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//===-- PartialSpecialization.cpp - Specialize for common constants--------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass finds function arguments that are often a common constant and
// specializes a version of the called function for that constant.
//
// This pass simply does the cloning for functions it specializes. It depends
// on IPSCCP and DAE to clean up the results.
//
// The initial heuristic favors constant arguments that are used in control
// flow.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "partialspecialization"
#include "llvm/Transforms/IPO.h"
#include "llvm/Constant.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Support/CallSite.h"
#include "llvm/ADT/DenseSet.h"
#include <map>
using namespace llvm;
STATISTIC(numSpecialized, "Number of specialized functions created");
STATISTIC(numReplaced, "Number of callers replaced by specialization");
// Maximum number of arguments markable interested
static const int MaxInterests = 6;
// Call must be used at least occasionally
static const int CallsMin = 5;
// Must have 10% of calls having the same constant to specialize on
static const double ConstValPercent = .1;
namespace {
typedef SmallVector<int, MaxInterests> InterestingArgVector;
class PartSpec : public ModulePass {
void scanForInterest(Function&, InterestingArgVector&);
int scanDistribution(Function&, int, std::map<Constant*, int>&);
public :
static char ID; // Pass identification, replacement for typeid
PartSpec() : ModulePass(ID) {}
bool runOnModule(Module &M);
};
}
char PartSpec::ID = 0;
INITIALIZE_PASS(PartSpec, "partialspecialization",
"Partial Specialization", false, false);
// Specialize F by replacing the arguments (keys) in replacements with the
// constants (values). Replace all calls to F with those constants with
// a call to the specialized function. Returns the specialized function
static Function*
SpecializeFunction(Function* F,
ValueMap<const Value*, Value*>& replacements) {
// arg numbers of deleted arguments
DenseMap<unsigned, const Argument*> deleted;
for (ValueMap<const Value*, Value*>::iterator
repb = replacements.begin(), repe = replacements.end();
repb != repe; ++repb) {
Argument const *arg = cast<const Argument>(repb->first);
deleted[arg->getArgNo()] = arg;
}
Function* NF = CloneFunction(F, replacements);
NF->setLinkage(GlobalValue::InternalLinkage);
F->getParent()->getFunctionList().push_back(NF);
for (Value::use_iterator ii = F->use_begin(), ee = F->use_end();
ii != ee; ) {
Value::use_iterator i = ii;
++ii;
User *U = *i;
CallSite CS(U);
if (CS) {
if (CS.getCalledFunction() == F) {
SmallVector<Value*, 6> args;
// Assemble the non-specialized arguments for the updated callsite.
// In the process, make sure that the specialized arguments are
// constant and match the specialization. If that's not the case,
// this callsite needs to call the original or some other
// specialization; don't change it here.
CallSite::arg_iterator as = CS.arg_begin(), ae = CS.arg_end();
for (CallSite::arg_iterator ai = as; ai != ae; ++ai) {
DenseMap<unsigned, const Argument*>::iterator delit = deleted.find(
std::distance(as, ai));
if (delit == deleted.end())
args.push_back(cast<Value>(ai));
else {
Constant *ci = dyn_cast<Constant>(ai);
if (!(ci && ci == replacements[delit->second]))
goto next_use;
}
}
Value* NCall;
if (CallInst *CI = dyn_cast<CallInst>(U)) {
NCall = CallInst::Create(NF, args.begin(), args.end(),
CI->getName(), CI);
cast<CallInst>(NCall)->setTailCall(CI->isTailCall());
cast<CallInst>(NCall)->setCallingConv(CI->getCallingConv());
} else {
InvokeInst *II = cast<InvokeInst>(U);
NCall = InvokeInst::Create(NF, II->getNormalDest(),
II->getUnwindDest(),
args.begin(), args.end(),
II->getName(), II);
cast<InvokeInst>(NCall)->setCallingConv(II->getCallingConv());
}
CS.getInstruction()->replaceAllUsesWith(NCall);
CS.getInstruction()->eraseFromParent();
++numReplaced;
}
}
next_use:;
}
return NF;
}
bool PartSpec::runOnModule(Module &M) {
bool Changed = false;
for (Module::iterator I = M.begin(); I != M.end(); ++I) {
Function &F = *I;
if (F.isDeclaration() || F.mayBeOverridden()) continue;
InterestingArgVector interestingArgs;
scanForInterest(F, interestingArgs);
// Find the first interesting Argument that we can specialize on
// If there are multiple interesting Arguments, then those will be found
// when processing the cloned function.
bool breakOuter = false;
for (unsigned int x = 0; !breakOuter && x < interestingArgs.size(); ++x) {
std::map<Constant*, int> distribution;
int total = scanDistribution(F, interestingArgs[x], distribution);
if (total > CallsMin)
for (std::map<Constant*, int>::iterator ii = distribution.begin(),
ee = distribution.end(); ii != ee; ++ii)
if (total > ii->second && ii->first &&
ii->second > total * ConstValPercent) {
ValueMap<const Value*, Value*> m;
Function::arg_iterator arg = F.arg_begin();
for (int y = 0; y < interestingArgs[x]; ++y)
++arg;
m[&*arg] = ii->first;
SpecializeFunction(&F, m);
++numSpecialized;
breakOuter = true;
Changed = true;
}
}
}
return Changed;
}
/// scanForInterest - This function decides which arguments would be worth
/// specializing on.
void PartSpec::scanForInterest(Function& F, InterestingArgVector& args) {
for(Function::arg_iterator ii = F.arg_begin(), ee = F.arg_end();
ii != ee; ++ii) {
for(Value::use_iterator ui = ii->use_begin(), ue = ii->use_end();
ui != ue; ++ui) {
bool interesting = false;
User *U = *ui;
if (isa<CmpInst>(U)) interesting = true;
else if (isa<CallInst>(U))
interesting = ui->getOperand(0) == ii;
else if (isa<InvokeInst>(U))
interesting = ui->getOperand(0) == ii;
else if (isa<SwitchInst>(U)) interesting = true;
else if (isa<BranchInst>(U)) interesting = true;
if (interesting) {
args.push_back(std::distance(F.arg_begin(), ii));
break;
}
}
}
}
/// scanDistribution - Construct a histogram of constants for arg of F at arg.
int PartSpec::scanDistribution(Function& F, int arg,
std::map<Constant*, int>& dist) {
bool hasIndirect = false;
int total = 0;
for (Value::use_iterator ii = F.use_begin(), ee = F.use_end();
ii != ee; ++ii) {
User *U = *ii;
CallSite CS(U);
if (CS && CS.getCalledFunction() == &F) {
++dist[dyn_cast<Constant>(CS.getArgument(arg))];
++total;
} else
hasIndirect = true;
}
// Preserve the original address taken function even if all other uses
// will be specialized.
if (hasIndirect) ++total;
return total;
}
ModulePass* llvm::createPartialSpecializationPass() { return new PartSpec(); }
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