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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__anon79dc68770111::ConstantPropagation40     ConstantPropagation() : FunctionPass(ID) {
41       initializeConstantPropagationPass(*PassRegistry::getPassRegistry());
42     }
43 
44     bool runOnFunction(Function &F) override;
45 
getAnalysisUsage__anon79dc68770111::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