1//===- CalledValuePropagation.cpp - Propagate called values -----*- C++ -*-===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file implements a transformation that attaches !callees metadata to
10// indirect call sites. For a given call site, the metadata, if present,
11// indicates the set of functions the call site could possibly target at
12// run-time. This metadata is added to indirect call sites when the set of
13// possible targets can be determined by analysis and is known to be small. The
14// analysis driving the transformation is similar to constant propagation and
15// makes uses of the generic sparse propagation solver.
16//
17//===----------------------------------------------------------------------===//
18
19#include "llvm/Transforms/IPO/CalledValuePropagation.h"
20#include "llvm/Analysis/SparsePropagation.h"
21#include "llvm/Analysis/ValueLatticeUtils.h"
22#include "llvm/IR/Constants.h"
23#include "llvm/IR/MDBuilder.h"
24#include "llvm/IR/Module.h"
25#include "llvm/Support/CommandLine.h"
26#include "llvm/Transforms/IPO.h"
27
28using namespace llvm;
29
30#define DEBUG_TYPE "called-value-propagation"
31
32/// The maximum number of functions to track per lattice value. Once the number
33/// of functions a call site can possibly target exceeds this threshold, it's
34/// lattice value becomes overdefined. The number of possible lattice values is
35/// bounded by Ch(F, M), where F is the number of functions in the module and M
36/// is MaxFunctionsPerValue. As such, this value should be kept very small. We
37/// likely can't do anything useful for call sites with a large number of
38/// possible targets, anyway.
39static cl::opt<unsigned> MaxFunctionsPerValue(
40 "cvp-max-functions-per-value", cl::Hidden, cl::init(Val: 4),
41 cl::desc("The maximum number of functions to track per lattice value"));
42
43namespace {
44/// To enable interprocedural analysis, we assign LLVM values to the following
45/// groups. The register group represents SSA registers, the return group
46/// represents the return values of functions, and the memory group represents
47/// in-memory values. An LLVM Value can technically be in more than one group.
48/// It's necessary to distinguish these groups so we can, for example, track a
49/// global variable separately from the value stored at its location.
50enum class IPOGrouping { Register, Return, Memory };
51
52/// Our LatticeKeys are PointerIntPairs composed of LLVM values and groupings.
53using CVPLatticeKey = PointerIntPair<Value *, 2, IPOGrouping>;
54
55/// The lattice value type used by our custom lattice function. It holds the
56/// lattice state, and a set of functions.
57class CVPLatticeVal {
58public:
59 /// The states of the lattice values. Only the FunctionSet state is
60 /// interesting. It indicates the set of functions to which an LLVM value may
61 /// refer.
62 enum CVPLatticeStateTy { Undefined, FunctionSet, Overdefined, Untracked };
63
64 /// Comparator for sorting the functions set. We want to keep the order
65 /// deterministic for testing, etc.
66 struct Compare {
67 bool operator()(const Function *LHS, const Function *RHS) const {
68 return LHS->getName() < RHS->getName();
69 }
70 };
71
72 CVPLatticeVal() = default;
73 CVPLatticeVal(CVPLatticeStateTy LatticeState) : LatticeState(LatticeState) {}
74 CVPLatticeVal(std::vector<Function *> &&Functions)
75 : LatticeState(FunctionSet), Functions(std::move(Functions)) {
76 assert(llvm::is_sorted(this->Functions, Compare()));
77 }
78
79 /// Get a reference to the functions held by this lattice value. The number
80 /// of functions will be zero for states other than FunctionSet.
81 const std::vector<Function *> &getFunctions() const {
82 return Functions;
83 }
84
85 /// Returns true if the lattice value is in the FunctionSet state.
86 bool isFunctionSet() const { return LatticeState == FunctionSet; }
87
88 bool operator==(const CVPLatticeVal &RHS) const {
89 return LatticeState == RHS.LatticeState && Functions == RHS.Functions;
90 }
91
92 bool operator!=(const CVPLatticeVal &RHS) const {
93 return LatticeState != RHS.LatticeState || Functions != RHS.Functions;
94 }
95
96private:
97 /// Holds the state this lattice value is in.
98 CVPLatticeStateTy LatticeState = Undefined;
99
100 /// Holds functions indicating the possible targets of call sites. This set
101 /// is empty for lattice values in the undefined, overdefined, and untracked
102 /// states. The maximum size of the set is controlled by
103 /// MaxFunctionsPerValue. Since most LLVM values are expected to be in
104 /// uninteresting states (i.e., overdefined), CVPLatticeVal objects should be
105 /// small and efficiently copyable.
106 // FIXME: This could be a TinyPtrVector and/or merge with LatticeState.
107 std::vector<Function *> Functions;
108};
109
110/// The custom lattice function used by the generic sparse propagation solver.
111/// It handles merging lattice values and computing new lattice values for
112/// constants, arguments, values returned from trackable functions, and values
113/// located in trackable global variables. It also computes the lattice values
114/// that change as a result of executing instructions.
115class CVPLatticeFunc
116 : public AbstractLatticeFunction<CVPLatticeKey, CVPLatticeVal> {
117public:
118 CVPLatticeFunc()
119 : AbstractLatticeFunction(CVPLatticeVal(CVPLatticeVal::Undefined),
120 CVPLatticeVal(CVPLatticeVal::Overdefined),
121 CVPLatticeVal(CVPLatticeVal::Untracked)) {}
122
123 /// Compute and return a CVPLatticeVal for the given CVPLatticeKey.
124 CVPLatticeVal ComputeLatticeVal(CVPLatticeKey Key) override {
125 switch (Key.getInt()) {
126 case IPOGrouping::Register:
127 if (isa<Instruction>(Val: Key.getPointer())) {
128 return getUndefVal();
129 } else if (auto *A = dyn_cast<Argument>(Val: Key.getPointer())) {
130 if (canTrackArgumentsInterprocedurally(F: A->getParent()))
131 return getUndefVal();
132 } else if (auto *C = dyn_cast<Constant>(Val: Key.getPointer())) {
133 return computeConstant(C);
134 }
135 return getOverdefinedVal();
136 case IPOGrouping::Memory:
137 case IPOGrouping::Return:
138 if (auto *GV = dyn_cast<GlobalVariable>(Val: Key.getPointer())) {
139 if (canTrackGlobalVariableInterprocedurally(GV))
140 return computeConstant(C: GV->getInitializer());
141 } else if (auto *F = cast<Function>(Val: Key.getPointer()))
142 if (canTrackReturnsInterprocedurally(F))
143 return getUndefVal();
144 }
145 return getOverdefinedVal();
146 }
147
148 /// Merge the two given lattice values. The interesting cases are merging two
149 /// FunctionSet values and a FunctionSet value with an Undefined value. For
150 /// these cases, we simply union the function sets. If the size of the union
151 /// is greater than the maximum functions we track, the merged value is
152 /// overdefined.
153 CVPLatticeVal MergeValues(CVPLatticeVal X, CVPLatticeVal Y) override {
154 if (X == getOverdefinedVal() || Y == getOverdefinedVal())
155 return getOverdefinedVal();
156 if (X == getUndefVal() && Y == getUndefVal())
157 return getUndefVal();
158 std::vector<Function *> Union;
159 std::set_union(first1: X.getFunctions().begin(), last1: X.getFunctions().end(),
160 first2: Y.getFunctions().begin(), last2: Y.getFunctions().end(),
161 result: std::back_inserter(x&: Union), comp: CVPLatticeVal::Compare{});
162 if (Union.size() > MaxFunctionsPerValue)
163 return getOverdefinedVal();
164 return CVPLatticeVal(std::move(Union));
165 }
166
167 /// Compute the lattice values that change as a result of executing the given
168 /// instruction. The changed values are stored in \p ChangedValues. We handle
169 /// just a few kinds of instructions since we're only propagating values that
170 /// can be called.
171 void ComputeInstructionState(
172 Instruction &I, DenseMap<CVPLatticeKey, CVPLatticeVal> &ChangedValues,
173 SparseSolver<CVPLatticeKey, CVPLatticeVal> &SS) override {
174 switch (I.getOpcode()) {
175 case Instruction::Call:
176 case Instruction::Invoke:
177 return visitCallBase(CB&: cast<CallBase>(Val&: I), ChangedValues, SS);
178 case Instruction::Load:
179 return visitLoad(I&: *cast<LoadInst>(Val: &I), ChangedValues, SS);
180 case Instruction::Ret:
181 return visitReturn(I&: *cast<ReturnInst>(Val: &I), ChangedValues, SS);
182 case Instruction::Select:
183 return visitSelect(I&: *cast<SelectInst>(Val: &I), ChangedValues, SS);
184 case Instruction::Store:
185 return visitStore(I&: *cast<StoreInst>(Val: &I), ChangedValues, SS);
186 default:
187 return visitInst(I, ChangedValues, SS);
188 }
189 }
190
191 /// Print the given CVPLatticeVal to the specified stream.
192 void PrintLatticeVal(CVPLatticeVal LV, raw_ostream &OS) override {
193 if (LV == getUndefVal())
194 OS << "Undefined ";
195 else if (LV == getOverdefinedVal())
196 OS << "Overdefined";
197 else if (LV == getUntrackedVal())
198 OS << "Untracked ";
199 else
200 OS << "FunctionSet";
201 }
202
203 /// Print the given CVPLatticeKey to the specified stream.
204 void PrintLatticeKey(CVPLatticeKey Key, raw_ostream &OS) override {
205 if (Key.getInt() == IPOGrouping::Register)
206 OS << "<reg> ";
207 else if (Key.getInt() == IPOGrouping::Memory)
208 OS << "<mem> ";
209 else if (Key.getInt() == IPOGrouping::Return)
210 OS << "<ret> ";
211 if (isa<Function>(Val: Key.getPointer()))
212 OS << Key.getPointer()->getName();
213 else
214 OS << *Key.getPointer();
215 }
216
217 /// We collect a set of indirect calls when visiting call sites. This method
218 /// returns a reference to that set.
219 SmallPtrSetImpl<CallBase *> &getIndirectCalls() { return IndirectCalls; }
220
221private:
222 /// Holds the indirect calls we encounter during the analysis. We will attach
223 /// metadata to these calls after the analysis indicating the functions the
224 /// calls can possibly target.
225 SmallPtrSet<CallBase *, 32> IndirectCalls;
226
227 /// Compute a new lattice value for the given constant. The constant, after
228 /// stripping any pointer casts, should be a Function. We ignore null
229 /// pointers as an optimization, since calling these values is undefined
230 /// behavior.
231 CVPLatticeVal computeConstant(Constant *C) {
232 if (isa<ConstantPointerNull>(Val: C))
233 return CVPLatticeVal(CVPLatticeVal::FunctionSet);
234 if (auto *F = dyn_cast<Function>(Val: C->stripPointerCasts()))
235 return CVPLatticeVal({F});
236 return getOverdefinedVal();
237 }
238
239 /// Handle return instructions. The function's return state is the merge of
240 /// the returned value state and the function's return state.
241 void visitReturn(ReturnInst &I,
242 DenseMap<CVPLatticeKey, CVPLatticeVal> &ChangedValues,
243 SparseSolver<CVPLatticeKey, CVPLatticeVal> &SS) {
244 Function *F = I.getParent()->getParent();
245 if (F->getReturnType()->isVoidTy())
246 return;
247 auto RegI = CVPLatticeKey(I.getReturnValue(), IPOGrouping::Register);
248 auto RetF = CVPLatticeKey(F, IPOGrouping::Return);
249 ChangedValues[RetF] =
250 MergeValues(X: SS.getValueState(Key: RegI), Y: SS.getValueState(Key: RetF));
251 }
252
253 /// Handle call sites. The state of a called function's formal arguments is
254 /// the merge of the argument state with the call sites corresponding actual
255 /// argument state. The call site state is the merge of the call site state
256 /// with the returned value state of the called function.
257 void visitCallBase(CallBase &CB,
258 DenseMap<CVPLatticeKey, CVPLatticeVal> &ChangedValues,
259 SparseSolver<CVPLatticeKey, CVPLatticeVal> &SS) {
260 Function *F = CB.getCalledFunction();
261 auto RegI = CVPLatticeKey(&CB, IPOGrouping::Register);
262
263 // If this is an indirect call, save it so we can quickly revisit it when
264 // attaching metadata.
265 if (!F)
266 IndirectCalls.insert(Ptr: &CB);
267
268 // If we can't track the function's return values, there's nothing to do.
269 if (!F || !canTrackReturnsInterprocedurally(F)) {
270 // Void return, No need to create and update CVPLattice state as no one
271 // can use it.
272 if (CB.getType()->isVoidTy())
273 return;
274 ChangedValues[RegI] = getOverdefinedVal();
275 return;
276 }
277
278 // Inform the solver that the called function is executable, and perform
279 // the merges for the arguments and return value.
280 SS.MarkBlockExecutable(BB: &F->front());
281 auto RetF = CVPLatticeKey(F, IPOGrouping::Return);
282 for (Argument &A : F->args()) {
283 auto RegFormal = CVPLatticeKey(&A, IPOGrouping::Register);
284 auto RegActual =
285 CVPLatticeKey(CB.getArgOperand(i: A.getArgNo()), IPOGrouping::Register);
286 ChangedValues[RegFormal] =
287 MergeValues(X: SS.getValueState(Key: RegFormal), Y: SS.getValueState(Key: RegActual));
288 }
289
290 // Void return, No need to create and update CVPLattice state as no one can
291 // use it.
292 if (CB.getType()->isVoidTy())
293 return;
294
295 ChangedValues[RegI] =
296 MergeValues(X: SS.getValueState(Key: RegI), Y: SS.getValueState(Key: RetF));
297 }
298
299 /// Handle select instructions. The select instruction state is the merge the
300 /// true and false value states.
301 void visitSelect(SelectInst &I,
302 DenseMap<CVPLatticeKey, CVPLatticeVal> &ChangedValues,
303 SparseSolver<CVPLatticeKey, CVPLatticeVal> &SS) {
304 auto RegI = CVPLatticeKey(&I, IPOGrouping::Register);
305 auto RegT = CVPLatticeKey(I.getTrueValue(), IPOGrouping::Register);
306 auto RegF = CVPLatticeKey(I.getFalseValue(), IPOGrouping::Register);
307 ChangedValues[RegI] =
308 MergeValues(X: SS.getValueState(Key: RegT), Y: SS.getValueState(Key: RegF));
309 }
310
311 /// Handle load instructions. If the pointer operand of the load is a global
312 /// variable, we attempt to track the value. The loaded value state is the
313 /// merge of the loaded value state with the global variable state.
314 void visitLoad(LoadInst &I,
315 DenseMap<CVPLatticeKey, CVPLatticeVal> &ChangedValues,
316 SparseSolver<CVPLatticeKey, CVPLatticeVal> &SS) {
317 auto RegI = CVPLatticeKey(&I, IPOGrouping::Register);
318 if (auto *GV = dyn_cast<GlobalVariable>(Val: I.getPointerOperand())) {
319 auto MemGV = CVPLatticeKey(GV, IPOGrouping::Memory);
320 ChangedValues[RegI] =
321 MergeValues(X: SS.getValueState(Key: RegI), Y: SS.getValueState(Key: MemGV));
322 } else {
323 ChangedValues[RegI] = getOverdefinedVal();
324 }
325 }
326
327 /// Handle store instructions. If the pointer operand of the store is a
328 /// global variable, we attempt to track the value. The global variable state
329 /// is the merge of the stored value state with the global variable state.
330 void visitStore(StoreInst &I,
331 DenseMap<CVPLatticeKey, CVPLatticeVal> &ChangedValues,
332 SparseSolver<CVPLatticeKey, CVPLatticeVal> &SS) {
333 auto *GV = dyn_cast<GlobalVariable>(Val: I.getPointerOperand());
334 if (!GV)
335 return;
336 auto RegI = CVPLatticeKey(I.getValueOperand(), IPOGrouping::Register);
337 auto MemGV = CVPLatticeKey(GV, IPOGrouping::Memory);
338 ChangedValues[MemGV] =
339 MergeValues(X: SS.getValueState(Key: RegI), Y: SS.getValueState(Key: MemGV));
340 }
341
342 /// Handle all other instructions. All other instructions are marked
343 /// overdefined.
344 void visitInst(Instruction &I,
345 DenseMap<CVPLatticeKey, CVPLatticeVal> &ChangedValues,
346 SparseSolver<CVPLatticeKey, CVPLatticeVal> &SS) {
347 // Simply bail if this instruction has no user.
348 if (I.use_empty())
349 return;
350 auto RegI = CVPLatticeKey(&I, IPOGrouping::Register);
351 ChangedValues[RegI] = getOverdefinedVal();
352 }
353};
354} // namespace
355
356namespace llvm {
357/// A specialization of LatticeKeyInfo for CVPLatticeKeys. The generic solver
358/// must translate between LatticeKeys and LLVM Values when adding Values to
359/// its work list and inspecting the state of control-flow related values.
360template <> struct LatticeKeyInfo<CVPLatticeKey> {
361 static inline Value *getValueFromLatticeKey(CVPLatticeKey Key) {
362 return Key.getPointer();
363 }
364 static inline CVPLatticeKey getLatticeKeyFromValue(Value *V) {
365 return CVPLatticeKey(V, IPOGrouping::Register);
366 }
367};
368} // namespace llvm
369
370static bool runCVP(Module &M) {
371 // Our custom lattice function and generic sparse propagation solver.
372 CVPLatticeFunc Lattice;
373 SparseSolver<CVPLatticeKey, CVPLatticeVal> Solver(&Lattice);
374
375 // For each function in the module, if we can't track its arguments, let the
376 // generic solver assume it is executable.
377 for (Function &F : M)
378 if (!F.isDeclaration() && !canTrackArgumentsInterprocedurally(F: &F))
379 Solver.MarkBlockExecutable(BB: &F.front());
380
381 // Solver our custom lattice. In doing so, we will also build a set of
382 // indirect call sites.
383 Solver.Solve();
384
385 // Attach metadata to the indirect call sites that were collected indicating
386 // the set of functions they can possibly target.
387 bool Changed = false;
388 MDBuilder MDB(M.getContext());
389 for (CallBase *C : Lattice.getIndirectCalls()) {
390 auto RegI = CVPLatticeKey(C->getCalledOperand(), IPOGrouping::Register);
391 CVPLatticeVal LV = Solver.getExistingValueState(Key: RegI);
392 if (!LV.isFunctionSet() || LV.getFunctions().empty())
393 continue;
394 MDNode *Callees = MDB.createCallees(Callees: LV.getFunctions());
395 C->setMetadata(KindID: LLVMContext::MD_callees, Node: Callees);
396 Changed = true;
397 }
398
399 return Changed;
400}
401
402PreservedAnalyses CalledValuePropagationPass::run(Module &M,
403 ModuleAnalysisManager &) {
404 runCVP(M);
405 return PreservedAnalyses::all();
406}
407