1//===- LoopVersioning.cpp - Utility to version a loop ---------------------===//
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 file defines a utility class to perform loop versioning. The versioned
10// loop speculates that otherwise may-aliasing memory accesses don't overlap and
11// emits checks to prove this.
12//
13//===----------------------------------------------------------------------===//
14
15#include "llvm/Transforms/Utils/LoopVersioning.h"
16#include "llvm/ADT/ArrayRef.h"
17#include "llvm/Analysis/AliasAnalysis.h"
18#include "llvm/Analysis/InstSimplifyFolder.h"
19#include "llvm/Analysis/LoopAccessAnalysis.h"
20#include "llvm/Analysis/LoopInfo.h"
21#include "llvm/Analysis/ScalarEvolution.h"
22#include "llvm/Analysis/TargetLibraryInfo.h"
23#include "llvm/IR/Dominators.h"
24#include "llvm/IR/MDBuilder.h"
25#include "llvm/IR/PassManager.h"
26#include "llvm/Support/CommandLine.h"
27#include "llvm/Transforms/Utils/BasicBlockUtils.h"
28#include "llvm/Transforms/Utils/Cloning.h"
29#include "llvm/Transforms/Utils/LoopUtils.h"
30#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
31
32using namespace llvm;
33
34#define DEBUG_TYPE "loop-versioning"
35
36static cl::opt<bool>
37 AnnotateNoAlias("loop-version-annotate-no-alias", cl::init(Val: true),
38 cl::Hidden,
39 cl::desc("Add no-alias annotation for instructions that "
40 "are disambiguated by memchecks"));
41
42LoopVersioning::LoopVersioning(const LoopAccessInfo &LAI,
43 ArrayRef<RuntimePointerCheck> Checks, Loop *L,
44 LoopInfo *LI, DominatorTree *DT,
45 ScalarEvolution *SE)
46 : VersionedLoop(L), AliasChecks(Checks), Preds(LAI.getPSE().getPredicate()),
47 LAI(LAI), LI(LI), DT(DT), SE(SE) {}
48
49void LoopVersioning::versionLoop(
50 const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
51 assert(VersionedLoop->getUniqueExitBlock() && "No single exit block");
52 assert(VersionedLoop->isLoopSimplifyForm() &&
53 "Loop is not in loop-simplify form");
54
55 Value *MemRuntimeCheck;
56 Value *SCEVRuntimeCheck;
57 Value *RuntimeCheck = nullptr;
58
59 // Add the memcheck in the original preheader (this is empty initially).
60 BasicBlock *RuntimeCheckBB = VersionedLoop->getLoopPreheader();
61 const auto &RtPtrChecking = *LAI.getRuntimePointerChecking();
62
63 SCEVExpander Exp2(*RtPtrChecking.getSE(),
64 VersionedLoop->getHeader()->getDataLayout(),
65 "induction");
66 MemRuntimeCheck = addRuntimeChecks(Loc: RuntimeCheckBB->getTerminator(),
67 TheLoop: VersionedLoop, PointerChecks: AliasChecks, Expander&: Exp2);
68
69 SCEVExpander Exp(*SE, RuntimeCheckBB->getDataLayout(),
70 "scev.check");
71 SCEVRuntimeCheck =
72 Exp.expandCodeForPredicate(Pred: &Preds, Loc: RuntimeCheckBB->getTerminator());
73
74 IRBuilder<InstSimplifyFolder> Builder(
75 RuntimeCheckBB->getContext(),
76 InstSimplifyFolder(RuntimeCheckBB->getDataLayout()));
77 if (MemRuntimeCheck && SCEVRuntimeCheck) {
78 Builder.SetInsertPoint(RuntimeCheckBB->getTerminator());
79 RuntimeCheck =
80 Builder.CreateOr(LHS: MemRuntimeCheck, RHS: SCEVRuntimeCheck, Name: "lver.safe");
81 } else
82 RuntimeCheck = MemRuntimeCheck ? MemRuntimeCheck : SCEVRuntimeCheck;
83
84 assert(RuntimeCheck && "called even though we don't need "
85 "any runtime checks");
86
87 // Rename the block to make the IR more readable.
88 RuntimeCheckBB->setName(VersionedLoop->getHeader()->getName() +
89 ".lver.check");
90
91 // Create empty preheader for the loop (and after cloning for the
92 // non-versioned loop).
93 BasicBlock *PH =
94 SplitBlock(Old: RuntimeCheckBB, SplitPt: RuntimeCheckBB->getTerminator(), DT, LI,
95 MSSAU: nullptr, BBName: VersionedLoop->getHeader()->getName() + ".ph");
96
97 // Clone the loop including the preheader.
98 //
99 // FIXME: This does not currently preserve SimplifyLoop because the exit
100 // block is a join between the two loops.
101 SmallVector<BasicBlock *, 8> NonVersionedLoopBlocks;
102 NonVersionedLoop =
103 cloneLoopWithPreheader(Before: PH, LoopDomBB: RuntimeCheckBB, OrigLoop: VersionedLoop, VMap,
104 NameSuffix: ".lver.orig", LI, DT, Blocks&: NonVersionedLoopBlocks);
105 remapInstructionsInBlocks(Blocks: NonVersionedLoopBlocks, VMap);
106
107 // Insert the conditional branch based on the result of the memchecks.
108 Instruction *OrigTerm = RuntimeCheckBB->getTerminator();
109 Builder.SetInsertPoint(OrigTerm);
110 Builder.CreateCondBr(Cond: RuntimeCheck, True: NonVersionedLoop->getLoopPreheader(),
111 False: VersionedLoop->getLoopPreheader());
112 OrigTerm->eraseFromParent();
113
114 // The loops merge in the original exit block. This is now dominated by the
115 // memchecking block.
116 DT->changeImmediateDominator(BB: VersionedLoop->getExitBlock(), NewBB: RuntimeCheckBB);
117
118 // Adds the necessary PHI nodes for the versioned loops based on the
119 // loop-defined values used outside of the loop.
120 addPHINodes(DefsUsedOutside);
121 formDedicatedExitBlocks(L: NonVersionedLoop, DT, LI, MSSAU: nullptr, PreserveLCSSA: true);
122 formDedicatedExitBlocks(L: VersionedLoop, DT, LI, MSSAU: nullptr, PreserveLCSSA: true);
123 assert(NonVersionedLoop->isLoopSimplifyForm() &&
124 VersionedLoop->isLoopSimplifyForm() &&
125 "The versioned loops should be in simplify form.");
126}
127
128void LoopVersioning::addPHINodes(
129 const SmallVectorImpl<Instruction *> &DefsUsedOutside) {
130 BasicBlock *PHIBlock = VersionedLoop->getExitBlock();
131 assert(PHIBlock && "No single successor to loop exit block");
132 PHINode *PN;
133
134 // First add a single-operand PHI for each DefsUsedOutside if one does not
135 // exists yet.
136 for (auto *Inst : DefsUsedOutside) {
137 // See if we have a single-operand PHI with the value defined by the
138 // original loop.
139 for (auto I = PHIBlock->begin(); (PN = dyn_cast<PHINode>(Val&: I)); ++I) {
140 if (PN->getIncomingValue(i: 0) == Inst) {
141 SE->forgetLcssaPhiWithNewPredecessor(L: VersionedLoop, V: PN);
142 break;
143 }
144 }
145 // If not create it.
146 if (!PN) {
147 PN = PHINode::Create(Ty: Inst->getType(), NumReservedValues: 2, NameStr: Inst->getName() + ".lver");
148 PN->insertBefore(InsertPos: PHIBlock->begin());
149 SmallVector<User*, 8> UsersToUpdate;
150 for (User *U : Inst->users())
151 if (!VersionedLoop->contains(BB: cast<Instruction>(Val: U)->getParent()))
152 UsersToUpdate.push_back(Elt: U);
153 for (User *U : UsersToUpdate)
154 U->replaceUsesOfWith(From: Inst, To: PN);
155 PN->addIncoming(V: Inst, BB: VersionedLoop->getExitingBlock());
156 }
157 }
158
159 // Then for each PHI add the operand for the edge from the cloned loop.
160 for (auto I = PHIBlock->begin(); (PN = dyn_cast<PHINode>(Val&: I)); ++I) {
161 assert(PN->getNumOperands() == 1 &&
162 "Exit block should only have on predecessor");
163
164 // If the definition was cloned used that otherwise use the same value.
165 Value *ClonedValue = PN->getIncomingValue(i: 0);
166 auto Mapped = VMap.find(Val: ClonedValue);
167 if (Mapped != VMap.end())
168 ClonedValue = Mapped->second;
169
170 PN->addIncoming(V: ClonedValue, BB: NonVersionedLoop->getExitingBlock());
171 }
172}
173
174void LoopVersioning::prepareNoAliasMetadata() {
175 // We need to turn the no-alias relation between pointer checking groups into
176 // no-aliasing annotations between instructions.
177 //
178 // We accomplish this by mapping each pointer checking group (a set of
179 // pointers memchecked together) to an alias scope and then also mapping each
180 // group to the list of scopes it can't alias.
181
182 const RuntimePointerChecking *RtPtrChecking = LAI.getRuntimePointerChecking();
183 LLVMContext &Context = VersionedLoop->getHeader()->getContext();
184
185 // First allocate an aliasing scope for each pointer checking group.
186 //
187 // While traversing through the checking groups in the loop, also create a
188 // reverse map from pointers to the pointer checking group they were assigned
189 // to.
190 MDBuilder MDB(Context);
191 MDNode *Domain = MDB.createAnonymousAliasScopeDomain(Name: "LVerDomain");
192
193 for (const auto &Group : RtPtrChecking->CheckingGroups) {
194 GroupToScope[&Group] = MDB.createAnonymousAliasScope(Domain);
195
196 for (unsigned PtrIdx : Group.Members)
197 PtrToGroup[RtPtrChecking->getPointerInfo(PtrIdx).PointerValue] = &Group;
198 }
199
200 // Go through the checks and for each pointer group, collect the scopes for
201 // each non-aliasing pointer group.
202 DenseMap<const RuntimeCheckingPtrGroup *, SmallVector<Metadata *, 4>>
203 GroupToNonAliasingScopes;
204
205 for (const auto &Check : AliasChecks)
206 GroupToNonAliasingScopes[Check.first].push_back(Elt: GroupToScope[Check.second]);
207
208 // Finally, transform the above to actually map to scope list which is what
209 // the metadata uses.
210
211 for (const auto &Pair : GroupToNonAliasingScopes)
212 GroupToNonAliasingScopeList[Pair.first] = MDNode::get(Context, MDs: Pair.second);
213}
214
215void LoopVersioning::annotateLoopWithNoAlias() {
216 if (!AnnotateNoAlias)
217 return;
218
219 // First prepare the maps.
220 prepareNoAliasMetadata();
221
222 // Add the scope and no-alias metadata to the instructions.
223 for (Instruction *I : LAI.getDepChecker().getMemoryInstructions()) {
224 annotateInstWithNoAlias(I);
225 }
226}
227
228std::pair<MDNode *, MDNode *>
229LoopVersioning::getNoAliasMetadataFor(const Instruction *OrigInst) const {
230 if (!AnnotateNoAlias)
231 return {nullptr, nullptr};
232
233 LLVMContext &Context = VersionedLoop->getHeader()->getContext();
234 const Value *Ptr = isa<LoadInst>(Val: OrigInst)
235 ? cast<LoadInst>(Val: OrigInst)->getPointerOperand()
236 : cast<StoreInst>(Val: OrigInst)->getPointerOperand();
237
238 MDNode *AliasScope = nullptr;
239 MDNode *NoAlias = nullptr;
240 // Find the group for the pointer and then add the scope metadata.
241 auto Group = PtrToGroup.find(Val: Ptr);
242 if (Group != PtrToGroup.end()) {
243 AliasScope = MDNode::concatenate(
244 A: OrigInst->getMetadata(KindID: LLVMContext::MD_alias_scope),
245 B: MDNode::get(Context, MDs: GroupToScope.lookup(Val: Group->second)));
246
247 // Add the no-alias metadata.
248 auto NonAliasingScopeList = GroupToNonAliasingScopeList.find(Val: Group->second);
249 if (NonAliasingScopeList != GroupToNonAliasingScopeList.end())
250 NoAlias =
251 MDNode::concatenate(A: OrigInst->getMetadata(KindID: LLVMContext::MD_noalias),
252 B: NonAliasingScopeList->second);
253 }
254 return {AliasScope, NoAlias};
255}
256
257void LoopVersioning::annotateInstWithNoAlias(Instruction *VersionedInst,
258 const Instruction *OrigInst) {
259 const auto &[AliasScopeMD, NoAliasMD] = getNoAliasMetadataFor(OrigInst);
260 if (AliasScopeMD)
261 VersionedInst->setMetadata(KindID: LLVMContext::MD_alias_scope, Node: AliasScopeMD);
262
263 if (NoAliasMD)
264 VersionedInst->setMetadata(KindID: LLVMContext::MD_noalias, Node: NoAliasMD);
265}
266
267namespace {
268bool runImpl(LoopInfo *LI, LoopAccessInfoManager &LAIs, DominatorTree *DT,
269 ScalarEvolution *SE) {
270 // Build up a worklist of inner-loops to version. This is necessary as the
271 // act of versioning a loop creates new loops and can invalidate iterators
272 // across the loops.
273 SmallVector<Loop *, 8> Worklist;
274
275 for (Loop *TopLevelLoop : *LI)
276 for (Loop *L : depth_first(G: TopLevelLoop))
277 // We only handle inner-most loops.
278 if (L->isInnermost())
279 Worklist.push_back(Elt: L);
280
281 // Now walk the identified inner loops.
282 bool Changed = false;
283 for (Loop *L : Worklist) {
284 if (!L->isLoopSimplifyForm() || !L->isRotatedForm() ||
285 !L->getExitingBlock())
286 continue;
287 const LoopAccessInfo &LAI = LAIs.getInfo(L&: *L);
288 if (!LAI.hasConvergentOp() &&
289 (LAI.getNumRuntimePointerChecks() ||
290 !LAI.getPSE().getPredicate().isAlwaysTrue())) {
291 if (!L->isLCSSAForm(DT: *DT))
292 formLCSSARecursively(L&: *L, DT: *DT, LI, SE);
293
294 LoopVersioning LVer(LAI, LAI.getRuntimePointerChecking()->getChecks(), L,
295 LI, DT, SE);
296 LVer.versionLoop();
297 LVer.annotateLoopWithNoAlias();
298 Changed = true;
299 LAIs.clear();
300 }
301 }
302
303 return Changed;
304}
305}
306
307PreservedAnalyses LoopVersioningPass::run(Function &F,
308 FunctionAnalysisManager &AM) {
309 auto &SE = AM.getResult<ScalarEvolutionAnalysis>(IR&: F);
310 auto &LI = AM.getResult<LoopAnalysis>(IR&: F);
311 LoopAccessInfoManager &LAIs = AM.getResult<LoopAccessAnalysis>(IR&: F);
312 auto &DT = AM.getResult<DominatorTreeAnalysis>(IR&: F);
313
314 if (runImpl(LI: &LI, LAIs, DT: &DT, SE: &SE))
315 return PreservedAnalyses::none();
316 return PreservedAnalyses::all();
317}
318