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/DataLayout.h"
  12 #include "llvm/IR/Function.h"
  13 #include "llvm/IR/Instructions.h"
  14 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  15 #include "llvm/Transforms/Utils/Local.h"
  16 using namespace llvm;
  17
  18 /// DemoteRegToStack - This function takes a virtual register computed by an
  19 /// Instruction and replaces it with a slot in the stack frame, allocated via
  20 /// alloca.  This allows the CFG to be changed around without fear of
  21 /// invalidating the SSA information for the value.  It returns the pointer to
  22 /// the alloca inserted to create a stack slot for I.
  23 AllocaInst *llvm::DemoteRegToStack(Instruction &I, bool VolatileLoads,
  24                                    std::optional<BasicBlock::iterator> AllocaPoint) {
  25   if (I.use_empty()) {
  26     I.eraseFromParent();
  27     return nullptr;
  28   }
  29
  30   Function *F = I.getParent()->getParent();
  31   const DataLayout &DL = F->getDataLayout();
  32
  33   // Create a stack slot to hold the value.
  34   AllocaInst *Slot;
  35   if (AllocaPoint) {
  36     Slot = new AllocaInst(I.getType(), DL.getAllocaAddrSpace(), nullptr,
  37                           I.getName()+".reg2mem", *AllocaPoint);
  38   } else {
  39     Slot = new AllocaInst(I.getType(), DL.getAllocaAddrSpace(), nullptr,
  40                           I.getName() + ".reg2mem", F->getEntryBlock().begin());
  41   }
  42
  43   // We cannot demote invoke instructions to the stack if their normal edge
  44   // is critical. Therefore, split the critical edge and create a basic block
  45   // into which the store can be inserted.
  46   if (InvokeInst *II = dyn_cast<InvokeInst>(&I)) {
  47     if (!II->getNormalDest()->getSinglePredecessor()) {
  48       unsigned SuccNum = GetSuccessorNumber(II->getParent(), II->getNormalDest());
  49       assert(isCriticalEdge(II, SuccNum) && "Expected a critical edge!");
  50       BasicBlock *BB = SplitCriticalEdge(II, SuccNum);
  51       assert(BB && "Unable to split critical edge.");
  52       (void)BB;
  53     }
  54   } else if (CallBrInst *CBI = dyn_cast<CallBrInst>(&I)) {
  55     for (unsigned i = 0; i < CBI->getNumSuccessors(); i++) {
  56       auto *Succ = CBI->getSuccessor(i);
  57       if (!Succ->getSinglePredecessor()) {
  58         assert(isCriticalEdge(CBI, i) && "Expected a critical edge!");
  59         [[maybe_unused]] BasicBlock *BB = SplitCriticalEdge(CBI, i);
  60         assert(BB && "Unable to split critical edge.");
  61       }
  62     }
  63   }
  64
  65   // Change all of the users of the instruction to read from the stack slot.
  66   while (!I.use_empty()) {
  67     Instruction *U = cast<Instruction>(I.user_back());
  68     if (PHINode *PN = dyn_cast<PHINode>(U)) {
  69       // If this is a PHI node, we can't insert a load of the value before the
  70       // use.  Instead insert the load in the predecessor block corresponding
  71       // to the incoming value.
  72       //
  73       // Note that if there are multiple edges from a basic block to this PHI
  74       // node that we cannot have multiple loads. The problem is that the
  75       // resulting PHI node will have multiple values (from each load) coming in
  76       // from the same block, which is illegal SSA form. For this reason, we
  77       // keep track of and reuse loads we insert.
  78       DenseMap<BasicBlock*, Value*> Loads;
  79       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
  80         if (PN->getIncomingValue(i) == &I) {
  81           Value *&V = Loads[PN->getIncomingBlock(i)];
  82           if (!V) {
  83             // Insert the load into the predecessor block
  84             V = new LoadInst(I.getType(), Slot, I.getName() + ".reload",
  85                              VolatileLoads,
  86                              PN->getIncomingBlock(i)->getTerminator()->getIterator());
  87             Loads[PN->getIncomingBlock(i)] = V;
  88           }
  89           PN->setIncomingValue(i, V);
  90         }
  91
  92     } else {
  93       // If this is a normal instruction, just insert a load.
  94       Value *V = new LoadInst(I.getType(), Slot, I.getName() + ".reload",
  95                               VolatileLoads, U->getIterator());
  96       U->replaceUsesOfWith(&I, V);
  97     }
  98   }
  99
 100   // Insert stores of the computed value into the stack slot. We have to be
 101   // careful if I is an invoke instruction, because we can't insert the store
 102   // AFTER the terminator instruction.
 103   BasicBlock::iterator InsertPt;
 104   if (!I.isTerminator()) {
 105     InsertPt = ++I.getIterator();
 106     // Don't insert before PHI nodes or landingpad instrs.
 107     for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
 108       if (isa<CatchSwitchInst>(InsertPt))
 109         break;
 110     if (isa<CatchSwitchInst>(InsertPt)) {
 111       for (BasicBlock *Handler : successors(&*InsertPt))
 112         new StoreInst(&I, Slot, Handler->getFirstInsertionPt());
 113       return Slot;
 114     }
 115   } else if (InvokeInst *II = dyn_cast<InvokeInst>(&I)) {
 116     InsertPt = II->getNormalDest()->getFirstInsertionPt();
 117   } else if (CallBrInst *CBI = dyn_cast<CallBrInst>(&I)) {
 118     for (BasicBlock *Succ : successors(CBI))
 119       new StoreInst(CBI, Slot, Succ->getFirstInsertionPt());
 120     return Slot;
 121   } else {
 122     llvm_unreachable("Unsupported terminator for Reg2Mem");
 123   }
 124
 125   new StoreInst(&I, Slot, InsertPt);
 126   return Slot;
 127 }
 128
 129 /// DemotePHIToStack - This function takes a virtual register computed by a PHI
 130 /// node and replaces it with a slot in the stack frame allocated via alloca.
 131 /// The PHI node is deleted. It returns the pointer to the alloca inserted.
 132 AllocaInst *llvm::DemotePHIToStack(PHINode *P, std::optional<BasicBlock::iterator> AllocaPoint) {
 133   if (P->use_empty()) {
 134     P->eraseFromParent();
 135     return nullptr;
 136   }
 137
 138   const DataLayout &DL = P->getDataLayout();
 139
 140   // Create a stack slot to hold the value.
 141   AllocaInst *Slot;
 142   if (AllocaPoint) {
 143     Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
 144                           P->getName()+".reg2mem", *AllocaPoint);
 145   } else {
 146     Function *F = P->getParent()->getParent();
 147     Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
 148                           P->getName() + ".reg2mem",
 149                           F->getEntryBlock().begin());
 150   }
 151
 152   // Iterate over each operand inserting a store in each predecessor.
 153   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
 154     if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
 155       assert(II->getParent() != P->getIncomingBlock(i) &&
 156              "Invoke edge not supported yet"); (void)II;
 157     }
 158     new StoreInst(P->getIncomingValue(i), Slot,
 159                   P->getIncomingBlock(i)->getTerminator()->getIterator());
 160   }
 161
 162   // Insert a load in place of the PHI and replace all uses.
 163   BasicBlock::iterator InsertPt = P->getIterator();
 164   // Don't insert before PHI nodes or landingpad instrs.
 165   for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
 166     if (isa<CatchSwitchInst>(InsertPt))
 167       break;
 168   if (isa<CatchSwitchInst>(InsertPt)) {
 169     // We need a separate load before each actual use of the PHI
 170     SmallVector<Instruction *, 4> Users;
 171     for (User *U : P->users()) {
 172       Instruction *User = cast<Instruction>(U);
 173       Users.push_back(User);
 174     }
 175     for (Instruction *User : Users) {
 176       Value *V =
 177           new LoadInst(P->getType(), Slot, P->getName() + ".reload", User->getIterator());
 178       User->replaceUsesOfWith(P, V);
 179     }
 180   } else {
 181     Value *V =
 182         new LoadInst(P->getType(), Slot, P->getName() + ".reload", InsertPt);
 183     P->replaceAllUsesWith(V);
 184   }
 185   // Delete PHI.
 186   P->eraseFromParent();
 187   return Slot;
 188 }