LLVM API Documentation

DemoteRegToStack.cpp

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00001 //===- DemoteRegToStack.cpp - Move a virtual register to the stack --------===//
00002 //
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file was developed by the LLVM research group and is distributed under
00006 // the University of Illinois Open Source License. See LICENSE.TXT for details.
00007 //
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This file provide the function DemoteRegToStack().  This function takes a
00011 // virtual register computed by an Instruction and replaces it with a slot in
00012 // the stack frame, allocated via alloca. It returns the pointer to the
00013 // AllocaInst inserted.  After this function is called on an instruction, we are
00014 // guaranteed that the only user of the instruction is a store that is
00015 // immediately after it.
00016 //
00017 //===----------------------------------------------------------------------===//
00018 
00019 #include "llvm/Transforms/Utils/Local.h"
00020 #include "llvm/Function.h"
00021 #include "llvm/Instructions.h"
00022 #include "llvm/Type.h"
00023 #include <map>
00024 using namespace llvm;
00025 
00026 /// DemoteRegToStack - This function takes a virtual register computed by an
00027 /// Instruction and replaces it with a slot in the stack frame, allocated via
00028 /// alloca.  This allows the CFG to be changed around without fear of
00029 /// invalidating the SSA information for the value.  It returns the pointer to
00030 /// the alloca inserted to create a stack slot for I.
00031 ///
00032 AllocaInst* llvm::DemoteRegToStack(Instruction &I, bool VolatileLoads) {
00033   if (I.use_empty()) return 0;                // nothing to do!
00034 
00035   // Create a stack slot to hold the value.
00036   Function *F = I.getParent()->getParent();
00037   AllocaInst *Slot = new AllocaInst(I.getType(), 0, I.getName(),
00038                                     F->getEntryBlock().begin());
00039 
00040   // Change all of the users of the instruction to read from the stack slot
00041   // instead.
00042   while (!I.use_empty()) {
00043     Instruction *U = cast<Instruction>(I.use_back());
00044     if (PHINode *PN = dyn_cast<PHINode>(U)) {
00045       // If this is a PHI node, we can't insert a load of the value before the
00046       // use.  Instead, insert the load in the predecessor block corresponding
00047       // to the incoming value.
00048       //
00049       // Note that if there are multiple edges from a basic block to this PHI
00050       // node that we cannot multiple loads.  The problem is that the resultant
00051       // PHI node will have multiple values (from each load) coming in from the
00052       // same block, which is illegal SSA form.  For this reason, we keep track
00053       // and reuse loads we insert.
00054       std::map<BasicBlock*, Value*> Loads;
00055       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
00056         if (PN->getIncomingValue(i) == &I) {
00057           Value *&V = Loads[PN->getIncomingBlock(i)];
00058           if (V == 0) {
00059             // Insert the load into the predecessor block
00060             V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, 
00061                              PN->getIncomingBlock(i)->getTerminator());
00062           }
00063           PN->setIncomingValue(i, V);
00064         }
00065 
00066     } else {
00067       // If this is a normal instruction, just insert a load.
00068       Value *V = new LoadInst(Slot, I.getName()+".reload", VolatileLoads, U);
00069       U->replaceUsesOfWith(&I, V);
00070     }
00071   }
00072 
00073 
00074   // Insert stores of the computed value into the stack slot.  We have to be
00075   // careful is I is an invoke instruction though, because we can't insert the
00076   // store AFTER the terminator instruction.
00077   BasicBlock::iterator InsertPt;
00078   if (!isa<TerminatorInst>(I)) {
00079     InsertPt = &I;
00080     ++InsertPt;
00081   } else {
00082     // We cannot demote invoke instructions to the stack if their normal edge
00083     // is critical.
00084     InvokeInst &II = cast<InvokeInst>(I);
00085     assert(II.getNormalDest()->getSinglePredecessor() &&
00086            "Cannot demote invoke with a critical successor!");
00087     InsertPt = II.getNormalDest()->begin();
00088   }
00089 
00090   for (; isa<PHINode>(InsertPt); ++InsertPt)
00091   /* empty */;   // Don't insert before any PHI nodes.
00092   new StoreInst(&I, Slot, InsertPt);
00093 
00094   return Slot;
00095 }