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/Compiler.h"
30 #include "llvm/Support/CFG.h"
31 #include <list>
32 using namespace llvm;
33 
34 STATISTIC(NumRegsDemoted, "Number of registers demoted");
35 STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted");
36 
37 namespace {
38   struct VISIBILITY_HIDDEN RegToMem : public FunctionPass {
39     static char ID; // Pass identification, replacement for typeid
40     RegToMem() : FunctionPass(&ID) {}
41 
42     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
43       AU.addRequiredID(BreakCriticalEdgesID);
44       AU.addPreservedID(BreakCriticalEdgesID);
45     }
46 
47    bool valueEscapes(Instruction* i) {
48       BasicBlock* bb = i->getParent();
49       for (Value::use_iterator ii = i->use_begin(), ie = i->use_end();
50            ii != ie; ++ii)
51         if (cast<Instruction>(*ii)->getParent() != bb ||
52             isa<PHINode>(*ii))
53           return true;
54       return false;
55     }
56 
57     virtual bool runOnFunction(Function &F) {
58       if (!F.isDeclaration()) {
59         // Insert all new allocas into entry block.
60         BasicBlock* BBEntry = &F.getEntryBlock();
61         assert(pred_begin(BBEntry) == pred_end(BBEntry) &&
62                "Entry block to function must not have predecessors!");
63 
64         // Find first non-alloca instruction and create insertion point. This is
65         // safe if block is well-formed: it always have terminator, otherwise
66         // we'll get and assertion.
67         BasicBlock::iterator I = BBEntry->begin();
68         while (isa<AllocaInst>(I)) ++I;
69 
70         CastInst *AllocaInsertionPoint =
71           CastInst::Create(Instruction::BitCast,
72                       Constant::getNullValue(Type::Int32Ty), Type::Int32Ty,
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 
117 char RegToMem::ID = 0;
118 static RegisterPass<RegToMem>
119 X("reg2mem", "Demote all values to stack slots");
120 
121 // createDemoteRegisterToMemory - Provide an entry point to create this pass.
122 //
123 const PassInfo *const llvm::DemoteRegisterToMemoryID = &X;
124 FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
125   return new RegToMem();
126 }
127