1 //===- MergeFunctions.cpp - Merge identical functions ---------------------===//
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 pass looks for equivalent functions that are mergable and folds them.
10 //
11 // Order relation is defined on set of functions. It was made through
12 // special function comparison procedure that returns
13 // 0 when functions are equal,
14 // -1 when Left function is less than right function, and
15 // 1 for opposite case. We need total-ordering, so we need to maintain
16 // four properties on the functions set:
17 // a <= a (reflexivity)
18 // if a <= b and b <= a then a = b (antisymmetry)
19 // if a <= b and b <= c then a <= c (transitivity).
20 // for all a and b: a <= b or b <= a (totality).
21 //
22 // Comparison iterates through each instruction in each basic block.
23 // Functions are kept on binary tree. For each new function F we perform
24 // lookup in binary tree.
25 // In practice it works the following way:
26 // -- We define Function* container class with custom "operator<" (FunctionPtr).
27 // -- "FunctionPtr" instances are stored in std::set collection, so every
28 // std::set::insert operation will give you result in log(N) time.
29 //
30 // As an optimization, a hash of the function structure is calculated first, and
31 // two functions are only compared if they have the same hash. This hash is
32 // cheap to compute, and has the property that if function F == G according to
33 // the comparison function, then hash(F) == hash(G). This consistency property
34 // is critical to ensuring all possible merging opportunities are exploited.
35 // Collisions in the hash affect the speed of the pass but not the correctness
36 // or determinism of the resulting transformation.
37 //
38 // When a match is found the functions are folded. If both functions are
39 // overridable, we move the functionality into a new internal function and
40 // leave two overridable thunks to it.
41 //
42 //===----------------------------------------------------------------------===//
43 //
44 // Future work:
45 //
46 // * virtual functions.
47 //
48 // Many functions have their address taken by the virtual function table for
49 // the object they belong to. However, as long as it's only used for a lookup
50 // and call, this is irrelevant, and we'd like to fold such functions.
51 //
52 // * be smarter about bitcasts.
53 //
54 // In order to fold functions, we will sometimes add either bitcast instructions
55 // or bitcast constant expressions. Unfortunately, this can confound further
56 // analysis since the two functions differ where one has a bitcast and the
57 // other doesn't. We should learn to look through bitcasts.
58 //
59 // * Compare complex types with pointer types inside.
60 // * Compare cross-reference cases.
61 // * Compare complex expressions.
62 //
63 // All the three issues above could be described as ability to prove that
64 // fA == fB == fC == fE == fF == fG in example below:
65 //
66 // void fA() {
67 // fB();
68 // }
69 // void fB() {
70 // fA();
71 // }
72 //
73 // void fE() {
74 // fF();
75 // }
76 // void fF() {
77 // fG();
78 // }
79 // void fG() {
80 // fE();
81 // }
82 //
83 // Simplest cross-reference case (fA <--> fB) was implemented in previous
84 // versions of MergeFunctions, though it presented only in two function pairs
85 // in test-suite (that counts >50k functions)
86 // Though possibility to detect complex cross-referencing (e.g.: A->B->C->D->A)
87 // could cover much more cases.
88 //
89 //===----------------------------------------------------------------------===//
90
91 #include "llvm/ADT/ArrayRef.h"
92 #include "llvm/ADT/SmallPtrSet.h"
93 #include "llvm/ADT/SmallVector.h"
94 #include "llvm/ADT/Statistic.h"
95 #include "llvm/IR/Argument.h"
96 #include "llvm/IR/Attributes.h"
97 #include "llvm/IR/BasicBlock.h"
98 #include "llvm/IR/Constant.h"
99 #include "llvm/IR/Constants.h"
100 #include "llvm/IR/DebugInfoMetadata.h"
101 #include "llvm/IR/DebugLoc.h"
102 #include "llvm/IR/DerivedTypes.h"
103 #include "llvm/IR/Function.h"
104 #include "llvm/IR/GlobalValue.h"
105 #include "llvm/IR/IRBuilder.h"
106 #include "llvm/IR/InstrTypes.h"
107 #include "llvm/IR/Instruction.h"
108 #include "llvm/IR/Instructions.h"
109 #include "llvm/IR/IntrinsicInst.h"
110 #include "llvm/IR/Module.h"
111 #include "llvm/IR/Type.h"
112 #include "llvm/IR/Use.h"
113 #include "llvm/IR/User.h"
114 #include "llvm/IR/Value.h"
115 #include "llvm/IR/ValueHandle.h"
116 #include "llvm/IR/ValueMap.h"
117 #include "llvm/InitializePasses.h"
118 #include "llvm/Pass.h"
119 #include "llvm/Support/Casting.h"
120 #include "llvm/Support/CommandLine.h"
121 #include "llvm/Support/Debug.h"
122 #include "llvm/Support/raw_ostream.h"
123 #include "llvm/Transforms/IPO.h"
124 #include "llvm/Transforms/IPO/MergeFunctions.h"
125 #include "llvm/Transforms/Utils/FunctionComparator.h"
126 #include <algorithm>
127 #include <cassert>
128 #include <iterator>
129 #include <set>
130 #include <utility>
131 #include <vector>
132
133 using namespace llvm;
134
135 #define DEBUG_TYPE "mergefunc"
136
137 STATISTIC(NumFunctionsMerged, "Number of functions merged");
138 STATISTIC(NumThunksWritten, "Number of thunks generated");
139 STATISTIC(NumAliasesWritten, "Number of aliases generated");
140 STATISTIC(NumDoubleWeak, "Number of new functions created");
141
142 static cl::opt<unsigned> NumFunctionsForSanityCheck(
143 "mergefunc-sanity",
144 cl::desc("How many functions in module could be used for "
145 "MergeFunctions pass sanity check. "
146 "'0' disables this check. Works only with '-debug' key."),
147 cl::init(0), cl::Hidden);
148
149 // Under option -mergefunc-preserve-debug-info we:
150 // - Do not create a new function for a thunk.
151 // - Retain the debug info for a thunk's parameters (and associated
152 // instructions for the debug info) from the entry block.
153 // Note: -debug will display the algorithm at work.
154 // - Create debug-info for the call (to the shared implementation) made by
155 // a thunk and its return value.
156 // - Erase the rest of the function, retaining the (minimally sized) entry
157 // block to create a thunk.
158 // - Preserve a thunk's call site to point to the thunk even when both occur
159 // within the same translation unit, to aid debugability. Note that this
160 // behaviour differs from the underlying -mergefunc implementation which
161 // modifies the thunk's call site to point to the shared implementation
162 // when both occur within the same translation unit.
163 static cl::opt<bool>
164 MergeFunctionsPDI("mergefunc-preserve-debug-info", cl::Hidden,
165 cl::init(false),
166 cl::desc("Preserve debug info in thunk when mergefunc "
167 "transformations are made."));
168
169 static cl::opt<bool>
170 MergeFunctionsAliases("mergefunc-use-aliases", cl::Hidden,
171 cl::init(false),
172 cl::desc("Allow mergefunc to create aliases"));
173
174 namespace {
175
176 class FunctionNode {
177 mutable AssertingVH<Function> F;
178 FunctionComparator::FunctionHash Hash;
179
180 public:
181 // Note the hash is recalculated potentially multiple times, but it is cheap.
FunctionNode(Function * F)182 FunctionNode(Function *F)
183 : F(F), Hash(FunctionComparator::functionHash(*F)) {}
184
getFunc() const185 Function *getFunc() const { return F; }
getHash() const186 FunctionComparator::FunctionHash getHash() const { return Hash; }
187
188 /// Replace the reference to the function F by the function G, assuming their
189 /// implementations are equal.
replaceBy(Function * G) const190 void replaceBy(Function *G) const {
191 F = G;
192 }
193 };
194
195 /// MergeFunctions finds functions which will generate identical machine code,
196 /// by considering all pointer types to be equivalent. Once identified,
197 /// MergeFunctions will fold them by replacing a call to one to a call to a
198 /// bitcast of the other.
199 class MergeFunctions {
200 public:
MergeFunctions()201 MergeFunctions() : FnTree(FunctionNodeCmp(&GlobalNumbers)) {
202 }
203
204 bool runOnModule(Module &M);
205
206 private:
207 // The function comparison operator is provided here so that FunctionNodes do
208 // not need to become larger with another pointer.
209 class FunctionNodeCmp {
210 GlobalNumberState* GlobalNumbers;
211
212 public:
FunctionNodeCmp(GlobalNumberState * GN)213 FunctionNodeCmp(GlobalNumberState* GN) : GlobalNumbers(GN) {}
214
operator ()(const FunctionNode & LHS,const FunctionNode & RHS) const215 bool operator()(const FunctionNode &LHS, const FunctionNode &RHS) const {
216 // Order first by hashes, then full function comparison.
217 if (LHS.getHash() != RHS.getHash())
218 return LHS.getHash() < RHS.getHash();
219 FunctionComparator FCmp(LHS.getFunc(), RHS.getFunc(), GlobalNumbers);
220 return FCmp.compare() == -1;
221 }
222 };
223 using FnTreeType = std::set<FunctionNode, FunctionNodeCmp>;
224
225 GlobalNumberState GlobalNumbers;
226
227 /// A work queue of functions that may have been modified and should be
228 /// analyzed again.
229 std::vector<WeakTrackingVH> Deferred;
230
231 #ifndef NDEBUG
232 /// Checks the rules of order relation introduced among functions set.
233 /// Returns true, if sanity check has been passed, and false if failed.
234 bool doSanityCheck(std::vector<WeakTrackingVH> &Worklist);
235 #endif
236
237 /// Insert a ComparableFunction into the FnTree, or merge it away if it's
238 /// equal to one that's already present.
239 bool insert(Function *NewFunction);
240
241 /// Remove a Function from the FnTree and queue it up for a second sweep of
242 /// analysis.
243 void remove(Function *F);
244
245 /// Find the functions that use this Value and remove them from FnTree and
246 /// queue the functions.
247 void removeUsers(Value *V);
248
249 /// Replace all direct calls of Old with calls of New. Will bitcast New if
250 /// necessary to make types match.
251 void replaceDirectCallers(Function *Old, Function *New);
252
253 /// Merge two equivalent functions. Upon completion, G may be deleted, or may
254 /// be converted into a thunk. In either case, it should never be visited
255 /// again.
256 void mergeTwoFunctions(Function *F, Function *G);
257
258 /// Fill PDIUnrelatedWL with instructions from the entry block that are
259 /// unrelated to parameter related debug info.
260 void filterInstsUnrelatedToPDI(BasicBlock *GEntryBlock,
261 std::vector<Instruction *> &PDIUnrelatedWL);
262
263 /// Erase the rest of the CFG (i.e. barring the entry block).
264 void eraseTail(Function *G);
265
266 /// Erase the instructions in PDIUnrelatedWL as they are unrelated to the
267 /// parameter debug info, from the entry block.
268 void eraseInstsUnrelatedToPDI(std::vector<Instruction *> &PDIUnrelatedWL);
269
270 /// Replace G with a simple tail call to bitcast(F). Also (unless
271 /// MergeFunctionsPDI holds) replace direct uses of G with bitcast(F),
272 /// delete G.
273 void writeThunk(Function *F, Function *G);
274
275 // Replace G with an alias to F (deleting function G)
276 void writeAlias(Function *F, Function *G);
277
278 // Replace G with an alias to F if possible, or a thunk to F if possible.
279 // Returns false if neither is the case.
280 bool writeThunkOrAlias(Function *F, Function *G);
281
282 /// Replace function F with function G in the function tree.
283 void replaceFunctionInTree(const FunctionNode &FN, Function *G);
284
285 /// The set of all distinct functions. Use the insert() and remove() methods
286 /// to modify it. The map allows efficient lookup and deferring of Functions.
287 FnTreeType FnTree;
288
289 // Map functions to the iterators of the FunctionNode which contains them
290 // in the FnTree. This must be updated carefully whenever the FnTree is
291 // modified, i.e. in insert(), remove(), and replaceFunctionInTree(), to avoid
292 // dangling iterators into FnTree. The invariant that preserves this is that
293 // there is exactly one mapping F -> FN for each FunctionNode FN in FnTree.
294 DenseMap<AssertingVH<Function>, FnTreeType::iterator> FNodesInTree;
295 };
296
297 class MergeFunctionsLegacyPass : public ModulePass {
298 public:
299 static char ID;
300
MergeFunctionsLegacyPass()301 MergeFunctionsLegacyPass(): ModulePass(ID) {
302 initializeMergeFunctionsLegacyPassPass(*PassRegistry::getPassRegistry());
303 }
304
runOnModule(Module & M)305 bool runOnModule(Module &M) override {
306 if (skipModule(M))
307 return false;
308
309 MergeFunctions MF;
310 return MF.runOnModule(M);
311 }
312 };
313
314 } // end anonymous namespace
315
316 char MergeFunctionsLegacyPass::ID = 0;
317 INITIALIZE_PASS(MergeFunctionsLegacyPass, "mergefunc",
318 "Merge Functions", false, false)
319
createMergeFunctionsPass()320 ModulePass *llvm::createMergeFunctionsPass() {
321 return new MergeFunctionsLegacyPass();
322 }
323
run(Module & M,ModuleAnalysisManager & AM)324 PreservedAnalyses MergeFunctionsPass::run(Module &M,
325 ModuleAnalysisManager &AM) {
326 MergeFunctions MF;
327 if (!MF.runOnModule(M))
328 return PreservedAnalyses::all();
329 return PreservedAnalyses::none();
330 }
331
332 #ifndef NDEBUG
doSanityCheck(std::vector<WeakTrackingVH> & Worklist)333 bool MergeFunctions::doSanityCheck(std::vector<WeakTrackingVH> &Worklist) {
334 if (const unsigned Max = NumFunctionsForSanityCheck) {
335 unsigned TripleNumber = 0;
336 bool Valid = true;
337
338 dbgs() << "MERGEFUNC-SANITY: Started for first " << Max << " functions.\n";
339
340 unsigned i = 0;
341 for (std::vector<WeakTrackingVH>::iterator I = Worklist.begin(),
342 E = Worklist.end();
343 I != E && i < Max; ++I, ++i) {
344 unsigned j = i;
345 for (std::vector<WeakTrackingVH>::iterator J = I; J != E && j < Max;
346 ++J, ++j) {
347 Function *F1 = cast<Function>(*I);
348 Function *F2 = cast<Function>(*J);
349 int Res1 = FunctionComparator(F1, F2, &GlobalNumbers).compare();
350 int Res2 = FunctionComparator(F2, F1, &GlobalNumbers).compare();
351
352 // If F1 <= F2, then F2 >= F1, otherwise report failure.
353 if (Res1 != -Res2) {
354 dbgs() << "MERGEFUNC-SANITY: Non-symmetric; triple: " << TripleNumber
355 << "\n";
356 dbgs() << *F1 << '\n' << *F2 << '\n';
357 Valid = false;
358 }
359
360 if (Res1 == 0)
361 continue;
362
363 unsigned k = j;
364 for (std::vector<WeakTrackingVH>::iterator K = J; K != E && k < Max;
365 ++k, ++K, ++TripleNumber) {
366 if (K == J)
367 continue;
368
369 Function *F3 = cast<Function>(*K);
370 int Res3 = FunctionComparator(F1, F3, &GlobalNumbers).compare();
371 int Res4 = FunctionComparator(F2, F3, &GlobalNumbers).compare();
372
373 bool Transitive = true;
374
375 if (Res1 != 0 && Res1 == Res4) {
376 // F1 > F2, F2 > F3 => F1 > F3
377 Transitive = Res3 == Res1;
378 } else if (Res3 != 0 && Res3 == -Res4) {
379 // F1 > F3, F3 > F2 => F1 > F2
380 Transitive = Res3 == Res1;
381 } else if (Res4 != 0 && -Res3 == Res4) {
382 // F2 > F3, F3 > F1 => F2 > F1
383 Transitive = Res4 == -Res1;
384 }
385
386 if (!Transitive) {
387 dbgs() << "MERGEFUNC-SANITY: Non-transitive; triple: "
388 << TripleNumber << "\n";
389 dbgs() << "Res1, Res3, Res4: " << Res1 << ", " << Res3 << ", "
390 << Res4 << "\n";
391 dbgs() << *F1 << '\n' << *F2 << '\n' << *F3 << '\n';
392 Valid = false;
393 }
394 }
395 }
396 }
397
398 dbgs() << "MERGEFUNC-SANITY: " << (Valid ? "Passed." : "Failed.") << "\n";
399 return Valid;
400 }
401 return true;
402 }
403 #endif
404
405 /// Check whether \p F is eligible for function merging.
isEligibleForMerging(Function & F)406 static bool isEligibleForMerging(Function &F) {
407 return !F.isDeclaration() && !F.hasAvailableExternallyLinkage();
408 }
409
runOnModule(Module & M)410 bool MergeFunctions::runOnModule(Module &M) {
411 bool Changed = false;
412
413 // All functions in the module, ordered by hash. Functions with a unique
414 // hash value are easily eliminated.
415 std::vector<std::pair<FunctionComparator::FunctionHash, Function *>>
416 HashedFuncs;
417 for (Function &Func : M) {
418 if (isEligibleForMerging(Func)) {
419 HashedFuncs.push_back({FunctionComparator::functionHash(Func), &Func});
420 }
421 }
422
423 llvm::stable_sort(HashedFuncs, less_first());
424
425 auto S = HashedFuncs.begin();
426 for (auto I = HashedFuncs.begin(), IE = HashedFuncs.end(); I != IE; ++I) {
427 // If the hash value matches the previous value or the next one, we must
428 // consider merging it. Otherwise it is dropped and never considered again.
429 if ((I != S && std::prev(I)->first == I->first) ||
430 (std::next(I) != IE && std::next(I)->first == I->first) ) {
431 Deferred.push_back(WeakTrackingVH(I->second));
432 }
433 }
434
435 do {
436 std::vector<WeakTrackingVH> Worklist;
437 Deferred.swap(Worklist);
438
439 LLVM_DEBUG(doSanityCheck(Worklist));
440
441 LLVM_DEBUG(dbgs() << "size of module: " << M.size() << '\n');
442 LLVM_DEBUG(dbgs() << "size of worklist: " << Worklist.size() << '\n');
443
444 // Insert functions and merge them.
445 for (WeakTrackingVH &I : Worklist) {
446 if (!I)
447 continue;
448 Function *F = cast<Function>(I);
449 if (!F->isDeclaration() && !F->hasAvailableExternallyLinkage()) {
450 Changed |= insert(F);
451 }
452 }
453 LLVM_DEBUG(dbgs() << "size of FnTree: " << FnTree.size() << '\n');
454 } while (!Deferred.empty());
455
456 FnTree.clear();
457 FNodesInTree.clear();
458 GlobalNumbers.clear();
459
460 return Changed;
461 }
462
463 // Replace direct callers of Old with New.
replaceDirectCallers(Function * Old,Function * New)464 void MergeFunctions::replaceDirectCallers(Function *Old, Function *New) {
465 Constant *BitcastNew = ConstantExpr::getBitCast(New, Old->getType());
466 for (auto UI = Old->use_begin(), UE = Old->use_end(); UI != UE;) {
467 Use *U = &*UI;
468 ++UI;
469 CallBase *CB = dyn_cast<CallBase>(U->getUser());
470 if (CB && CB->isCallee(U)) {
471 // Do not copy attributes from the called function to the call-site.
472 // Function comparison ensures that the attributes are the same up to
473 // type congruences in byval(), in which case we need to keep the byval
474 // type of the call-site, not the callee function.
475 remove(CB->getFunction());
476 U->set(BitcastNew);
477 }
478 }
479 }
480
481 // Helper for writeThunk,
482 // Selects proper bitcast operation,
483 // but a bit simpler then CastInst::getCastOpcode.
createCast(IRBuilder<> & Builder,Value * V,Type * DestTy)484 static Value *createCast(IRBuilder<> &Builder, Value *V, Type *DestTy) {
485 Type *SrcTy = V->getType();
486 if (SrcTy->isStructTy()) {
487 assert(DestTy->isStructTy());
488 assert(SrcTy->getStructNumElements() == DestTy->getStructNumElements());
489 Value *Result = UndefValue::get(DestTy);
490 for (unsigned int I = 0, E = SrcTy->getStructNumElements(); I < E; ++I) {
491 Value *Element = createCast(
492 Builder, Builder.CreateExtractValue(V, makeArrayRef(I)),
493 DestTy->getStructElementType(I));
494
495 Result =
496 Builder.CreateInsertValue(Result, Element, makeArrayRef(I));
497 }
498 return Result;
499 }
500 assert(!DestTy->isStructTy());
501 if (SrcTy->isIntegerTy() && DestTy->isPointerTy())
502 return Builder.CreateIntToPtr(V, DestTy);
503 else if (SrcTy->isPointerTy() && DestTy->isIntegerTy())
504 return Builder.CreatePtrToInt(V, DestTy);
505 else
506 return Builder.CreateBitCast(V, DestTy);
507 }
508
509 // Erase the instructions in PDIUnrelatedWL as they are unrelated to the
510 // parameter debug info, from the entry block.
eraseInstsUnrelatedToPDI(std::vector<Instruction * > & PDIUnrelatedWL)511 void MergeFunctions::eraseInstsUnrelatedToPDI(
512 std::vector<Instruction *> &PDIUnrelatedWL) {
513 LLVM_DEBUG(
514 dbgs() << " Erasing instructions (in reverse order of appearance in "
515 "entry block) unrelated to parameter debug info from entry "
516 "block: {\n");
517 while (!PDIUnrelatedWL.empty()) {
518 Instruction *I = PDIUnrelatedWL.back();
519 LLVM_DEBUG(dbgs() << " Deleting Instruction: ");
520 LLVM_DEBUG(I->print(dbgs()));
521 LLVM_DEBUG(dbgs() << "\n");
522 I->eraseFromParent();
523 PDIUnrelatedWL.pop_back();
524 }
525 LLVM_DEBUG(dbgs() << " } // Done erasing instructions unrelated to parameter "
526 "debug info from entry block. \n");
527 }
528
529 // Reduce G to its entry block.
eraseTail(Function * G)530 void MergeFunctions::eraseTail(Function *G) {
531 std::vector<BasicBlock *> WorklistBB;
532 for (Function::iterator BBI = std::next(G->begin()), BBE = G->end();
533 BBI != BBE; ++BBI) {
534 BBI->dropAllReferences();
535 WorklistBB.push_back(&*BBI);
536 }
537 while (!WorklistBB.empty()) {
538 BasicBlock *BB = WorklistBB.back();
539 BB->eraseFromParent();
540 WorklistBB.pop_back();
541 }
542 }
543
544 // We are interested in the following instructions from the entry block as being
545 // related to parameter debug info:
546 // - @llvm.dbg.declare
547 // - stores from the incoming parameters to locations on the stack-frame
548 // - allocas that create these locations on the stack-frame
549 // - @llvm.dbg.value
550 // - the entry block's terminator
551 // The rest are unrelated to debug info for the parameters; fill up
552 // PDIUnrelatedWL with such instructions.
filterInstsUnrelatedToPDI(BasicBlock * GEntryBlock,std::vector<Instruction * > & PDIUnrelatedWL)553 void MergeFunctions::filterInstsUnrelatedToPDI(
554 BasicBlock *GEntryBlock, std::vector<Instruction *> &PDIUnrelatedWL) {
555 std::set<Instruction *> PDIRelated;
556 for (BasicBlock::iterator BI = GEntryBlock->begin(), BIE = GEntryBlock->end();
557 BI != BIE; ++BI) {
558 if (auto *DVI = dyn_cast<DbgValueInst>(&*BI)) {
559 LLVM_DEBUG(dbgs() << " Deciding: ");
560 LLVM_DEBUG(BI->print(dbgs()));
561 LLVM_DEBUG(dbgs() << "\n");
562 DILocalVariable *DILocVar = DVI->getVariable();
563 if (DILocVar->isParameter()) {
564 LLVM_DEBUG(dbgs() << " Include (parameter): ");
565 LLVM_DEBUG(BI->print(dbgs()));
566 LLVM_DEBUG(dbgs() << "\n");
567 PDIRelated.insert(&*BI);
568 } else {
569 LLVM_DEBUG(dbgs() << " Delete (!parameter): ");
570 LLVM_DEBUG(BI->print(dbgs()));
571 LLVM_DEBUG(dbgs() << "\n");
572 }
573 } else if (auto *DDI = dyn_cast<DbgDeclareInst>(&*BI)) {
574 LLVM_DEBUG(dbgs() << " Deciding: ");
575 LLVM_DEBUG(BI->print(dbgs()));
576 LLVM_DEBUG(dbgs() << "\n");
577 DILocalVariable *DILocVar = DDI->getVariable();
578 if (DILocVar->isParameter()) {
579 LLVM_DEBUG(dbgs() << " Parameter: ");
580 LLVM_DEBUG(DILocVar->print(dbgs()));
581 AllocaInst *AI = dyn_cast_or_null<AllocaInst>(DDI->getAddress());
582 if (AI) {
583 LLVM_DEBUG(dbgs() << " Processing alloca users: ");
584 LLVM_DEBUG(dbgs() << "\n");
585 for (User *U : AI->users()) {
586 if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
587 if (Value *Arg = SI->getValueOperand()) {
588 if (dyn_cast<Argument>(Arg)) {
589 LLVM_DEBUG(dbgs() << " Include: ");
590 LLVM_DEBUG(AI->print(dbgs()));
591 LLVM_DEBUG(dbgs() << "\n");
592 PDIRelated.insert(AI);
593 LLVM_DEBUG(dbgs() << " Include (parameter): ");
594 LLVM_DEBUG(SI->print(dbgs()));
595 LLVM_DEBUG(dbgs() << "\n");
596 PDIRelated.insert(SI);
597 LLVM_DEBUG(dbgs() << " Include: ");
598 LLVM_DEBUG(BI->print(dbgs()));
599 LLVM_DEBUG(dbgs() << "\n");
600 PDIRelated.insert(&*BI);
601 } else {
602 LLVM_DEBUG(dbgs() << " Delete (!parameter): ");
603 LLVM_DEBUG(SI->print(dbgs()));
604 LLVM_DEBUG(dbgs() << "\n");
605 }
606 }
607 } else {
608 LLVM_DEBUG(dbgs() << " Defer: ");
609 LLVM_DEBUG(U->print(dbgs()));
610 LLVM_DEBUG(dbgs() << "\n");
611 }
612 }
613 } else {
614 LLVM_DEBUG(dbgs() << " Delete (alloca NULL): ");
615 LLVM_DEBUG(BI->print(dbgs()));
616 LLVM_DEBUG(dbgs() << "\n");
617 }
618 } else {
619 LLVM_DEBUG(dbgs() << " Delete (!parameter): ");
620 LLVM_DEBUG(BI->print(dbgs()));
621 LLVM_DEBUG(dbgs() << "\n");
622 }
623 } else if (BI->isTerminator() && &*BI == GEntryBlock->getTerminator()) {
624 LLVM_DEBUG(dbgs() << " Will Include Terminator: ");
625 LLVM_DEBUG(BI->print(dbgs()));
626 LLVM_DEBUG(dbgs() << "\n");
627 PDIRelated.insert(&*BI);
628 } else {
629 LLVM_DEBUG(dbgs() << " Defer: ");
630 LLVM_DEBUG(BI->print(dbgs()));
631 LLVM_DEBUG(dbgs() << "\n");
632 }
633 }
634 LLVM_DEBUG(
635 dbgs()
636 << " Report parameter debug info related/related instructions: {\n");
637 for (BasicBlock::iterator BI = GEntryBlock->begin(), BE = GEntryBlock->end();
638 BI != BE; ++BI) {
639
640 Instruction *I = &*BI;
641 if (PDIRelated.find(I) == PDIRelated.end()) {
642 LLVM_DEBUG(dbgs() << " !PDIRelated: ");
643 LLVM_DEBUG(I->print(dbgs()));
644 LLVM_DEBUG(dbgs() << "\n");
645 PDIUnrelatedWL.push_back(I);
646 } else {
647 LLVM_DEBUG(dbgs() << " PDIRelated: ");
648 LLVM_DEBUG(I->print(dbgs()));
649 LLVM_DEBUG(dbgs() << "\n");
650 }
651 }
652 LLVM_DEBUG(dbgs() << " }\n");
653 }
654
655 /// Whether this function may be replaced by a forwarding thunk.
canCreateThunkFor(Function * F)656 static bool canCreateThunkFor(Function *F) {
657 if (F->isVarArg())
658 return false;
659
660 // Don't merge tiny functions using a thunk, since it can just end up
661 // making the function larger.
662 if (F->size() == 1) {
663 if (F->front().size() <= 2) {
664 LLVM_DEBUG(dbgs() << "canCreateThunkFor: " << F->getName()
665 << " is too small to bother creating a thunk for\n");
666 return false;
667 }
668 }
669 return true;
670 }
671
672 // Replace G with a simple tail call to bitcast(F). Also (unless
673 // MergeFunctionsPDI holds) replace direct uses of G with bitcast(F),
674 // delete G. Under MergeFunctionsPDI, we use G itself for creating
675 // the thunk as we preserve the debug info (and associated instructions)
676 // from G's entry block pertaining to G's incoming arguments which are
677 // passed on as corresponding arguments in the call that G makes to F.
678 // For better debugability, under MergeFunctionsPDI, we do not modify G's
679 // call sites to point to F even when within the same translation unit.
writeThunk(Function * F,Function * G)680 void MergeFunctions::writeThunk(Function *F, Function *G) {
681 BasicBlock *GEntryBlock = nullptr;
682 std::vector<Instruction *> PDIUnrelatedWL;
683 BasicBlock *BB = nullptr;
684 Function *NewG = nullptr;
685 if (MergeFunctionsPDI) {
686 LLVM_DEBUG(dbgs() << "writeThunk: (MergeFunctionsPDI) Do not create a new "
687 "function as thunk; retain original: "
688 << G->getName() << "()\n");
689 GEntryBlock = &G->getEntryBlock();
690 LLVM_DEBUG(
691 dbgs() << "writeThunk: (MergeFunctionsPDI) filter parameter related "
692 "debug info for "
693 << G->getName() << "() {\n");
694 filterInstsUnrelatedToPDI(GEntryBlock, PDIUnrelatedWL);
695 GEntryBlock->getTerminator()->eraseFromParent();
696 BB = GEntryBlock;
697 } else {
698 NewG = Function::Create(G->getFunctionType(), G->getLinkage(),
699 G->getAddressSpace(), "", G->getParent());
700 NewG->setComdat(G->getComdat());
701 BB = BasicBlock::Create(F->getContext(), "", NewG);
702 }
703
704 IRBuilder<> Builder(BB);
705 Function *H = MergeFunctionsPDI ? G : NewG;
706 SmallVector<Value *, 16> Args;
707 unsigned i = 0;
708 FunctionType *FFTy = F->getFunctionType();
709 for (Argument &AI : H->args()) {
710 Args.push_back(createCast(Builder, &AI, FFTy->getParamType(i)));
711 ++i;
712 }
713
714 CallInst *CI = Builder.CreateCall(F, Args);
715 ReturnInst *RI = nullptr;
716 CI->setTailCall();
717 CI->setCallingConv(F->getCallingConv());
718 CI->setAttributes(F->getAttributes());
719 if (H->getReturnType()->isVoidTy()) {
720 RI = Builder.CreateRetVoid();
721 } else {
722 RI = Builder.CreateRet(createCast(Builder, CI, H->getReturnType()));
723 }
724
725 if (MergeFunctionsPDI) {
726 DISubprogram *DIS = G->getSubprogram();
727 if (DIS) {
728 DebugLoc CIDbgLoc = DebugLoc::get(DIS->getScopeLine(), 0, DIS);
729 DebugLoc RIDbgLoc = DebugLoc::get(DIS->getScopeLine(), 0, DIS);
730 CI->setDebugLoc(CIDbgLoc);
731 RI->setDebugLoc(RIDbgLoc);
732 } else {
733 LLVM_DEBUG(
734 dbgs() << "writeThunk: (MergeFunctionsPDI) No DISubprogram for "
735 << G->getName() << "()\n");
736 }
737 eraseTail(G);
738 eraseInstsUnrelatedToPDI(PDIUnrelatedWL);
739 LLVM_DEBUG(
740 dbgs() << "} // End of parameter related debug info filtering for: "
741 << G->getName() << "()\n");
742 } else {
743 NewG->copyAttributesFrom(G);
744 NewG->takeName(G);
745 removeUsers(G);
746 G->replaceAllUsesWith(NewG);
747 G->eraseFromParent();
748 }
749
750 LLVM_DEBUG(dbgs() << "writeThunk: " << H->getName() << '\n');
751 ++NumThunksWritten;
752 }
753
754 // Whether this function may be replaced by an alias
canCreateAliasFor(Function * F)755 static bool canCreateAliasFor(Function *F) {
756 if (!MergeFunctionsAliases || !F->hasGlobalUnnamedAddr())
757 return false;
758
759 // We should only see linkages supported by aliases here
760 assert(F->hasLocalLinkage() || F->hasExternalLinkage()
761 || F->hasWeakLinkage() || F->hasLinkOnceLinkage());
762 return true;
763 }
764
765 // Replace G with an alias to F (deleting function G)
writeAlias(Function * F,Function * G)766 void MergeFunctions::writeAlias(Function *F, Function *G) {
767 Constant *BitcastF = ConstantExpr::getBitCast(F, G->getType());
768 PointerType *PtrType = G->getType();
769 auto *GA = GlobalAlias::create(
770 PtrType->getElementType(), PtrType->getAddressSpace(),
771 G->getLinkage(), "", BitcastF, G->getParent());
772
773 F->setAlignment(MaybeAlign(std::max(F->getAlignment(), G->getAlignment())));
774 GA->takeName(G);
775 GA->setVisibility(G->getVisibility());
776 GA->setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
777
778 removeUsers(G);
779 G->replaceAllUsesWith(GA);
780 G->eraseFromParent();
781
782 LLVM_DEBUG(dbgs() << "writeAlias: " << GA->getName() << '\n');
783 ++NumAliasesWritten;
784 }
785
786 // Replace G with an alias to F if possible, or a thunk to F if
787 // profitable. Returns false if neither is the case.
writeThunkOrAlias(Function * F,Function * G)788 bool MergeFunctions::writeThunkOrAlias(Function *F, Function *G) {
789 if (canCreateAliasFor(G)) {
790 writeAlias(F, G);
791 return true;
792 }
793 if (canCreateThunkFor(F)) {
794 writeThunk(F, G);
795 return true;
796 }
797 return false;
798 }
799
800 // Merge two equivalent functions. Upon completion, Function G is deleted.
mergeTwoFunctions(Function * F,Function * G)801 void MergeFunctions::mergeTwoFunctions(Function *F, Function *G) {
802 if (F->isInterposable()) {
803 assert(G->isInterposable());
804
805 // Both writeThunkOrAlias() calls below must succeed, either because we can
806 // create aliases for G and NewF, or because a thunk for F is profitable.
807 // F here has the same signature as NewF below, so that's what we check.
808 if (!canCreateThunkFor(F) &&
809 (!canCreateAliasFor(F) || !canCreateAliasFor(G)))
810 return;
811
812 // Make them both thunks to the same internal function.
813 Function *NewF = Function::Create(F->getFunctionType(), F->getLinkage(),
814 F->getAddressSpace(), "", F->getParent());
815 NewF->copyAttributesFrom(F);
816 NewF->takeName(F);
817 removeUsers(F);
818 F->replaceAllUsesWith(NewF);
819
820 MaybeAlign MaxAlignment(std::max(G->getAlignment(), NewF->getAlignment()));
821
822 writeThunkOrAlias(F, G);
823 writeThunkOrAlias(F, NewF);
824
825 F->setAlignment(MaxAlignment);
826 F->setLinkage(GlobalValue::PrivateLinkage);
827 ++NumDoubleWeak;
828 ++NumFunctionsMerged;
829 } else {
830 // For better debugability, under MergeFunctionsPDI, we do not modify G's
831 // call sites to point to F even when within the same translation unit.
832 if (!G->isInterposable() && !MergeFunctionsPDI) {
833 if (G->hasGlobalUnnamedAddr()) {
834 // G might have been a key in our GlobalNumberState, and it's illegal
835 // to replace a key in ValueMap<GlobalValue *> with a non-global.
836 GlobalNumbers.erase(G);
837 // If G's address is not significant, replace it entirely.
838 Constant *BitcastF = ConstantExpr::getBitCast(F, G->getType());
839 removeUsers(G);
840 G->replaceAllUsesWith(BitcastF);
841 } else {
842 // Redirect direct callers of G to F. (See note on MergeFunctionsPDI
843 // above).
844 replaceDirectCallers(G, F);
845 }
846 }
847
848 // If G was internal then we may have replaced all uses of G with F. If so,
849 // stop here and delete G. There's no need for a thunk. (See note on
850 // MergeFunctionsPDI above).
851 if (G->isDiscardableIfUnused() && G->use_empty() && !MergeFunctionsPDI) {
852 G->eraseFromParent();
853 ++NumFunctionsMerged;
854 return;
855 }
856
857 if (writeThunkOrAlias(F, G)) {
858 ++NumFunctionsMerged;
859 }
860 }
861 }
862
863 /// Replace function F by function G.
replaceFunctionInTree(const FunctionNode & FN,Function * G)864 void MergeFunctions::replaceFunctionInTree(const FunctionNode &FN,
865 Function *G) {
866 Function *F = FN.getFunc();
867 assert(FunctionComparator(F, G, &GlobalNumbers).compare() == 0 &&
868 "The two functions must be equal");
869
870 auto I = FNodesInTree.find(F);
871 assert(I != FNodesInTree.end() && "F should be in FNodesInTree");
872 assert(FNodesInTree.count(G) == 0 && "FNodesInTree should not contain G");
873
874 FnTreeType::iterator IterToFNInFnTree = I->second;
875 assert(&(*IterToFNInFnTree) == &FN && "F should map to FN in FNodesInTree.");
876 // Remove F -> FN and insert G -> FN
877 FNodesInTree.erase(I);
878 FNodesInTree.insert({G, IterToFNInFnTree});
879 // Replace F with G in FN, which is stored inside the FnTree.
880 FN.replaceBy(G);
881 }
882
883 // Ordering for functions that are equal under FunctionComparator
isFuncOrderCorrect(const Function * F,const Function * G)884 static bool isFuncOrderCorrect(const Function *F, const Function *G) {
885 if (F->isInterposable() != G->isInterposable()) {
886 // Strong before weak, because the weak function may call the strong
887 // one, but not the other way around.
888 return !F->isInterposable();
889 }
890 if (F->hasLocalLinkage() != G->hasLocalLinkage()) {
891 // External before local, because we definitely have to keep the external
892 // function, but may be able to drop the local one.
893 return !F->hasLocalLinkage();
894 }
895 // Impose a total order (by name) on the replacement of functions. This is
896 // important when operating on more than one module independently to prevent
897 // cycles of thunks calling each other when the modules are linked together.
898 return F->getName() <= G->getName();
899 }
900
901 // Insert a ComparableFunction into the FnTree, or merge it away if equal to one
902 // that was already inserted.
insert(Function * NewFunction)903 bool MergeFunctions::insert(Function *NewFunction) {
904 std::pair<FnTreeType::iterator, bool> Result =
905 FnTree.insert(FunctionNode(NewFunction));
906
907 if (Result.second) {
908 assert(FNodesInTree.count(NewFunction) == 0);
909 FNodesInTree.insert({NewFunction, Result.first});
910 LLVM_DEBUG(dbgs() << "Inserting as unique: " << NewFunction->getName()
911 << '\n');
912 return false;
913 }
914
915 const FunctionNode &OldF = *Result.first;
916
917 if (!isFuncOrderCorrect(OldF.getFunc(), NewFunction)) {
918 // Swap the two functions.
919 Function *F = OldF.getFunc();
920 replaceFunctionInTree(*Result.first, NewFunction);
921 NewFunction = F;
922 assert(OldF.getFunc() != F && "Must have swapped the functions.");
923 }
924
925 LLVM_DEBUG(dbgs() << " " << OldF.getFunc()->getName()
926 << " == " << NewFunction->getName() << '\n');
927
928 Function *DeleteF = NewFunction;
929 mergeTwoFunctions(OldF.getFunc(), DeleteF);
930 return true;
931 }
932
933 // Remove a function from FnTree. If it was already in FnTree, add
934 // it to Deferred so that we'll look at it in the next round.
remove(Function * F)935 void MergeFunctions::remove(Function *F) {
936 auto I = FNodesInTree.find(F);
937 if (I != FNodesInTree.end()) {
938 LLVM_DEBUG(dbgs() << "Deferred " << F->getName() << ".\n");
939 FnTree.erase(I->second);
940 // I->second has been invalidated, remove it from the FNodesInTree map to
941 // preserve the invariant.
942 FNodesInTree.erase(I);
943 Deferred.emplace_back(F);
944 }
945 }
946
947 // For each instruction used by the value, remove() the function that contains
948 // the instruction. This should happen right before a call to RAUW.
removeUsers(Value * V)949 void MergeFunctions::removeUsers(Value *V) {
950 for (User *U : V->users())
951 if (auto *I = dyn_cast<Instruction>(U))
952 remove(I->getFunction());
953 }
954