1 //===- ConstantProp.cpp - Code to perform Simple Constant Propagation -----===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements constant propagation and merging:
11 //
12 // Specifically, this:
13 // * Converts instructions like "add int 1, 2" into 3
14 //
15 // Notice that:
16 // * This pass has a habit of making definitions be dead. It is a good idea
17 // to run a DIE pass sometime after running this pass.
18 //
19 //===----------------------------------------------------------------------===//
20
21 #include "llvm/Transforms/Scalar.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/Analysis/ConstantFolding.h"
24 #include "llvm/IR/Constant.h"
25 #include "llvm/IR/DataLayout.h"
26 #include "llvm/IR/InstIterator.h"
27 #include "llvm/IR/Instruction.h"
28 #include "llvm/Pass.h"
29 #include "llvm/Target/TargetLibraryInfo.h"
30 #include <set>
31 using namespace llvm;
32
33 #define DEBUG_TYPE "constprop"
34
35 STATISTIC(NumInstKilled, "Number of instructions killed");
36
37 namespace {
38 struct ConstantPropagation : public FunctionPass {
39 static char ID; // Pass identification, replacement for typeid
ConstantPropagation__anon0602ed290111::ConstantPropagation40 ConstantPropagation() : FunctionPass(ID) {
41 initializeConstantPropagationPass(*PassRegistry::getPassRegistry());
42 }
43
44 bool runOnFunction(Function &F) override;
45
getAnalysisUsage__anon0602ed290111::ConstantPropagation46 void getAnalysisUsage(AnalysisUsage &AU) const override {
47 AU.setPreservesCFG();
48 AU.addRequired<TargetLibraryInfo>();
49 }
50 };
51 }
52
53 char ConstantPropagation::ID = 0;
54 INITIALIZE_PASS_BEGIN(ConstantPropagation, "constprop",
55 "Simple constant propagation", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)56 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
57 INITIALIZE_PASS_END(ConstantPropagation, "constprop",
58 "Simple constant propagation", false, false)
59
60 FunctionPass *llvm::createConstantPropagationPass() {
61 return new ConstantPropagation();
62 }
63
runOnFunction(Function & F)64 bool ConstantPropagation::runOnFunction(Function &F) {
65 // Initialize the worklist to all of the instructions ready to process...
66 std::set<Instruction*> WorkList;
67 for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) {
68 WorkList.insert(&*i);
69 }
70 bool Changed = false;
71 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
72 const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr;
73 TargetLibraryInfo *TLI = &getAnalysis<TargetLibraryInfo>();
74
75 while (!WorkList.empty()) {
76 Instruction *I = *WorkList.begin();
77 WorkList.erase(WorkList.begin()); // Get an element from the worklist...
78
79 if (!I->use_empty()) // Don't muck with dead instructions...
80 if (Constant *C = ConstantFoldInstruction(I, DL, TLI)) {
81 // Add all of the users of this instruction to the worklist, they might
82 // be constant propagatable now...
83 for (User *U : I->users())
84 WorkList.insert(cast<Instruction>(U));
85
86 // Replace all of the uses of a variable with uses of the constant.
87 I->replaceAllUsesWith(C);
88
89 // Remove the dead instruction.
90 WorkList.erase(I);
91 I->eraseFromParent();
92
93 // We made a change to the function...
94 Changed = true;
95 ++NumInstKilled;
96 }
97 }
98 return Changed;
99 }
100