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//===- DemoteRegToStack.cpp - Move a virtual register to the stack --------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/Analysis/CFG.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Type.h"
#include "llvm/Transforms/Utils/Local.h"
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, bool VolatileLoads,
Instruction *AllocaPoint) {
if (I.use_empty()) {
I.eraseFromParent();
return nullptr;
}
// Create a stack slot to hold the value.
AllocaInst *Slot;
if (AllocaPoint) {
Slot = new AllocaInst(I.getType(), nullptr,
I.getName()+".reg2mem", AllocaPoint);
} else {
Function *F = I.getParent()->getParent();
Slot = new AllocaInst(I.getType(), nullptr, I.getName()+".reg2mem",
F->getEntryBlock().begin());
}
// We cannot demote invoke instructions to the stack if their normal edge
// is critical. Therefore, split the critical edge and create a basic block
// into which the store can be inserted.
if (InvokeInst *II = dyn_cast<InvokeInst>(&I)) {
if (!II->getNormalDest()->getSinglePredecessor()) {
unsigned SuccNum = GetSuccessorNumber(II->getParent(), II->getNormalDest());
assert(isCriticalEdge(II, SuccNum) && "Expected a critical edge!");
BasicBlock *BB = SplitCriticalEdge(II, SuccNum);
assert(BB && "Unable to split critical edge.");
(void)BB;
}
}
// Change all of the users of the instruction to read from the stack slot.
while (!I.use_empty()) {
Instruction *U = cast<Instruction>(I.user_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 have multiple loads. The problem is that the
// resulting 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 of and reuse loads we insert.
DenseMap<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) {
// Insert the load into the predecessor block
V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads,
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", VolatileLoads, U);
U->replaceUsesOfWith(&I, V);
}
}
// Insert stores of the computed value into the stack slot. We have to be
// careful if I is an invoke instruction, because we can't insert the store
// AFTER the terminator instruction.
BasicBlock::iterator InsertPt;
if (!isa<TerminatorInst>(I)) {
InsertPt = &I;
++InsertPt;
for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt)
/* empty */; // Don't insert before PHI nodes or landingpad instrs.
} else {
InvokeInst &II = cast<InvokeInst>(I);
InsertPt = II.getNormalDest()->getFirstInsertionPt();
}
new StoreInst(&I, Slot, InsertPt);
return Slot;
}
/// DemotePHIToStack - This function takes a virtual register computed by a PHI
/// node and replaces it with a slot in the stack frame allocated via alloca.
/// The PHI node is deleted. It returns the pointer to the alloca inserted.
AllocaInst *llvm::DemotePHIToStack(PHINode *P, Instruction *AllocaPoint) {
if (P->use_empty()) {
P->eraseFromParent();
return nullptr;
}
// Create a stack slot to hold the value.
AllocaInst *Slot;
if (AllocaPoint) {
Slot = new AllocaInst(P->getType(), nullptr,
P->getName()+".reg2mem", AllocaPoint);
} else {
Function *F = P->getParent()->getParent();
Slot = new AllocaInst(P->getType(), nullptr, P->getName()+".reg2mem",
F->getEntryBlock().begin());
}
// Iterate over each operand inserting a store in each predecessor.
for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
if (InvokeInst *II = dyn_cast<InvokeInst>(P->getIncomingValue(i))) {
assert(II->getParent() != P->getIncomingBlock(i) &&
"Invoke edge not supported yet"); (void)II;
}
new StoreInst(P->getIncomingValue(i), Slot,
P->getIncomingBlock(i)->getTerminator());
}
// Insert a load in place of the PHI and replace all uses.
BasicBlock::iterator InsertPt = P;
for (; isa<PHINode>(InsertPt) || isa<LandingPadInst>(InsertPt); ++InsertPt)
/* empty */; // Don't insert before PHI nodes or landingpad instrs.
Value *V = new LoadInst(Slot, P->getName()+".reload", InsertPt);
P->replaceAllUsesWith(V);
// Delete PHI.
P->eraseFromParent();
return Slot;
}
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