LLVM API Documentation

DecomposeMultiDimRefs.cpp

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00001 //===- llvm/Transforms/DecomposeMultiDimRefs.cpp - Lower array refs to 1D -===//
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 // DecomposeMultiDimRefs - Convert multi-dimensional references consisting of
00011 // any combination of 2 or more array and structure indices into a sequence of
00012 // instructions (using getelementpr and cast) so that each instruction has at
00013 // most one index (except structure references, which need an extra leading
00014 // index of [0]).
00015 //
00016 //===----------------------------------------------------------------------===//
00017 
00018 #include "SparcV9Internals.h"
00019 #include "llvm/DerivedTypes.h"
00020 #include "llvm/Constants.h"
00021 #include "llvm/Constant.h"
00022 #include "llvm/Instructions.h"
00023 #include "llvm/BasicBlock.h"
00024 #include "llvm/Pass.h"
00025 #include "llvm/ADT/Statistic.h"
00026 #include "llvm/Support/Debug.h"
00027 #include <iostream>
00028 using namespace llvm;
00029 
00030 namespace {
00031   Statistic<> NumAdded("lowerrefs", "# of getelementptr instructions added");
00032 
00033   struct DecomposePass : public BasicBlockPass {
00034     virtual bool runOnBasicBlock(BasicBlock &BB);
00035   };
00036   RegisterOpt<DecomposePass> X("lowerrefs", "Decompose multi-dimensional "
00037                                "structure/array references");
00038 }
00039 
00040 // runOnBasicBlock - Entry point for array or structure references with multiple
00041 // indices.
00042 //
00043 bool DecomposePass::runOnBasicBlock(BasicBlock &BB) {
00044   bool changed = false;
00045   for (BasicBlock::iterator II = BB.begin(); II != BB.end(); )
00046     if (GetElementPtrInst *gep = dyn_cast<GetElementPtrInst>(II++)) // pre-inc
00047       if (gep->getNumIndices() >= 2)
00048         changed |= DecomposeArrayRef(gep); // always modifies II
00049   return changed;
00050 }
00051 
00052 FunctionPass *llvm::createDecomposeMultiDimRefsPass() {
00053   return new DecomposePass();
00054 }
00055 
00056 static inline bool isZeroConst (Value *V) {
00057   return isa<Constant> (V) && (cast<Constant>(V)->isNullValue());
00058 }
00059 
00060 // Function: DecomposeArrayRef()
00061 //
00062 // For any GetElementPtrInst with 2 or more array and structure indices:
00063 //
00064 //      opCode CompositeType* P, [uint|ubyte] idx1, ..., [uint|ubyte] idxN
00065 //
00066 // this function generates the following sequence:
00067 //
00068 //      ptr1   = getElementPtr P,         idx1
00069 //      ptr2   = getElementPtr ptr1,   0, idx2
00070 //      ...
00071 //      ptrN-1 = getElementPtr ptrN-2, 0, idxN-1
00072 //      opCode                 ptrN-1, 0, idxN  // New-MAI
00073 //
00074 // Then it replaces the original instruction with this sequence,
00075 // and replaces all uses of the original instruction with New-MAI.
00076 // If idx1 is 0, we simply omit the first getElementPtr instruction.
00077 //
00078 // On return: BBI points to the instruction after the current one
00079 //            (whether or not *BBI was replaced).
00080 //
00081 // Return value: true if the instruction was replaced; false otherwise.
00082 //
00083 bool llvm::DecomposeArrayRef(GetElementPtrInst* GEP) {
00084   if (GEP->getNumIndices() < 2
00085       || (GEP->getNumIndices() == 2
00086           && isZeroConst(GEP->getOperand(1)))) {
00087     DEBUG (std::cerr << "DecomposeArrayRef: Skipping " << *GEP);
00088     return false;
00089   } else {
00090     DEBUG (std::cerr << "DecomposeArrayRef: Decomposing " << *GEP);
00091   }
00092 
00093   BasicBlock *BB = GEP->getParent();
00094   Value *LastPtr = GEP->getPointerOperand();
00095   Instruction *InsertPoint = GEP->getNext(); // Insert before the next insn
00096 
00097   // Process each index except the last one.
00098   User::const_op_iterator OI = GEP->idx_begin(), OE = GEP->idx_end();
00099   for (; OI+1 != OE; ++OI) {
00100     std::vector<Value*> Indices;
00101 
00102     // If this is the first index and is 0, skip it and move on!
00103     if (OI == GEP->idx_begin()) {
00104       if (isZeroConst (*OI))
00105         continue;
00106     }
00107     else // Not the first index: include initial [0] to deref the last ptr
00108       Indices.push_back(Constant::getNullValue(Type::LongTy));
00109 
00110     Indices.push_back(*OI);
00111 
00112     // New Instruction: nextPtr1 = GetElementPtr LastPtr, Indices
00113     LastPtr = new GetElementPtrInst(LastPtr, Indices, "ptr1", InsertPoint);
00114     ++NumAdded;
00115   }
00116 
00117   // Now create a new instruction to replace the original one
00118   //
00119   const PointerType *PtrTy = cast<PointerType>(LastPtr->getType());
00120 
00121   // Get the final index vector, including an initial [0] as before.
00122   std::vector<Value*> Indices;
00123   Indices.push_back(Constant::getNullValue(Type::LongTy));
00124   Indices.push_back(*OI);
00125 
00126   Value *NewVal = new GetElementPtrInst(LastPtr, Indices, GEP->getName(),
00127                                         InsertPoint);
00128 
00129   // Replace all uses of the old instruction with the new
00130   GEP->replaceAllUsesWith(NewVal);
00131 
00132   // Now remove and delete the old instruction...
00133   BB->getInstList().erase(GEP);
00134 
00135   return true;
00136 }
00137