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//===-- PartialSpecialization.cpp - Specialize for common constants--------===//
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// The LLVM Compiler Infrastructure
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//===----------------------------------------------------------------------===//
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// This pass finds function arguments that are often a common constant and
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// specializes a version of the called function for that constant.
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// This pass simply does the cloning for functions it specializes. It depends
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// on IPSCCP and DAE to clean up the results.
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// The initial heuristic favors constant arguments that are used in control
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "partialspecialization"
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#include "llvm/Transforms/IPO.h"
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#include "llvm/Constant.h"
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#include "llvm/Instructions.h"
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#include "llvm/Module.h"
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#include "llvm/Pass.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Transforms/Utils/Cloning.h"
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#include "llvm/Support/CallSite.h"
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#include "llvm/ADT/DenseSet.h"
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STATISTIC(numSpecialized, "Number of specialized functions created");
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// Call must be used at least occasionally
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static const int CallsMin = 5;
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// Must have 10% of calls having the same constant to specialize on
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static const double ConstValPercent = .1;
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class PartSpec : public ModulePass {
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void scanForInterest(Function&, SmallVector<int, 6>&);
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int scanDistribution(Function&, int, std::map<Constant*, int>&);
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static char ID; // Pass identification, replacement for typeid
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PartSpec() : ModulePass(&ID) {}
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bool runOnModule(Module &M);
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char PartSpec::ID = 0;
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static RegisterPass<PartSpec>
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X("partialspecialization", "Partial Specialization");
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// Specialize F by replacing the arguments (keys) in replacements with the
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// constants (values). Replace all calls to F with those constants with
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// a call to the specialized function. Returns the specialized function
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SpecializeFunction(Function* F,
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DenseMap<const Value*, Value*>& replacements) {
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// arg numbers of deleted arguments
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DenseSet<unsigned> deleted;
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for (DenseMap<const Value*, Value*>::iterator
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repb = replacements.begin(), repe = replacements.end();
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deleted.insert(cast<Argument>(repb->first)->getArgNo());
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Function* NF = CloneFunction(F, replacements);
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NF->setLinkage(GlobalValue::InternalLinkage);
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F->getParent()->getFunctionList().push_back(NF);
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for (Value::use_iterator ii = F->use_begin(), ee = F->use_end();
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Value::use_iterator i = ii;
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if (isa<CallInst>(i) || isa<InvokeInst>(i)) {
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CallSite CS(cast<Instruction>(i));
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if (CS.getCalledFunction() == F) {
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SmallVector<Value*, 6> args;
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for (unsigned x = 0; x < CS.arg_size(); ++x)
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if (!deleted.count(x))
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args.push_back(CS.getArgument(x));
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if (CallInst *CI = dyn_cast<CallInst>(i)) {
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NCall = CallInst::Create(NF, args.begin(), args.end(),
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cast<CallInst>(NCall)->setTailCall(CI->isTailCall());
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cast<CallInst>(NCall)->setCallingConv(CI->getCallingConv());
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InvokeInst *II = cast<InvokeInst>(i);
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NCall = InvokeInst::Create(NF, II->getNormalDest(),
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args.begin(), args.end(),
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cast<InvokeInst>(NCall)->setCallingConv(II->getCallingConv());
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CS.getInstruction()->replaceAllUsesWith(NCall);
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CS.getInstruction()->eraseFromParent();
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bool PartSpec::runOnModule(Module &M) {
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bool Changed = false;
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for (Module::iterator I = M.begin(); I != M.end(); ++I) {
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if (F.isDeclaration() || F.mayBeOverridden()) continue;
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SmallVector<int, 6> interestingArgs;
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scanForInterest(F, interestingArgs);
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// Find the first interesting Argument that we can specialize on
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// If there are multiple interesting Arguments, then those will be found
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// when processing the cloned function.
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bool breakOuter = false;
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for (unsigned int x = 0; !breakOuter && x < interestingArgs.size(); ++x) {
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std::map<Constant*, int> distribution;
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int total = scanDistribution(F, interestingArgs[x], distribution);
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if (total > CallsMin)
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for (std::map<Constant*, int>::iterator ii = distribution.begin(),
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ee = distribution.end(); ii != ee; ++ii)
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if (total > ii->second && ii->first &&
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ii->second > total * ConstValPercent) {
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DenseMap<const Value*, Value*> m;
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Function::arg_iterator arg = F.arg_begin();
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for (int y = 0; y < interestingArgs[x]; ++y)
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m[&*arg] = ii->first;
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SpecializeFunction(&F, m);
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/// scanForInterest - This function decides which arguments would be worth
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void PartSpec::scanForInterest(Function& F, SmallVector<int, 6>& args) {
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for(Function::arg_iterator ii = F.arg_begin(), ee = F.arg_end();
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for(Value::use_iterator ui = ii->use_begin(), ue = ii->use_end();
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bool interesting = false;
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if (isa<CmpInst>(ui)) interesting = true;
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else if (isa<CallInst>(ui))
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interesting = ui->getOperand(0) == ii;
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else if (isa<InvokeInst>(ui))
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interesting = ui->getOperand(0) == ii;
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else if (isa<SwitchInst>(ui)) interesting = true;
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else if (isa<BranchInst>(ui)) interesting = true;
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args.push_back(std::distance(F.arg_begin(), ii));
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/// scanDistribution - Construct a histogram of constants for arg of F at arg.
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int PartSpec::scanDistribution(Function& F, int arg,
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std::map<Constant*, int>& dist) {
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bool hasIndirect = false;
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for(Value::use_iterator ii = F.use_begin(), ee = F.use_end();
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if ((isa<CallInst>(ii) || isa<InvokeInst>(ii))
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&& ii->getOperand(0) == &F) {
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++dist[dyn_cast<Constant>(ii->getOperand(arg + 1))];
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// Preserve the original address taken function even if all other uses
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// will be specialized.
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if (hasIndirect) ++total;
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ModulePass* llvm::createPartialSpecializationPass() { return new PartSpec(); }