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/Transforms/Utils/Local.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/Analysis/ConstantFolding.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/Analysis/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
40     ConstantPropagation() : FunctionPass(ID) {
41       initializeConstantPropagationPass(*PassRegistry::getPassRegistry());
42     }
43 
44     bool runOnFunction(Function &F) override;
45 
46     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)
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 
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