379 lines
14 KiB
C++
379 lines
14 KiB
C++
//===- ScalarReplAggregates.cpp - Scalar Replacement of Aggregates --------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file was developed by the LLVM research group and is distributed under
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// the University of Illinois Open Source License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This transformation implements the well known scalar replacement of
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// aggregates transformation. This xform breaks up alloca instructions of
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// aggregate type (structure or array) into individual alloca instructions for
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// each member (if possible). Then, if possible, it transforms the individual
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// alloca instructions into nice clean scalar SSA form.
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//
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// This combines a simple SRoA algorithm with the Mem2Reg algorithm because
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// often interact, especially for C++ programs. As such, iterating between
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// SRoA, then Mem2Reg until we run out of things to promote works well.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Constants.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Function.h"
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#include "llvm/Pass.h"
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#include "llvm/Instructions.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Support/GetElementPtrTypeIterator.h"
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#include "llvm/Target/TargetData.h"
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#include "llvm/Transforms/Utils/PromoteMemToReg.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/ADT/StringExtras.h"
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using namespace llvm;
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namespace {
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Statistic<> NumReplaced("scalarrepl", "Number of allocas broken up");
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Statistic<> NumPromoted("scalarrepl", "Number of allocas promoted");
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struct SROA : public FunctionPass {
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bool runOnFunction(Function &F);
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bool performScalarRepl(Function &F);
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bool performPromotion(Function &F);
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// getAnalysisUsage - This pass does not require any passes, but we know it
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// will not alter the CFG, so say so.
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<DominatorTree>();
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AU.addRequired<DominanceFrontier>();
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AU.addRequired<TargetData>();
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AU.setPreservesCFG();
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}
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private:
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int isSafeElementUse(Value *Ptr);
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int isSafeUseOfAllocation(Instruction *User);
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int isSafeAllocaToScalarRepl(AllocationInst *AI);
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void CanonicalizeAllocaUsers(AllocationInst *AI);
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AllocaInst *AddNewAlloca(Function &F, const Type *Ty, AllocationInst *Base);
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};
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RegisterOpt<SROA> X("scalarrepl", "Scalar Replacement of Aggregates");
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}
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// Public interface to the ScalarReplAggregates pass
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FunctionPass *llvm::createScalarReplAggregatesPass() { return new SROA(); }
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bool SROA::runOnFunction(Function &F) {
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bool Changed = performPromotion(F);
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while (1) {
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bool LocalChange = performScalarRepl(F);
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if (!LocalChange) break; // No need to repromote if no scalarrepl
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Changed = true;
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LocalChange = performPromotion(F);
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if (!LocalChange) break; // No need to re-scalarrepl if no promotion
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}
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return Changed;
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}
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bool SROA::performPromotion(Function &F) {
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std::vector<AllocaInst*> Allocas;
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const TargetData &TD = getAnalysis<TargetData>();
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DominatorTree &DT = getAnalysis<DominatorTree>();
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DominanceFrontier &DF = getAnalysis<DominanceFrontier>();
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BasicBlock &BB = F.getEntryBlock(); // Get the entry node for the function
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bool Changed = false;
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while (1) {
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Allocas.clear();
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// Find allocas that are safe to promote, by looking at all instructions in
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// the entry node
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for (BasicBlock::iterator I = BB.begin(), E = --BB.end(); I != E; ++I)
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if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) // Is it an alloca?
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if (isAllocaPromotable(AI, TD))
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Allocas.push_back(AI);
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if (Allocas.empty()) break;
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PromoteMemToReg(Allocas, DT, DF, TD);
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NumPromoted += Allocas.size();
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Changed = true;
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}
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return Changed;
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}
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// performScalarRepl - This algorithm is a simple worklist driven algorithm,
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// which runs on all of the malloc/alloca instructions in the function, removing
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// them if they are only used by getelementptr instructions.
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//
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bool SROA::performScalarRepl(Function &F) {
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std::vector<AllocationInst*> WorkList;
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// Scan the entry basic block, adding any alloca's and mallocs to the worklist
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BasicBlock &BB = F.getEntryBlock();
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for (BasicBlock::iterator I = BB.begin(), E = BB.end(); I != E; ++I)
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if (AllocationInst *A = dyn_cast<AllocationInst>(I))
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WorkList.push_back(A);
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// Process the worklist
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bool Changed = false;
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while (!WorkList.empty()) {
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AllocationInst *AI = WorkList.back();
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WorkList.pop_back();
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// We cannot transform the allocation instruction if it is an array
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// allocation (allocations OF arrays are ok though), and an allocation of a
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// scalar value cannot be decomposed at all.
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//
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if (AI->isArrayAllocation() ||
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(!isa<StructType>(AI->getAllocatedType()) &&
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!isa<ArrayType>(AI->getAllocatedType()))) continue;
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// Check that all of the users of the allocation are capable of being
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// transformed.
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switch (isSafeAllocaToScalarRepl(AI)) {
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default: assert(0 && "Unexpected value!");
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case 0: // Not safe to scalar replace.
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continue;
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case 1: // Safe, but requires cleanup/canonicalizations first
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CanonicalizeAllocaUsers(AI);
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case 3: // Safe to scalar replace.
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break;
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}
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DEBUG(std::cerr << "Found inst to xform: " << *AI);
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Changed = true;
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std::vector<AllocaInst*> ElementAllocas;
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if (const StructType *ST = dyn_cast<StructType>(AI->getAllocatedType())) {
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ElementAllocas.reserve(ST->getNumContainedTypes());
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for (unsigned i = 0, e = ST->getNumContainedTypes(); i != e; ++i) {
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AllocaInst *NA = new AllocaInst(ST->getContainedType(i), 0,
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AI->getName() + "." + utostr(i), AI);
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ElementAllocas.push_back(NA);
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WorkList.push_back(NA); // Add to worklist for recursive processing
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}
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} else {
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const ArrayType *AT = cast<ArrayType>(AI->getAllocatedType());
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ElementAllocas.reserve(AT->getNumElements());
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const Type *ElTy = AT->getElementType();
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for (unsigned i = 0, e = AT->getNumElements(); i != e; ++i) {
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AllocaInst *NA = new AllocaInst(ElTy, 0,
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AI->getName() + "." + utostr(i), AI);
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ElementAllocas.push_back(NA);
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WorkList.push_back(NA); // Add to worklist for recursive processing
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}
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}
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// Now that we have created the alloca instructions that we want to use,
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// expand the getelementptr instructions to use them.
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//
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while (!AI->use_empty()) {
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Instruction *User = cast<Instruction>(AI->use_back());
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GetElementPtrInst *GEPI = cast<GetElementPtrInst>(User);
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// We now know that the GEP is of the form: GEP <ptr>, 0, <cst>
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uint64_t Idx = cast<ConstantInt>(GEPI->getOperand(2))->getRawValue();
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assert(Idx < ElementAllocas.size() && "Index out of range?");
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AllocaInst *AllocaToUse = ElementAllocas[Idx];
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Value *RepValue;
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if (GEPI->getNumOperands() == 3) {
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// Do not insert a new getelementptr instruction with zero indices, only
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// to have it optimized out later.
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RepValue = AllocaToUse;
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} else {
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// We are indexing deeply into the structure, so we still need a
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// getelement ptr instruction to finish the indexing. This may be
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// expanded itself once the worklist is rerun.
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//
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std::string OldName = GEPI->getName(); // Steal the old name.
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std::vector<Value*> NewArgs;
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NewArgs.push_back(Constant::getNullValue(Type::IntTy));
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NewArgs.insert(NewArgs.end(), GEPI->op_begin()+3, GEPI->op_end());
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GEPI->setName("");
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RepValue = new GetElementPtrInst(AllocaToUse, NewArgs, OldName, GEPI);
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}
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// Move all of the users over to the new GEP.
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GEPI->replaceAllUsesWith(RepValue);
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// Delete the old GEP
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GEPI->eraseFromParent();
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}
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// Finally, delete the Alloca instruction
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AI->getParent()->getInstList().erase(AI);
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NumReplaced++;
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}
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return Changed;
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}
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/// isSafeElementUse - Check to see if this use is an allowed use for a
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/// getelementptr instruction of an array aggregate allocation.
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///
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int SROA::isSafeElementUse(Value *Ptr) {
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for (Value::use_iterator I = Ptr->use_begin(), E = Ptr->use_end();
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I != E; ++I) {
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Instruction *User = cast<Instruction>(*I);
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switch (User->getOpcode()) {
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case Instruction::Load: break;
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case Instruction::Store:
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// Store is ok if storing INTO the pointer, not storing the pointer
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if (User->getOperand(0) == Ptr) return 0;
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break;
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case Instruction::GetElementPtr: {
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GetElementPtrInst *GEP = cast<GetElementPtrInst>(User);
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if (GEP->getNumOperands() > 1) {
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if (!isa<Constant>(GEP->getOperand(1)) ||
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!cast<Constant>(GEP->getOperand(1))->isNullValue())
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return 0; // Using pointer arithmetic to navigate the array...
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}
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if (!isSafeElementUse(GEP)) return 0;
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break;
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}
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default:
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DEBUG(std::cerr << " Transformation preventing inst: " << *User);
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return 0;
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}
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}
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return 3; // All users look ok :)
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}
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/// AllUsersAreLoads - Return true if all users of this value are loads.
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static bool AllUsersAreLoads(Value *Ptr) {
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for (Value::use_iterator I = Ptr->use_begin(), E = Ptr->use_end();
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I != E; ++I)
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if (cast<Instruction>(*I)->getOpcode() != Instruction::Load)
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return false;
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return true;
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}
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/// isSafeUseOfAllocation - Check to see if this user is an allowed use for an
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/// aggregate allocation.
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///
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int SROA::isSafeUseOfAllocation(Instruction *User) {
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if (!isa<GetElementPtrInst>(User)) return 0;
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GetElementPtrInst *GEPI = cast<GetElementPtrInst>(User);
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gep_type_iterator I = gep_type_begin(GEPI), E = gep_type_end(GEPI);
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// The GEP is safe to transform if it is of the form GEP <ptr>, 0, <cst>
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if (I == E ||
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I.getOperand() != Constant::getNullValue(I.getOperand()->getType()))
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return 0;
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++I;
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if (I == E) return 0; // ran out of GEP indices??
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// If this is a use of an array allocation, do a bit more checking for sanity.
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if (const ArrayType *AT = dyn_cast<ArrayType>(*I)) {
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uint64_t NumElements = AT->getNumElements();
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if (ConstantInt *CI = dyn_cast<ConstantInt>(I.getOperand())) {
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// Check to make sure that index falls within the array. If not,
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// something funny is going on, so we won't do the optimization.
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//
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if (cast<ConstantInt>(GEPI->getOperand(2))->getRawValue() >= NumElements)
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return 0;
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} else {
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// If this is an array index and the index is not constant, we cannot
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// promote... that is unless the array has exactly one or two elements in
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// it, in which case we CAN promote it, but we have to canonicalize this
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// out if this is the only problem.
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if (NumElements == 1 || NumElements == 2)
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return AllUsersAreLoads(GEPI) ? 1 : 0; // Canonicalization required!
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return 0;
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}
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}
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// If there are any non-simple uses of this getelementptr, make sure to reject
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// them.
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return isSafeElementUse(GEPI);
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}
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/// isSafeStructAllocaToScalarRepl - Check to see if the specified allocation of
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/// an aggregate can be broken down into elements. Return 0 if not, 3 if safe,
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/// or 1 if safe after canonicalization has been performed.
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///
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int SROA::isSafeAllocaToScalarRepl(AllocationInst *AI) {
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// Loop over the use list of the alloca. We can only transform it if all of
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// the users are safe to transform.
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//
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int isSafe = 3;
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for (Value::use_iterator I = AI->use_begin(), E = AI->use_end();
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I != E; ++I) {
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isSafe &= isSafeUseOfAllocation(cast<Instruction>(*I));
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if (isSafe == 0) {
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DEBUG(std::cerr << "Cannot transform: " << *AI << " due to user: "
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<< **I);
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return 0;
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}
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}
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// If we require cleanup, isSafe is now 1, otherwise it is 3.
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return isSafe;
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}
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/// CanonicalizeAllocaUsers - If SROA reported that it can promote the specified
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/// allocation, but only if cleaned up, perform the cleanups required.
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void SROA::CanonicalizeAllocaUsers(AllocationInst *AI) {
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// At this point, we know that the end result will be SROA'd and promoted, so
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// we can insert ugly code if required so long as sroa+mem2reg will clean it
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// up.
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for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end();
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UI != E; ) {
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GetElementPtrInst *GEPI = cast<GetElementPtrInst>(*UI++);
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gep_type_iterator I = gep_type_begin(GEPI);
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++I;
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if (const ArrayType *AT = dyn_cast<ArrayType>(*I)) {
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uint64_t NumElements = AT->getNumElements();
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if (!isa<ConstantInt>(I.getOperand())) {
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if (NumElements == 1) {
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GEPI->setOperand(2, Constant::getNullValue(Type::IntTy));
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} else {
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assert(NumElements == 2 && "Unhandled case!");
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// All users of the GEP must be loads. At each use of the GEP, insert
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// two loads of the appropriate indexed GEP and select between them.
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Value *IsOne = BinaryOperator::createSetNE(I.getOperand(),
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Constant::getNullValue(I.getOperand()->getType()),
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"isone", GEPI);
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// Insert the new GEP instructions, which are properly indexed.
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std::vector<Value*> Indices(GEPI->op_begin()+1, GEPI->op_end());
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Indices[1] = Constant::getNullValue(Type::IntTy);
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Value *ZeroIdx = new GetElementPtrInst(GEPI->getOperand(0), Indices,
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GEPI->getName()+".0", GEPI);
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Indices[1] = ConstantInt::get(Type::IntTy, 1);
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Value *OneIdx = new GetElementPtrInst(GEPI->getOperand(0), Indices,
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GEPI->getName()+".1", GEPI);
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// Replace all loads of the variable index GEP with loads from both
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// indexes and a select.
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while (!GEPI->use_empty()) {
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LoadInst *LI = cast<LoadInst>(GEPI->use_back());
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Value *Zero = new LoadInst(ZeroIdx, LI->getName()+".0", LI);
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Value *One = new LoadInst(OneIdx , LI->getName()+".1", LI);
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Value *R = new SelectInst(IsOne, One, Zero, LI->getName(), LI);
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LI->replaceAllUsesWith(R);
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LI->eraseFromParent();
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}
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GEPI->eraseFromParent();
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}
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}
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}
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}
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}
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