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