1 //===- LoopInstSimplify.cpp - Loop Instruction Simplification Pass --------===//
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 performs lightweight instruction simplification on loop bodies.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #include "llvm/Transforms/Scalar/LoopInstSimplify.h"
14 #include "llvm/ADT/PointerIntPair.h"
15 #include "llvm/ADT/STLExtras.h"
16 #include "llvm/ADT/SmallPtrSet.h"
17 #include "llvm/ADT/SmallVector.h"
18 #include "llvm/ADT/Statistic.h"
19 #include "llvm/Analysis/AssumptionCache.h"
20 #include "llvm/Analysis/InstructionSimplify.h"
21 #include "llvm/Analysis/LoopInfo.h"
22 #include "llvm/Analysis/LoopIterator.h"
23 #include "llvm/Analysis/LoopPass.h"
24 #include "llvm/Analysis/MemorySSA.h"
25 #include "llvm/Analysis/MemorySSAUpdater.h"
26 #include "llvm/Analysis/TargetLibraryInfo.h"
27 #include "llvm/IR/BasicBlock.h"
28 #include "llvm/IR/CFG.h"
29 #include "llvm/IR/DataLayout.h"
30 #include "llvm/IR/Dominators.h"
31 #include "llvm/IR/Instruction.h"
32 #include "llvm/IR/Instructions.h"
33 #include "llvm/IR/Module.h"
34 #include "llvm/IR/PassManager.h"
35 #include "llvm/IR/User.h"
36 #include "llvm/InitializePasses.h"
37 #include "llvm/Pass.h"
38 #include "llvm/Support/Casting.h"
39 #include "llvm/Transforms/Scalar.h"
40 #include "llvm/Transforms/Utils/Local.h"
41 #include "llvm/Transforms/Utils/LoopUtils.h"
42 #include <algorithm>
43 #include <utility>
44 
45 using namespace llvm;
46 
47 #define DEBUG_TYPE "loop-instsimplify"
48 
49 STATISTIC(NumSimplified, "Number of redundant instructions simplified");
50 
51 static bool simplifyLoopInst(Loop &L, DominatorTree &DT, LoopInfo &LI,
52                              AssumptionCache &AC, const TargetLibraryInfo &TLI,
53                              MemorySSAUpdater *MSSAU) {
54   const DataLayout &DL = L.getHeader()->getModule()->getDataLayout();
55   SimplifyQuery SQ(DL, &TLI, &DT, &AC);
56 
57   // On the first pass over the loop body we try to simplify every instruction.
58   // On subsequent passes, we can restrict this to only simplifying instructions
59   // where the inputs have been updated. We end up needing two sets: one
60   // containing the instructions we are simplifying in *this* pass, and one for
61   // the instructions we will want to simplify in the *next* pass. We use
62   // pointers so we can swap between two stably allocated sets.
63   SmallPtrSet<const Instruction *, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
64 
65   // Track the PHI nodes that have already been visited during each iteration so
66   // that we can identify when it is necessary to iterate.
67   SmallPtrSet<PHINode *, 4> VisitedPHIs;
68 
69   // While simplifying we may discover dead code or cause code to become dead.
70   // Keep track of all such instructions and we will delete them at the end.
71   SmallVector<Instruction *, 8> DeadInsts;
72 
73   // First we want to create an RPO traversal of the loop body. By processing in
74   // RPO we can ensure that definitions are processed prior to uses (for non PHI
75   // uses) in all cases. This ensures we maximize the simplifications in each
76   // iteration over the loop and minimizes the possible causes for continuing to
77   // iterate.
78   LoopBlocksRPO RPOT(&L);
79   RPOT.perform(&LI);
80   MemorySSA *MSSA = MSSAU ? MSSAU->getMemorySSA() : nullptr;
81 
82   bool Changed = false;
83   for (;;) {
84     if (MSSAU && VerifyMemorySSA)
85       MSSA->verifyMemorySSA();
86     for (BasicBlock *BB : RPOT) {
87       for (Instruction &I : *BB) {
88         if (auto *PI = dyn_cast<PHINode>(&I))
89           VisitedPHIs.insert(PI);
90 
91         if (I.use_empty()) {
92           if (isInstructionTriviallyDead(&I, &TLI))
93             DeadInsts.push_back(&I);
94           continue;
95         }
96 
97         // We special case the first iteration which we can detect due to the
98         // empty `ToSimplify` set.
99         bool IsFirstIteration = ToSimplify->empty();
100 
101         if (!IsFirstIteration && !ToSimplify->count(&I))
102           continue;
103 
104         Value *V = SimplifyInstruction(&I, SQ.getWithInstruction(&I));
105         if (!V || !LI.replacementPreservesLCSSAForm(&I, V))
106           continue;
107 
108         for (Value::use_iterator UI = I.use_begin(), UE = I.use_end();
109              UI != UE;) {
110           Use &U = *UI++;
111           auto *UserI = cast<Instruction>(U.getUser());
112           U.set(V);
113 
114           // If the instruction is used by a PHI node we have already processed
115           // we'll need to iterate on the loop body to converge, so add it to
116           // the next set.
117           if (auto *UserPI = dyn_cast<PHINode>(UserI))
118             if (VisitedPHIs.count(UserPI)) {
119               Next->insert(UserPI);
120               continue;
121             }
122 
123           // If we are only simplifying targeted instructions and the user is an
124           // instruction in the loop body, add it to our set of targeted
125           // instructions. Because we process defs before uses (outside of PHIs)
126           // we won't have visited it yet.
127           //
128           // We also skip any uses outside of the loop being simplified. Those
129           // should always be PHI nodes due to LCSSA form, and we don't want to
130           // try to simplify those away.
131           assert((L.contains(UserI) || isa<PHINode>(UserI)) &&
132                  "Uses outside the loop should be PHI nodes due to LCSSA!");
133           if (!IsFirstIteration && L.contains(UserI))
134             ToSimplify->insert(UserI);
135         }
136 
137         if (MSSAU)
138           if (Instruction *SimpleI = dyn_cast_or_null<Instruction>(V))
139             if (MemoryAccess *MA = MSSA->getMemoryAccess(&I))
140               if (MemoryAccess *ReplacementMA = MSSA->getMemoryAccess(SimpleI))
141                 MA->replaceAllUsesWith(ReplacementMA);
142 
143         assert(I.use_empty() && "Should always have replaced all uses!");
144         if (isInstructionTriviallyDead(&I, &TLI))
145           DeadInsts.push_back(&I);
146         ++NumSimplified;
147         Changed = true;
148       }
149     }
150 
151     // Delete any dead instructions found thus far now that we've finished an
152     // iteration over all instructions in all the loop blocks.
153     if (!DeadInsts.empty()) {
154       Changed = true;
155       RecursivelyDeleteTriviallyDeadInstructions(DeadInsts, &TLI, MSSAU);
156     }
157 
158     if (MSSAU && VerifyMemorySSA)
159       MSSA->verifyMemorySSA();
160 
161     // If we never found a PHI that needs to be simplified in the next
162     // iteration, we're done.
163     if (Next->empty())
164       break;
165 
166     // Otherwise, put the next set in place for the next iteration and reset it
167     // and the visited PHIs for that iteration.
168     std::swap(Next, ToSimplify);
169     Next->clear();
170     VisitedPHIs.clear();
171     DeadInsts.clear();
172   }
173 
174   return Changed;
175 }
176 
177 namespace {
178 
179 class LoopInstSimplifyLegacyPass : public LoopPass {
180 public:
181   static char ID; // Pass ID, replacement for typeid
182 
183   LoopInstSimplifyLegacyPass() : LoopPass(ID) {
184     initializeLoopInstSimplifyLegacyPassPass(*PassRegistry::getPassRegistry());
185   }
186 
187   bool runOnLoop(Loop *L, LPPassManager &LPM) override {
188     if (skipLoop(L))
189       return false;
190     DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
191     LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
192     AssumptionCache &AC =
193         getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
194             *L->getHeader()->getParent());
195     const TargetLibraryInfo &TLI =
196         getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(
197             *L->getHeader()->getParent());
198     MemorySSA *MSSA = nullptr;
199     Optional<MemorySSAUpdater> MSSAU;
200     if (EnableMSSALoopDependency) {
201       MSSA = &getAnalysis<MemorySSAWrapperPass>().getMSSA();
202       MSSAU = MemorySSAUpdater(MSSA);
203     }
204 
205     return simplifyLoopInst(*L, DT, LI, AC, TLI,
206                             MSSAU.hasValue() ? MSSAU.getPointer() : nullptr);
207   }
208 
209   void getAnalysisUsage(AnalysisUsage &AU) const override {
210     AU.addRequired<AssumptionCacheTracker>();
211     AU.addRequired<DominatorTreeWrapperPass>();
212     AU.addRequired<TargetLibraryInfoWrapperPass>();
213     AU.setPreservesCFG();
214     if (EnableMSSALoopDependency) {
215       AU.addRequired<MemorySSAWrapperPass>();
216       AU.addPreserved<MemorySSAWrapperPass>();
217     }
218     getLoopAnalysisUsage(AU);
219   }
220 };
221 
222 } // end anonymous namespace
223 
224 PreservedAnalyses LoopInstSimplifyPass::run(Loop &L, LoopAnalysisManager &AM,
225                                             LoopStandardAnalysisResults &AR,
226                                             LPMUpdater &) {
227   Optional<MemorySSAUpdater> MSSAU;
228   if (AR.MSSA) {
229     MSSAU = MemorySSAUpdater(AR.MSSA);
230     if (VerifyMemorySSA)
231       AR.MSSA->verifyMemorySSA();
232   }
233   if (!simplifyLoopInst(L, AR.DT, AR.LI, AR.AC, AR.TLI,
234                         MSSAU.hasValue() ? MSSAU.getPointer() : nullptr))
235     return PreservedAnalyses::all();
236 
237   auto PA = getLoopPassPreservedAnalyses();
238   PA.preserveSet<CFGAnalyses>();
239   if (AR.MSSA)
240     PA.preserve<MemorySSAAnalysis>();
241   return PA;
242 }
243 
244 char LoopInstSimplifyLegacyPass::ID = 0;
245 
246 INITIALIZE_PASS_BEGIN(LoopInstSimplifyLegacyPass, "loop-instsimplify",
247                       "Simplify instructions in loops", false, false)
248 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
249 INITIALIZE_PASS_DEPENDENCY(LoopPass)
250 INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
251 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
252 INITIALIZE_PASS_END(LoopInstSimplifyLegacyPass, "loop-instsimplify",
253                     "Simplify instructions in loops", false, false)
254 
255 Pass *llvm::createLoopInstSimplifyPass() {
256   return new LoopInstSimplifyLegacyPass();
257 }
258