1 //==- AArch64PromoteConstant.cpp - Promote constant to global for AArch64 --==//
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
7 //===----------------------------------------------------------------------===//
9 // This file implements the AArch64PromoteConstant pass which promotes constants
10 // to global variables when this is likely to be more efficient. Currently only
11 // types related to constant vector (i.e., constant vector, array of constant
12 // vectors, constant structure with a constant vector field, etc.) are promoted
13 // to global variables. Constant vectors are likely to be lowered in target
14 // constant pool during instruction selection already; therefore, the access
15 // will remain the same (memory load), but the structure types are not split
16 // into different constant pool accesses for each field. A bonus side effect is
17 // that created globals may be merged by the global merge pass.
19 // FIXME: This pass may be useful for other targets too.
20 //===----------------------------------------------------------------------===//
23 #include "llvm/ADT/DenseMap.h"
24 #include "llvm/ADT/SmallVector.h"
25 #include "llvm/ADT/Statistic.h"
26 #include "llvm/IR/BasicBlock.h"
27 #include "llvm/IR/Constant.h"
28 #include "llvm/IR/Constants.h"
29 #include "llvm/IR/Dominators.h"
30 #include "llvm/IR/Function.h"
31 #include "llvm/IR/GlobalValue.h"
32 #include "llvm/IR/GlobalVariable.h"
33 #include "llvm/IR/IRBuilder.h"
34 #include "llvm/IR/InlineAsm.h"
35 #include "llvm/IR/InstIterator.h"
36 #include "llvm/IR/Instruction.h"
37 #include "llvm/IR/Instructions.h"
38 #include "llvm/IR/IntrinsicInst.h"
39 #include "llvm/IR/Module.h"
40 #include "llvm/IR/Type.h"
41 #include "llvm/InitializePasses.h"
42 #include "llvm/Pass.h"
43 #include "llvm/Support/Casting.h"
44 #include "llvm/Support/CommandLine.h"
45 #include "llvm/Support/Debug.h"
46 #include "llvm/Support/raw_ostream.h"
53 #define DEBUG_TYPE "aarch64-promote-const"
55 // Stress testing mode - disable heuristics.
56 static cl::opt<bool> Stress("aarch64-stress-promote-const", cl::Hidden,
57 cl::desc("Promote all vector constants"));
59 STATISTIC(NumPromoted, "Number of promoted constants");
60 STATISTIC(NumPromotedUses, "Number of promoted constants uses");
62 //===----------------------------------------------------------------------===//
63 // AArch64PromoteConstant
64 //===----------------------------------------------------------------------===//
68 /// Promotes interesting constant into global variables.
69 /// The motivating example is:
70 /// static const uint16_t TableA[32] = {
71 /// 41944, 40330, 38837, 37450, 36158, 34953, 33826, 32768,
72 /// 31776, 30841, 29960, 29128, 28340, 27595, 26887, 26215,
73 /// 25576, 24967, 24386, 23832, 23302, 22796, 22311, 21846,
74 /// 21400, 20972, 20561, 20165, 19785, 19419, 19066, 18725,
77 /// uint8x16x4_t LoadStatic(void) {
79 /// ret.val[0] = vld1q_u16(TableA + 0);
80 /// ret.val[1] = vld1q_u16(TableA + 8);
81 /// ret.val[2] = vld1q_u16(TableA + 16);
82 /// ret.val[3] = vld1q_u16(TableA + 24);
86 /// The constants in this example are folded into the uses. Thus, 4 different
87 /// constants are created.
89 /// As their type is vector the cheapest way to create them is to load them
92 /// Therefore the final assembly final has 4 different loads. With this pass
93 /// enabled, only one load is issued for the constants.
94 class AArch64PromoteConstant : public ModulePass {
96 struct PromotedConstant {
97 bool ShouldConvert = false;
98 GlobalVariable *GV = nullptr;
100 using PromotionCacheTy = SmallDenseMap<Constant *, PromotedConstant, 16>;
102 struct UpdateRecord {
107 UpdateRecord(Constant *C, Instruction *User, unsigned Op)
108 : C(C), User(User), Op(Op) {}
113 AArch64PromoteConstant() : ModulePass(ID) {
114 initializeAArch64PromoteConstantPass(*PassRegistry::getPassRegistry());
117 StringRef getPassName() const override { return "AArch64 Promote Constant"; }
119 /// Iterate over the functions and promote the interesting constants into
120 /// global variables with module scope.
121 bool runOnModule(Module &M) override {
122 LLVM_DEBUG(dbgs() << getPassName() << '\n');
125 bool Changed = false;
126 PromotionCacheTy PromotionCache;
128 Changed |= runOnFunction(MF, PromotionCache);
134 /// Look for interesting constants used within the given function.
135 /// Promote them into global variables, load these global variables within
136 /// the related function, so that the number of inserted load is minimal.
137 bool runOnFunction(Function &F, PromotionCacheTy &PromotionCache);
139 // This transformation requires dominator info
140 void getAnalysisUsage(AnalysisUsage &AU) const override {
141 AU.setPreservesCFG();
142 AU.addRequired<DominatorTreeWrapperPass>();
143 AU.addPreserved<DominatorTreeWrapperPass>();
146 /// Type to store a list of Uses.
147 using Uses = SmallVector<std::pair<Instruction *, unsigned>, 4>;
148 /// Map an insertion point to all the uses it dominates.
149 using InsertionPoints = DenseMap<Instruction *, Uses>;
151 /// Find the closest point that dominates the given Use.
152 Instruction *findInsertionPoint(Instruction &User, unsigned OpNo);
154 /// Check if the given insertion point is dominated by an existing
156 /// If true, the given use is added to the list of dominated uses for
157 /// the related existing point.
158 /// \param NewPt the insertion point to be checked
159 /// \param User the user of the constant
160 /// \param OpNo the operand number of the use
161 /// \param InsertPts existing insertion points
162 /// \pre NewPt and all instruction in InsertPts belong to the same function
163 /// \return true if one of the insertion point in InsertPts dominates NewPt,
165 bool isDominated(Instruction *NewPt, Instruction *User, unsigned OpNo,
166 InsertionPoints &InsertPts);
168 /// Check if the given insertion point can be merged with an existing
169 /// insertion point in a common dominator.
170 /// If true, the given use is added to the list of the created insertion
172 /// \param NewPt the insertion point to be checked
173 /// \param User the user of the constant
174 /// \param OpNo the operand number of the use
175 /// \param InsertPts existing insertion points
176 /// \pre NewPt and all instruction in InsertPts belong to the same function
177 /// \pre isDominated returns false for the exact same parameters.
178 /// \return true if it exists an insertion point in InsertPts that could
179 /// have been merged with NewPt in a common dominator,
181 bool tryAndMerge(Instruction *NewPt, Instruction *User, unsigned OpNo,
182 InsertionPoints &InsertPts);
184 /// Compute the minimal insertion points to dominates all the interesting
186 /// Insertion points are group per function and each insertion point
187 /// contains a list of all the uses it dominates within the related function
188 /// \param User the user of the constant
189 /// \param OpNo the operand number of the constant
190 /// \param[out] InsertPts output storage of the analysis
191 void computeInsertionPoint(Instruction *User, unsigned OpNo,
192 InsertionPoints &InsertPts);
194 /// Insert a definition of a new global variable at each point contained in
195 /// InsPtsPerFunc and update the related uses (also contained in
197 void insertDefinitions(Function &F, GlobalVariable &GV,
198 InsertionPoints &InsertPts);
200 /// Do the constant promotion indicated by the Updates records, keeping track
201 /// of globals in PromotionCache.
202 void promoteConstants(Function &F, SmallVectorImpl<UpdateRecord> &Updates,
203 PromotionCacheTy &PromotionCache);
205 /// Transfer the list of dominated uses of IPI to NewPt in InsertPts.
206 /// Append Use to this list and delete the entry of IPI in InsertPts.
207 static void appendAndTransferDominatedUses(Instruction *NewPt,
208 Instruction *User, unsigned OpNo,
209 InsertionPoints::iterator &IPI,
210 InsertionPoints &InsertPts) {
211 // Record the dominated use.
212 IPI->second.emplace_back(User, OpNo);
213 // Transfer the dominated uses of IPI to NewPt
214 // Inserting into the DenseMap may invalidate existing iterator.
215 // Keep a copy of the key to find the iterator to erase. Keep a copy of the
216 // value so that we don't have to dereference IPI->second.
217 Instruction *OldInstr = IPI->first;
218 Uses OldUses = std::move(IPI->second);
219 InsertPts[NewPt] = std::move(OldUses);
221 InsertPts.erase(OldInstr);
225 } // end anonymous namespace
227 char AArch64PromoteConstant::ID = 0;
229 INITIALIZE_PASS_BEGIN(AArch64PromoteConstant, "aarch64-promote-const",
230 "AArch64 Promote Constant Pass", false, false)
231 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
232 INITIALIZE_PASS_END(AArch64PromoteConstant, "aarch64-promote-const",
233 "AArch64 Promote Constant Pass", false, false)
235 ModulePass *llvm::createAArch64PromoteConstantPass() {
236 return new AArch64PromoteConstant();
239 /// Check if the given type uses a vector type.
240 static bool isConstantUsingVectorTy(const Type *CstTy) {
241 if (CstTy->isVectorTy())
243 if (CstTy->isStructTy()) {
244 for (unsigned EltIdx = 0, EndEltIdx = CstTy->getStructNumElements();
245 EltIdx < EndEltIdx; ++EltIdx)
246 if (isConstantUsingVectorTy(CstTy->getStructElementType(EltIdx)))
248 } else if (CstTy->isArrayTy())
249 return isConstantUsingVectorTy(CstTy->getArrayElementType());
253 // Returns true if \p C contains only ConstantData leafs and no global values,
254 // block addresses or constant expressions. Traverses ConstantAggregates.
255 static bool containsOnlyConstantData(const Constant *C) {
256 if (isa<ConstantData>(C))
259 if (isa<GlobalValue>(C) || isa<BlockAddress>(C) || isa<ConstantExpr>(C))
262 return all_of(C->operands(), [](const Use &U) {
263 return containsOnlyConstantData(cast<Constant>(&U));
267 /// Check if the given use (Instruction + OpIdx) of Cst should be converted into
268 /// a load of a global variable initialized with Cst.
269 /// A use should be converted if it is legal to do so.
270 /// For instance, it is not legal to turn the mask operand of a shuffle vector
271 /// into a load of a global variable.
272 static bool shouldConvertUse(const Constant *Cst, const Instruction *Instr,
274 // shufflevector instruction expects a const for the mask argument, i.e., the
275 // third argument. Do not promote this use in that case.
276 if (isa<const ShuffleVectorInst>(Instr) && OpIdx == 2)
279 // extractvalue instruction expects a const idx.
280 if (isa<const ExtractValueInst>(Instr) && OpIdx > 0)
283 // extractvalue instruction expects a const idx.
284 if (isa<const InsertValueInst>(Instr) && OpIdx > 1)
287 if (isa<const AllocaInst>(Instr) && OpIdx > 0)
290 // Alignment argument must be constant.
291 if (isa<const LoadInst>(Instr) && OpIdx > 0)
294 // Alignment argument must be constant.
295 if (isa<const StoreInst>(Instr) && OpIdx > 1)
298 // Index must be constant.
299 if (isa<const GetElementPtrInst>(Instr) && OpIdx > 0)
302 // Personality function and filters must be constant.
303 // Give up on that instruction.
304 if (isa<const LandingPadInst>(Instr))
307 // Switch instruction expects constants to compare to.
308 if (isa<const SwitchInst>(Instr))
311 // Expected address must be a constant.
312 if (isa<const IndirectBrInst>(Instr))
315 // Do not mess with intrinsics.
316 if (isa<const IntrinsicInst>(Instr))
319 // Do not mess with inline asm.
320 const CallInst *CI = dyn_cast<const CallInst>(Instr);
321 return !(CI && CI->isInlineAsm());
324 /// Check if the given Cst should be converted into
325 /// a load of a global variable initialized with Cst.
326 /// A constant should be converted if it is likely that the materialization of
327 /// the constant will be tricky. Thus, we give up on zero or undef values.
329 /// \todo Currently, accept only vector related types.
330 /// Also we give up on all simple vector type to keep the existing
331 /// behavior. Otherwise, we should push here all the check of the lowering of
332 /// BUILD_VECTOR. By giving up, we lose the potential benefit of merging
333 /// constant via global merge and the fact that the same constant is stored
334 /// only once with this method (versus, as many function that uses the constant
335 /// for the regular approach, even for float).
336 /// Again, the simplest solution would be to promote every
337 /// constant and rematerialize them when they are actually cheap to create.
338 static bool shouldConvertImpl(const Constant *Cst) {
339 if (isa<const UndefValue>(Cst))
342 // FIXME: In some cases, it may be interesting to promote in memory
343 // a zero initialized constant.
344 // E.g., when the type of Cst require more instructions than the
345 // adrp/add/load sequence or when this sequence can be shared by several
347 // Ideally, we could promote this into a global and rematerialize the constant
348 // when it was a bad idea.
349 if (Cst->isZeroValue())
355 // FIXME: see function \todo
356 if (Cst->getType()->isVectorTy())
358 return isConstantUsingVectorTy(Cst->getType());
362 shouldConvert(Constant &C,
363 AArch64PromoteConstant::PromotionCacheTy &PromotionCache) {
364 auto Converted = PromotionCache.insert(
365 std::make_pair(&C, AArch64PromoteConstant::PromotedConstant()));
366 if (Converted.second)
367 Converted.first->second.ShouldConvert = shouldConvertImpl(&C);
368 return Converted.first->second.ShouldConvert;
371 Instruction *AArch64PromoteConstant::findInsertionPoint(Instruction &User,
373 // If this user is a phi, the insertion point is in the related
374 // incoming basic block.
375 if (PHINode *PhiInst = dyn_cast<PHINode>(&User))
376 return PhiInst->getIncomingBlock(OpNo)->getTerminator();
381 bool AArch64PromoteConstant::isDominated(Instruction *NewPt, Instruction *User,
383 InsertionPoints &InsertPts) {
384 DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>(
385 *NewPt->getParent()->getParent()).getDomTree();
387 // Traverse all the existing insertion points and check if one is dominating
388 // NewPt. If it is, remember that.
389 for (auto &IPI : InsertPts) {
390 if (NewPt == IPI.first || DT.dominates(IPI.first, NewPt) ||
391 // When IPI.first is a terminator instruction, DT may think that
392 // the result is defined on the edge.
393 // Here we are testing the insertion point, not the definition.
394 (IPI.first->getParent() != NewPt->getParent() &&
395 DT.dominates(IPI.first->getParent(), NewPt->getParent()))) {
396 // No need to insert this point. Just record the dominated use.
397 LLVM_DEBUG(dbgs() << "Insertion point dominated by:\n");
398 LLVM_DEBUG(IPI.first->print(dbgs()));
399 LLVM_DEBUG(dbgs() << '\n');
400 IPI.second.emplace_back(User, OpNo);
407 bool AArch64PromoteConstant::tryAndMerge(Instruction *NewPt, Instruction *User,
409 InsertionPoints &InsertPts) {
410 DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>(
411 *NewPt->getParent()->getParent()).getDomTree();
412 BasicBlock *NewBB = NewPt->getParent();
414 // Traverse all the existing insertion point and check if one is dominated by
415 // NewPt and thus useless or can be combined with NewPt into a common
417 for (InsertionPoints::iterator IPI = InsertPts.begin(),
418 EndIPI = InsertPts.end();
419 IPI != EndIPI; ++IPI) {
420 BasicBlock *CurBB = IPI->first->getParent();
421 if (NewBB == CurBB) {
422 // Instructions are in the same block.
423 // By construction, NewPt is dominating the other.
424 // Indeed, isDominated returned false with the exact same arguments.
425 LLVM_DEBUG(dbgs() << "Merge insertion point with:\n");
426 LLVM_DEBUG(IPI->first->print(dbgs()));
427 LLVM_DEBUG(dbgs() << "\nat considered insertion point.\n");
428 appendAndTransferDominatedUses(NewPt, User, OpNo, IPI, InsertPts);
432 // Look for a common dominator
433 BasicBlock *CommonDominator = DT.findNearestCommonDominator(NewBB, CurBB);
434 // If none exists, we cannot merge these two points.
435 if (!CommonDominator)
438 if (CommonDominator != NewBB) {
439 // By construction, the CommonDominator cannot be CurBB.
440 assert(CommonDominator != CurBB &&
441 "Instruction has not been rejected during isDominated check!");
442 // Take the last instruction of the CommonDominator as insertion point
443 NewPt = CommonDominator->getTerminator();
445 // else, CommonDominator is the block of NewBB, hence NewBB is the last
446 // possible insertion point in that block.
447 LLVM_DEBUG(dbgs() << "Merge insertion point with:\n");
448 LLVM_DEBUG(IPI->first->print(dbgs()));
449 LLVM_DEBUG(dbgs() << '\n');
450 LLVM_DEBUG(NewPt->print(dbgs()));
451 LLVM_DEBUG(dbgs() << '\n');
452 appendAndTransferDominatedUses(NewPt, User, OpNo, IPI, InsertPts);
458 void AArch64PromoteConstant::computeInsertionPoint(
459 Instruction *User, unsigned OpNo, InsertionPoints &InsertPts) {
460 LLVM_DEBUG(dbgs() << "Considered use, opidx " << OpNo << ":\n");
461 LLVM_DEBUG(User->print(dbgs()));
462 LLVM_DEBUG(dbgs() << '\n');
464 Instruction *InsertionPoint = findInsertionPoint(*User, OpNo);
466 LLVM_DEBUG(dbgs() << "Considered insertion point:\n");
467 LLVM_DEBUG(InsertionPoint->print(dbgs()));
468 LLVM_DEBUG(dbgs() << '\n');
470 if (isDominated(InsertionPoint, User, OpNo, InsertPts))
472 // This insertion point is useful, check if we can merge some insertion
473 // point in a common dominator or if NewPt dominates an existing one.
474 if (tryAndMerge(InsertionPoint, User, OpNo, InsertPts))
477 LLVM_DEBUG(dbgs() << "Keep considered insertion point\n");
479 // It is definitely useful by its own
480 InsertPts[InsertionPoint].emplace_back(User, OpNo);
483 static void ensurePromotedGV(Function &F, Constant &C,
484 AArch64PromoteConstant::PromotedConstant &PC) {
485 assert(PC.ShouldConvert &&
486 "Expected that we should convert this to a global");
489 PC.GV = new GlobalVariable(
490 *F.getParent(), C.getType(), true, GlobalValue::InternalLinkage, nullptr,
491 "_PromotedConst", nullptr, GlobalVariable::NotThreadLocal);
492 PC.GV->setInitializer(&C);
493 LLVM_DEBUG(dbgs() << "Global replacement: ");
494 LLVM_DEBUG(PC.GV->print(dbgs()));
495 LLVM_DEBUG(dbgs() << '\n');
499 void AArch64PromoteConstant::insertDefinitions(Function &F,
500 GlobalVariable &PromotedGV,
501 InsertionPoints &InsertPts) {
503 // Do more checking for debug purposes.
504 DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>(F).getDomTree();
506 assert(!InsertPts.empty() && "Empty uses does not need a definition");
508 for (const auto &IPI : InsertPts) {
509 // Create the load of the global variable.
510 IRBuilder<> Builder(IPI.first);
511 LoadInst *LoadedCst =
512 Builder.CreateLoad(PromotedGV.getValueType(), &PromotedGV);
513 LLVM_DEBUG(dbgs() << "**********\n");
514 LLVM_DEBUG(dbgs() << "New def: ");
515 LLVM_DEBUG(LoadedCst->print(dbgs()));
516 LLVM_DEBUG(dbgs() << '\n');
518 // Update the dominated uses.
519 for (auto Use : IPI.second) {
521 assert(DT.dominates(LoadedCst,
522 findInsertionPoint(*Use.first, Use.second)) &&
523 "Inserted definition does not dominate all its uses!");
526 dbgs() << "Use to update " << Use.second << ":";
527 Use.first->print(dbgs());
530 Use.first->setOperand(Use.second, LoadedCst);
536 void AArch64PromoteConstant::promoteConstants(
537 Function &F, SmallVectorImpl<UpdateRecord> &Updates,
538 PromotionCacheTy &PromotionCache) {
539 // Promote the constants.
540 for (auto U = Updates.begin(), E = Updates.end(); U != E;) {
541 LLVM_DEBUG(dbgs() << "** Compute insertion points **\n");
543 Constant *C = First->C;
544 InsertionPoints InsertPts;
546 computeInsertionPoint(U->User, U->Op, InsertPts);
547 } while (++U != E && U->C == C);
549 auto &Promotion = PromotionCache[C];
550 ensurePromotedGV(F, *C, Promotion);
551 insertDefinitions(F, *Promotion.GV, InsertPts);
555 bool AArch64PromoteConstant::runOnFunction(Function &F,
556 PromotionCacheTy &PromotionCache) {
557 // Look for instructions using constant vector. Promote that constant to a
558 // global variable. Create as few loads of this variable as possible and
559 // update the uses accordingly.
560 SmallVector<UpdateRecord, 64> Updates;
561 for (Instruction &I : instructions(&F)) {
562 // Traverse the operand, looking for constant vectors. Replace them by a
563 // load of a global variable of constant vector type.
564 for (Use &U : I.operands()) {
565 Constant *Cst = dyn_cast<Constant>(U);
566 // There is no point in promoting global values as they are already
567 // global. Do not promote constants containing constant expression, global
568 // values or blockaddresses either, as they may require some code
570 if (!Cst || isa<GlobalValue>(Cst) || !containsOnlyConstantData(Cst))
573 // Check if this constant is worth promoting.
574 if (!shouldConvert(*Cst, PromotionCache))
577 // Check if this use should be promoted.
578 unsigned OpNo = &U - I.op_begin();
579 if (!shouldConvertUse(Cst, &I, OpNo))
582 Updates.emplace_back(Cst, &I, OpNo);
589 promoteConstants(F, Updates, PromotionCache);