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