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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/Compiler.h"
#include <map>
using namespace llvm;
STATISTIC(numSpecialized, "Number of specialized functions created");
// 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 {
class VISIBILITY_HIDDEN PartSpec : public ModulePass {
void scanForInterest(Function&, SmallVector<int, 6>&);
void replaceUsersFor(Function&, int, Constant*, Function*);
int scanDistribution(Function&, int, std::map<Constant*, int>&);
public :
static char ID; // Pass identification, replacement for typeid
PartSpec() : ModulePass((intptr_t)&ID) {}
bool runOnModule(Module &M);
};
}
char PartSpec::ID = 0;
static RegisterPass<PartSpec>
X("partialspecialization", "Partial Specialization");
bool PartSpec::runOnModule(Module &M) {
bool Changed = false;
for (Module::iterator I = M.begin(); I != M.end(); ++I) {
Function &F = *I;
if (F.isDeclaration()) continue;
SmallVector<int, 6> 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) {
Function* NF = CloneFunction(&F);
NF->setLinkage(GlobalValue::InternalLinkage);
M.getFunctionList().push_back(NF);
replaceUsersFor(F, interestingArgs[x], ii->first, NF);
breakOuter = true;
Changed = true;
}
}
}
return Changed;
}
/// scanForInterest - This function decides which arguments would be worth
/// specializing on.
void PartSpec::scanForInterest(Function& F, SmallVector<int, 6>& 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) {
// As an initial proxy for control flow, specialize on arguments
// that are used in comparisons.
if (isa<CmpInst>(ui)) {
args.push_back(std::distance(F.arg_begin(), ii));
break;
}
}
}
}
/// replaceUsersFor - Replace direct calls to F with NF if the arg argnum is
/// the constant val
void PartSpec::replaceUsersFor(Function& F , int argnum, Constant* val,
Function* NF) {
++numSpecialized;
for(Value::use_iterator ii = F.use_begin(), ee = F.use_end();
ii != ee; ++ii)
if (CallInst* CI = dyn_cast<CallInst>(ii))
if (CI->getOperand(0) == &F && CI->getOperand(argnum + 1) == val)
CI->setOperand(0, NF);
}
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)
if (CallInst* CI = dyn_cast<CallInst>(ii)) {
++dist[dyn_cast<Constant>(CI->getOperand(arg + 1))];
++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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