llvm/lib/Transforms/Utils/DemoteRegToStack.cpp

   1 //===- DemoteRegToStack.cpp - Move a virtual register to the stack --------===//
   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 #include "llvm/ADT/DenseMap.h"
  10 #include "llvm/Analysis/CFG.h"
  11 #include "llvm/IR/Function.h"
  12 #include "llvm/IR/Instructions.h"
  13 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  14 #include "llvm/Transforms/Utils/Local.h"
  15 using namespace llvm;
  16
  17 /// DemoteRegToStack - This function takes a virtual register computed by an
  18 /// Instruction and replaces it with a slot in the stack frame, allocated via
  19 /// alloca.  This allows the CFG to be changed around without fear of
  20 /// invalidating the SSA information for the value.  It returns the pointer to
  21 /// the alloca inserted to create a stack slot for I.
  22 AllocaInst *llvm::DemoteRegToStack(Instruction &I, bool VolatileLoads,
  23                                    Instruction *AllocaPoint) {
  24   if (I.use_empty()) {
  25     I.eraseFromParent();
  26     return nullptr;
  27   }
  28
  29   Function *F = I.getParent()->getParent();
  30   const DataLayout &DL = F->getParent()->getDataLayout();
  31
  32   // Create a stack slot to hold the value.
  33   AllocaInst *Slot;
  34   if (AllocaPoint) {
  35     Slot = new AllocaInst(I.getType(), DL.getAllocaAddrSpace(), nullptr,
  36                           I.getName()+".reg2mem", AllocaPoint);
  37   } else {
  38     Slot = new AllocaInst(I.getType(), DL.getAllocaAddrSpace(), nullptr,
  39                           I.getName() + ".reg2mem", &F->getEntryBlock().front());
  40   }
  41
  42   // We cannot demote invoke instructions to the stack if their normal edge
  43   // is critical. Therefore, split the critical edge and create a basic block
  44   // into which the store can be inserted.
  45   if (InvokeInst *II = dyn_cast<InvokeInst>(&I)) {
  46     if (!II->getNormalDest()->getSinglePredecessor()) {
  47       unsigned SuccNum = GetSuccessorNumber(II->getParent(), II->getNormalDest());
  48       assert(isCriticalEdge(II, SuccNum) && "Expected a critical edge!");
  49       BasicBlock *BB = SplitCriticalEdge(II, SuccNum);
  50       assert(BB && "Unable to split critical edge.");
  51       (void)BB;
  52     }
  53   }
  54
  55   // Change all of the users of the instruction to read from the stack slot.
  56   while (!I.use_empty()) {
  57     Instruction *U = cast<Instruction>(I.user_back());
  58     if (PHINode *PN = dyn_cast<PHINode>(U)) {
  59       // If this is a PHI node, we can't insert a load of the value before the
  60       // use.  Instead insert the load in the predecessor block corresponding
  61       // to the incoming value.
  62       //
  63       // Note that if there are multiple edges from a basic block to this PHI
  64       // node that we cannot have multiple loads. The problem is that the
  65       // resulting PHI node will have multiple values (from each load) coming in
  66       // from the same block, which is illegal SSA form. For this reason, we
  67       // keep track of and reuse loads we insert.
  68       DenseMap<BasicBlock*, Value*> Loads;
  69       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
  70         if (PN->getIncomingValue(i) == &I) {
  71           Value *&V = Loads[PN->getIncomingBlock(i)];
  72           if (!V) {
  73             // Insert the load into the predecessor block
  74             V = new LoadInst(I.getType(), Slot, I.getName() + ".reload",
  75                              VolatileLoads,
  76                              PN->getIncomingBlock(i)->getTerminator());
  77             Loads[PN->getIncomingBlock(i)] = V;
  78           }
  79           PN->setIncomingValue(i, V);
  80         }
  81
  82     } else {
  83       // If this is a normal instruction, just insert a load.
  84       Value *V = new LoadInst(I.getType(), Slot, I.getName() + ".reload",
  85                               VolatileLoads, U);
  86       U->replaceUsesOfWith(&I, V);
  87     }
  88   }
  89
  90   // Insert stores of the computed value into the stack slot. We have to be
  91   // careful if I is an invoke instruction, because we can't insert the store
  92   // AFTER the terminator instruction.
  93   BasicBlock::iterator InsertPt;
  94   if (!I.isTerminator()) {
  95     InsertPt = ++I.getIterator();
  96     // Don't insert before PHI nodes or landingpad instrs.
  97     for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
  98       if (isa<CatchSwitchInst>(InsertPt))
  99         break;
 100     if (isa<CatchSwitchInst>(InsertPt)) {
 101       for (BasicBlock *Handler : successors(&*InsertPt))
 102         new StoreInst(&I, Slot, &*Handler->getFirstInsertionPt());
 103       return Slot;
 104     }
 105   } else {
 106     InvokeInst &II = cast<InvokeInst>(I);
 107     InsertPt = II.getNormalDest()->getFirstInsertionPt();
 108   }
 109
 110   new StoreInst(&I, Slot, &*InsertPt);
 111   return Slot;
 112 }
 113
 114 /// DemotePHIToStack - This function takes a virtual register computed by a PHI
 115 /// node and replaces it with a slot in the stack frame allocated via alloca.
 116 /// The PHI node is deleted. It returns the pointer to the alloca inserted.
 117 AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {
 118   if (P->use_empty()) {
 119     P->eraseFromParent();
 120     return nullptr;
 121   }
 122
 123   const DataLayout &DL = P->getModule()->getDataLayout();
 124
 125   // Create a stack slot to hold the value.
 126   AllocaInst *Slot;
 127   if (AllocaPoint) {
 128     Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
 129                           P->getName()+".reg2mem", AllocaPoint);
 130   } else {
 131     Function *F = P->getParent()->getParent();
 132     Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
 133                           P->getName() + ".reg2mem",
 134                           &F->getEntryBlock().front());
 135   }
 136
 137   // Iterate over each operand inserting a store in each predecessor.
 138   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
 139     if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
 140       assert(II->getParent() != P->getIncomingBlock(i) &&
 141              "Invoke edge not supported yet"); (void)II;
 142     }
 143     new StoreInst(P->getIncomingValue(i), Slot,
 144                   P->getIncomingBlock(i)->getTerminator());
 145   }
 146
 147   // Insert a load in place of the PHI and replace all uses.
 148   BasicBlock::iterator InsertPt = P->getIterator();
 149   // Don't insert before PHI nodes or landingpad instrs.
 150   for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
 151     if (isa<CatchSwitchInst>(InsertPt))
 152       break;
 153   if (isa<CatchSwitchInst>(InsertPt)) {
 154     // We need a separate load before each actual use of the PHI
 155     SmallVector<Instruction *, 4> Users;
 156     for (User *U : P->users()) {
 157       Instruction *User = cast<Instruction>(U);
 158       Users.push_back(User);
 159     }
 160     for (Instruction *User : Users) {
 161       Value *V =
 162           new LoadInst(P->getType(), Slot, P->getName() + ".reload", User);
 163       User->replaceUsesOfWith(P, V);
 164     }
 165   } else {
 166     Value *V =
 167         new LoadInst(P->getType(), Slot, P->getName() + ".reload", &*InsertPt);
 168     P->replaceAllUsesWith(V);
 169   }
 170   // Delete PHI.
 171   P->eraseFromParent();
 172   return Slot;
 173 }