1//===- FixIrreducible.cpp - Convert irreducible control-flow into loops ---===//
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// INPUT CFG: The blocks H and B form an irreducible cycle with two headers.
10//
11// Entry
12// / \
13// v v
14// H ----> B
15// ^ /|
16// `----' |
17// v
18// Exit
19//
20// OUTPUT CFG: Converted to a natural loop with a new header N.
21//
22// Entry
23// |
24// v
25// N <---.
26// / \ \
27// / \ |
28// v v /
29// H --> B --'
30// |
31// v
32// Exit
33//
34// To convert an irreducible cycle C to a natural loop L:
35//
36// 1. Add a new node N to C.
37// 2. Redirect all external incoming edges through N.
38// 3. Redirect all edges incident on header H through N.
39//
40// This is sufficient to ensure that:
41//
42// a. Every closed path in C also exists in L, with the modification that any
43// path passing through H now passes through N before reaching H.
44// b. Every external path incident on any entry of C is now incident on N and
45// then redirected to the entry.
46//
47// Thus, L is a strongly connected component dominated by N, and hence L is a
48// natural loop with header N.
49//
50// When an irreducible cycle C with header H is transformed into a loop, the
51// following invariants hold:
52//
53// 1. No new subcycles are "discovered" in the set (C-H). The only internal
54// edges that are redirected by the transform are incident on H. Any subcycle
55// S in (C-H), already existed prior to this transform, and is already in the
56// list of children for this cycle C.
57//
58// 2. Subcycles of C are not modified by the transform. For some subcycle S of
59// C, edges incident on the entries of S are either internal to C, or they
60// are now redirected through N, which is outside of S. So the list of
61// entries to S does not change. Since the transform only adds a block
62// outside S, and redirects edges that are not internal to S, the list of
63// blocks in S does not change.
64//
65// 3. Similarly, any natural loop L included in C is not affected, with one
66// exception: L is "destroyed" by the transform iff its header is H. The
67// backedges of such a loop are now redirected to N instead, and hence the
68// body of this loop gets merged into the new loop with header N.
69//
70// The actual transformation is handled by the ControlFlowHub, which redirects
71// specified control flow edges through a set of guard blocks. This also moves
72// every PHINode in an outgoing block to the hub. Since the hub dominates all
73// the outgoing blocks, each such PHINode continues to dominate its uses. Since
74// every header in an SCC has at least two predecessors, every value used in the
75// header (or later) but defined in a predecessor (or earlier) is represented by
76// a PHINode in a header. Hence the above handling of PHINodes is sufficient and
77// no further processing is required to restore SSA.
78//
79// Limitation: The pass cannot handle switch statements and indirect
80// branches. Both must be lowered to plain branches first.
81//
82// CallBr support: CallBr is handled as a more general branch instruction which
83// can have multiple successors. The pass redirects the edges to intermediate
84// target blocks that unconditionally branch to the original callbr target
85// blocks. This allows the control flow hub to know to which of the original
86// target blocks to jump to.
87// Example input CFG:
88// Entry (callbr)
89// / \
90// v v
91// H ----> B
92// ^ /|
93// `----' |
94// v
95// Exit
96//
97// becomes:
98// Entry (callbr)
99// / \
100// v v
101// target.H target.B
102// | |
103// v v
104// H ----> B
105// ^ /|
106// `----' |
107// v
108// Exit
109//
110// Note
111// OUTPUT CFG: Converted to a natural loop with a new header N.
112//
113// Entry (callbr)
114// / \
115// v v
116// target.H target.B
117// \ /
118// \ /
119// v v
120// N <---.
121// / \ \
122// / \ |
123// v v /
124// H --> B --'
125// |
126// v
127// Exit
128//
129//===----------------------------------------------------------------------===//
130
131#include "llvm/Transforms/Utils/FixIrreducible.h"
132#include "llvm/Analysis/CycleAnalysis.h"
133#include "llvm/Analysis/DomTreeUpdater.h"
134#include "llvm/Analysis/LoopInfo.h"
135#include "llvm/InitializePasses.h"
136#include "llvm/Pass.h"
137#include "llvm/Transforms/Utils.h"
138#include "llvm/Transforms/Utils/BasicBlockUtils.h"
139#include "llvm/Transforms/Utils/ControlFlowUtils.h"
140
141#define DEBUG_TYPE "fix-irreducible"
142
143using namespace llvm;
144
145namespace {
146struct FixIrreducible : public FunctionPass {
147 static char ID;
148 FixIrreducible() : FunctionPass(ID) {
149 initializeFixIrreduciblePass(*PassRegistry::getPassRegistry());
150 }
151
152 void getAnalysisUsage(AnalysisUsage &AU) const override {
153 AU.addRequired<DominatorTreeWrapperPass>();
154 AU.addRequired<CycleInfoWrapperPass>();
155 AU.addPreserved<DominatorTreeWrapperPass>();
156 AU.addPreserved<CycleInfoWrapperPass>();
157 AU.addPreserved<LoopInfoWrapperPass>();
158 }
159
160 bool runOnFunction(Function &F) override;
161};
162} // namespace
163
164char FixIrreducible::ID = 0;
165
166FunctionPass *llvm::createFixIrreduciblePass() { return new FixIrreducible(); }
167
168INITIALIZE_PASS_BEGIN(FixIrreducible, "fix-irreducible",
169 "Convert irreducible control-flow into natural loops",
170 false /* Only looks at CFG */, false /* Analysis Pass */)
171INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
172INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
173INITIALIZE_PASS_END(FixIrreducible, "fix-irreducible",
174 "Convert irreducible control-flow into natural loops",
175 false /* Only looks at CFG */, false /* Analysis Pass */)
176
177// When a new loop is created, existing children of the parent loop may now be
178// fully inside the new loop. Reconnect these as children of the new loop.
179static void reconnectChildLoops(LoopInfo &LI, Loop *ParentLoop, Loop *NewLoop,
180 BasicBlock *OldHeader) {
181 auto &CandidateLoops = ParentLoop ? ParentLoop->getSubLoopsVector()
182 : LI.getTopLevelLoopsVector();
183 // Any candidate is a child iff its header is owned by the new loop. Move all
184 // the children to a new vector.
185 auto FirstChild = llvm::partition(Range&: CandidateLoops, P: [&](Loop *L) {
186 return NewLoop == L || !NewLoop->contains(BB: L->getHeader());
187 });
188 SmallVector<Loop *, 8> ChildLoops(FirstChild, CandidateLoops.end());
189 CandidateLoops.erase(first: FirstChild, last: CandidateLoops.end());
190
191 for (Loop *Child : ChildLoops) {
192 LLVM_DEBUG(dbgs() << "child loop: " << Child->getHeader()->getName()
193 << "\n");
194 // A child loop whose header was the old cycle header gets destroyed since
195 // its backedges are removed.
196 if (Child->getHeader() == OldHeader) {
197 for (auto *BB : Child->blocks()) {
198 if (LI.getLoopFor(BB) != Child)
199 continue;
200 LI.changeLoopFor(BB, L: NewLoop);
201 LLVM_DEBUG(dbgs() << "moved block from child: " << BB->getName()
202 << "\n");
203 }
204 std::vector<Loop *> GrandChildLoops;
205 std::swap(x&: GrandChildLoops, y&: Child->getSubLoopsVector());
206 for (auto *GrandChildLoop : GrandChildLoops) {
207 GrandChildLoop->setParentLoop(nullptr);
208 NewLoop->addChildLoop(NewChild: GrandChildLoop);
209 }
210 LI.destroy(L: Child);
211 LLVM_DEBUG(dbgs() << "subsumed child loop (common header)\n");
212 continue;
213 }
214
215 Child->setParentLoop(nullptr);
216 NewLoop->addChildLoop(NewChild: Child);
217 LLVM_DEBUG(dbgs() << "added child loop to new loop\n");
218 }
219}
220
221static void updateLoopInfo(LoopInfo &LI, Cycle &C,
222 ArrayRef<BasicBlock *> GuardBlocks) {
223 // The parent loop is a natural loop L mapped to the cycle header H as long as
224 // H is not also the header of L. In the latter case, L is destroyed and we
225 // seek its parent instead.
226 BasicBlock *CycleHeader = C.getHeader();
227 Loop *ParentLoop = LI.getLoopFor(BB: CycleHeader);
228 if (ParentLoop && ParentLoop->getHeader() == CycleHeader)
229 ParentLoop = ParentLoop->getParentLoop();
230
231 // Create a new loop from the now-transformed cycle
232 auto *NewLoop = LI.AllocateLoop();
233 if (ParentLoop) {
234 ParentLoop->addChildLoop(NewChild: NewLoop);
235 } else {
236 LI.addTopLevelLoop(New: NewLoop);
237 }
238
239 // Add the guard blocks to the new loop. The first guard block is
240 // the head of all the backedges, and it is the first to be inserted
241 // in the loop. This ensures that it is recognized as the
242 // header. Since the new loop is already in LoopInfo, the new blocks
243 // are also propagated up the chain of parent loops.
244 for (auto *G : GuardBlocks) {
245 LLVM_DEBUG(dbgs() << "added guard block to loop: " << G->getName() << "\n");
246 NewLoop->addBasicBlockToLoop(NewBB: G, LI);
247 }
248
249 for (auto *BB : C.blocks()) {
250 NewLoop->addBlockEntry(BB);
251 if (LI.getLoopFor(BB) == ParentLoop) {
252 LLVM_DEBUG(dbgs() << "moved block from parent: " << BB->getName()
253 << "\n");
254 LI.changeLoopFor(BB, L: NewLoop);
255 } else {
256 LLVM_DEBUG(dbgs() << "added block from child: " << BB->getName() << "\n");
257 }
258 }
259 LLVM_DEBUG(dbgs() << "header for new loop: "
260 << NewLoop->getHeader()->getName() << "\n");
261
262 reconnectChildLoops(LI, ParentLoop, NewLoop, OldHeader: C.getHeader());
263
264 LLVM_DEBUG(dbgs() << "Verify new loop.\n"; NewLoop->print(dbgs()));
265 NewLoop->verifyLoop();
266 if (ParentLoop) {
267 LLVM_DEBUG(dbgs() << "Verify parent loop.\n"; ParentLoop->print(dbgs()));
268 ParentLoop->verifyLoop();
269 }
270}
271
272// Given a set of blocks and headers in an irreducible SCC, convert it into a
273// natural loop. Also insert this new loop at its appropriate place in the
274// hierarchy of loops.
275static bool fixIrreducible(Cycle &C, CycleInfo &CI, DominatorTree &DT,
276 LoopInfo *LI) {
277 if (C.isReducible())
278 return false;
279 LLVM_DEBUG(dbgs() << "Processing cycle:\n" << CI.print(&C) << "\n";);
280
281 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
282 ControlFlowHub CHub;
283 SetVector<BasicBlock *> Predecessors;
284
285 // Redirect internal edges incident on the header.
286 BasicBlock *Header = C.getHeader();
287 for (BasicBlock *P : predecessors(BB: Header)) {
288 if (C.contains(Block: P))
289 Predecessors.insert(X: P);
290 }
291
292 for (BasicBlock *P : Predecessors) {
293 if (BranchInst *Branch = dyn_cast<BranchInst>(Val: P->getTerminator())) {
294 // Exactly one of the two successors is the header.
295 BasicBlock *Succ0 = Branch->getSuccessor(i: 0) == Header ? Header : nullptr;
296 BasicBlock *Succ1 = Succ0 ? nullptr : Header;
297 assert(Succ0 || Branch->getSuccessor(1) == Header);
298 assert(Succ0 || Succ1);
299 CHub.addBranch(BB: P, Succ0, Succ1);
300
301 LLVM_DEBUG(dbgs() << "Added internal branch: " << printBasicBlock(P)
302 << " -> " << printBasicBlock(Succ0)
303 << (Succ0 && Succ1 ? " " : "") << printBasicBlock(Succ1)
304 << '\n');
305 } else if (CallBrInst *CallBr = dyn_cast<CallBrInst>(Val: P->getTerminator())) {
306 for (unsigned I = 0; I < CallBr->getNumSuccessors(); ++I) {
307 BasicBlock *Succ = CallBr->getSuccessor(i: I);
308 if (Succ != Header)
309 continue;
310 BasicBlock *NewSucc = SplitCallBrEdge(CallBrBlock: P, Succ, SuccIdx: I, DTU: &DTU, CI: &CI, LI);
311 CHub.addBranch(BB: NewSucc, Succ0: Succ);
312 LLVM_DEBUG(dbgs() << "Added internal branch: "
313 << printBasicBlock(NewSucc) << " -> "
314 << printBasicBlock(Succ) << '\n');
315 }
316 } else {
317 llvm_unreachable("unsupported block terminator");
318 }
319 }
320
321 // Redirect external incoming edges. This includes the edges on the header.
322 Predecessors.clear();
323 for (BasicBlock *E : C.entries()) {
324 for (BasicBlock *P : predecessors(BB: E)) {
325 if (!C.contains(Block: P))
326 Predecessors.insert(X: P);
327 }
328 }
329
330 for (BasicBlock *P : Predecessors) {
331 if (BranchInst *Branch = dyn_cast<BranchInst>(Val: P->getTerminator()); Branch) {
332 BasicBlock *Succ0 = Branch->getSuccessor(i: 0);
333 Succ0 = C.contains(Block: Succ0) ? Succ0 : nullptr;
334 BasicBlock *Succ1 =
335 Branch->isUnconditional() ? nullptr : Branch->getSuccessor(i: 1);
336 Succ1 = Succ1 && C.contains(Block: Succ1) ? Succ1 : nullptr;
337 CHub.addBranch(BB: P, Succ0, Succ1);
338
339 LLVM_DEBUG(dbgs() << "Added external branch: " << printBasicBlock(P)
340 << " -> " << printBasicBlock(Succ0)
341 << (Succ0 && Succ1 ? " " : "") << printBasicBlock(Succ1)
342 << '\n');
343 } else if (CallBrInst *CallBr = dyn_cast<CallBrInst>(Val: P->getTerminator())) {
344 for (unsigned I = 0; I < CallBr->getNumSuccessors(); ++I) {
345 BasicBlock *Succ = CallBr->getSuccessor(i: I);
346 if (!C.contains(Block: Succ))
347 continue;
348 BasicBlock *NewSucc = SplitCallBrEdge(CallBrBlock: P, Succ, SuccIdx: I, DTU: &DTU, CI: &CI, LI);
349 CHub.addBranch(BB: NewSucc, Succ0: Succ);
350 LLVM_DEBUG(dbgs() << "Added external branch: "
351 << printBasicBlock(NewSucc) << " -> "
352 << printBasicBlock(Succ) << '\n');
353 }
354 } else {
355 llvm_unreachable("unsupported block terminator");
356 }
357 }
358
359 // Redirect all the backedges through a "hub" consisting of a series
360 // of guard blocks that manage the flow of control from the
361 // predecessors to the headers.
362 SmallVector<BasicBlock *> GuardBlocks;
363
364 // Minor optimization: The cycle entries are discovered in an order that is
365 // the opposite of the order in which these blocks appear as branch targets.
366 // This results in a lot of condition inversions in the control flow out of
367 // the new ControlFlowHub, which can be mitigated if the orders match. So we
368 // reverse the entries when adding them to the hub.
369 SetVector<BasicBlock *> Entries;
370 Entries.insert(Start: C.entry_rbegin(), End: C.entry_rend());
371
372 CHub.finalize(DTU: &DTU, GuardBlocks, Prefix: "irr");
373#if defined(EXPENSIVE_CHECKS)
374 assert(DT.verify(DominatorTree::VerificationLevel::Full));
375#else
376 assert(DT.verify(DominatorTree::VerificationLevel::Fast));
377#endif
378
379 // If we are updating LoopInfo, do that now before modifying the cycle. This
380 // ensures that the first guard block is the header of a new natural loop.
381 if (LI)
382 updateLoopInfo(LI&: *LI, C, GuardBlocks);
383
384 for (auto *G : GuardBlocks) {
385 LLVM_DEBUG(dbgs() << "added guard block to cycle: " << G->getName()
386 << "\n");
387 CI.addBlockToCycle(Block: G, Cycle: &C);
388 }
389 C.setSingleEntry(GuardBlocks[0]);
390
391 C.verifyCycle();
392 if (Cycle *Parent = C.getParentCycle())
393 Parent->verifyCycle();
394
395 LLVM_DEBUG(dbgs() << "Finished one cycle:\n"; CI.print(dbgs()););
396 return true;
397}
398
399static bool FixIrreducibleImpl(Function &F, CycleInfo &CI, DominatorTree &DT,
400 LoopInfo *LI) {
401 LLVM_DEBUG(dbgs() << "===== Fix irreducible control-flow in function: "
402 << F.getName() << "\n");
403
404 bool Changed = false;
405 for (Cycle *TopCycle : CI.toplevel_cycles()) {
406 for (Cycle *C : depth_first(G: TopCycle)) {
407 Changed |= fixIrreducible(C&: *C, CI, DT, LI);
408 }
409 }
410
411 if (!Changed)
412 return false;
413
414#if defined(EXPENSIVE_CHECKS)
415 CI.verify();
416 if (LI) {
417 LI->verify(DT);
418 }
419#endif // EXPENSIVE_CHECKS
420
421 return true;
422}
423
424bool FixIrreducible::runOnFunction(Function &F) {
425 auto *LIWP = getAnalysisIfAvailable<LoopInfoWrapperPass>();
426 LoopInfo *LI = LIWP ? &LIWP->getLoopInfo() : nullptr;
427 auto &CI = getAnalysis<CycleInfoWrapperPass>().getResult();
428 auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
429 return FixIrreducibleImpl(F, CI, DT, LI);
430}
431
432PreservedAnalyses FixIrreduciblePass::run(Function &F,
433 FunctionAnalysisManager &AM) {
434 auto *LI = AM.getCachedResult<LoopAnalysis>(IR&: F);
435 auto &CI = AM.getResult<CycleAnalysis>(IR&: F);
436 auto &DT = AM.getResult<DominatorTreeAnalysis>(IR&: F);
437
438 if (!FixIrreducibleImpl(F, CI, DT, LI))
439 return PreservedAnalyses::all();
440
441 PreservedAnalyses PA;
442 PA.preserve<LoopAnalysis>();
443 PA.preserve<CycleAnalysis>();
444 PA.preserve<DominatorTreeAnalysis>();
445 return PA;
446}
447