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 intended to be
11// the inverse of PromoteMemoryToRegister.  By converting to loads, the only
12// values live across 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#include "llvm/Transforms/Scalar.h"
20#include "llvm/ADT/Statistic.h"
21#include "llvm/IR/BasicBlock.h"
22#include "llvm/IR/CFG.h"
23#include "llvm/IR/Function.h"
24#include "llvm/IR/Instructions.h"
25#include "llvm/IR/LLVMContext.h"
26#include "llvm/IR/Module.h"
27#include "llvm/Pass.h"
28#include "llvm/Transforms/Utils/Local.h"
29#include <list>
30using namespace llvm;
31
32#define DEBUG_TYPE "reg2mem"
33
34STATISTIC(NumRegsDemoted, "Number of registers demoted");
35STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted");
36
37namespace {
38  struct RegToMem : public FunctionPass {
39    static char ID; // Pass identification, replacement for typeid
40    RegToMem() : FunctionPass(ID) {
41      initializeRegToMemPass(*PassRegistry::getPassRegistry());
42    }
43
44    void getAnalysisUsage(AnalysisUsage &AU) const override {
45      AU.addRequiredID(BreakCriticalEdgesID);
46      AU.addPreservedID(BreakCriticalEdgesID);
47    }
48
49    bool valueEscapes(const Instruction *Inst) const {
50      const BasicBlock *BB = Inst->getParent();
51      for (const User *U : Inst->users()) {
52        const Instruction *UI = cast<Instruction>(U);
53        if (UI->getParent() != BB || isa<PHINode>(UI))
54          return true;
55      }
56      return false;
57    }
58
59    bool runOnFunction(Function &F) override;
60  };
61}
62
63char RegToMem::ID = 0;
64INITIALIZE_PASS_BEGIN(RegToMem, "reg2mem", "Demote all values to stack slots",
65                false, false)
66INITIALIZE_PASS_DEPENDENCY(BreakCriticalEdges)
67INITIALIZE_PASS_END(RegToMem, "reg2mem", "Demote all values to stack slots",
68                false, false)
69
70bool RegToMem::runOnFunction(Function &F) {
71  if (F.isDeclaration())
72    return false;
73
74  // Insert all new allocas into entry block.
75  BasicBlock *BBEntry = &F.getEntryBlock();
76  assert(pred_begin(BBEntry) == pred_end(BBEntry) &&
77         "Entry block to function must not have predecessors!");
78
79  // Find first non-alloca instruction and create insertion point. This is
80  // safe if block is well-formed: it always have terminator, otherwise
81  // we'll get and assertion.
82  BasicBlock::iterator I = BBEntry->begin();
83  while (isa<AllocaInst>(I)) ++I;
84
85  CastInst *AllocaInsertionPoint =
86    new BitCastInst(Constant::getNullValue(Type::getInt32Ty(F.getContext())),
87                    Type::getInt32Ty(F.getContext()),
88                    "reg2mem alloca point", I);
89
90  // Find the escaped instructions. But don't create stack slots for
91  // allocas in entry block.
92  std::list<Instruction*> WorkList;
93  for (Function::iterator ibb = F.begin(), ibe = F.end();
94       ibb != ibe; ++ibb)
95    for (BasicBlock::iterator iib = ibb->begin(), iie = ibb->end();
96         iib != iie; ++iib) {
97      if (!(isa<AllocaInst>(iib) && iib->getParent() == BBEntry) &&
98          valueEscapes(iib)) {
99        WorkList.push_front(&*iib);
100      }
101    }
102
103  // Demote escaped instructions
104  NumRegsDemoted += WorkList.size();
105  for (std::list<Instruction*>::iterator ilb = WorkList.begin(),
106       ile = WorkList.end(); ilb != ile; ++ilb)
107    DemoteRegToStack(**ilb, false, AllocaInsertionPoint);
108
109  WorkList.clear();
110
111  // Find all phi's
112  for (Function::iterator ibb = F.begin(), ibe = F.end();
113       ibb != ibe; ++ibb)
114    for (BasicBlock::iterator iib = ibb->begin(), iie = ibb->end();
115         iib != iie; ++iib)
116      if (isa<PHINode>(iib))
117        WorkList.push_front(&*iib);
118
119  // Demote phi nodes
120  NumPhisDemoted += WorkList.size();
121  for (std::list<Instruction*>::iterator ilb = WorkList.begin(),
122       ile = WorkList.end(); ilb != ile; ++ilb)
123    DemotePHIToStack(cast<PHINode>(*ilb), AllocaInsertionPoint);
124
125  return true;
126}
127
128
129// createDemoteRegisterToMemory - Provide an entry point to create this pass.
130char &llvm::DemoteRegisterToMemoryID = RegToMem::ID;
131FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
132  return new RegToMem();
133}
134