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>
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 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 
61 char RegToMem::ID = 0;
62 static RegisterPass<RegToMem>
63 X("reg2mem", "Demote all values to stack slots");
64 
65 
66 bool 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 //
127 const PassInfo *const llvm::DemoteRegisterToMemoryID = &X;
128 FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
129   return new RegToMem();
130 }
131