Reg2Mem.cpp revision 67894a3d02caa0b4628a942bef923907bbd97c47 
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/LLVMContext.h"
25#include "llvm/Module.h"
26#include "llvm/BasicBlock.h"
27#include "llvm/Instructions.h"
28#include "llvm/ADT/Statistic.h"
29#include "llvm/Support/CFG.h"
30#include <list>
31using namespace llvm;
32
33STATISTIC(NumRegsDemoted, "Number of registers demoted");
34STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted");
35
36namespace {
37  struct 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(const Instruction *Inst) const {
47     const BasicBlock *BB = Inst->getParent();
48      for (Value::const_use_iterator UI = Inst->use_begin(),E = Inst->use_end();
49           UI != E; ++UI) {
50        const Instruction *I = cast<Instruction>(*UI);
51        if (I->getParent() != BB || isa<PHINode>(I))
52          return true;
53      }
54      return false;
55    }
56
57    virtual bool runOnFunction(Function &F);
58  };
59}
60
61char RegToMem::ID = 0;
62static RegisterPass<RegToMem>
63X("reg2mem", "Demote all values to stack slots");
64
65
66bool RegToMem::runOnFunction(Function &F) {
67  if (F.isDeclaration())
68    return false;
69
70  // Insert all new allocas into entry block.
71  BasicBlock *BBEntry = &F.getEntryBlock();
72  assert(pred_begin(BBEntry) == pred_end(BBEntry) &&
73         "Entry block to function must not have predecessors!");
74
75  // Find first non-alloca instruction and create insertion point. This is
76  // safe if block is well-formed: it always have terminator, otherwise
77  // we'll get and assertion.
78  BasicBlock::iterator I = BBEntry->begin();
79  while (isa<AllocaInst>(I)) ++I;
80
81  CastInst *AllocaInsertionPoint =
82    new BitCastInst(Constant::getNullValue(Type::getInt32Ty(F.getContext())),
83                    Type::getInt32Ty(F.getContext()),
84                    "reg2mem alloca point", I);
85
86  // Find the escaped instructions. But don't create stack slots for
87  // allocas in entry block.
88  std::list<Instruction*> WorkList;
89  for (Function::iterator ibb = F.begin(), ibe = F.end();
90       ibb != ibe; ++ibb)
91    for (BasicBlock::iterator iib = ibb->begin(), iie = ibb->end();
92         iib != iie; ++iib) {
93      if (!(isa<AllocaInst>(iib) && iib->getParent() == BBEntry) &&
94          valueEscapes(iib)) {
95        WorkList.push_front(&*iib);
96      }
97    }
98
99  // Demote escaped instructions
100  NumRegsDemoted += WorkList.size();
101  for (std::list<Instruction*>::iterator ilb = WorkList.begin(),
102       ile = WorkList.end(); ilb != ile; ++ilb)
103    DemoteRegToStack(**ilb, false, AllocaInsertionPoint);
104
105  WorkList.clear();
106
107  // Find all phi's
108  for (Function::iterator ibb = F.begin(), ibe = F.end();
109       ibb != ibe; ++ibb)
110    for (BasicBlock::iterator iib = ibb->begin(), iie = ibb->end();
111         iib != iie; ++iib)
112      if (isa<PHINode>(iib))
113        WorkList.push_front(&*iib);
114
115  // Demote phi nodes
116  NumPhisDemoted += WorkList.size();
117  for (std::list<Instruction*>::iterator ilb = WorkList.begin(),
118       ile = WorkList.end(); ilb != ile; ++ilb)
119    DemotePHIToStack(cast<PHINode>(*ilb), AllocaInsertionPoint);
120
121  return true;
122}
123
124
125// createDemoteRegisterToMemory - Provide an entry point to create this pass.
126//
127const PassInfo *const llvm::DemoteRegisterToMemoryID = &X;
128FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
129  return new RegToMem();
130}
131