1 //===- Reg2Mem.cpp - Convert registers to allocas -------------------------===//
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 demotes all registers to memory references.  It is intented to be
11 // the inverse of PromoteMemoryToRegister.  By converting to loads, the only
12 // values live accross basic blocks are allocas and loads before phi nodes.
13 // It is intended that this should make CFG hacking much easier.
14 // To make later hacking easier, the entry block is split into two, such that
15 // all introduced allocas and nothing else are in the entry block.
16 //
17 //===----------------------------------------------------------------------===//
18 
19 #define DEBUG_TYPE "reg2mem"
20 #include "llvm/Transforms/Scalar.h"
21 #include "llvm/Transforms/Utils/Local.h"
22 #include "llvm/Pass.h"
23 #include "llvm/Function.h"
24 #include "llvm/Module.h"
25 #include "llvm/BasicBlock.h"
26 #include "llvm/Instructions.h"
27 #include "llvm/ADT/Statistic.h"
28 #include "llvm/Support/Compiler.h"
29 #include "llvm/Support/CFG.h"
30 #include <list>
31 using namespace llvm;
32 
33 STATISTIC(NumRegsDemoted, "Number of registers demoted");
34 STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted");
35 
36 namespace {
37   struct VISIBILITY_HIDDEN RegToMem : public FunctionPass {
38     static char ID; // Pass identification, replacement for typeid
39     RegToMem() : FunctionPass(&ID) {}
40 
41     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
42       AU.addRequiredID(BreakCriticalEdgesID);
43       AU.addPreservedID(BreakCriticalEdgesID);
44     }
45 
46    bool valueEscapes(Instruction* i) {
47       BasicBlock* bb = i->getParent();
48       for (Value::use_iterator ii = i->use_begin(), ie = i->use_end();
49            ii != ie; ++ii)
50         if (cast<Instruction>(*ii)->getParent() != bb ||
51             isa<PHINode>(*ii))
52           return true;
53       return false;
54     }
55 
56     virtual bool runOnFunction(Function &F) {
57       if (!F.isDeclaration()) {
58         // Insert all new allocas into entry block.
59         BasicBlock* BBEntry = &F.getEntryBlock();
60         assert(pred_begin(BBEntry) == pred_end(BBEntry) &&
61                "Entry block to function must not have predecessors!");
62 
63         // Find first non-alloca instruction and create insertion point. This is
64         // safe if block is well-formed: it always have terminator, otherwise
65         // we'll get and assertion.
66         BasicBlock::iterator I = BBEntry->begin();
67         while (isa<AllocaInst>(I)) ++I;
68 
69         CastInst *AllocaInsertionPoint =
70           CastInst::Create(Instruction::BitCast,
71                            Constant::getNullValue(Type::Int32Ty), Type::Int32Ty,
72                            "reg2mem alloca point", I);
73 
74         // Find the escaped instructions. But don't create stack slots for
75         // allocas in entry block.
76         std::list<Instruction*> worklist;
77         for (Function::iterator ibb = F.begin(), ibe = F.end();
78              ibb != ibe; ++ibb)
79           for (BasicBlock::iterator iib = ibb->begin(), iie = ibb->end();
80                iib != iie; ++iib) {
81             if (!(isa<AllocaInst>(iib) && iib->getParent() == BBEntry) &&
82                 valueEscapes(iib)) {
83               worklist.push_front(&*iib);
84             }
85           }
86 
87         // Demote escaped instructions
88         NumRegsDemoted += worklist.size();
89         for (std::list<Instruction*>::iterator ilb = worklist.begin(),
90                ile = worklist.end(); ilb != ile; ++ilb)
91           DemoteRegToStack(**ilb, false, AllocaInsertionPoint);
92 
93         worklist.clear();
94 
95         // Find all phi's
96         for (Function::iterator ibb = F.begin(), ibe = F.end();
97              ibb != ibe; ++ibb)
98           for (BasicBlock::iterator iib = ibb->begin(), iie = ibb->end();
99                iib != iie; ++iib)
100             if (isa<PHINode>(iib))
101               worklist.push_front(&*iib);
102 
103         // Demote phi nodes
104         NumPhisDemoted += worklist.size();
105         for (std::list<Instruction*>::iterator ilb = worklist.begin(),
106                ile = worklist.end(); ilb != ile; ++ilb)
107           DemotePHIToStack(cast<PHINode>(*ilb), AllocaInsertionPoint);
108 
109         return true;
110       }
111       return false;
112     }
113   };
114 }
115 
116 char RegToMem::ID = 0;
117 static RegisterPass<RegToMem>
118 X("reg2mem", "Demote all values to stack slots");
119 
120 // createDemoteRegisterToMemory - Provide an entry point to create this pass.
121 //
122 const PassInfo *const llvm::DemoteRegisterToMemoryID = &X;
123 FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
124   return new RegToMem();
125 }
126