lib/Transforms/IPO/PartialSpecialization.cpp

   1 //===-- PartialSpecialization.cpp - Specialize for common constants--------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This pass finds function arguments that are often a common constant and
  11 // specializes a version of the called function for that constant.
  12 //
  13 // This pass simply does the cloning for functions it specializes.  It depends
  14 // on IPSCCP and DAE to clean up the results.
  15 //
  16 // The initial heuristic favors constant arguments that are used in control
  17 // flow.
  18 //
  19 //===----------------------------------------------------------------------===//
  20
  21 #define DEBUG_TYPE "partialspecialization"
  22 #include "llvm/Transforms/IPO.h"
  23 #include "llvm/Constant.h"
  24 #include "llvm/Instructions.h"
  25 #include "llvm/Module.h"
  26 #include "llvm/Pass.h"
  27 #include "llvm/ADT/Statistic.h"
  28 #include "llvm/Transforms/Utils/Cloning.h"
  29 #include "llvm/Support/CallSite.h"
  30 #include "llvm/ADT/DenseSet.h"
  31 #include <map>
  32 using namespace llvm;
  33
  34 STATISTIC(numSpecialized, "Number of specialized functions created");
  35
  36 // Call must be used at least occasionally
  37 static const int CallsMin = 5;
  38
  39 // Must have 10% of calls having the same constant to specialize on
  40 static const double ConstValPercent = .1;
  41
  42 namespace {
  43   class PartSpec : public ModulePass {
  44     void scanForInterest(Function&, SmallVector<int, 6>&);
  45     int scanDistribution(Function&, int, std::map<Constant*, int>&);
  46   public :
  47     static char ID; // Pass identification, replacement for typeid
  48     PartSpec() : ModulePass(&ID) {}
  49     bool runOnModule(Module &M);
  50   };
  51 }
  52
  53 char PartSpec::ID = 0;
  54 static RegisterPass<PartSpec>
  55 X("partialspecialization", "Partial Specialization");
  56
  57 // Specialize F by replacing the arguments (keys) in replacements with the
  58 // constants (values).  Replace all calls to F with those constants with
  59 // a call to the specialized function.  Returns the specialized function
  60 static Function*
  61 SpecializeFunction(Function* F,
  62                    DenseMap<const Value*, Value*>& replacements) {
  63   // arg numbers of deleted arguments
  64   DenseSet<unsigned> deleted;
  65   for (DenseMap<const Value*, Value*>::iterator
  66          repb = replacements.begin(), repe = replacements.end();
  67        repb != repe; ++repb)
  68     deleted.insert(cast<Argument>(repb->first)->getArgNo());
  69
  70   Function* NF = CloneFunction(F, replacements);
  71   NF->setLinkage(GlobalValue::InternalLinkage);
  72   F->getParent()->getFunctionList().push_back(NF);
  73
  74   for (Value::use_iterator ii = F->use_begin(), ee = F->use_end();
  75        ii != ee; ) {
  76     Value::use_iterator i = ii;
  77     ++ii;
  78     if (isa<CallInst>(i) || isa<InvokeInst>(i)) {
  79       CallSite CS(cast<Instruction>(i));
  80       if (CS.getCalledFunction() == F) {
  81
  82         SmallVector<Value*, 6> args;
  83         for (unsigned x = 0; x < CS.arg_size(); ++x)
  84           if (!deleted.count(x))
  85             args.push_back(CS.getArgument(x));
  86         Value* NCall;
  87         if (CallInst *CI = dyn_cast<CallInst>(i)) {
  88           NCall = CallInst::Create(NF, args.begin(), args.end(),
  89                                    CI->getName(), CI);
  90           cast<CallInst>(NCall)->setTailCall(CI->isTailCall());
  91           cast<CallInst>(NCall)->setCallingConv(CI->getCallingConv());
  92         } else {
  93           InvokeInst *II = cast<InvokeInst>(i);
  94           NCall = InvokeInst::Create(NF, II->getNormalDest(),
  95                                      II->getUnwindDest(),
  96                                      args.begin(), args.end(),
  97                                      II->getName(), II);
  98           cast<InvokeInst>(NCall)->setCallingConv(II->getCallingConv());
  99         }
 100         CS.getInstruction()->replaceAllUsesWith(NCall);
 101         CS.getInstruction()->eraseFromParent();
 102       }
 103     }
 104   }
 105   return NF;
 106 }
 107
 108
 109 bool PartSpec::runOnModule(Module &M) {
 110   bool Changed = false;
 111   for (Module::iterator I = M.begin(); I != M.end(); ++I) {
 112     Function &F = *I;
 113     if (F.isDeclaration() || F.mayBeOverridden()) continue;
 114     SmallVector<int, 6> interestingArgs;
 115     scanForInterest(F, interestingArgs);
 116
 117     // Find the first interesting Argument that we can specialize on
 118     // If there are multiple interesting Arguments, then those will be found
 119     // when processing the cloned function.
 120     bool breakOuter = false;
 121     for (unsigned int x = 0; !breakOuter && x < interestingArgs.size(); ++x) {
 122       std::map<Constant*, int> distribution;
 123       int total = scanDistribution(F, interestingArgs[x], distribution);
 124       if (total > CallsMin)
 125         for (std::map<Constant*, int>::iterator ii = distribution.begin(),
 126                ee = distribution.end(); ii != ee; ++ii)
 127           if (total > ii->second && ii->first &&
 128                ii->second > total * ConstValPercent) {
 129             DenseMap<const Value*, Value*> m;
 130             Function::arg_iterator arg = F.arg_begin();
 131             for (int y = 0; y < interestingArgs[x]; ++y)
 132               ++arg;
 133             m[&*arg] = ii->first;
 134             SpecializeFunction(&F, m);
 135             ++numSpecialized;
 136             breakOuter = true;
 137             Changed = true;
 138           }
 139     }
 140   }
 141   return Changed;
 142 }
 143
 144 /// scanForInterest - This function decides which arguments would be worth
 145 /// specializing on.
 146 void PartSpec::scanForInterest(Function& F, SmallVector<int, 6>& args) {
 147   for(Function::arg_iterator ii = F.arg_begin(), ee = F.arg_end();
 148       ii != ee; ++ii) {
 149     for(Value::use_iterator ui = ii->use_begin(), ue = ii->use_end();
 150         ui != ue; ++ui) {
 151
 152       bool interesting = false;
 153
 154       if (isa<CmpInst>(ui)) interesting = true;
 155       else if (isa<CallInst>(ui))
 156         interesting = ui->getOperand(0) == ii;
 157       else if (isa<InvokeInst>(ui))
 158         interesting = ui->getOperand(0) == ii;
 159       else if (isa<SwitchInst>(ui)) interesting = true;
 160       else if (isa<BranchInst>(ui)) interesting = true;
 161
 162       if (interesting) {
 163         args.push_back(std::distance(F.arg_begin(), ii));
 164         break;
 165       }
 166     }
 167   }
 168 }
 169
 170 /// scanDistribution - Construct a histogram of constants for arg of F at arg.
 171 int PartSpec::scanDistribution(Function& F, int arg,
 172                                std::map<Constant*, int>& dist) {
 173   bool hasIndirect = false;
 174   int total = 0;
 175   for(Value::use_iterator ii = F.use_begin(), ee = F.use_end();
 176       ii != ee; ++ii)
 177     if ((isa<CallInst>(ii) || isa<InvokeInst>(ii))
 178         && ii->getOperand(0) == &F) {
 179       ++dist[dyn_cast<Constant>(ii->getOperand(arg + 1))];
 180       ++total;
 181     } else
 182       hasIndirect = true;
 183
 184   // Preserve the original address taken function even if all other uses
 185   // will be specialized.
 186   if (hasIndirect) ++total;
 187   return total;
 188 }
 189
 190 ModulePass* llvm::createPartialSpecializationPass() { return new PartSpec(); }