New pass to decompose multi-dimensional array references into

a sequence of 1-D references, using a sequence of getElementPtrs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@1948 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/DecomposeMultiDimRefs.cpp b/lib/Transforms/Scalar/DecomposeMultiDimRefs.cpp
new file mode 100644 (file)
index 0000000..a2d49c8
--- /dev/null
+++ b/lib/Transforms/Scalar/DecomposeMultiDimRefs.cpp
@@ -0,0 +1,148 @@
+//===- llvm/Transforms/DecomposeArrayRefs.cpp - Lower array refs to 1D -----=//
+//
+// DecomposeArrayRefs - 
+// Convert multi-dimensional array references into a sequence of
+// instructions (using getelementpr and cast) so that each instruction
+// has at most one array offset.
+//
+//===---------------------------------------------------------------------===//
+
+#include "llvm/Transforms/DecomposeArrayRefs.h"
+#include "llvm/iMemory.h"
+#include "llvm/iOther.h"
+#include "llvm/BasicBlock.h"
+#include "llvm/Method.h"
+#include "llvm/Pass.h"
+
+
+// 
+// This function repeats until we have a one-dim. reference: {
+//      // For an N-dim array ref, where N > 1, insert:
+//      aptr1 = getElementPtr [N-dim array] * lastPtr, uint firstIndex
+//      aptr2 = cast [N-dim-arry] * aptr to [<N-1>-dim-array] *
+// }
+// Then it replaces the original instruction with an equivalent one that
+// uses the last aptr2 generated in the loop and a single index.
+// 
+static BasicBlock::reverse_iterator
+decomposeArrayRef(BasicBlock::reverse_iterator& BBI)
+{
+  MemAccessInst *memI = cast<MemAccessInst>(*BBI);
+  BasicBlock* BB = memI->getParent();
+  Value* lastPtr = memI->getPointerOperand();
+  vector<Instruction*> newIvec;
+  
+  MemAccessInst::const_op_iterator OI = memI->idx_begin();
+  for (MemAccessInst::const_op_iterator OE = memI->idx_end(); OI != OE; ++OI)
+    {
+      if (OI+1 == OE)                     // skip the last operand
+        break;
+      
+      assert(isa<PointerType>(lastPtr->getType()));
+      vector<Value*> idxVec(1, *OI);
+
+      // The first index does not change the type of the pointer
+      // since all pointers are treated as potential arrays (i.e.,
+      // int *X is either a scalar X[0] or an array at X[i]).
+      // 
+      const Type* nextPtrType;
+      // if (OI == memI->idx_begin())
+      //   nextPtrType = lastPtr->getType();
+      // else
+      //   {
+             const Type* nextArrayType =  
+               MemAccessInst::getIndexedType(lastPtr->getType(), idxVec,
+                                             /*allowCompositeLeaf*/ true);
+             nextPtrType = PointerType::get(cast<SequentialType>(nextArrayType)
+                                            ->getElementType());
+      //   }
+      
+      Instruction* gepInst  = new GetElementPtrInst(lastPtr, idxVec, "aptr1");
+      Instruction* castInst = new CastInst(gepInst, nextPtrType, "aptr2");
+      lastPtr  = castInst;
+      
+      newIvec.push_back(gepInst);
+      newIvec.push_back(castInst);
+    }
+  
+  // Now create a new instruction to replace the original one
+  assert(lastPtr != memI->getPointerOperand() && "the above loop did not execute?");
+  assert(isa<PointerType>(lastPtr->getType()));
+  vector<Value*> idxVec(1, *OI);
+  const std::string newInstName = memI->hasName()? memI->getName()
+                                                 : string("oneDimRef");
+  Instruction* newInst = NULL;
+  
+  switch(memI->getOpcode())
+    {
+    case Instruction::Load:
+      newInst = new LoadInst(lastPtr, idxVec /*, newInstName */); break;
+    case Instruction::Store:
+      newInst = new StoreInst(memI->getOperand(0),
+                              lastPtr, idxVec /*, newInstName */); break;
+      break;
+    case Instruction::GetElementPtr:
+      newInst = new GetElementPtrInst(lastPtr, idxVec /*, newInstName */); break;
+    default:
+      assert(0 && "Unrecognized memory access instruction"); break;
+    }
+  
+  newIvec.push_back(newInst);
+  
+  // Replace all uses of the old instruction with the new
+  memI->replaceAllUsesWith(newInst);
+  
+  // Insert the instructions created in reverse order.  insert is destructive
+  // so we always have to use the new pointer returned by insert.
+  BasicBlock::iterator newI = BBI.base(); // gives ptr to instr. after memI
+  --newI;                                 // step back to memI
+  for (int i = newIvec.size()-1; i >= 0; i--)
+    newI = BB->getInstList().insert(newI, newIvec[i]);
+  
+  // Now delete the old instruction and return a pointer to the first new one
+  BB->getInstList().remove(memI);
+  delete memI;
+  
+  BasicBlock::reverse_iterator retI(newI); // reverse ptr to instr before newI
+  return --retI;                           // reverse pointer to newI
+}
+
+
+//---------------------------------------------------------------------------
+// Entry point for decomposing multi-dimensional array references
+//---------------------------------------------------------------------------
+
+static bool
+doDecomposeArrayRefs(Method *M)
+{
+  bool changed = false;
+  
+  for (Method::iterator BI = M->begin(), BE = M->end(); BI != BE; ++BI)
+    for (BasicBlock::reverse_iterator newI, II=(*BI)->rbegin();
+         II != (*BI)->rend(); II = ++newI)
+      {
+        newI = II;
+        if (MemAccessInst *memI = dyn_cast<MemAccessInst>(*II))
+          { // Check for a multi-dimensional array access
+            const PointerType* ptrType =
+              cast<PointerType>(memI->getPointerOperand()->getType()); 
+            if (isa<ArrayType>(ptrType->getElementType()) &&
+                memI->getNumOperands() > 1+ memI->getFirstIndexOperandNumber())
+              {
+                newI = decomposeArrayRef(II);
+                changed = true;
+              }
+          }
+      }
+  
+  return changed;
+}
+
+
+namespace {
+  struct DecomposeArrayRefsPass : public MethodPass {
+    virtual bool runOnMethod(Method *M) { return doDecomposeArrayRefs(M); }
+  };
+}
+
+Pass *createDecomposeArrayRefsPass() { return new DecomposeArrayRefsPass(); }