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 #define DEBUG_TYPE "constprop"
22 #include "llvm/Transforms/Scalar.h"
23 #include "llvm/Analysis/ConstantFolding.h"
24 #include "llvm/Constant.h"
25 #include "llvm/Instruction.h"
26 #include "llvm/Pass.h"
27 #include "llvm/Support/InstIterator.h"
28 #include "llvm/ADT/Statistic.h"
29 #include <set>
30 using namespace llvm;
31
32 STATISTIC(NumInstKilled, "Number of instructions killed");
33
34 namespace {
35 struct ConstantPropagation : public FunctionPass {
36 static char ID; // Pass identification, replacement for typeid
ConstantPropagation__anon0f4ebd4a0111::ConstantPropagation37 ConstantPropagation() : FunctionPass(ID) {}
38
39 bool runOnFunction(Function &F);
40
getAnalysisUsage__anon0f4ebd4a0111::ConstantPropagation41 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
42 AU.setPreservesCFG();
43 }
44 };
45 }
46
47 char ConstantPropagation::ID = 0;
48 INITIALIZE_PASS(ConstantPropagation, "constprop",
49 "Simple constant propagation", false, false);
50
createConstantPropagationPass()51 FunctionPass *llvm::createConstantPropagationPass() {
52 return new ConstantPropagation();
53 }
54
55
runOnFunction(Function & F)56 bool ConstantPropagation::runOnFunction(Function &F) {
57 // Initialize the worklist to all of the instructions ready to process...
58 std::set<Instruction*> WorkList;
59 for(inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) {
60 WorkList.insert(&*i);
61 }
62 bool Changed = false;
63
64 while (!WorkList.empty()) {
65 Instruction *I = *WorkList.begin();
66 WorkList.erase(WorkList.begin()); // Get an element from the worklist...
67
68 if (!I->use_empty()) // Don't muck with dead instructions...
69 if (Constant *C = ConstantFoldInstruction(I)) {
70 // Add all of the users of this instruction to the worklist, they might
71 // be constant propagatable now...
72 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
73 UI != UE; ++UI)
74 WorkList.insert(cast<Instruction>(*UI));
75
76 // Replace all of the uses of a variable with uses of the constant.
77 I->replaceAllUsesWith(C);
78
79 // Remove the dead instruction.
80 WorkList.erase(I);
81 I->eraseFromParent();
82
83 // We made a change to the function...
84 Changed = true;
85 ++NumInstKilled;
86 }
87 }
88 return Changed;
89 }
90