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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/Analysis/InlineCost.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;
namespace {
typedef SmallVector<int, MaxInterests> InterestingArgVector;
class PartSpec : public ModulePass {
void scanForInterest(Function&, InterestingArgVector&);
int scanDistribution(Function&, int, std::map<Constant*, int>&);
InlineCostAnalyzer CA;
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,
ValueToValueMapTy& replacements) {
// arg numbers of deleted arguments
DenseMap<unsigned, const Argument*> deleted;
for (ValueToValueMapTy::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,
/*ModuleLevelChanges=*/false);
NF->setLinkage(GlobalValue::InternalLinkage);
F->getParent()->getFunctionList().push_back(NF);
// FIXME: Specialized versions getting the same constants should also get
// the same name. That way, specializations for public functions can be
// marked linkonce_odr and reused across modules.
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;
scanDistribution(F, interestingArgs[x], distribution);
for (std::map<Constant*, int>::iterator ii = distribution.begin(),
ee = distribution.end(); ii != ee; ++ii) {
// The distribution map might have an entry for NULL (i.e., one or more
// callsites were passing a non-constant there). We allow that to
// happen so that we can see whether any callsites pass a non-constant;
// if none do and the function is internal, we might have an opportunity
// to kill the original function.
if (!ii->first) continue;
int bonus = ii->second;
SmallVector<unsigned, 1> argnos;
argnos.push_back(interestingArgs[x]);
InlineCost cost = CA.getSpecializationCost(&F, argnos);
// FIXME: If this is the last constant entry, and no non-constant
// entries exist, and the target function is internal, the cost should
// be reduced by the original size of the target function, almost
// certainly making it negative and causing a specialization that will
// leave the original function dead and removable.
if (cost.isAlways() ||
(cost.isVariable() && cost.getValue() < bonus)) {
ValueToValueMapTy 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) {
int argno = std::distance(F.arg_begin(), ii);
SmallVector<unsigned, 1> argnos;
argnos.push_back(argno);
int bonus = CA.getSpecializationBonus(&F, argnos);
if (bonus > 0) {
args.push_back(argno);
}
}
}
/// scanDistribution - Construct a histogram of constants for arg of F at arg.
/// For each distinct constant, we'll compute the total of the specialization
/// bonus across all callsites passing that constant; if that total exceeds
/// the specialization cost, we will create the specialization.
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) {
SmallVector<unsigned, 1> argnos;
argnos.push_back(arg);
dist[dyn_cast<Constant>(CS.getArgument(arg))] +=
CA.getSpecializationBonus(&F, argnos);
++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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