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      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>
     30 using namespace llvm;
     31 
     32 #define DEBUG_TYPE "reg2mem"
     33 
     34 STATISTIC(NumRegsDemoted, "Number of registers demoted");
     35 STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted");
     36 
     37 namespace {
     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 
     63 char RegToMem::ID = 0;
     64 INITIALIZE_PASS_BEGIN(RegToMem, "reg2mem", "Demote all values to stack slots",
     65                 false, false)
     66 INITIALIZE_PASS_DEPENDENCY(BreakCriticalEdges)
     67 INITIALIZE_PASS_END(RegToMem, "reg2mem", "Demote all values to stack slots",
     68                 false, false)
     69 
     70 bool 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_empty(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.
    130 char &llvm::DemoteRegisterToMemoryID = RegToMem::ID;
    131 FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
    132   return new RegToMem();
    133 }
    134