Value *Inverted = invertCondition(Condition);
179 DeletionCandidates.push_back(Elt: Condition);
180 Phi->addIncoming(V: Inverted, BB);
181 }
182 OneSuccessorDone = true;
183 }
184 }
185 }
186}
187
188// Capture the existing control flow as guard predicates, and redirect
189// control flow from \p Incoming block through the \p GuardBlocks to the
190// \p Outgoing blocks.
191//
192// There is one guard predicate for each outgoing block OutBB. The
193// predicate represents whether the hub should transfer control flow
194// to OutBB. These predicates are NOT ORTHOGONAL. The Hub evaluates
195// them in the same order as the Outgoing set-vector, and control
196// branches to the first outgoing block whose predicate evaluates to true.
197//
198// The last guard block has two outgoing blocks as successors since the
199// condition for the final outgoing block is trivially true. So we create one
200// less block (including the first guard block) than the number of outgoing
201// blocks.
202static void convertToGuardPredicates(
203 ArrayRef<EdgeDescriptor> Branches, ArrayRef<BasicBlock *> Outgoing,
204 SmallVectorImpl<BasicBlock *> &GuardBlocks,
205 SmallVectorImpl<WeakVH> &DeletionCandidates, const StringRef Prefix,
206 std::optional<unsigned> MaxControlFlowBooleans) {
207 BBPredicates GuardPredicates;
208 Function *F = Outgoing.front()->getParent();
209
210 for (int I = 0, E = Outgoing.size() - 1; I != E; ++I)
211 GuardBlocks.push_back(
212 Elt: BasicBlock::Create(Context&: F->getContext(), Name: Prefix + ".guard", Parent: F));
213
214 // When we are using an integer to record which target block to jump to, we
215 // are creating less live values, actually we are using one single integer to
216 // store the index of the target block. When we are using booleans to store
217 // the branching information, we need (N-1) boolean values, where N is the
218 // number of outgoing block.
219 if (!MaxControlFlowBooleans || Outgoing.size() <= *MaxControlFlowBooleans)
220 calcPredicateUsingBooleans(Branches, Outgoing, GuardBlocks, GuardPredicates,
221 DeletionCandidates);
222 else
223 calcPredicateUsingInteger(Branches, Outgoing, GuardBlocks, GuardPredicates);
224
225 setupBranchForGuard(GuardBlocks, Outgoing, GuardPredicates);
226}
227
228// After creating a control flow hub, the operands of PHINodes in an outgoing
229// block Out no longer match the predecessors of that block. Predecessors of Out
230// that are incoming blocks to the hub are now replaced by just one edge from
231// the hub. To match this new control flow, the corresponding values from each
232// PHINode must now be moved a new PHINode in the first guard block of the hub.
233//
234// This operation cannot be performed with SSAUpdater, because it involves one
235// new use: If the block Out is in the list of Incoming blocks, then the newly
236// created PHI in the Hub will use itself along that edge from Out to Hub.
237static void reconnectPhis(BasicBlock *Out, BasicBlock *GuardBlock,
238 ArrayRef<EdgeDescriptor> Incoming,
239 BasicBlock *FirstGuardBlock) {
240 auto I = Out->begin();
241 while (I != Out->end() && isa<PHINode>(Val: I)) {
242 auto *Phi = cast<PHINode>(Val&: I);
243 auto *NewPhi =
244 PHINode::Create(Ty: Phi->getType(), NumReservedValues: Incoming.size(),
245 NameStr: Phi->getName() + ".moved", InsertBefore: FirstGuardBlock->begin());
246 bool AllUndef = true;
247 for (auto [BB, Succ0, Succ1] : Incoming) {
248 Value *V = PoisonValue::get(T: Phi->getType());
249 if (Phi->getBasicBlockIndex(BB) != -1) {
250 V = Phi->removeIncomingValue(BB, DeletePHIIfEmpty: false);
251 if (BB == Out) {
252 V = NewPhi;
253 }
254 AllUndef &= isa<UndefValue>(Val: V);
255 }
256
257 NewPhi->addIncoming(V, BB);
258 }
259 assert(NewPhi->getNumIncomingValues() == Incoming.size());
260 Value *NewV = NewPhi;
261 if (AllUndef) {
262 NewPhi->eraseFromParent();
263 NewV = PoisonValue::get(T: Phi->getType());
264 }
265 if (Phi->getNumOperands() == 0) {
266 Phi->replaceAllUsesWith(V: NewV);
267 I = Phi->eraseFromParent();
268 continue;
269 }
270 Phi->addIncoming(V: NewV, BB: GuardBlock);
271 ++I;
272 }
273}
274
275std::pair<BasicBlock *, bool> ControlFlowHub::finalize(
276 DomTreeUpdater *DTU, SmallVectorImpl<BasicBlock *> &GuardBlocks,
277 const StringRef Prefix, std::optional<unsigned> MaxControlFlowBooleans) {
278#ifndef NDEBUG
279 SmallSet<BasicBlock *, 8> Incoming;
280#endif
281 SetVector<BasicBlock *> Outgoing;
282
283 for (auto [BB, Succ0, Succ1] : Branches) {
284#ifndef NDEBUG
285 assert(Incoming.insert(BB).second && "Duplicate entry for incoming block.");
286#endif
287 if (Succ0)
288 Outgoing.insert(X: Succ0);
289 if (Succ1)
290 Outgoing.insert(X: Succ1);
291 }
292
293 if (Outgoing.size() < 2)
294 return {Outgoing.front(), false};
295
296 SmallVector<DominatorTree::UpdateType, 16> Updates;
297 if (DTU) {
298 for (auto [BB, Succ0, Succ1] : Branches) {
299 if (Succ0)
300 Updates.push_back(Elt: {DominatorTree::Delete, BB, Succ0});
301 if (Succ1)
302 Updates.push_back(Elt: {DominatorTree::Delete, BB, Succ1});
303 }
304 }
305
306 SmallVector<WeakVH, 8> DeletionCandidates;
307 convertToGuardPredicates(Branches, Outgoing: Outgoing.getArrayRef(), GuardBlocks,
308 DeletionCandidates, Prefix, MaxControlFlowBooleans);
309 BasicBlock *FirstGuardBlock = GuardBlocks.front();
310
311 // Update the PHINodes in each outgoing block to match the new control flow.
312 for (int I = 0, E = GuardBlocks.size(); I != E; ++I)
313 reconnectPhis(Out: Outgoing[I], GuardBlock: GuardBlocks[I], Incoming: Branches, FirstGuardBlock);
314 // Process the Nth (last) outgoing block with the (N-1)th (last) guard block.
315 reconnectPhis(Out: Outgoing.back(), GuardBlock: GuardBlocks.back(), Incoming: Branches, FirstGuardBlock);
316
317 if (DTU) {
318 int NumGuards = GuardBlocks.size();
319
320 for (auto [BB, Succ0, Succ1] : Branches)
321 Updates.push_back(Elt: {DominatorTree::Insert, BB, FirstGuardBlock});
322
323 for (int I = 0; I != NumGuards - 1; ++I) {
324 Updates.push_back(Elt: {DominatorTree::Insert, GuardBlocks[I], Outgoing[I]});
325 Updates.push_back(
326 Elt: {DominatorTree::Insert, GuardBlocks[I], GuardBlocks[I + 1]});
327 }
328 // The second successor of the last guard block is an outgoing block instead
329 // of having a "next" guard block.
330 Updates.push_back(Elt: {DominatorTree::Insert, GuardBlocks[NumGuards - 1],
331 Outgoing[NumGuards - 1]});
332 Updates.push_back(Elt: {DominatorTree::Insert, GuardBlocks[NumGuards - 1],
333 Outgoing[NumGuards]});
334 DTU->applyUpdates(Updates);
335 }
336
337 for (auto I : DeletionCandidates) {
338 if (I->use_empty())
339 if (auto *Inst = dyn_cast_or_null<Instruction>(Val&: I))
340 Inst->eraseFromParent();
341 }
342
343 return {FirstGuardBlock, true};
344}
345