lib/Transforms/Scalar/Reg2Mem.cpp

   1 //===- Reg2Mem.cpp - Convert registers to allocas -------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file demotes all registers to memory references.  It is intended to be
  10 // the inverse of PromoteMemoryToRegister.  By converting to loads, the only
  11 // values live across basic blocks are allocas and loads before phi nodes.
  12 // It is intended that this should make CFG hacking much easier.
  13 // To make later hacking easier, the entry block is split into two, such that
  14 // all introduced allocas and nothing else are in the entry block.
  15 //
  16 //===----------------------------------------------------------------------===//
  17
  18 #include "llvm/ADT/Statistic.h"
  19 #include "llvm/Transforms/Utils/Local.h"
  20 #include "llvm/IR/BasicBlock.h"
  21 #include "llvm/IR/CFG.h"
  22 #include "llvm/IR/Function.h"
  23 #include "llvm/IR/Instructions.h"
  24 #include "llvm/IR/LLVMContext.h"
  25 #include "llvm/IR/Module.h"
  26 #include "llvm/Pass.h"
  27 #include "llvm/Transforms/Scalar.h"
  28 #include "llvm/Transforms/Utils.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() || skipFunction(F))
  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 = new BitCastInst(
  86       Constant::getNullValue(Type::getInt32Ty(F.getContext())),
  87       Type::getInt32Ty(F.getContext()), "reg2mem alloca point", &*I);
  88
  89   // Find the escaped instructions. But don't create stack slots for
  90   // allocas in entry block.
  91   std::list<Instruction*> WorkList;
  92   for (BasicBlock &ibb : F)
  93     for (BasicBlock::iterator iib = ibb.begin(), iie = ibb.end(); iib != iie;
  94          ++iib) {
  95       if (!(isa<AllocaInst>(iib) && iib->getParent() == BBEntry) &&
  96           valueEscapes(&*iib)) {
  97         WorkList.push_front(&*iib);
  98       }
  99     }
 100
 101   // Demote escaped instructions
 102   NumRegsDemoted += WorkList.size();
 103   for (Instruction *ilb : WorkList)
 104     DemoteRegToStack(*ilb, false, AllocaInsertionPoint);
 105
 106   WorkList.clear();
 107
 108   // Find all phi's
 109   for (BasicBlock &ibb : F)
 110     for (BasicBlock::iterator iib = ibb.begin(), iie = ibb.end(); iib != iie;
 111          ++iib)
 112       if (isa<PHINode>(iib))
 113         WorkList.push_front(&*iib);
 114
 115   // Demote phi nodes
 116   NumPhisDemoted += WorkList.size();
 117   for (Instruction *ilb : WorkList)
 118     DemotePHIToStack(cast<PHINode>(ilb), AllocaInsertionPoint);
 119
 120   return true;
 121 }
 122
 123
 124 // createDemoteRegisterToMemory - Provide an entry point to create this pass.
 125 char &llvm::DemoteRegisterToMemoryID = RegToMem::ID;
 126 FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
 127   return new RegToMem();
 128 }