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/PassManager.h"
  28 #include "llvm/Transforms/Scalar.h"
  29 #include "llvm/Transforms/Utils.h"
  30 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  31 #include "llvm/Transforms/Utils/Local.h"
  32 #include <list>
  33 using namespace llvm;
  34
  35 #define DEBUG_TYPE "reg2mem"
  36
  37 STATISTIC(NumRegsDemoted, "Number of registers demoted");
  38 STATISTIC(NumPhisDemoted, "Number of phi-nodes demoted");
  39
  40 static bool valueEscapes(const Instruction &Inst) {
  41   if (!Inst.getType()->isSized())
  42     return false;
  43
  44   const BasicBlock *BB = Inst.getParent();
  45   for (const User *U : Inst.users()) {
  46     const Instruction *UI = cast<Instruction>(U);
  47     if (UI->getParent() != BB || isa<PHINode>(UI))
  48       return true;
  49   }
  50   return false;
  51 }
  52
  53 static bool runPass(Function &F) {
  54   // Insert all new allocas into entry block.
  55   BasicBlock *BBEntry = &F.getEntryBlock();
  56   assert(pred_empty(BBEntry) &&
  57          "Entry block to function must not have predecessors!");
  58
  59   // Find first non-alloca instruction and create insertion point. This is
  60   // safe if block is well-formed: it always have terminator, otherwise
  61   // we'll get and assertion.
  62   BasicBlock::iterator I = BBEntry->begin();
  63   while (isa<AllocaInst>(I)) ++I;
  64
  65   CastInst *AllocaInsertionPoint = new BitCastInst(
  66       Constant::getNullValue(Type::getInt32Ty(F.getContext())),
  67       Type::getInt32Ty(F.getContext()), "reg2mem alloca point", &*I);
  68
  69   // Find the escaped instructions. But don't create stack slots for
  70   // allocas in entry block.
  71   std::list<Instruction*> WorkList;
  72   for (Instruction &I : instructions(F))
  73     if (!(isa<AllocaInst>(I) && I.getParent() == BBEntry) && valueEscapes(I))
  74       WorkList.push_front(&I);
  75
  76   // Demote escaped instructions
  77   NumRegsDemoted += WorkList.size();
  78   for (Instruction *I : WorkList)
  79     DemoteRegToStack(*I, false, AllocaInsertionPoint);
  80
  81   WorkList.clear();
  82
  83   // Find all phi's
  84   for (BasicBlock &BB : F)
  85     for (auto &Phi : BB.phis())
  86       WorkList.push_front(&Phi);
  87
  88   // Demote phi nodes
  89   NumPhisDemoted += WorkList.size();
  90   for (Instruction *I : WorkList)
  91     DemotePHIToStack(cast<PHINode>(I), AllocaInsertionPoint);
  92
  93   return true;
  94 }
  95
  96 PreservedAnalyses RegToMemPass::run(Function &F, FunctionAnalysisManager &AM) {
  97   auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
  98   auto *LI = &AM.getResult<LoopAnalysis>(F);
  99   unsigned N = SplitAllCriticalEdges(F, CriticalEdgeSplittingOptions(DT, LI));
 100   bool Changed = runPass(F);
 101   if (N == 0 && !Changed)
 102     return PreservedAnalyses::all();
 103   PreservedAnalyses PA;
 104   PA.preserve<DominatorTreeAnalysis>();
 105   PA.preserve<LoopAnalysis>();
 106   return PA;
 107 }