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
37     ConstantPropagation() : FunctionPass(ID) {}
38 
39     bool runOnFunction(Function &F);
40 
41     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 
51 FunctionPass *llvm::createConstantPropagationPass() {
52   return new ConstantPropagation();
53 }
54 
55 
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