1 //===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
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 implements sparse conditional constant propagation and merging:
10 //
11 // Specifically, this:
12 //   * Assumes values are constant unless proven otherwise
13 //   * Assumes BasicBlocks are dead unless proven otherwise
14 //   * Proves values to be constant, and replaces them with constants
15 //   * Proves conditional branches to be unconditional
16 //
17 //===----------------------------------------------------------------------===//
18 
19 #include "llvm/Transforms/Scalar/SCCP.h"
20 #include "llvm/ADT/DenseMap.h"
21 #include "llvm/ADT/MapVector.h"
22 #include "llvm/ADT/STLExtras.h"
23 #include "llvm/ADT/SetVector.h"
24 #include "llvm/ADT/SmallPtrSet.h"
25 #include "llvm/ADT/SmallVector.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/Analysis/DomTreeUpdater.h"
28 #include "llvm/Analysis/GlobalsModRef.h"
29 #include "llvm/Analysis/TargetLibraryInfo.h"
30 #include "llvm/Analysis/ValueTracking.h"
31 #include "llvm/IR/BasicBlock.h"
32 #include "llvm/IR/Constant.h"
33 #include "llvm/IR/DerivedTypes.h"
34 #include "llvm/IR/Function.h"
35 #include "llvm/IR/GlobalVariable.h"
36 #include "llvm/IR/InstrTypes.h"
37 #include "llvm/IR/Instruction.h"
38 #include "llvm/IR/Instructions.h"
39 #include "llvm/IR/Module.h"
40 #include "llvm/IR/PassManager.h"
41 #include "llvm/IR/Type.h"
42 #include "llvm/IR/User.h"
43 #include "llvm/IR/Value.h"
44 #include "llvm/InitializePasses.h"
45 #include "llvm/Pass.h"
46 #include "llvm/Support/Casting.h"
47 #include "llvm/Support/Debug.h"
48 #include "llvm/Support/ErrorHandling.h"
49 #include "llvm/Support/raw_ostream.h"
50 #include "llvm/Transforms/Scalar.h"
51 #include "llvm/Transforms/Utils/Local.h"
52 #include "llvm/Transforms/Utils/SCCPSolver.h"
53 #include <cassert>
54 #include <utility>
55 #include <vector>
56 
57 using namespace llvm;
58 
59 #define DEBUG_TYPE "sccp"
60 
61 STATISTIC(NumInstRemoved, "Number of instructions removed");
62 STATISTIC(NumDeadBlocks , "Number of basic blocks unreachable");
63 STATISTIC(NumInstReplaced,
64           "Number of instructions replaced with (simpler) instruction");
65 
66 // runSCCP() - Run the Sparse Conditional Constant Propagation algorithm,
67 // and return true if the function was modified.
68 static bool runSCCP(Function &F, const DataLayout &DL,
69                     const TargetLibraryInfo *TLI, DomTreeUpdater &DTU) {
70   LLVM_DEBUG(dbgs() << "SCCP on function '" << F.getName() << "'\n");
71   SCCPSolver Solver(
72       DL, [TLI](Function &F) -> const TargetLibraryInfo & { return *TLI; },
73       F.getContext());
74 
75   // Mark the first block of the function as being executable.
76   Solver.markBlockExecutable(&F.front());
77 
78   // Mark all arguments to the function as being overdefined.
79   for (Argument &AI : F.args())
80     Solver.markOverdefined(&AI);
81 
82   // Solve for constants.
83   bool ResolvedUndefs = true;
84   while (ResolvedUndefs) {
85     Solver.solve();
86     LLVM_DEBUG(dbgs() << "RESOLVING UNDEFs\n");
87     ResolvedUndefs = Solver.resolvedUndefsIn(F);
88   }
89 
90   bool MadeChanges = false;
91 
92   // If we decided that there are basic blocks that are dead in this function,
93   // delete their contents now.  Note that we cannot actually delete the blocks,
94   // as we cannot modify the CFG of the function.
95 
96   SmallPtrSet<Value *, 32> InsertedValues;
97   SmallVector<BasicBlock *, 8> BlocksToErase;
98   for (BasicBlock &BB : F) {
99     if (!Solver.isBlockExecutable(&BB)) {
100       LLVM_DEBUG(dbgs() << "  BasicBlock Dead:" << BB);
101       ++NumDeadBlocks;
102       BlocksToErase.push_back(&BB);
103       MadeChanges = true;
104       continue;
105     }
106 
107     MadeChanges |= Solver.simplifyInstsInBlock(BB, InsertedValues,
108                                                NumInstRemoved, NumInstReplaced);
109   }
110 
111   // Remove unreachable blocks and non-feasible edges.
112   for (BasicBlock *DeadBB : BlocksToErase)
113     NumInstRemoved += changeToUnreachable(DeadBB->getFirstNonPHI(),
114                                           /*PreserveLCSSA=*/false, &DTU);
115 
116   BasicBlock *NewUnreachableBB = nullptr;
117   for (BasicBlock &BB : F)
118     MadeChanges |= Solver.removeNonFeasibleEdges(&BB, DTU, NewUnreachableBB);
119 
120   for (BasicBlock *DeadBB : BlocksToErase)
121     if (!DeadBB->hasAddressTaken())
122       DTU.deleteBB(DeadBB);
123 
124   return MadeChanges;
125 }
126 
127 PreservedAnalyses SCCPPass::run(Function &F, FunctionAnalysisManager &AM) {
128   const DataLayout &DL = F.getParent()->getDataLayout();
129   auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
130   auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
131   DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
132   if (!runSCCP(F, DL, &TLI, DTU))
133     return PreservedAnalyses::all();
134 
135   auto PA = PreservedAnalyses();
136   PA.preserve<DominatorTreeAnalysis>();
137   return PA;
138 }
139 
140 namespace {
141 
142 //===--------------------------------------------------------------------===//
143 //
144 /// SCCP Class - This class uses the SCCPSolver to implement a per-function
145 /// Sparse Conditional Constant Propagator.
146 ///
147 class SCCPLegacyPass : public FunctionPass {
148 public:
149   // Pass identification, replacement for typeid
150   static char ID;
151 
152   SCCPLegacyPass() : FunctionPass(ID) {
153     initializeSCCPLegacyPassPass(*PassRegistry::getPassRegistry());
154   }
155 
156   void getAnalysisUsage(AnalysisUsage &AU) const override {
157     AU.addRequired<TargetLibraryInfoWrapperPass>();
158     AU.addPreserved<GlobalsAAWrapperPass>();
159     AU.addPreserved<DominatorTreeWrapperPass>();
160   }
161 
162   // runOnFunction - Run the Sparse Conditional Constant Propagation
163   // algorithm, and return true if the function was modified.
164   bool runOnFunction(Function &F) override {
165     if (skipFunction(F))
166       return false;
167     const DataLayout &DL = F.getParent()->getDataLayout();
168     const TargetLibraryInfo *TLI =
169         &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
170     auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
171     DomTreeUpdater DTU(DTWP ? &DTWP->getDomTree() : nullptr,
172                        DomTreeUpdater::UpdateStrategy::Lazy);
173     return runSCCP(F, DL, TLI, DTU);
174   }
175 };
176 
177 } // end anonymous namespace
178 
179 char SCCPLegacyPass::ID = 0;
180 
181 INITIALIZE_PASS_BEGIN(SCCPLegacyPass, "sccp",
182                       "Sparse Conditional Constant Propagation", false, false)
183 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
184 INITIALIZE_PASS_END(SCCPLegacyPass, "sccp",
185                     "Sparse Conditional Constant Propagation", false, false)
186 
187 // createSCCPPass - This is the public interface to this file.
188 FunctionPass *llvm::createSCCPPass() { return new SCCPLegacyPass(); }
189 
190