Reg2Mem.cpp revision 3e8b6631e67e01e4960a7ba4668a50c596607473 
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(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::getInt32Ty(F.getContext())),
72                                             Type::getInt32Ty(F.getContext()),
73                           "reg2mem alloca point", I);
74
75        // Find the escaped instructions. But don't create stack slots for
76        // allocas in entry block.
77        std::list<Instruction*> worklist;
78        for (Function::iterator ibb = F.begin(), ibe = F.end();
79             ibb != ibe; ++ibb)
80          for (BasicBlock::iterator iib = ibb->begin(), iie = ibb->end();
81               iib != iie; ++iib) {
82            if (!(isa<AllocaInst>(iib) && iib->getParent() == BBEntry) &&
83                valueEscapes(iib)) {
84              worklist.push_front(&*iib);
85            }
86          }
87
88        // Demote escaped instructions
89        NumRegsDemoted += worklist.size();
90        for (std::list<Instruction*>::iterator ilb = worklist.begin(),
91               ile = worklist.end(); ilb != ile; ++ilb)
92          DemoteRegToStack(**ilb, false, AllocaInsertionPoint);
93
94        worklist.clear();
95
96        // Find all phi's
97        for (Function::iterator ibb = F.begin(), ibe = F.end();
98             ibb != ibe; ++ibb)
99          for (BasicBlock::iterator iib = ibb->begin(), iie = ibb->end();
100               iib != iie; ++iib)
101            if (isa<PHINode>(iib))
102              worklist.push_front(&*iib);
103
104        // Demote phi nodes
105        NumPhisDemoted += worklist.size();
106        for (std::list<Instruction*>::iterator ilb = worklist.begin(),
107               ile = worklist.end(); ilb != ile; ++ilb)
108          DemotePHIToStack(cast<PHINode>(*ilb), AllocaInsertionPoint);
109
110        return true;
111      }
112      return false;
113    }
114  };
115}
116
117char RegToMem::ID = 0;
118static RegisterPass<RegToMem>
119X("reg2mem", "Demote all values to stack slots");
120
121// createDemoteRegisterToMemory - Provide an entry point to create this pass.
122//
123const PassInfo *const llvm::DemoteRegisterToMemoryID = &X;
124FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
125  return new RegToMem();
126}
127