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//===- DemoteRegToStack.cpp - Move a virtual register to the stack --------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file provide the function DemoteRegToStack(). This function takes a
// virtual register computed by an Instruction and replaces it with a slot in
// the stack frame, allocated via alloca. It returns the pointer to the
// AllocaInst inserted. After this function is called on an instruction, we are
// guaranteed that the only user of the instruction is a store that is
// immediately after it.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include <map>
using namespace llvm;
/// DemoteRegToStack - This function takes a virtual register computed by an
/// Instruction and replaces it with a slot in the stack frame, allocated via
/// alloca. This allows the CFG to be changed around without fear of
/// invalidating the SSA information for the value. It returns the pointer to
/// the alloca inserted to create a stack slot for I.
///
AllocaInst* llvm::DemoteRegToStack(Instruction &I) {
if (I.use_empty()) return 0; // nothing to do!
// Create a stack slot to hold the value.
Function *F = I.getParent()->getParent();
AllocaInst *Slot = new AllocaInst(I.getType(), 0, I.getName(),
F->getEntryBlock().begin());
// Change all of the users of the instruction to read from the stack slot
// instead.
while (!I.use_empty()) {
Instruction *U = cast<Instruction>(I.use_back());
if (PHINode *PN = dyn_cast<PHINode>(U)) {
// If this is a PHI node, we can't insert a load of the value before the
// use. Instead, insert the load in the predecessor block corresponding
// to the incoming value.
//
// Note that if there are multiple edges from a basic block to this PHI
// node that we cannot multiple loads. The problem is that the resultant
// PHI node will have multiple values (from each load) coming in from the
// same block, which is illegal SSA form. For this reason, we keep track
// and reuse loads we insert.
std::map<BasicBlock*, Value*> Loads;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (PN->getIncomingValue(i) == &I) {
Value *&V = Loads[PN->getIncomingBlock(i)];
if (V == 0) {
// Insert the load into the predecessor block
V = new LoadInst(Slot, I.getName()+".reload",
PN->getIncomingBlock(i)->getTerminator());
}
PN->setIncomingValue(i, V);
}
} else {
// If this is a normal instruction, just insert a load.
Value *V = new LoadInst(Slot, I.getName()+".reload", U);
U->replaceUsesOfWith(&I, V);
}
}
// Insert stores of the computed value into the stack slot. We have to be
// careful is I is an invoke instruction though, because we can't insert the
// store AFTER the terminator instruction.
if (!isa<TerminatorInst>(I)) {
BasicBlock::iterator InsertPt = &I;
for (++InsertPt; isa<PHINode>(InsertPt); ++InsertPt)
/* empty */; // Don't insert before any PHI nodes.
new StoreInst(&I, Slot, InsertPt);
} else {
// FIXME: We cannot yet demote invoke instructions to the stack, because
// doing so would require breaking critical edges. This should be fixed
// eventually.
assert(0 &&
"Cannot demote the value computed by an invoke instruction yet!");
}
return Slot;
}
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