lib/Transforms/Utils/DemoteRegToStack.cpp

   1 //===- DemoteRegToStack.cpp - Move a virtual register to the stack --------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file was developed by the LLVM research group and is distributed under
   6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file provide the function DemoteRegToStack().  This function takes a
  11 // virtual register computed by an Instruction and replaces it with a slot in
  12 // the stack frame, allocated via alloca. It returns the pointer to the
  13 // AllocaInst inserted.  After this function is called on an instruction, we are
  14 // guaranteed that the only user of the instruction is a store that is
  15 // immediately after it.
  16 //
  17 //===----------------------------------------------------------------------===//
  18
  19 #include "llvm/Transforms/Utils/Local.h"
  20 #include "llvm/Function.h"
  21 #include "llvm/Instructions.h"
  22 #include "llvm/Type.h"
  23 #include <map>
  24 using namespace llvm;
  25
  26 /// DemoteRegToStack - This function takes a virtual register computed by an
  27 /// Instruction and replaces it with a slot in the stack frame, allocated via
  28 /// alloca.  This allows the CFG to be changed around without fear of
  29 /// invalidating the SSA information for the value.  It returns the pointer to
  30 /// the alloca inserted to create a stack slot for I.
  31 ///
  32 AllocaInst* llvm::DemoteRegToStack(Instruction &I, bool VolatileLoads) {
  33   if (I.use_empty()) return 0;                // nothing to do!
  34
  35   // Create a stack slot to hold the value.
  36   Function *F = I.getParent()->getParent();
  37   AllocaInst *Slot = new AllocaInst(I.getType(), 0, I.getName(),
  38                                     F->getEntryBlock().begin());
  39
  40   // Change all of the users of the instruction to read from the stack slot
  41   // instead.
  42   while (!I.use_empty()) {
  43     Instruction *U = cast<Instruction>(I.use_back());
  44     if (PHINode *PN = dyn_cast<PHINode>(U)) {
  45       // If this is a PHI node, we can't insert a load of the value before the
  46       // use.  Instead, insert the load in the predecessor block corresponding
  47       // to the incoming value.
  48       //
  49       // Note that if there are multiple edges from a basic block to this PHI
  50       // node that we cannot multiple loads.  The problem is that the resultant
  51       // PHI node will have multiple values (from each load) coming in from the
  52       // same block, which is illegal SSA form.  For this reason, we keep track
  53       // and reuse loads we insert.
  54       std::map<BasicBlock*, Value*> Loads;
  55       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
  56         if (PN->getIncomingValue(i) == &I) {
  57           Value *&V = Loads[PN->getIncomingBlock(i)];
  58           if (V == 0) {
  59             // Insert the load into the predecessor block
  60             V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads,
  61                              PN->getIncomingBlock(i)->getTerminator());
  62           }
  63           PN->setIncomingValue(i, V);
  64         }
  65
  66     } else {
  67       // If this is a normal instruction, just insert a load.
  68       Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
  69       U->replaceUsesOfWith(&I, V);
  70     }
  71   }
  72
  73
  74   // Insert stores of the computed value into the stack slot.  We have to be
  75   // careful is I is an invoke instruction though, because we can't insert the
  76   // store AFTER the terminator instruction.
  77   BasicBlock::iterator InsertPt;
  78   if (!isa<TerminatorInst>(I)) {
  79     InsertPt = &I;
  80     ++InsertPt;
  81   } else {
  82     // We cannot demote invoke instructions to the stack if their normal edge
  83     // is critical.
  84     InvokeInst &II = cast<InvokeInst>(I);
  85     assert(II.getNormalDest()->getSinglePredecessor() &&
  86            "Cannot demote invoke with a critical successor!");
  87     InsertPt = II.getNormalDest()->begin();
  88   }
  89
  90   for (; isa<PHINode>(InsertPt); ++InsertPt)
  91   /* empty */;   // Don't insert before any PHI nodes.
  92   new StoreInst(&I, Slot, InsertPt);
  93
  94   return Slot;
  95 }