1//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
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// This pass transforms loops by placing phi nodes at the end of the loops for
10// all values that are live across the loop boundary. For example, it turns
11// the left into the right code:
12//
13// for (...) for (...)
14// if (c) if (c)
15// X1 = ... X1 = ...
16// else else
17// X2 = ... X2 = ...
18// X3 = phi(X1, X2) X3 = phi(X1, X2)
19// ... = X3 + 4 X4 = phi(X3)
20// ... = X4 + 4
21//
22// This is still valid LLVM; the extra phi nodes are purely redundant, and will
23// be trivially eliminated by InstCombine. The major benefit of this
24// transformation is that it makes many other loop optimizations, such as
25// LoopUnswitching, simpler.
26//
27//===----------------------------------------------------------------------===//
28
29#include "llvm/Transforms/Utils/LCSSA.h"
30#include "llvm/ADT/STLExtras.h"
31#include "llvm/ADT/Statistic.h"
32#include "llvm/Analysis/AliasAnalysis.h"
33#include "llvm/Analysis/BasicAliasAnalysis.h"
34#include "llvm/Analysis/BranchProbabilityInfo.h"
35#include "llvm/Analysis/GlobalsModRef.h"
36#include "llvm/Analysis/LoopInfo.h"
37#include "llvm/Analysis/LoopPass.h"
38#include "llvm/Analysis/MemorySSA.h"
39#include "llvm/Analysis/ScalarEvolution.h"
40#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
41#include "llvm/IR/DebugInfo.h"
42#include "llvm/IR/Dominators.h"
43#include "llvm/IR/Instructions.h"
44#include "llvm/IR/IntrinsicInst.h"
45#include "llvm/IR/PredIteratorCache.h"
46#include "llvm/InitializePasses.h"
47#include "llvm/Pass.h"
48#include "llvm/Support/CommandLine.h"
49#include "llvm/Transforms/Utils.h"
50#include "llvm/Transforms/Utils/LoopUtils.h"
51#include "llvm/Transforms/Utils/SSAUpdater.h"
52using namespace llvm;
53
54#define DEBUG_TYPE "lcssa"
55
56STATISTIC(NumLCSSA, "Number of live out of a loop variables");
57
58#ifdef EXPENSIVE_CHECKS
59static bool VerifyLoopLCSSA = true;
60#else
61static bool VerifyLoopLCSSA = false;
62#endif
63static cl::opt<bool, true>
64 VerifyLoopLCSSAFlag("verify-loop-lcssa", cl::location(L&: VerifyLoopLCSSA),
65 cl::Hidden,
66 cl::desc("Verify loop lcssa form (time consuming)"));
67
68/// Return true if the specified block is in the list.
69static bool isExitBlock(BasicBlock *BB,
70 const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
71 return is_contained(Range: ExitBlocks, Element: BB);
72}
73
74/// For every instruction from the worklist, check to see if it has any uses
75/// that are outside the current loop. If so, insert LCSSA PHI nodes and
76/// rewrite the uses.
77bool llvm::formLCSSAForInstructions(SmallVectorImpl<Instruction *> &Worklist,
78 const DominatorTree &DT, const LoopInfo &LI,
79 ScalarEvolution *SE,
80 SmallVectorImpl<PHINode *> *PHIsToRemove,
81 SmallVectorImpl<PHINode *> *InsertedPHIs) {
82 SmallVector<Use *, 16> UsesToRewrite;
83 SmallSetVector<PHINode *, 16> LocalPHIsToRemove;
84 PredIteratorCache PredCache;
85 bool Changed = false;
86
87 // Cache the Loop ExitBlocks across this loop. We expect to get a lot of
88 // instructions within the same loops, computing the exit blocks is
89 // expensive, and we're not mutating the loop structure.
90 SmallDenseMap<Loop*, SmallVector<BasicBlock *,1>> LoopExitBlocks;
91
92 while (!Worklist.empty()) {
93 UsesToRewrite.clear();
94
95 Instruction *I = Worklist.pop_back_val();
96 assert(!I->getType()->isTokenTy() && "Tokens shouldn't be in the worklist");
97 BasicBlock *InstBB = I->getParent();
98 Loop *L = LI.getLoopFor(BB: InstBB);
99 assert(L && "Instruction belongs to a BB that's not part of a loop");
100 if (!LoopExitBlocks.count(Val: L))
101 L->getExitBlocks(ExitBlocks&: LoopExitBlocks[L]);
102 assert(LoopExitBlocks.count(L));
103 const SmallVectorImpl<BasicBlock *> &ExitBlocks = LoopExitBlocks[L];
104
105 if (ExitBlocks.empty())
106 continue;
107
108 for (Use &U : make_early_inc_range(Range: I->uses())) {
109 Instruction *User = cast<Instruction>(Val: U.getUser());
110 BasicBlock *UserBB = User->getParent();
111
112 // Skip uses in unreachable blocks.
113 if (!DT.isReachableFromEntry(A: UserBB)) {
114 U.set(PoisonValue::get(T: I->getType()));
115 continue;
116 }
117
118 // For practical purposes, we consider that the use in a PHI
119 // occurs in the respective predecessor block. For more info,
120 // see the `phi` doc in LangRef and the LCSSA doc.
121 if (auto *PN = dyn_cast<PHINode>(Val: User))
122 UserBB = PN->getIncomingBlock(U);
123
124 if (InstBB != UserBB && !L->contains(BB: UserBB))
125 UsesToRewrite.push_back(Elt: &U);
126 }
127
128 // If there are no uses outside the loop, exit with no change.
129 if (UsesToRewrite.empty())
130 continue;
131
132 ++NumLCSSA; // We are applying the transformation
133
134 // Invoke instructions are special in that their result value is not
135 // available along their unwind edge. The code below tests to see whether
136 // DomBB dominates the value, so adjust DomBB to the normal destination
137 // block, which is effectively where the value is first usable.
138 BasicBlock *DomBB = InstBB;
139 if (auto *Inv = dyn_cast<InvokeInst>(Val: I))
140 DomBB = Inv->getNormalDest();
141
142 const DomTreeNode *DomNode = DT.getNode(BB: DomBB);
143
144 SmallVector<PHINode *, 16> AddedPHIs;
145 SmallVector<PHINode *, 8> PostProcessPHIs;
146
147 SmallVector<PHINode *, 4> LocalInsertedPHIs;
148 SSAUpdater SSAUpdate(&LocalInsertedPHIs);
149 SSAUpdate.Initialize(Ty: I->getType(), Name: I->getName());
150
151 // Insert the LCSSA phi's into all of the exit blocks dominated by the
152 // value, and add them to the Phi's map.
153 bool HasSCEV = SE && SE->isSCEVable(Ty: I->getType()) &&
154 SE->getExistingSCEV(V: I) != nullptr;
155 for (BasicBlock *ExitBB : ExitBlocks) {
156 if (!DT.dominates(A: DomNode, B: DT.getNode(BB: ExitBB)))
157 continue;
158
159 // If we already inserted something for this BB, don't reprocess it.
160 if (SSAUpdate.HasValueForBlock(BB: ExitBB))
161 continue;
162 PHINode *PN = PHINode::Create(Ty: I->getType(), NumReservedValues: PredCache.size(BB: ExitBB),
163 NameStr: I->getName() + ".lcssa");
164 PN->insertBefore(InsertPos: ExitBB->begin());
165 if (InsertedPHIs)
166 InsertedPHIs->push_back(Elt: PN);
167 // Get the debug location from the original instruction.
168 PN->setDebugLoc(I->getDebugLoc());
169
170 // Add inputs from inside the loop for this PHI. This is valid
171 // because `I` dominates `ExitBB` (checked above). This implies
172 // that every incoming block/edge is dominated by `I` as well,
173 // i.e. we can add uses of `I` to those incoming edges/append to the incoming
174 // blocks without violating the SSA dominance property.
175 for (BasicBlock *Pred : PredCache.get(BB: ExitBB)) {
176 PN->addIncoming(V: I, BB: Pred);
177
178 // If the exit block has a predecessor not within the loop, arrange for
179 // the incoming value use corresponding to that predecessor to be
180 // rewritten in terms of a different LCSSA PHI.
181 if (!L->contains(BB: Pred))
182 UsesToRewrite.push_back(
183 Elt: &PN->getOperandUse(i: PN->getOperandNumForIncomingValue(
184 i: PN->getNumIncomingValues() - 1)));
185 }
186
187 AddedPHIs.push_back(Elt: PN);
188
189 // Remember that this phi makes the value alive in this block.
190 SSAUpdate.AddAvailableValue(BB: ExitBB, V: PN);
191
192 // LoopSimplify might fail to simplify some loops (e.g. when indirect
193 // branches are involved). In such situations, it might happen that an
194 // exit for Loop L1 is the header of a disjoint Loop L2. Thus, when we
195 // create PHIs in such an exit block, we are also inserting PHIs into L2's
196 // header. This could break LCSSA form for L2 because these inserted PHIs
197 // can also have uses outside of L2. Remember all PHIs in such situation
198 // as to revisit than later on. FIXME: Remove this if indirectbr support
199 // into LoopSimplify gets improved.
200 if (auto *OtherLoop = LI.getLoopFor(BB: ExitBB))
201 if (!L->contains(L: OtherLoop))
202 PostProcessPHIs.push_back(Elt: PN);
203
204 // If we have a cached SCEV for the original instruction, make sure the
205 // new LCSSA phi node is also cached. This makes sures that BECounts
206 // based on it will be invalidated when the LCSSA phi node is invalidated,
207 // which some passes rely on.
208 if (HasSCEV)
209 SE->getSCEV(V: PN);
210 }
211
212 // Rewrite all uses outside the loop in terms of the new PHIs we just
213 // inserted.
214 for (Use *UseToRewrite : UsesToRewrite) {
215 Instruction *User = cast<Instruction>(Val: UseToRewrite->getUser());
216 BasicBlock *UserBB = User->getParent();
217
218 // For practical purposes, we consider that the use in a PHI
219 // occurs in the respective predecessor block. For more info,
220 // see the `phi` doc in LangRef and the LCSSA doc.
221 if (auto *PN = dyn_cast<PHINode>(Val: User))
222 UserBB = PN->getIncomingBlock(U: *UseToRewrite);
223
224 // If this use is in an exit block, rewrite to use the newly inserted PHI.
225 // This is required for correctness because SSAUpdate doesn't handle uses
226 // in the same block. It assumes the PHI we inserted is at the end of the
227 // block.
228 if (isa<PHINode>(Val: UserBB->begin()) && isExitBlock(BB: UserBB, ExitBlocks)) {
229 UseToRewrite->set(&UserBB->front());
230 continue;
231 }
232
233 // If we added a single PHI, it must dominate all uses and we can directly
234 // rename it.
235 if (AddedPHIs.size() == 1) {
236 UseToRewrite->set(AddedPHIs[0]);
237 continue;
238 }
239
240 // Otherwise, do full PHI insertion.
241 SSAUpdate.RewriteUse(U&: *UseToRewrite);
242 }
243
244 SmallVector<DbgValueInst *, 4> DbgValues;
245 SmallVector<DbgVariableRecord *, 4> DbgVariableRecords;
246 llvm::findDbgValues(DbgValues, V: I, DbgVariableRecords: &DbgVariableRecords);
247
248 // Update pre-existing debug value uses that reside outside the loop.
249 for (auto *DVI : DbgValues) {
250 BasicBlock *UserBB = DVI->getParent();
251 if (InstBB == UserBB || L->contains(BB: UserBB))
252 continue;
253 // We currently only handle debug values residing in blocks that were
254 // traversed while rewriting the uses. If we inserted just a single PHI,
255 // we will handle all relevant debug values.
256 Value *V = AddedPHIs.size() == 1 ? AddedPHIs[0]
257 : SSAUpdate.FindValueForBlock(BB: UserBB);
258 if (V)
259 DVI->replaceVariableLocationOp(OldValue: I, NewValue: V);
260 }
261
262 // RemoveDIs: copy-paste of block above, using non-instruction debug-info
263 // records.
264 for (DbgVariableRecord *DVR : DbgVariableRecords) {
265 BasicBlock *UserBB = DVR->getMarker()->getParent();
266 if (InstBB == UserBB || L->contains(BB: UserBB))
267 continue;
268 // We currently only handle debug values residing in blocks that were
269 // traversed while rewriting the uses. If we inserted just a single PHI,
270 // we will handle all relevant debug values.
271 Value *V = AddedPHIs.size() == 1 ? AddedPHIs[0]
272 : SSAUpdate.FindValueForBlock(BB: UserBB);
273 if (V)
274 DVR->replaceVariableLocationOp(OldValue: I, NewValue: V);
275 }
276
277 // SSAUpdater might have inserted phi-nodes inside other loops. We'll need
278 // to post-process them to keep LCSSA form.
279 for (PHINode *InsertedPN : LocalInsertedPHIs) {
280 if (auto *OtherLoop = LI.getLoopFor(BB: InsertedPN->getParent()))
281 if (!L->contains(L: OtherLoop))
282 PostProcessPHIs.push_back(Elt: InsertedPN);
283 if (InsertedPHIs)
284 InsertedPHIs->push_back(Elt: InsertedPN);
285 }
286
287 // Post process PHI instructions that were inserted into another disjoint
288 // loop and update their exits properly.
289 for (auto *PostProcessPN : PostProcessPHIs)
290 if (!PostProcessPN->use_empty())
291 Worklist.push_back(Elt: PostProcessPN);
292
293 // Keep track of PHI nodes that we want to remove because they did not have
294 // any uses rewritten.
295 for (PHINode *PN : AddedPHIs)
296 if (PN->use_empty())
297 LocalPHIsToRemove.insert(X: PN);
298
299 Changed = true;
300 }
301
302 // Remove PHI nodes that did not have any uses rewritten or add them to
303 // PHIsToRemove, so the caller can remove them after some additional cleanup.
304 // We need to redo the use_empty() check here, because even if the PHI node
305 // wasn't used when added to LocalPHIsToRemove, later added PHI nodes can be
306 // using it. This cleanup is not guaranteed to handle trees/cycles of PHI
307 // nodes that only are used by each other. Such situations has only been
308 // noticed when the input IR contains unreachable code, and leaving some extra
309 // redundant PHI nodes in such situations is considered a minor problem.
310 if (PHIsToRemove) {
311 PHIsToRemove->append(in_start: LocalPHIsToRemove.begin(), in_end: LocalPHIsToRemove.end());
312 } else {
313 for (PHINode *PN : LocalPHIsToRemove)
314 if (PN->use_empty())
315 PN->eraseFromParent();
316 }
317 return Changed;
318}
319
320// Compute the set of BasicBlocks in the loop `L` dominating at least one exit.
321static void computeBlocksDominatingExits(
322 Loop &L, const DominatorTree &DT, SmallVector<BasicBlock *, 8> &ExitBlocks,
323 SmallSetVector<BasicBlock *, 8> &BlocksDominatingExits) {
324 // We start from the exit blocks, as every block trivially dominates itself
325 // (not strictly).
326 SmallVector<BasicBlock *, 8> BBWorklist(ExitBlocks);
327
328 while (!BBWorklist.empty()) {
329 BasicBlock *BB = BBWorklist.pop_back_val();
330
331 // Check if this is a loop header. If this is the case, we're done.
332 if (L.getHeader() == BB)
333 continue;
334
335 // Otherwise, add its immediate predecessor in the dominator tree to the
336 // worklist, unless we visited it already.
337 BasicBlock *IDomBB = DT.getNode(BB)->getIDom()->getBlock();
338
339 // Exit blocks can have an immediate dominator not belonging to the
340 // loop. For an exit block to be immediately dominated by another block
341 // outside the loop, it implies not all paths from that dominator, to the
342 // exit block, go through the loop.
343 // Example:
344 //
345 // |---- A
346 // | |
347 // | B<--
348 // | | |
349 // |---> C --
350 // |
351 // D
352 //
353 // C is the exit block of the loop and it's immediately dominated by A,
354 // which doesn't belong to the loop.
355 if (!L.contains(BB: IDomBB))
356 continue;
357
358 if (BlocksDominatingExits.insert(X: IDomBB))
359 BBWorklist.push_back(Elt: IDomBB);
360 }
361}
362
363bool llvm::formLCSSA(Loop &L, const DominatorTree &DT, const LoopInfo *LI,
364 ScalarEvolution *SE) {
365 bool Changed = false;
366
367#ifdef EXPENSIVE_CHECKS
368 // Verify all sub-loops are in LCSSA form already.
369 for (Loop *SubLoop: L) {
370 (void)SubLoop; // Silence unused variable warning.
371 assert(SubLoop->isRecursivelyLCSSAForm(DT, *LI) && "Subloop not in LCSSA!");
372 }
373#endif
374
375 SmallVector<BasicBlock *, 8> ExitBlocks;
376 L.getExitBlocks(ExitBlocks);
377 if (ExitBlocks.empty())
378 return false;
379
380 SmallSetVector<BasicBlock *, 8> BlocksDominatingExits;
381
382 // We want to avoid use-scanning leveraging dominance informations.
383 // If a block doesn't dominate any of the loop exits, the none of the values
384 // defined in the loop can be used outside.
385 // We compute the set of blocks fullfilling the conditions in advance
386 // walking the dominator tree upwards until we hit a loop header.
387 computeBlocksDominatingExits(L, DT, ExitBlocks, BlocksDominatingExits);
388
389 SmallVector<Instruction *, 8> Worklist;
390
391 // Look at all the instructions in the loop, checking to see if they have uses
392 // outside the loop. If so, put them into the worklist to rewrite those uses.
393 for (BasicBlock *BB : BlocksDominatingExits) {
394 // Skip blocks that are part of any sub-loops, they must be in LCSSA
395 // already.
396 if (LI->getLoopFor(BB) != &L)
397 continue;
398 for (Instruction &I : *BB) {
399 // Reject two common cases fast: instructions with no uses (like stores)
400 // and instructions with one use that is in the same block as this.
401 if (I.use_empty() ||
402 (I.hasOneUse() && I.user_back()->getParent() == BB &&
403 !isa<PHINode>(Val: I.user_back())))
404 continue;
405
406 // Tokens cannot be used in PHI nodes, so we skip over them.
407 // We can run into tokens which are live out of a loop with catchswitch
408 // instructions in Windows EH if the catchswitch has one catchpad which
409 // is inside the loop and another which is not.
410 if (I.getType()->isTokenTy())
411 continue;
412
413 Worklist.push_back(Elt: &I);
414 }
415 }
416
417 Changed = formLCSSAForInstructions(Worklist, DT, LI: *LI, SE);
418
419 assert(L.isLCSSAForm(DT));
420
421 return Changed;
422}
423
424/// Process a loop nest depth first.
425bool llvm::formLCSSARecursively(Loop &L, const DominatorTree &DT,
426 const LoopInfo *LI, ScalarEvolution *SE) {
427 bool Changed = false;
428
429 // Recurse depth-first through inner loops.
430 for (Loop *SubLoop : L.getSubLoops())
431 Changed |= formLCSSARecursively(L&: *SubLoop, DT, LI, SE);
432
433 Changed |= formLCSSA(L, DT, LI, SE);
434 return Changed;
435}
436
437/// Process all loops in the function, inner-most out.
438static bool formLCSSAOnAllLoops(const LoopInfo *LI, const DominatorTree &DT,
439 ScalarEvolution *SE) {
440 bool Changed = false;
441 for (const auto &L : *LI)
442 Changed |= formLCSSARecursively(L&: *L, DT, LI, SE);
443 return Changed;
444}
445
446namespace {
447struct LCSSAWrapperPass : public FunctionPass {
448 static char ID; // Pass identification, replacement for typeid
449 LCSSAWrapperPass() : FunctionPass(ID) {
450 initializeLCSSAWrapperPassPass(*PassRegistry::getPassRegistry());
451 }
452
453 // Cached analysis information for the current function.
454 DominatorTree *DT;
455 LoopInfo *LI;
456 ScalarEvolution *SE;
457
458 bool runOnFunction(Function &F) override;
459 void verifyAnalysis() const override {
460 // This check is very expensive. On the loop intensive compiles it may cause
461 // up to 10x slowdown. Currently it's disabled by default. LPPassManager
462 // always does limited form of the LCSSA verification. Similar reasoning
463 // was used for the LoopInfo verifier.
464 if (VerifyLoopLCSSA) {
465 assert(all_of(*LI,
466 [&](Loop *L) {
467 return L->isRecursivelyLCSSAForm(*DT, *LI);
468 }) &&
469 "LCSSA form is broken!");
470 }
471 };
472
473 /// This transformation requires natural loop information & requires that
474 /// loop preheaders be inserted into the CFG. It maintains both of these,
475 /// as well as the CFG. It also requires dominator information.
476 void getAnalysisUsage(AnalysisUsage &AU) const override {
477 AU.setPreservesCFG();
478
479 AU.addRequired<DominatorTreeWrapperPass>();
480 AU.addRequired<LoopInfoWrapperPass>();
481 AU.addPreservedID(ID&: LoopSimplifyID);
482 AU.addPreserved<AAResultsWrapperPass>();
483 AU.addPreserved<BasicAAWrapperPass>();
484 AU.addPreserved<GlobalsAAWrapperPass>();
485 AU.addPreserved<ScalarEvolutionWrapperPass>();
486 AU.addPreserved<SCEVAAWrapperPass>();
487 AU.addPreserved<BranchProbabilityInfoWrapperPass>();
488 AU.addPreserved<MemorySSAWrapperPass>();
489
490 // This is needed to perform LCSSA verification inside LPPassManager
491 AU.addRequired<LCSSAVerificationPass>();
492 AU.addPreserved<LCSSAVerificationPass>();
493 }
494};
495}
496
497char LCSSAWrapperPass::ID = 0;
498INITIALIZE_PASS_BEGIN(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
499 false, false)
500INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
501INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
502INITIALIZE_PASS_DEPENDENCY(LCSSAVerificationPass)
503INITIALIZE_PASS_END(LCSSAWrapperPass, "lcssa", "Loop-Closed SSA Form Pass",
504 false, false)
505
506Pass *llvm::createLCSSAPass() { return new LCSSAWrapperPass(); }
507char &llvm::LCSSAID = LCSSAWrapperPass::ID;
508
509/// Transform \p F into loop-closed SSA form.
510bool LCSSAWrapperPass::runOnFunction(Function &F) {
511 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
512 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
513 auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
514 SE = SEWP ? &SEWP->getSE() : nullptr;
515
516 return formLCSSAOnAllLoops(LI, DT: *DT, SE);
517}
518
519PreservedAnalyses LCSSAPass::run(Function &F, FunctionAnalysisManager &AM) {
520 auto &LI = AM.getResult<LoopAnalysis>(IR&: F);
521 auto &DT = AM.getResult<DominatorTreeAnalysis>(IR&: F);
522 auto *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(IR&: F);
523 if (!formLCSSAOnAllLoops(LI: &LI, DT, SE))
524 return PreservedAnalyses::all();
525
526 PreservedAnalyses PA;
527 PA.preserveSet<CFGAnalyses>();
528 PA.preserve<ScalarEvolutionAnalysis>();
529 // BPI maps terminators to probabilities, since we don't modify the CFG, no
530 // updates are needed to preserve it.
531 PA.preserve<BranchProbabilityAnalysis>();
532 PA.preserve<MemorySSAAnalysis>();
533 return PA;
534}
535

source code of llvm/lib/Transforms/Utils/LCSSA.cpp