Line data Source code
1 : //===- GlobalMerge.cpp - Internal globals merging -------------------------===//
2 : //
3 : // The LLVM Compiler Infrastructure
4 : //
5 : // This file is distributed under the University of Illinois Open Source
6 : // License. See LICENSE.TXT for details.
7 : //
8 : //===----------------------------------------------------------------------===//
9 : //
10 : // This pass merges globals with internal linkage into one. This way all the
11 : // globals which were merged into a biggest one can be addressed using offsets
12 : // from the same base pointer (no need for separate base pointer for each of the
13 : // global). Such a transformation can significantly reduce the register pressure
14 : // when many globals are involved.
15 : //
16 : // For example, consider the code which touches several global variables at
17 : // once:
18 : //
19 : // static int foo[N], bar[N], baz[N];
20 : //
21 : // for (i = 0; i < N; ++i) {
22 : // foo[i] = bar[i] * baz[i];
23 : // }
24 : //
25 : // On ARM the addresses of 3 arrays should be kept in the registers, thus
26 : // this code has quite large register pressure (loop body):
27 : //
28 : // ldr r1, [r5], #4
29 : // ldr r2, [r6], #4
30 : // mul r1, r2, r1
31 : // str r1, [r0], #4
32 : //
33 : // Pass converts the code to something like:
34 : //
35 : // static struct {
36 : // int foo[N];
37 : // int bar[N];
38 : // int baz[N];
39 : // } merged;
40 : //
41 : // for (i = 0; i < N; ++i) {
42 : // merged.foo[i] = merged.bar[i] * merged.baz[i];
43 : // }
44 : //
45 : // and in ARM code this becomes:
46 : //
47 : // ldr r0, [r5, #40]
48 : // ldr r1, [r5, #80]
49 : // mul r0, r1, r0
50 : // str r0, [r5], #4
51 : //
52 : // note that we saved 2 registers here almostly "for free".
53 : //
54 : // However, merging globals can have tradeoffs:
55 : // - it confuses debuggers, tools, and users
56 : // - it makes linker optimizations less useful (order files, LOHs, ...)
57 : // - it forces usage of indexed addressing (which isn't necessarily "free")
58 : // - it can increase register pressure when the uses are disparate enough.
59 : //
60 : // We use heuristics to discover the best global grouping we can (cf cl::opts).
61 : //
62 : // ===---------------------------------------------------------------------===//
63 :
64 : #include "llvm/ADT/BitVector.h"
65 : #include "llvm/ADT/DenseMap.h"
66 : #include "llvm/ADT/SmallPtrSet.h"
67 : #include "llvm/ADT/SmallVector.h"
68 : #include "llvm/ADT/Statistic.h"
69 : #include "llvm/ADT/StringRef.h"
70 : #include "llvm/ADT/Triple.h"
71 : #include "llvm/ADT/Twine.h"
72 : #include "llvm/CodeGen/Passes.h"
73 : #include "llvm/IR/BasicBlock.h"
74 : #include "llvm/IR/Constants.h"
75 : #include "llvm/IR/DataLayout.h"
76 : #include "llvm/IR/DerivedTypes.h"
77 : #include "llvm/IR/Function.h"
78 : #include "llvm/IR/GlobalAlias.h"
79 : #include "llvm/IR/GlobalValue.h"
80 : #include "llvm/IR/GlobalVariable.h"
81 : #include "llvm/IR/Instruction.h"
82 : #include "llvm/IR/Module.h"
83 : #include "llvm/IR/Type.h"
84 : #include "llvm/IR/Use.h"
85 : #include "llvm/IR/User.h"
86 : #include "llvm/Pass.h"
87 : #include "llvm/Support/Casting.h"
88 : #include "llvm/Support/CommandLine.h"
89 : #include "llvm/Support/Debug.h"
90 : #include "llvm/Support/raw_ostream.h"
91 : #include "llvm/Target/TargetLoweringObjectFile.h"
92 : #include "llvm/Target/TargetMachine.h"
93 : #include <algorithm>
94 : #include <cassert>
95 : #include <cstddef>
96 : #include <cstdint>
97 : #include <string>
98 : #include <vector>
99 :
100 : using namespace llvm;
101 :
102 : #define DEBUG_TYPE "global-merge"
103 :
104 : // FIXME: This is only useful as a last-resort way to disable the pass.
105 : static cl::opt<bool>
106 : EnableGlobalMerge("enable-global-merge", cl::Hidden,
107 : cl::desc("Enable the global merge pass"),
108 : cl::init(true));
109 :
110 : static cl::opt<unsigned>
111 : GlobalMergeMaxOffset("global-merge-max-offset", cl::Hidden,
112 : cl::desc("Set maximum offset for global merge pass"),
113 : cl::init(0));
114 :
115 : static cl::opt<bool> GlobalMergeGroupByUse(
116 : "global-merge-group-by-use", cl::Hidden,
117 : cl::desc("Improve global merge pass to look at uses"), cl::init(true));
118 :
119 : static cl::opt<bool> GlobalMergeIgnoreSingleUse(
120 : "global-merge-ignore-single-use", cl::Hidden,
121 : cl::desc("Improve global merge pass to ignore globals only used alone"),
122 : cl::init(true));
123 :
124 : static cl::opt<bool>
125 : EnableGlobalMergeOnConst("global-merge-on-const", cl::Hidden,
126 : cl::desc("Enable global merge pass on constants"),
127 : cl::init(false));
128 :
129 : // FIXME: this could be a transitional option, and we probably need to remove
130 : // it if only we are sure this optimization could always benefit all targets.
131 : static cl::opt<cl::boolOrDefault>
132 : EnableGlobalMergeOnExternal("global-merge-on-external", cl::Hidden,
133 : cl::desc("Enable global merge pass on external linkage"));
134 :
135 : STATISTIC(NumMerged, "Number of globals merged");
136 :
137 : namespace {
138 :
139 : class GlobalMerge : public FunctionPass {
140 : const TargetMachine *TM = nullptr;
141 :
142 : // FIXME: Infer the maximum possible offset depending on the actual users
143 : // (these max offsets are different for the users inside Thumb or ARM
144 : // functions), see the code that passes in the offset in the ARM backend
145 : // for more information.
146 : unsigned MaxOffset;
147 :
148 : /// Whether we should try to optimize for size only.
149 : /// Currently, this applies a dead simple heuristic: only consider globals
150 : /// used in minsize functions for merging.
151 : /// FIXME: This could learn about optsize, and be used in the cost model.
152 : bool OnlyOptimizeForSize = false;
153 :
154 : /// Whether we should merge global variables that have external linkage.
155 : bool MergeExternalGlobals = false;
156 :
157 : bool IsMachO;
158 :
159 : bool doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
160 : Module &M, bool isConst, unsigned AddrSpace) const;
161 :
162 : /// Merge everything in \p Globals for which the corresponding bit
163 : /// in \p GlobalSet is set.
164 : bool doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
165 : const BitVector &GlobalSet, Module &M, bool isConst,
166 : unsigned AddrSpace) const;
167 :
168 : /// Check if the given variable has been identified as must keep
169 : /// \pre setMustKeepGlobalVariables must have been called on the Module that
170 : /// contains GV
171 : bool isMustKeepGlobalVariable(const GlobalVariable *GV) const {
172 496 : return MustKeepGlobalVariables.count(GV);
173 : }
174 :
175 : /// Collect every variables marked as "used" or used in a landing pad
176 : /// instruction for this Module.
177 : void setMustKeepGlobalVariables(Module &M);
178 :
179 : /// Collect every variables marked as "used"
180 : void collectUsedGlobalVariables(Module &M, StringRef Name);
181 :
182 : /// Keep track of the GlobalVariable that must not be merged away
183 : SmallPtrSet<const GlobalVariable *, 16> MustKeepGlobalVariables;
184 :
185 : public:
186 : static char ID; // Pass identification, replacement for typeid.
187 :
188 5 : explicit GlobalMerge()
189 15 : : FunctionPass(ID), MaxOffset(GlobalMergeMaxOffset) {
190 5 : initializeGlobalMergePass(*PassRegistry::getPassRegistry());
191 5 : }
192 :
193 3688 : explicit GlobalMerge(const TargetMachine *TM, unsigned MaximalOffset,
194 : bool OnlyOptimizeForSize, bool MergeExternalGlobals)
195 3688 : : FunctionPass(ID), TM(TM), MaxOffset(MaximalOffset),
196 : OnlyOptimizeForSize(OnlyOptimizeForSize),
197 3688 : MergeExternalGlobals(MergeExternalGlobals) {
198 3688 : initializeGlobalMergePass(*PassRegistry::getPassRegistry());
199 3688 : }
200 :
201 : bool doInitialization(Module &M) override;
202 : bool runOnFunction(Function &F) override;
203 : bool doFinalization(Module &M) override;
204 :
205 7 : StringRef getPassName() const override { return "Merge internal globals"; }
206 :
207 3682 : void getAnalysisUsage(AnalysisUsage &AU) const override {
208 3682 : AU.setPreservesCFG();
209 3682 : FunctionPass::getAnalysisUsage(AU);
210 3682 : }
211 : };
212 :
213 : } // end anonymous namespace
214 :
215 : char GlobalMerge::ID = 0;
216 :
217 28859 : INITIALIZE_PASS(GlobalMerge, DEBUG_TYPE, "Merge global variables", false, false)
218 :
219 101 : bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
220 : Module &M, bool isConst, unsigned AddrSpace) const {
221 101 : auto &DL = M.getDataLayout();
222 : // FIXME: Find better heuristics
223 : std::stable_sort(Globals.begin(), Globals.end(),
224 : [&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
225 : return DL.getTypeAllocSize(GV1->getValueType()) <
226 : DL.getTypeAllocSize(GV2->getValueType());
227 : });
228 :
229 : // If we want to just blindly group all globals together, do so.
230 101 : if (!GlobalMergeGroupByUse) {
231 8 : BitVector AllGlobals(Globals.size());
232 8 : AllGlobals.set();
233 8 : return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
234 : }
235 :
236 : // If we want to be smarter, look at all uses of each global, to try to
237 : // discover all sets of globals used together, and how many times each of
238 : // these sets occurred.
239 : //
240 : // Keep this reasonably efficient, by having an append-only list of all sets
241 : // discovered so far (UsedGlobalSet), and mapping each "together-ness" unit of
242 : // code (currently, a Function) to the set of globals seen so far that are
243 : // used together in that unit (GlobalUsesByFunction).
244 : //
245 : // When we look at the Nth global, we know that any new set is either:
246 : // - the singleton set {N}, containing this global only, or
247 : // - the union of {N} and a previously-discovered set, containing some
248 : // combination of the previous N-1 globals.
249 : // Using that knowledge, when looking at the Nth global, we can keep:
250 : // - a reference to the singleton set {N} (CurGVOnlySetIdx)
251 : // - a list mapping each previous set to its union with {N} (EncounteredUGS),
252 : // if it actually occurs.
253 :
254 : // We keep track of the sets of globals used together "close enough".
255 1351 : struct UsedGlobalSet {
256 : BitVector Globals;
257 : unsigned UsageCount = 1;
258 :
259 210 : UsedGlobalSet(size_t Size) : Globals(Size) {}
260 : };
261 :
262 : // Each set is unique in UsedGlobalSets.
263 93 : std::vector<UsedGlobalSet> UsedGlobalSets;
264 :
265 : // Avoid repeating the create-global-set pattern.
266 : auto CreateGlobalSet = [&]() -> UsedGlobalSet & {
267 363 : UsedGlobalSets.emplace_back(Globals.size());
268 : return UsedGlobalSets.back();
269 : };
270 :
271 : // The first set is the empty set.
272 93 : CreateGlobalSet().UsageCount = 0;
273 :
274 : // We define "close enough" to be "in the same function".
275 : // FIXME: Grouping uses by function is way too aggressive, so we should have
276 : // a better metric for distance between uses.
277 : // The obvious alternative would be to group by BasicBlock, but that's in
278 : // turn too conservative..
279 : // Anything in between wouldn't be trivial to compute, so just stick with
280 : // per-function grouping.
281 :
282 : // The value type is an index into UsedGlobalSets.
283 : // The default (0) conveniently points to the empty set.
284 : DenseMap<Function *, size_t /*UsedGlobalSetIdx*/> GlobalUsesByFunction;
285 :
286 : // Now, look at each merge-eligible global in turn.
287 :
288 : // Keep track of the sets we already encountered to which we added the
289 : // current global.
290 : // Each element matches the same-index element in UsedGlobalSets.
291 : // This lets us efficiently tell whether a set has already been expanded to
292 : // include the current global.
293 : std::vector<size_t> EncounteredUGS;
294 :
295 379 : for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) {
296 286 : GlobalVariable *GV = Globals[GI];
297 :
298 : // Reset the encountered sets for this global...
299 : std::fill(EncounteredUGS.begin(), EncounteredUGS.end(), 0);
300 : // ...and grow it in case we created new sets for the previous global.
301 572 : EncounteredUGS.resize(UsedGlobalSets.size());
302 :
303 : // We might need to create a set that only consists of the current global.
304 : // Keep track of its index into UsedGlobalSets.
305 : size_t CurGVOnlySetIdx = 0;
306 :
307 : // For each global, look at all its Uses.
308 1050 : for (auto &U : GV->uses()) {
309 : // This Use might be a ConstantExpr. We're interested in Instruction
310 : // users, so look through ConstantExpr...
311 : Use *UI, *UE;
312 764 : if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U.getUser())) {
313 139 : if (CE->use_empty())
314 : continue;
315 : UI = &*CE->use_begin();
316 : UE = nullptr;
317 625 : } else if (isa<Instruction>(U.getUser())) {
318 : UI = &U;
319 625 : UE = UI->getNext();
320 : } else {
321 : continue;
322 : }
323 :
324 : // ...to iterate on all the instruction users of the global.
325 : // Note that we iterate on Uses and not on Users to be able to getNext().
326 1549 : for (; UI != UE; UI = UI->getNext()) {
327 785 : Instruction *I = dyn_cast<Instruction>(UI->getUser());
328 : if (!I)
329 728 : continue;
330 :
331 785 : Function *ParentFn = I->getParent()->getParent();
332 :
333 : // If we're only optimizing for size, ignore non-minsize functions.
334 1383 : if (OnlyOptimizeForSize && !ParentFn->optForMinSize())
335 : continue;
336 :
337 195 : size_t UGSIdx = GlobalUsesByFunction[ParentFn];
338 :
339 : // If this is the first global the basic block uses, map it to the set
340 : // consisting of this global only.
341 195 : if (!UGSIdx) {
342 : // If that set doesn't exist yet, create it.
343 66 : if (!CurGVOnlySetIdx) {
344 60 : CurGVOnlySetIdx = UsedGlobalSets.size();
345 60 : CreateGlobalSet().Globals.set(GI);
346 : } else {
347 12 : ++UsedGlobalSets[CurGVOnlySetIdx].UsageCount;
348 : }
349 :
350 66 : GlobalUsesByFunction[ParentFn] = CurGVOnlySetIdx;
351 66 : continue;
352 : }
353 :
354 : // If we already encountered this BB, just increment the counter.
355 387 : if (UsedGlobalSets[UGSIdx].Globals.test(GI)) {
356 66 : ++UsedGlobalSets[UGSIdx].UsageCount;
357 66 : continue;
358 : }
359 :
360 : // If not, the previous set wasn't actually used in this function.
361 63 : --UsedGlobalSets[UGSIdx].UsageCount;
362 :
363 : // If we already expanded the previous set to include this global, just
364 : // reuse that expanded set.
365 126 : if (size_t ExpandedIdx = EncounteredUGS[UGSIdx]) {
366 6 : ++UsedGlobalSets[ExpandedIdx].UsageCount;
367 6 : GlobalUsesByFunction[ParentFn] = ExpandedIdx;
368 6 : continue;
369 : }
370 :
371 : // If not, create a new set consisting of the union of the previous set
372 : // and this global. Mark it as encountered, so we can reuse it later.
373 57 : GlobalUsesByFunction[ParentFn] = EncounteredUGS[UGSIdx] =
374 57 : UsedGlobalSets.size();
375 :
376 : UsedGlobalSet &NewUGS = CreateGlobalSet();
377 : NewUGS.Globals.set(GI);
378 114 : NewUGS.Globals |= UsedGlobalSets[UGSIdx].Globals;
379 : }
380 : }
381 : }
382 :
383 : // Now we found a bunch of sets of globals used together. We accumulated
384 : // the number of times we encountered the sets (i.e., the number of blocks
385 : // that use that exact set of globals).
386 : //
387 : // Multiply that by the size of the set to give us a crude profitability
388 : // metric.
389 93 : std::stable_sort(UsedGlobalSets.begin(), UsedGlobalSets.end(),
390 : [](const UsedGlobalSet &UGS1, const UsedGlobalSet &UGS2) {
391 : return UGS1.Globals.count() * UGS1.UsageCount <
392 : UGS2.Globals.count() * UGS2.UsageCount;
393 : });
394 :
395 : // We can choose to merge all globals together, but ignore globals never used
396 : // with another global. This catches the obviously non-profitable cases of
397 : // having a single global, but is aggressive enough for any other case.
398 93 : if (GlobalMergeIgnoreSingleUse) {
399 92 : BitVector AllGlobals(Globals.size());
400 290 : for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
401 198 : const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
402 198 : if (UGS.UsageCount == 0)
403 : continue;
404 66 : if (UGS.Globals.count() > 1)
405 50 : AllGlobals |= UGS.Globals;
406 : }
407 92 : return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
408 : }
409 :
410 : // Starting from the sets with the best (=biggest) profitability, find a
411 : // good combination.
412 : // The ideal (and expensive) solution can only be found by trying all
413 : // combinations, looking for the one with the best profitability.
414 : // Don't be smart about it, and just pick the first compatible combination,
415 : // starting with the sets with the best profitability.
416 1 : BitVector PickedGlobals(Globals.size());
417 : bool Changed = false;
418 :
419 13 : for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
420 12 : const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
421 12 : if (UGS.UsageCount == 0)
422 : continue;
423 5 : if (PickedGlobals.anyCommon(UGS.Globals))
424 : continue;
425 4 : PickedGlobals |= UGS.Globals;
426 : // If the set only contains one global, there's no point in merging.
427 : // Ignore the global for inclusion in other sets though, so keep it in
428 : // PickedGlobals.
429 4 : if (UGS.Globals.count() < 2)
430 : continue;
431 3 : Changed |= doMerge(Globals, UGS.Globals, M, isConst, AddrSpace);
432 : }
433 :
434 : return Changed;
435 : }
436 :
437 103 : bool GlobalMerge::doMerge(const SmallVectorImpl<GlobalVariable *> &Globals,
438 : const BitVector &GlobalSet, Module &M, bool isConst,
439 : unsigned AddrSpace) const {
440 : assert(Globals.size() > 1);
441 :
442 103 : Type *Int32Ty = Type::getInt32Ty(M.getContext());
443 103 : Type *Int8Ty = Type::getInt8Ty(M.getContext());
444 103 : auto &DL = M.getDataLayout();
445 :
446 : LLVM_DEBUG(dbgs() << " Trying to merge set, starts with #"
447 : << GlobalSet.find_first() << "\n");
448 :
449 : bool Changed = false;
450 103 : ssize_t i = GlobalSet.find_first();
451 151 : while (i != -1) {
452 : ssize_t j = 0;
453 : uint64_t MergedSize = 0;
454 : std::vector<Type*> Tys;
455 : std::vector<Constant*> Inits;
456 : std::vector<unsigned> StructIdxs;
457 :
458 : bool HasExternal = false;
459 48 : StringRef FirstExternalName;
460 48 : unsigned MaxAlign = 1;
461 : unsigned CurIdx = 0;
462 288 : for (j = i; j != -1; j = GlobalSet.find_next(j)) {
463 121 : Type *Ty = Globals[j]->getValueType();
464 :
465 : // Make sure we use the same alignment AsmPrinter would use.
466 121 : unsigned Align = DL.getPreferredAlignment(Globals[j]);
467 121 : unsigned Padding = alignTo(MergedSize, Align) - MergedSize;
468 121 : MergedSize += Padding;
469 121 : MergedSize += DL.getTypeAllocSize(Ty);
470 121 : if (MergedSize > MaxOffset) {
471 : break;
472 : }
473 120 : if (Padding) {
474 1 : Tys.push_back(ArrayType::get(Int8Ty, Padding));
475 1 : Inits.push_back(ConstantAggregateZero::get(Tys.back()));
476 1 : ++CurIdx;
477 : }
478 120 : Tys.push_back(Ty);
479 240 : Inits.push_back(Globals[j]->getInitializer());
480 120 : StructIdxs.push_back(CurIdx++);
481 :
482 120 : MaxAlign = std::max(MaxAlign, Align);
483 :
484 240 : if (Globals[j]->hasExternalLinkage() && !HasExternal) {
485 : HasExternal = true;
486 15 : FirstExternalName = Globals[j]->getName();
487 : }
488 : }
489 :
490 : // Exit early if there is only one global to merge.
491 96 : if (Tys.size() < 2) {
492 : i = j;
493 : continue;
494 : }
495 :
496 : // If merged variables doesn't have external linkage, we needn't to expose
497 : // the symbol after merging.
498 : GlobalValue::LinkageTypes Linkage = HasExternal
499 47 : ? GlobalValue::ExternalLinkage
500 : : GlobalValue::InternalLinkage;
501 : // Use a packed struct so we can control alignment.
502 47 : StructType *MergedTy = StructType::get(M.getContext(), Tys, true);
503 47 : Constant *MergedInit = ConstantStruct::get(MergedTy, Inits);
504 :
505 : // On Darwin external linkage needs to be preserved, otherwise
506 : // dsymutil cannot preserve the debug info for the merged
507 : // variables. If they have external linkage, use the symbol name
508 : // of the first variable merged as the suffix of global symbol
509 : // name. This avoids a link-time naming conflict for the
510 : // _MergedGlobals symbols.
511 : Twine MergedName =
512 22 : (IsMachO && HasExternal)
513 47 : ? "_MergedGlobals_" + FirstExternalName
514 : : "_MergedGlobals";
515 47 : auto MergedLinkage = IsMachO ? Linkage : GlobalValue::PrivateLinkage;
516 : auto *MergedGV = new GlobalVariable(
517 : M, MergedTy, isConst, MergedLinkage, MergedInit, MergedName, nullptr,
518 47 : GlobalVariable::NotThreadLocal, AddrSpace);
519 :
520 47 : MergedGV->setAlignment(MaxAlign);
521 95 : MergedGV->setSection(Globals[i]->getSection());
522 :
523 47 : const StructLayout *MergedLayout = DL.getStructLayout(MergedTy);
524 285 : for (ssize_t k = i, idx = 0; k != j; k = GlobalSet.find_next(k), ++idx) {
525 238 : GlobalValue::LinkageTypes Linkage = Globals[k]->getLinkage();
526 238 : std::string Name = Globals[k]->getName();
527 : GlobalValue::DLLStorageClassTypes DLLStorage =
528 119 : Globals[k]->getDLLStorageClass();
529 :
530 : // Copy metadata while adjusting any debug info metadata by the original
531 : // global's offset within the merged global.
532 119 : MergedGV->copyMetadata(Globals[k],
533 238 : MergedLayout->getElementOffset(StructIdxs[idx]));
534 :
535 : Constant *Idx[2] = {
536 119 : ConstantInt::get(Int32Ty, 0),
537 119 : ConstantInt::get(Int32Ty, StructIdxs[idx]),
538 119 : };
539 : Constant *GEP =
540 : ConstantExpr::getInBoundsGetElementPtr(MergedTy, MergedGV, Idx);
541 119 : Globals[k]->replaceAllUsesWith(GEP);
542 119 : Globals[k]->eraseFromParent();
543 :
544 : // When the linkage is not internal we must emit an alias for the original
545 : // variable name as it may be accessed from another object. On non-Mach-O
546 : // we can also emit an alias for internal linkage as it's safe to do so.
547 : // It's not safe on Mach-O as the alias (and thus the portion of the
548 : // MergedGlobals variable) may be dead stripped at link time.
549 119 : if (Linkage != GlobalValue::InternalLinkage || !IsMachO) {
550 186 : GlobalAlias *GA = GlobalAlias::create(Tys[StructIdxs[idx]], AddrSpace,
551 : Linkage, Name, GEP, &M);
552 : GA->setDLLStorageClass(DLLStorage);
553 : }
554 :
555 : NumMerged++;
556 : }
557 : Changed = true;
558 : i = j;
559 : }
560 :
561 103 : return Changed;
562 : }
563 :
564 7366 : void GlobalMerge::collectUsedGlobalVariables(Module &M, StringRef Name) {
565 : // Extract global variables from llvm.used array
566 7366 : const GlobalVariable *GV = M.getGlobalVariable(Name);
567 7382 : if (!GV || !GV->hasInitializer()) return;
568 :
569 : // Should be an array of 'i8*'.
570 : const ConstantArray *InitList = cast<ConstantArray>(GV->getInitializer());
571 :
572 32 : for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i)
573 : if (const GlobalVariable *G =
574 17 : dyn_cast<GlobalVariable>(InitList->getOperand(i)->stripPointerCasts()))
575 6 : MustKeepGlobalVariables.insert(G);
576 : }
577 :
578 3683 : void GlobalMerge::setMustKeepGlobalVariables(Module &M) {
579 3683 : collectUsedGlobalVariables(M, "llvm.used");
580 3683 : collectUsedGlobalVariables(M, "llvm.compiler.used");
581 :
582 39263 : for (Function &F : M) {
583 71197 : for (BasicBlock &BB : F) {
584 : Instruction *Pad = BB.getFirstNonPHI();
585 : if (!Pad->isEHPad())
586 : continue;
587 :
588 : // Keep globals used by landingpads and catchpads.
589 498 : for (const Use &U : Pad->operands()) {
590 : if (const GlobalVariable *GV =
591 104 : dyn_cast<GlobalVariable>(U->stripPointerCasts()))
592 18 : MustKeepGlobalVariables.insert(GV);
593 : }
594 : }
595 : }
596 3683 : }
597 :
598 3683 : bool GlobalMerge::doInitialization(Module &M) {
599 3683 : if (!EnableGlobalMerge)
600 : return false;
601 :
602 3683 : IsMachO = Triple(M.getTargetTriple()).isOSBinFormatMachO();
603 :
604 3683 : auto &DL = M.getDataLayout();
605 : DenseMap<std::pair<unsigned, StringRef>, SmallVector<GlobalVariable *, 16>>
606 : Globals, ConstGlobals, BSSGlobals;
607 : bool Changed = false;
608 3683 : setMustKeepGlobalVariables(M);
609 :
610 : // Grab all non-const globals.
611 6173 : for (auto &GV : M.globals()) {
612 : // Merge is safe for "normal" internal or external globals only
613 2490 : if (GV.isDeclaration() || GV.isThreadLocal() || GV.hasImplicitSection())
614 972 : continue;
615 :
616 : // It's not safe to merge globals that may be preempted
617 1518 : if (TM && !TM->shouldAssumeDSOLocal(M, &GV))
618 : continue;
619 :
620 1430 : if (!(MergeExternalGlobals && GV.hasExternalLinkage()) &&
621 : !GV.hasInternalLinkage())
622 : continue;
623 :
624 : PointerType *PT = dyn_cast<PointerType>(GV.getType());
625 : assert(PT && "Global variable is not a pointer!");
626 :
627 : unsigned AddressSpace = PT->getAddressSpace();
628 496 : StringRef Section = GV.getSection();
629 :
630 : // Ignore all 'special' globals.
631 496 : if (GV.getName().startswith("llvm.") ||
632 992 : GV.getName().startswith(".llvm."))
633 0 : continue;
634 :
635 : // Ignore all "required" globals:
636 496 : if (isMustKeepGlobalVariable(&GV))
637 : continue;
638 :
639 489 : Type *Ty = GV.getValueType();
640 489 : if (DL.getTypeAllocSize(Ty) < MaxOffset) {
641 906 : if (TM &&
642 906 : TargetLoweringObjectFile::getKindForGlobal(&GV, *TM).isBSS())
643 287 : BSSGlobals[{AddressSpace, Section}].push_back(&GV);
644 172 : else if (GV.isConstant())
645 51 : ConstGlobals[{AddressSpace, Section}].push_back(&GV);
646 : else
647 121 : Globals[{AddressSpace, Section}].push_back(&GV);
648 : }
649 : }
650 :
651 7444 : for (auto &P : Globals)
652 78 : if (P.second.size() > 1)
653 31 : Changed |= doMerge(P.second, M, false, P.first.first);
654 :
655 7489 : for (auto &P : BSSGlobals)
656 123 : if (P.second.size() > 1)
657 69 : Changed |= doMerge(P.second, M, false, P.first.first);
658 :
659 3683 : if (EnableGlobalMergeOnConst)
660 3 : for (auto &P : ConstGlobals)
661 1 : if (P.second.size() > 1)
662 1 : Changed |= doMerge(P.second, M, true, P.first.first);
663 :
664 : return Changed;
665 : }
666 :
667 27606 : bool GlobalMerge::runOnFunction(Function &F) {
668 27606 : return false;
669 : }
670 :
671 3647 : bool GlobalMerge::doFinalization(Module &M) {
672 3647 : MustKeepGlobalVariables.clear();
673 3647 : return false;
674 : }
675 :
676 3688 : Pass *llvm::createGlobalMergePass(const TargetMachine *TM, unsigned Offset,
677 : bool OnlyOptimizeForSize,
678 : bool MergeExternalByDefault) {
679 3688 : bool MergeExternal = (EnableGlobalMergeOnExternal == cl::BOU_UNSET) ?
680 : MergeExternalByDefault : (EnableGlobalMergeOnExternal == cl::BOU_TRUE);
681 3688 : return new GlobalMerge(TM, Offset, OnlyOptimizeForSize, MergeExternal);
682 : }
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