1 //===- llvm/Transforms/DecomposeMultiDimRefs.cpp - Lower array refs to 1D -===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // DecomposeMultiDimRefs - Convert multi-dimensional references consisting of
11 // any combination of 2 or more array and structure indices into a sequence of
12 // instructions (using getelementpr and cast) so that each instruction has at
13 // most one index (except structure references, which need an extra leading
16 //===----------------------------------------------------------------------===//
18 #include "llvm/Transforms/Scalar.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Constants.h"
21 #include "llvm/Constant.h"
22 #include "llvm/Instructions.h"
23 #include "llvm/BasicBlock.h"
24 #include "llvm/Pass.h"
25 #include "llvm/ADT/Statistic.h"
26 #include "llvm/Support/Debug.h"
30 Statistic<> NumAdded("lowerrefs", "# of getelementptr instructions added");
32 struct DecomposePass : public BasicBlockPass {
33 virtual bool runOnBasicBlock(BasicBlock &BB);
35 RegisterOpt<DecomposePass> X("lowerrefs", "Decompose multi-dimensional "
36 "structure/array references");
39 // runOnBasicBlock - Entry point for array or structure references with multiple
42 bool DecomposePass::runOnBasicBlock(BasicBlock &BB) {
44 for (BasicBlock::iterator II = BB.begin(); II != BB.end(); )
45 if (GetElementPtrInst *gep = dyn_cast<GetElementPtrInst>(II++)) // pre-inc
46 if (gep->getNumIndices() >= 2)
47 changed |= DecomposeArrayRef(gep); // always modifies II
51 FunctionPass *llvm::createDecomposeMultiDimRefsPass() {
52 return new DecomposePass();
55 static inline bool isZeroConst (Value *V) {
56 return isa<Constant> (V) && (cast<Constant>(V)->isNullValue());
59 // Function: DecomposeArrayRef()
61 // For any GetElementPtrInst with 2 or more array and structure indices:
63 // opCode CompositeType* P, [uint|ubyte] idx1, ..., [uint|ubyte] idxN
65 // this function generates the following sequence:
67 // ptr1 = getElementPtr P, idx1
68 // ptr2 = getElementPtr ptr1, 0, idx2
70 // ptrN-1 = getElementPtr ptrN-2, 0, idxN-1
71 // opCode ptrN-1, 0, idxN // New-MAI
73 // Then it replaces the original instruction with this sequence,
74 // and replaces all uses of the original instruction with New-MAI.
75 // If idx1 is 0, we simply omit the first getElementPtr instruction.
77 // On return: BBI points to the instruction after the current one
78 // (whether or not *BBI was replaced).
80 // Return value: true if the instruction was replaced; false otherwise.
82 bool llvm::DecomposeArrayRef(GetElementPtrInst* GEP) {
83 if (GEP->getNumIndices() < 2
84 || (GEP->getNumIndices() == 2
85 && isZeroConst(GEP->getOperand(1)))) {
86 DEBUG (std::cerr << "DecomposeArrayRef: Skipping " << *GEP);
89 DEBUG (std::cerr << "DecomposeArrayRef: Decomposing " << *GEP);
92 BasicBlock *BB = GEP->getParent();
93 Value *LastPtr = GEP->getPointerOperand();
94 Instruction *InsertPoint = GEP->getNext(); // Insert before the next insn
96 // Process each index except the last one.
97 User::const_op_iterator OI = GEP->idx_begin(), OE = GEP->idx_end();
98 for (; OI+1 != OE; ++OI) {
99 std::vector<Value*> Indices;
101 // If this is the first index and is 0, skip it and move on!
102 if (OI == GEP->idx_begin()) {
103 if (isZeroConst (*OI))
106 else // Not the first index: include initial [0] to deref the last ptr
107 Indices.push_back(Constant::getNullValue(Type::LongTy));
109 Indices.push_back(*OI);
111 // New Instruction: nextPtr1 = GetElementPtr LastPtr, Indices
112 LastPtr = new GetElementPtrInst(LastPtr, Indices, "ptr1", InsertPoint);
116 // Now create a new instruction to replace the original one
118 const PointerType *PtrTy = cast<PointerType>(LastPtr->getType());
120 // Get the final index vector, including an initial [0] as before.
121 std::vector<Value*> Indices;
122 Indices.push_back(Constant::getNullValue(Type::LongTy));
123 Indices.push_back(*OI);
125 Value *NewVal = new GetElementPtrInst(LastPtr, Indices, GEP->getName(),
128 // Replace all uses of the old instruction with the new
129 GEP->replaceAllUsesWith(NewVal);
131 // Now remove and delete the old instruction...
132 BB->getInstList().erase(GEP);