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