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/ADT/Statistic.h"
22 #include "llvm/Analysis/ConstantFolding.h"
23 #include "llvm/Analysis/TargetLibraryInfo.h"
24 #include "llvm/Transforms/Utils/Local.h"
25 #include "llvm/IR/Constant.h"
26 #include "llvm/IR/InstIterator.h"
27 #include "llvm/IR/Instruction.h"
28 #include "llvm/Pass.h"
29 #include "llvm/Transforms/Scalar.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__anoncceda3580111::ConstantPropagation40 ConstantPropagation() : FunctionPass(ID) {
41 initializeConstantPropagationPass(*PassRegistry::getPassRegistry());
42 }
43
44 bool runOnFunction(Function &F) override;
45
getAnalysisUsage__anoncceda3580111::ConstantPropagation46 void getAnalysisUsage(AnalysisUsage &AU) const override {
47 AU.setPreservesCFG();
48 AU.addRequired<TargetLibraryInfoWrapperPass>();
49 }
50 };
51 }
52
53 char ConstantPropagation::ID = 0;
54 INITIALIZE_PASS_BEGIN(ConstantPropagation, "constprop",
55 "Simple constant propagation", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)56 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
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 if (skipFunction(F))
66 return false;
67
68 // Initialize the worklist to all of the instructions ready to process...
69 std::set<Instruction*> WorkList;
70 for (Instruction &I: instructions(&F))
71 WorkList.insert(&I);
72
73 bool Changed = false;
74 const DataLayout &DL = F.getParent()->getDataLayout();
75 TargetLibraryInfo *TLI =
76 &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
77
78 while (!WorkList.empty()) {
79 Instruction *I = *WorkList.begin();
80 WorkList.erase(WorkList.begin()); // Get an element from the worklist...
81
82 if (!I->use_empty()) // Don't muck with dead instructions...
83 if (Constant *C = ConstantFoldInstruction(I, DL, TLI)) {
84 // Add all of the users of this instruction to the worklist, they might
85 // be constant propagatable now...
86 for (User *U : I->users())
87 WorkList.insert(cast<Instruction>(U));
88
89 // Replace all of the uses of a variable with uses of the constant.
90 I->replaceAllUsesWith(C);
91
92 // Remove the dead instruction.
93 WorkList.erase(I);
94 if (isInstructionTriviallyDead(I, TLI)) {
95 I->eraseFromParent();
96 ++NumInstKilled;
97 }
98
99 // We made a change to the function...
100 Changed = true;
101 }
102 }
103 return Changed;
104 }
105