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/BasicBlockUtils.h"
11#include "llvm/ADT/DenseMap.h"
12#include "llvm/Analysis/CFG.h"
13#include "llvm/IR/Function.h"
14#include "llvm/IR/Instructions.h"
15#include "llvm/IR/Type.h"
16#include "llvm/Transforms/Utils/Local.h"
17using namespace llvm;
18
19/// DemoteRegToStack - This function takes a virtual register computed by an
20/// Instruction and replaces it with a slot in the stack frame, allocated via
21/// alloca.  This allows the CFG to be changed around without fear of
22/// invalidating the SSA information for the value.  It returns the pointer to
23/// the alloca inserted to create a stack slot for I.
24AllocaInst *llvm::DemoteRegToStack(Instruction &I, bool VolatileLoads,
25                                   Instruction *AllocaPoint) {
26  if (I.use_empty()) {
27    I.eraseFromParent();
28    return nullptr;
29  }
30
31  // Create a stack slot to hold the value.
32  AllocaInst *Slot;
33  if (AllocaPoint) {
34    Slot = new AllocaInst(I.getType(), nullptr,
35                          I.getName()+".reg2mem", AllocaPoint);
36  } else {
37    Function *F = I.getParent()->getParent();
38    Slot = new AllocaInst(I.getType(), nullptr, I.getName() + ".reg2mem",
39                          &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(Slot, I.getName()+".reload", VolatileLoads,
75                             PN->getIncomingBlock(i)->getTerminator());
76          }
77          PN->setIncomingValue(i, V);
78        }
79
80    } else {
81      // If this is a normal instruction, just insert a load.
82      Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
83      U->replaceUsesOfWith(&I, V);
84    }
85  }
86
87  // Insert stores of the computed value into the stack slot. We have to be
88  // careful if I is an invoke instruction, because we can't insert the store
89  // AFTER the terminator instruction.
90  BasicBlock::iterator InsertPt;
91  if (!isa<TerminatorInst>(I)) {
92    InsertPt = ++I.getIterator();
93    for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
94      /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
95  } else {
96    InvokeInst &II = cast<InvokeInst>(I);
97    InsertPt = II.getNormalDest()->getFirstInsertionPt();
98  }
99
100  new StoreInst(&I, Slot, &*InsertPt);
101  return Slot;
102}
103
104/// DemotePHIToStack - This function takes a virtual register computed by a PHI
105/// node and replaces it with a slot in the stack frame allocated via alloca.
106/// The PHI node is deleted. It returns the pointer to the alloca inserted.
107AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {
108  if (P->use_empty()) {
109    P->eraseFromParent();
110    return nullptr;
111  }
112
113  // Create a stack slot to hold the value.
114  AllocaInst *Slot;
115  if (AllocaPoint) {
116    Slot = new AllocaInst(P->getType(), nullptr,
117                          P->getName()+".reg2mem", AllocaPoint);
118  } else {
119    Function *F = P->getParent()->getParent();
120    Slot = new AllocaInst(P->getType(), nullptr, P->getName() + ".reg2mem",
121                          &F->getEntryBlock().front());
122  }
123
124  // Iterate over each operand inserting a store in each predecessor.
125  for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
126    if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
127      assert(II->getParent() != P->getIncomingBlock(i) &&
128             "Invoke edge not supported yet"); (void)II;
129    }
130    new StoreInst(P->getIncomingValue(i), Slot,
131                  P->getIncomingBlock(i)->getTerminator());
132  }
133
134  // Insert a load in place of the PHI and replace all uses.
135  BasicBlock::iterator InsertPt = P->getIterator();
136
137  for (; isa<PHINode>(InsertPt) || InsertPt->isEHPad(); ++InsertPt)
138    /* empty */;   // Don't insert before PHI nodes or landingpad instrs.
139
140  Value *V = new LoadInst(Slot, P->getName() + ".reload", &*InsertPt);
141  P->replaceAllUsesWith(V);
142
143  // Delete PHI.
144  P->eraseFromParent();
145  return Slot;
146}
147