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
diff --git a/lib/Transforms/Scalar/ScalarReplAggregates.cpp b/lib/Transforms/Scalar/ScalarReplAggregates.cpp
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+++ b/lib/Transforms/Scalar/ScalarReplAggregates.cpp
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+//===- 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;
+}