1 //===-- PartialSpecialization.cpp - Specialize for common constants--------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass finds function arguments that are often a common constant and
11 // specializes a version of the called function for that constant.
13 // This pass simply does the cloning for functions it specializes. It depends
14 // on IPSCCP and DAE to clean up the results.
16 // The initial heuristic favors constant arguments that are used in control
19 //===----------------------------------------------------------------------===//
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/Analysis/InlineCost.h"
29 #include "llvm/Transforms/Utils/Cloning.h"
30 #include "llvm/Support/CallSite.h"
31 #include "llvm/ADT/DenseSet.h"
35 STATISTIC(numSpecialized, "Number of specialized functions created");
36 STATISTIC(numReplaced, "Number of callers replaced by specialization");
38 // Maximum number of arguments markable interested
39 static const int MaxInterests = 6;
42 typedef SmallVector<int, MaxInterests> InterestingArgVector;
43 class PartSpec : public ModulePass {
44 void scanForInterest(Function&, InterestingArgVector&);
45 int scanDistribution(Function&, int, std::map<Constant*, int>&);
46 InlineCostAnalyzer CA;
48 static char ID; // Pass identification, replacement for typeid
49 PartSpec() : ModulePass(ID) {}
50 bool runOnModule(Module &M);
54 char PartSpec::ID = 0;
55 INITIALIZE_PASS(PartSpec, "partialspecialization",
56 "Partial Specialization", false, false)
58 // Specialize F by replacing the arguments (keys) in replacements with the
59 // constants (values). Replace all calls to F with those constants with
60 // a call to the specialized function. Returns the specialized function
62 SpecializeFunction(Function* F,
63 ValueMap<const Value*, Value*>& replacements) {
64 // arg numbers of deleted arguments
65 DenseMap<unsigned, const Argument*> deleted;
66 for (ValueMap<const Value*, Value*>::iterator
67 repb = replacements.begin(), repe = replacements.end();
68 repb != repe; ++repb) {
69 Argument const *arg = cast<const Argument>(repb->first);
70 deleted[arg->getArgNo()] = arg;
73 Function* NF = CloneFunction(F, replacements,
74 /*ModuleLevelChanges=*/false);
75 NF->setLinkage(GlobalValue::InternalLinkage);
76 F->getParent()->getFunctionList().push_back(NF);
78 // FIXME: Specialized versions getting the same constants should also get
79 // the same name. That way, specializations for public functions can be
80 // marked linkonce_odr and reused across modules.
82 for (Value::use_iterator ii = F->use_begin(), ee = F->use_end();
84 Value::use_iterator i = ii;
89 if (CS.getCalledFunction() == F) {
90 SmallVector<Value*, 6> args;
91 // Assemble the non-specialized arguments for the updated callsite.
92 // In the process, make sure that the specialized arguments are
93 // constant and match the specialization. If that's not the case,
94 // this callsite needs to call the original or some other
95 // specialization; don't change it here.
96 CallSite::arg_iterator as = CS.arg_begin(), ae = CS.arg_end();
97 for (CallSite::arg_iterator ai = as; ai != ae; ++ai) {
98 DenseMap<unsigned, const Argument*>::iterator delit = deleted.find(
99 std::distance(as, ai));
100 if (delit == deleted.end())
101 args.push_back(cast<Value>(ai));
103 Constant *ci = dyn_cast<Constant>(ai);
104 if (!(ci && ci == replacements[delit->second]))
109 if (CallInst *CI = dyn_cast<CallInst>(U)) {
110 NCall = CallInst::Create(NF, args.begin(), args.end(),
112 cast<CallInst>(NCall)->setTailCall(CI->isTailCall());
113 cast<CallInst>(NCall)->setCallingConv(CI->getCallingConv());
115 InvokeInst *II = cast<InvokeInst>(U);
116 NCall = InvokeInst::Create(NF, II->getNormalDest(),
118 args.begin(), args.end(),
120 cast<InvokeInst>(NCall)->setCallingConv(II->getCallingConv());
122 CS.getInstruction()->replaceAllUsesWith(NCall);
123 CS.getInstruction()->eraseFromParent();
133 bool PartSpec::runOnModule(Module &M) {
134 bool Changed = false;
135 for (Module::iterator I = M.begin(); I != M.end(); ++I) {
137 if (F.isDeclaration() || F.mayBeOverridden()) continue;
138 InterestingArgVector interestingArgs;
139 scanForInterest(F, interestingArgs);
141 // Find the first interesting Argument that we can specialize on
142 // If there are multiple interesting Arguments, then those will be found
143 // when processing the cloned function.
144 bool breakOuter = false;
145 for (unsigned int x = 0; !breakOuter && x < interestingArgs.size(); ++x) {
146 std::map<Constant*, int> distribution;
147 scanDistribution(F, interestingArgs[x], distribution);
148 for (std::map<Constant*, int>::iterator ii = distribution.begin(),
149 ee = distribution.end(); ii != ee; ++ii) {
150 // The distribution map might have an entry for NULL (i.e., one or more
151 // callsites were passing a non-constant there). We allow that to
152 // happen so that we can see whether any callsites pass a non-constant;
153 // if none do and the function is internal, we might have an opportunity
154 // to kill the original function.
155 if (!ii->first) continue;
156 int bonus = ii->second;
157 SmallVector<unsigned, 1> argnos;
158 argnos.push_back(interestingArgs[x]);
159 InlineCost cost = CA.getSpecializationCost(&F, argnos);
160 // FIXME: If this is the last constant entry, and no non-constant
161 // entries exist, and the target function is internal, the cost should
162 // be reduced by the original size of the target function, almost
163 // certainly making it negative and causing a specialization that will
164 // leave the original function dead and removable.
165 if (cost.isAlways() ||
166 (cost.isVariable() && cost.getValue() < bonus)) {
167 ValueMap<const Value*, Value*> m;
168 Function::arg_iterator arg = F.arg_begin();
169 for (int y = 0; y < interestingArgs[x]; ++y)
171 m[&*arg] = ii->first;
172 SpecializeFunction(&F, m);
183 /// scanForInterest - This function decides which arguments would be worth
185 void PartSpec::scanForInterest(Function& F, InterestingArgVector& args) {
186 for(Function::arg_iterator ii = F.arg_begin(), ee = F.arg_end();
188 int argno = std::distance(F.arg_begin(), ii);
189 SmallVector<unsigned, 1> argnos;
190 argnos.push_back(argno);
191 int bonus = CA.getSpecializationBonus(&F, argnos);
193 args.push_back(argno);
198 /// scanDistribution - Construct a histogram of constants for arg of F at arg.
199 /// For each distinct constant, we'll compute the total of the specialization
200 /// bonus across all callsites passing that constant; if that total exceeds
201 /// the specialization cost, we will create the specialization.
202 int PartSpec::scanDistribution(Function& F, int arg,
203 std::map<Constant*, int>& dist) {
204 bool hasIndirect = false;
206 for (Value::use_iterator ii = F.use_begin(), ee = F.use_end();
210 if (CS && CS.getCalledFunction() == &F) {
211 SmallVector<unsigned, 1> argnos;
212 argnos.push_back(arg);
213 dist[dyn_cast<Constant>(CS.getArgument(arg))] +=
214 CA.getSpecializationBonus(&F, argnos);
220 // Preserve the original address taken function even if all other uses
221 // will be specialized.
222 if (hasIndirect) ++total;
226 ModulePass* llvm::createPartialSpecializationPass() { return new PartSpec(); }