1 //===- DemoteRegToStack.cpp - Move a virtual register to the stack --------===//
3 // The LLVM Compiler Infrastructure
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.
8 //===----------------------------------------------------------------------===//
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.
17 //===----------------------------------------------------------------------===//
19 #include "llvm/Transforms/Utils/Local.h"
20 #include "llvm/Function.h"
21 #include "llvm/Instructions.h"
25 /// DemoteRegToStack - This function takes a virtual register computed by an
26 /// Instruction and replaces it with a slot in the stack frame, allocated via
27 /// alloca. This allows the CFG to be changed around without fear of
28 /// invalidating the SSA information for the value. It returns the pointer to
29 /// the alloca inserted to create a stack slot for I.
31 AllocaInst* llvm::DemoteRegToStack(Instruction &I) {
32 if (I.use_empty()) return 0; // nothing to do!
34 // Create a stack slot to hold the value.
35 Function *F = I.getParent()->getParent();
36 AllocaInst *Slot = new AllocaInst(I.getType(), 0, I.getName(),
37 F->getEntryBlock().begin());
39 // 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.
48 // Note that if there are multiple edges from a basic block to this PHI
49 // node that we cannot multiple loads. The problem is that the resultant
50 // PHI node will have multiple values (from each load) coming in from the
51 // same block, which is illegal SSA form. For this reason, we keep track
52 // and reuse loads we insert.
53 std::map<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)];
58 // Insert the load into the predecessor block
59 V = new LoadInst(Slot, I.getName()+".reload",
60 PN->getIncomingBlock(i)->getTerminator());
62 PN->setIncomingValue(i, V);
66 // If this is a normal instruction, just insert a load.
67 Value *V = new LoadInst(Slot, I.getName()+".reload", U);
68 U->replaceUsesOfWith(&I, V);
73 // Insert stores of the computed value into the stack slot. We have to be
74 // careful is I is an invoke instruction though, because we can't insert the
75 // store AFTER the terminator instruction.
76 if (!isa<TerminatorInst>(I)) {
77 BasicBlock::iterator InsertPt = &I;
78 for (++InsertPt; isa<PHINode>(InsertPt); ++InsertPt)
79 /* empty */; // Don't insert before any PHI nodes.
80 new StoreInst(&I, Slot, InsertPt);
82 // FIXME: We cannot yet demote invoke instructions to the stack, because
83 // doing so would require breaking critical edges. This should be fixed
86 "Cannot demote the value computed by an invoke instruction yet!");