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/Transforms/Utils/Local.h"
  12 #include "llvm/IR/Function.h"
  13 #include "llvm/IR/Instructions.h"
  14 #include "llvm/IR/Type.h"
  15 #include "llvm/Transforms/Utils/BasicBlockUtils.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                                    Instruction *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->getParent()->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().front());
  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   }
  55
  56   // Change all of the users of the instruction to read from the stack slot.
  57   while (!I.use_empty()) {
  58     Instruction *U = cast<Instruction>(I.user_back());
  59     if (PHINode *PN = dyn_cast<PHINode>(U)) {
  60       // If this is a PHI node, we can't insert a load of the value before the
  61       // use.  Instead insert the load in the predecessor block corresponding
  62       // to the incoming value.
  63       //
  64       // Note that if there are multiple edges from a basic block to this PHI
  65       // node that we cannot have multiple loads. The problem is that the
  66       // resulting PHI node will have multiple values (from each load) coming in
  67       // from the same block, which is illegal SSA form. For this reason, we
  68       // keep track of and reuse loads we insert.
  69       DenseMap<BasicBlock*, Value*> Loads;
  70       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
  71         if (PN->getIncomingValue(i) == &I) {
  72           Value *&V = Loads[PN->getIncomingBlock(i)];
  73           if (!V) {
  74             // Insert the load into the predecessor block
  75             V = new LoadInst(I.getType(), Slot, I.getName() + ".reload",
  76                              VolatileLoads,
  77                              PN->getIncomingBlock(i)->getTerminator());
  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     for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
  97       /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
  98   } else {
  99     InvokeInst &II = cast<InvokeInst>(I);
 100     InsertPt = II.getNormalDest()->getFirstInsertionPt();
 101   }
 102
 103   new StoreInst(&I, Slot, &*InsertPt);
 104   return Slot;
 105 }
 106
 107 /// DemotePHIToStack - This function takes a virtual register computed by a PHI
 108 /// node and replaces it with a slot in the stack frame allocated via alloca.
 109 /// The PHI node is deleted. It returns the pointer to the alloca inserted.
 110 AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {
 111   if (P->use_empty()) {
 112     P->eraseFromParent();
 113     return nullptr;
 114   }
 115
 116   const DataLayout &DL = P->getModule()->getDataLayout();
 117
 118   // Create a stack slot to hold the value.
 119   AllocaInst *Slot;
 120   if (AllocaPoint) {
 121     Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
 122                           P->getName()+".reg2mem", AllocaPoint);
 123   } else {
 124     Function *F = P->getParent()->getParent();
 125     Slot = new AllocaInst(P->getType(), DL.getAllocaAddrSpace(), nullptr,
 126                           P->getName() + ".reg2mem",
 127                           &F->getEntryBlock().front());
 128   }
 129
 130   // Iterate over each operand inserting a store in each predecessor.
 131   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
 132     if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
 133       assert(II->getParent() != P->getIncomingBlock(i) &&
 134              "Invoke edge not supported yet"); (void)II;
 135     }
 136     new StoreInst(P->getIncomingValue(i), Slot,
 137                   P->getIncomingBlock(i)->getTerminator());
 138   }
 139
 140   // Insert a load in place of the PHI and replace all uses.
 141   BasicBlock::iterator InsertPt = P->getIterator();
 142
 143   for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
 144     /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
 145
 146   Value *V =
 147       new LoadInst(P->getType(), Slot, P->getName() + ".reload", &*InsertPt);
 148   P->replaceAllUsesWith(V);
 149
 150   // Delete PHI.
 151   P->eraseFromParent();
 152   return Slot;
 153 }