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      1 //===- DemoteRegToStack.cpp - Move a virtual register to the stack --------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 
     10 #include "llvm/Transforms/Utils/Local.h"
     11 #include "llvm/Function.h"
     12 #include "llvm/Instructions.h"
     13 #include "llvm/Type.h"
     14 #include "llvm/ADT/DenseMap.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 0;
     27   }
     28 
     29   // Create a stack slot to hold the value.
     30   AllocaInst *Slot;
     31   if (AllocaPoint) {
     32     Slot = new AllocaInst(I.getType(), 0,
     33                           I.getName()+".reg2mem", AllocaPoint);
     34   } else {
     35     Function *F = I.getParent()->getParent();
     36     Slot = new AllocaInst(I.getType(), 0, I.getName()+".reg2mem",
     37                           F->getEntryBlock().begin());
     38   }
     39 
     40   // Change all of the users of the instruction to read from the stack slot.
     41   while (!I.use_empty()) {
     42     Instruction *U = cast<Instruction>(I.use_back());
     43     if (PHINode *PN = dyn_cast<PHINode>(U)) {
     44       // If this is a PHI node, we can't insert a load of the value before the
     45       // use.  Instead insert the load in the predecessor block corresponding
     46       // to the incoming value.
     47       //
     48       // Note that if there are multiple edges from a basic block to this PHI
     49       // node that we cannot have multiple loads. The problem is that the
     50       // resulting PHI node will have multiple values (from each load) coming in
     51       // from the same block, which is illegal SSA form. For this reason, we
     52       // keep track of and reuse loads we insert.
     53       DenseMap<BasicBlock*, Value*> Loads;
     54       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
     55         if (PN->getIncomingValue(i) == &I) {
     56           Value *&V = Loads[PN->getIncomingBlock(i)];
     57           if (V == 0) {
     58             // Insert the load into the predecessor block
     59             V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads,
     60                              PN->getIncomingBlock(i)->getTerminator());
     61           }
     62           PN->setIncomingValue(i, V);
     63         }
     64 
     65     } else {
     66       // If this is a normal instruction, just insert a load.
     67       Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
     68       U->replaceUsesOfWith(&I, V);
     69     }
     70   }
     71 
     72 
     73   // Insert stores of the computed value into the stack slot. We have to be
     74   // careful if I is an invoke instruction, because we can't insert the store
     75   // AFTER the terminator instruction.
     76   BasicBlock::iterator InsertPt;
     77   if (!isa<TerminatorInst>(I)) {
     78     InsertPt = &I;
     79     ++InsertPt;
     80   } else {
     81     // We cannot demote invoke instructions to the stack if their normal edge
     82     // is critical.
     83     InvokeInst &II = cast<InvokeInst>(I);
     84     assert(II.getNormalDest()->getSinglePredecessor() &&
     85            "Cannot demote invoke with a critical successor!");
     86     InsertPt = II.getNormalDest()->begin();
     87   }
     88 
     89   for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt)
     90     /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
     91 
     92   new StoreInst(&I, Slot, InsertPt);
     93   return Slot;
     94 }
     95 
     96 /// DemotePHIToStack - This function takes a virtual register computed by a PHI
     97 /// node and replaces it with a slot in the stack frame allocated via alloca.
     98 /// The PHI node is deleted. It returns the pointer to the alloca inserted.
     99 AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {
    100   if (P->use_empty()) {
    101     P->eraseFromParent();
    102     return 0;
    103   }
    104 
    105   // Create a stack slot to hold the value.
    106   AllocaInst *Slot;
    107   if (AllocaPoint) {
    108     Slot = new AllocaInst(P->getType(), 0,
    109                           P->getName()+".reg2mem", AllocaPoint);
    110   } else {
    111     Function *F = P->getParent()->getParent();
    112     Slot = new AllocaInst(P->getType(), 0, P->getName()+".reg2mem",
    113                           F->getEntryBlock().begin());
    114   }
    115 
    116   // Iterate over each operand inserting a store in each predecessor.
    117   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
    118     if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
    119       assert(II->getParent() != P->getIncomingBlock(i) &&
    120              "Invoke edge not supported yet"); (void)II;
    121     }
    122     new StoreInst(P->getIncomingValue(i), Slot,
    123                   P->getIncomingBlock(i)->getTerminator());
    124   }
    125 
    126   // Insert a load in place of the PHI and replace all uses.
    127   Value *V = new LoadInst(Slot, P->getName()+".reload", P);
    128   P->replaceAllUsesWith(V);
    129 
    130   // Delete PHI.
    131   P->eraseFromParent();
    132   return Slot;
    133 }
    134