llvm/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/Transforms/Scalar/Reg2Mem.h"
  19 #include "llvm/ADT/Statistic.h"
  20 #include "llvm/Analysis/LoopInfo.h"
  21 #include "llvm/IR/BasicBlock.h"
  22 #include "llvm/IR/CFG.h"
  23 #include "llvm/IR/Dominators.h"
  24 #include "llvm/IR/Function.h"
  25 #include "llvm/IR/InstIterator.h"
  26 #include "llvm/IR/Instructions.h"
  27 #include "llvm/IR/LLVMContext.h"
  28 #include "llvm/IR/Module.h"
  29 #include "llvm/IR/PassManager.h"
  30 #include "llvm/InitializePasses.h"
  31 #include "llvm/Pass.h"
  32 #include "llvm/Transforms/Scalar.h"
  33 #include "llvm/Transforms/Utils.h"
  34 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  35 #include "llvm/Transforms/Utils/Local.h"
  36 #include <list>
  37 using namespace llvm;
  38
  39 #define DEBUG_TYPE "reg2mem"
  40
  41 STATISTIC(NumRegsDemoted, "Number of registers demoted");
  42 STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted");
  43
  44 static bool valueEscapes(const Instruction &Inst) {
  45   const BasicBlock *BB = Inst.getParent();
  46   for (const User *U : Inst.users()) {
  47     const Instruction *UI = cast<Instruction>(U);
  48     if (UI->getParent() != BB || isa<PHINode>(UI))
  49       return true;
  50   }
  51   return false;
  52 }
  53
  54 static bool runPass(Function &F) {
  55   // Insert all new allocas into entry block.
  56   BasicBlock *BBEntry = &F.getEntryBlock();
  57   assert(pred_empty(BBEntry) &&
  58          "Entry block to function must not have predecessors!");
  59
  60   // Find first non-alloca instruction and create insertion point. This is
  61   // safe if block is well-formed: it always have terminator, otherwise
  62   // we'll get and assertion.
  63   BasicBlock::iterator I = BBEntry->begin();
  64   while (isa<AllocaInst>(I)) ++I;
  65
  66   CastInst *AllocaInsertionPoint = new BitCastInst(
  67       Constant::getNullValue(Type::getInt32Ty(F.getContext())),
  68       Type::getInt32Ty(F.getContext()), "reg2mem alloca point", &*I);
  69
  70   // Find the escaped instructions. But don't create stack slots for
  71   // allocas in entry block.
  72   std::list<Instruction*> WorkList;
  73   for (Instruction &I : instructions(F))
  74     if (!(isa<AllocaInst>(I) && I.getParent() == BBEntry) && valueEscapes(I))
  75       WorkList.push_front(&I);
  76
  77   // Demote escaped instructions
  78   NumRegsDemoted += WorkList.size();
  79   for (Instruction *I : WorkList)
  80     DemoteRegToStack(*I, false, AllocaInsertionPoint);
  81
  82   WorkList.clear();
  83
  84   // Find all phi's
  85   for (BasicBlock &BB : F)
  86     for (auto &Phi : BB.phis())
  87       WorkList.push_front(&Phi);
  88
  89   // Demote phi nodes
  90   NumPhisDemoted += WorkList.size();
  91   for (Instruction *I : WorkList)
  92     DemotePHIToStack(cast<PHINode>(I), AllocaInsertionPoint);
  93
  94   return true;
  95 }
  96
  97 PreservedAnalyses RegToMemPass::run(Function &F, FunctionAnalysisManager &AM) {
  98   auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
  99   auto *LI = &AM.getResult<LoopAnalysis>(F);
 100   unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI));
 101   bool Changed = runPass(F);
 102   if (N == 0 && !Changed)
 103     return PreservedAnalyses::all();
 104   PreservedAnalyses PA;
 105   PA.preserve<DominatorTreeAnalysis>();
 106   PA.preserve<LoopAnalysis>();
 107   return PA;
 108 }
 109
 110 namespace {
 111 struct RegToMemLegacy : public FunctionPass {
 112   static char ID; // Pass identification, replacement for typeid
 113   RegToMemLegacy() : FunctionPass(ID) {
 114     initializeRegToMemLegacyPass(*PassRegistry::getPassRegistry());
 115   }
 116
 117   void getAnalysisUsage(AnalysisUsage &AU) const override {
 118     AU.addRequiredID(BreakCriticalEdgesID);
 119     AU.addPreservedID(BreakCriticalEdgesID);
 120   }
 121
 122   bool runOnFunction(Function &F) override {
 123     if (F.isDeclaration() || skipFunction(F))
 124       return false;
 125     return runPass(F);
 126   }
 127 };
 128 } // namespace
 129
 130 char RegToMemLegacy::ID = 0;
 131 INITIALIZE_PASS_BEGIN(RegToMemLegacy, "reg2mem",
 132                       "Demote all values to stack slots", false, false)
 133 INITIALIZE_PASS_DEPENDENCY(BreakCriticalEdges)
 134 INITIALIZE_PASS_END(RegToMemLegacy, "reg2mem",
 135                     "Demote all values to stack slots", false, false)
 136
 137 // createDemoteRegisterToMemory - Provide an entry point to create this pass.
 138 char &llvm::DemoteRegisterToMemoryID = RegToMemLegacy::ID;
 139 FunctionPass *llvm::createDemoteRegisterToMemoryPass() {
 140   return new RegToMemLegacy();
 141 }