Implementation of the simple "scalar replacement of aggregates" transformation

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@6346 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/ScalarReplAggregates.cpp

@@ -0,0 +1,164 @@
+//===- ScalarReplAggregates.cpp - Scalar Replacement of Aggregates --------===//
+//
+// This transformation implements the well known scalar replacement of
+// aggregates transformation.  This xform breaks up alloca instructions of
+// aggregate type (structure or array) into individual alloca instructions for
+// each member (if possible).
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Function.h"
+#include "llvm/Pass.h"
+#include "llvm/iMemory.h"
+#include "llvm/DerivedTypes.h"
+#include "llvm/Constants.h"
+#include "Support/StringExtras.h"
+#include "Support/Statistic.h"
+
+namespace {
+  Statistic<> NumReplaced("scalarrepl", "Number of alloca's broken up");
+
+  struct SROA : public FunctionPass {
+    bool runOnFunction(Function &F);
+
+  private:
+    AllocaInst *AddNewAlloca(Function &F, const Type *Ty, AllocationInst *Base);
+  };
+
+  RegisterOpt<SROA> X("scalarrepl", "Scalar Replacement of Aggregates");
+}
+
+Pass *createScalarReplAggregatesPass() { return new SROA(); }
+
+
+// runOnFunction - This algorithm is a simple worklist driven algorithm, which
+// runs on all of the malloc/alloca instructions in the function, removing them
+// if they are only used by getelementptr instructions.
+//
+bool SROA::runOnFunction(Function &F) {
+  std::vector<AllocationInst*> WorkList;
+
+  // Scan the entry basic block, adding any alloca's and mallocs to the worklist
+  BasicBlock &BB = F.getEntryNode();
+  for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I)
+    if (AllocationInst *A = dyn_cast<AllocationInst>(I))
+      WorkList.push_back(A);
+
+  // Process the worklist
+  bool Changed = false;
+  while (!WorkList.empty()) {
+    AllocationInst *AI = WorkList.back();
+    WorkList.pop_back();
+
+    // We cannot transform the allocation instruction if it is an array
+    // allocation, and an allocation of a scalar value cannot be decomposed
+    if (AI->isArrayAllocation() ||
+        (!isa<StructType>(AI->getAllocatedType()) /*&&
+                                                    !isa<ArrayType>(AI->getAllocatedType())*/
+         )) continue;
+    
+    // Loop over the use list of the alloca.  We can only transform it if there
+    // are only getelementptr instructions (with a zero first index) and free
+    // instructions.
+    //
+    bool CannotTransform = false;
+    for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
+         I != E; ++I) {
+      Instruction *User = cast<Instruction>(*I);
+      if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) {
+        // The GEP is safe to transform if it is of the form GEP <ptr>, 0, <cst>
+        if (GEPI->getNumOperands() <= 2 ||
+            GEPI->getOperand(1) != Constant::getNullValue(Type::LongTy) ||
+            !isa<Constant>(GEPI->getOperand(2)) ||
+            isa<ConstantExpr>(GEPI->getOperand(2))) {
+          DEBUG(std::cerr << "Cannot transform: " << *AI << "  due to user: "
+                          << User);
+          CannotTransform = true;
+          break;
+        }
+      } else {
+        DEBUG(std::cerr << "Cannot transform: " << *AI << "  due to user: "
+                        << User);
+        CannotTransform = true;
+        break;
+      }
+    }
+
+    if (CannotTransform) continue;
+
+    DEBUG(std::cerr << "Found inst to xform: " << *AI);
+    Changed = true;
+    
+    std::vector<AllocaInst*> ElementAllocas;
+    if (const StructType *ST = dyn_cast<StructType>(AI->getAllocatedType())) {
+      ElementAllocas.reserve(ST->getNumContainedTypes());
+      for (unsigned i = 0, e = ST->getNumContainedTypes(); i != e; ++i) {
+        AllocaInst *NA = new AllocaInst(ST->getContainedType(i), 0,
+                                        AI->getName() + "." + utostr(i), AI);
+        ElementAllocas.push_back(NA);
+        WorkList.push_back(NA);  // Add to worklist for recursive processing
+      }
+    } else {
+      const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType());
+      ElementAllocas.reserve(AT->getNumElements());
+      const Type *ElTy = AT->getElementType();
+      for (unsigned i = 0, e = AT->getNumElements(); i != e; ++i) {
+        AllocaInst *NA = new AllocaInst(ElTy, 0,
+                                        AI->getName() + "." + utostr(i), AI);
+        ElementAllocas.push_back(NA);
+        WorkList.push_back(NA);  // Add to worklist for recursive processing
+      }
+    }
+    
+    // Now that we have created the alloca instructions that we want to use,
+    // expand the getelementptr instructions to use them.
+    //
+    for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
+         I != E; ++I) {
+      Instruction *User = cast<Instruction>(*I);
+      if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(User)) {
+        // We now know that the GEP is of the form: GEP <ptr>, 0, <cst>
+        uint64_t Idx;
+        if (ConstantSInt *CSI = dyn_cast<ConstantSInt>(GEPI->getOperand(2)))
+          Idx = CSI->getValue();
+        else
+          Idx = cast<ConstantUInt>(GEPI->getOperand(2))->getValue();
+        
+        assert(Idx < ElementAllocas.size() && "Index out of range?");
+        AllocaInst *AllocaToUse = ElementAllocas[Idx];
+
+        Value *RepValue;
+        if (GEPI->getNumOperands() == 3) {
+          // Do not insert a new getelementptr instruction with zero indices,
+          // only to have it optimized out later.
+          RepValue = AllocaToUse;
+        } else {
+          // We are indexing deeply into the structure, so we still need a
+          // getelement ptr instruction to finish the indexing.  This may be
+          // expanded itself once the worklist is rerun.
+          //
+          std::string OldName = GEPI->getName();  // Steal the old name...
+          GEPI->setName("");
+          RepValue =
+            new GetElementPtrInst(AllocaToUse, 
+                                  std::vector<Value*>(GEPI->op_begin()+3, 
+                                                      GEPI->op_end()),
+                                  OldName, GEPI);
+        }
+
+        // Move all of the users over to the new GEP.
+        GEPI->replaceAllUsesWith(RepValue);
+        // Delete the old GEP
+        GEPI->getParent()->getInstList().erase(GEPI);
+      } else {
+        assert(0 && "Unexpected instruction type!");
+      }
+    }
+
+    // Finally, delete the Alloca instruction
+    AI->getParent()->getInstList().erase(AI);
+  }
+
+  return Changed;
+}