1 //===- ConstantProp.cpp - Code to perform Simple 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 constant propagation and merging:
10 //
11 // Specifically, this:
12 // * Converts instructions like "add int 1, 2" into 3
13 //
14 // Notice that:
15 // * This pass has a habit of making definitions be dead. It is a good idea
16 // to run a DIE pass sometime after running this pass.
17 //
18 //===----------------------------------------------------------------------===//
19
20 #include "llvm/ADT/SmallPtrSet.h"
21 #include "llvm/ADT/SmallVector.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/Analysis/ConstantFolding.h"
24 #include "llvm/Analysis/TargetLibraryInfo.h"
25 #include "llvm/IR/Constant.h"
26 #include "llvm/IR/InstIterator.h"
27 #include "llvm/IR/Instruction.h"
28 #include "llvm/InitializePasses.h"
29 #include "llvm/Pass.h"
30 #include "llvm/Support/DebugCounter.h"
31 #include "llvm/Transforms/Scalar.h"
32 #include "llvm/Transforms/Utils/Local.h"
33 using namespace llvm;
34
35 #define DEBUG_TYPE "constprop"
36
37 STATISTIC(NumInstKilled, "Number of instructions killed");
38 DEBUG_COUNTER(CPCounter, "constprop-transform",
39 "Controls which instructions are killed");
40
41 namespace {
42 struct ConstantPropagation : public FunctionPass {
43 static char ID; // Pass identification, replacement for typeid
ConstantPropagation__anon2657544c0111::ConstantPropagation44 ConstantPropagation() : FunctionPass(ID) {
45 initializeConstantPropagationPass(*PassRegistry::getPassRegistry());
46 }
47
48 bool runOnFunction(Function &F) override;
49
getAnalysisUsage__anon2657544c0111::ConstantPropagation50 void getAnalysisUsage(AnalysisUsage &AU) const override {
51 AU.setPreservesCFG();
52 AU.addRequired<TargetLibraryInfoWrapperPass>();
53 }
54 };
55 }
56
57 char ConstantPropagation::ID = 0;
58 INITIALIZE_PASS_BEGIN(ConstantPropagation, "constprop",
59 "Simple constant propagation", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)60 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
61 INITIALIZE_PASS_END(ConstantPropagation, "constprop",
62 "Simple constant propagation", false, false)
63
64 FunctionPass *llvm::createConstantPropagationPass() {
65 return new ConstantPropagation();
66 }
67
runOnFunction(Function & F)68 bool ConstantPropagation::runOnFunction(Function &F) {
69 if (skipFunction(F))
70 return false;
71
72 // Initialize the worklist to all of the instructions ready to process...
73 SmallPtrSet<Instruction *, 16> WorkList;
74 // The SmallVector of WorkList ensures that we do iteration at stable order.
75 // We use two containers rather than one SetVector, since remove is
76 // linear-time, and we don't care enough to remove from Vec.
77 SmallVector<Instruction *, 16> WorkListVec;
78 for (Instruction &I : instructions(&F)) {
79 WorkList.insert(&I);
80 WorkListVec.push_back(&I);
81 }
82
83 bool Changed = false;
84 const DataLayout &DL = F.getParent()->getDataLayout();
85 TargetLibraryInfo *TLI =
86 &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
87
88 while (!WorkList.empty()) {
89 SmallVector<Instruction*, 16> NewWorkListVec;
90 for (auto *I : WorkListVec) {
91 WorkList.erase(I); // Remove element from the worklist...
92
93 if (!I->use_empty()) // Don't muck with dead instructions...
94 if (Constant *C = ConstantFoldInstruction(I, DL, TLI)) {
95 if (!DebugCounter::shouldExecute(CPCounter))
96 continue;
97
98 // Add all of the users of this instruction to the worklist, they might
99 // be constant propagatable now...
100 for (User *U : I->users()) {
101 // If user not in the set, then add it to the vector.
102 if (WorkList.insert(cast<Instruction>(U)).second)
103 NewWorkListVec.push_back(cast<Instruction>(U));
104 }
105
106 // Replace all of the uses of a variable with uses of the constant.
107 I->replaceAllUsesWith(C);
108
109 if (isInstructionTriviallyDead(I, TLI)) {
110 I->eraseFromParent();
111 ++NumInstKilled;
112 }
113
114 // We made a change to the function...
115 Changed = true;
116 }
117 }
118 WorkListVec = std::move(NewWorkListVec);
119 }
120 return Changed;
121 }
122