1//===- ConstantHoisting.cpp - Prepare code for expensive constants --------===//
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 identifies expensive constants to hoist and coalesces them to
10// better prepare it for SelectionDAG-based code generation. This works around
11// the limitations of the basic-block-at-a-time approach.
12//
13// First it scans all instructions for integer constants and calculates its
14// cost. If the constant can be folded into the instruction (the cost is
15// TCC_Free) or the cost is just a simple operation (TCC_BASIC), then we don't
16// consider it expensive and leave it alone. This is the default behavior and
17// the default implementation of getIntImmCostInst will always return TCC_Free.
18//
19// If the cost is more than TCC_BASIC, then the integer constant can't be folded
20// into the instruction and it might be beneficial to hoist the constant.
21// Similar constants are coalesced to reduce register pressure and
22// materialization code.
23//
24// When a constant is hoisted, it is also hidden behind a bitcast to force it to
25// be live-out of the basic block. Otherwise the constant would be just
26// duplicated and each basic block would have its own copy in the SelectionDAG.
27// The SelectionDAG recognizes such constants as opaque and doesn't perform
28// certain transformations on them, which would create a new expensive constant.
29//
30// This optimization is only applied to integer constants in instructions and
31// simple (this means not nested) constant cast expressions. For example:
32// %0 = load i64* inttoptr (i64 big_constant to i64*)
33//===----------------------------------------------------------------------===//
34
35#include "llvm/Transforms/Scalar/ConstantHoisting.h"
36#include "llvm/ADT/APInt.h"
37#include "llvm/ADT/DenseMap.h"
38#include "llvm/ADT/SmallPtrSet.h"
39#include "llvm/ADT/SmallVector.h"
40#include "llvm/ADT/Statistic.h"
41#include "llvm/Analysis/BlockFrequencyInfo.h"
42#include "llvm/Analysis/ProfileSummaryInfo.h"
43#include "llvm/Analysis/TargetTransformInfo.h"
44#include "llvm/IR/BasicBlock.h"
45#include "llvm/IR/Constants.h"
46#include "llvm/IR/DataLayout.h"
47#include "llvm/IR/DebugInfoMetadata.h"
48#include "llvm/IR/Dominators.h"
49#include "llvm/IR/Function.h"
50#include "llvm/IR/InstrTypes.h"
51#include "llvm/IR/Instruction.h"
52#include "llvm/IR/Instructions.h"
53#include "llvm/IR/IntrinsicInst.h"
54#include "llvm/IR/Operator.h"
55#include "llvm/IR/Value.h"
56#include "llvm/InitializePasses.h"
57#include "llvm/Pass.h"
58#include "llvm/Support/BlockFrequency.h"
59#include "llvm/Support/Casting.h"
60#include "llvm/Support/CommandLine.h"
61#include "llvm/Support/Debug.h"
62#include "llvm/Support/raw_ostream.h"
63#include "llvm/Transforms/Scalar.h"
64#include "llvm/Transforms/Utils/Local.h"
65#include "llvm/Transforms/Utils/SizeOpts.h"
66#include <algorithm>
67#include <cassert>
68#include <cstdint>
69#include <iterator>
70#include <tuple>
71#include <utility>
72
73using namespace llvm;
74using namespace consthoist;
75
76#define DEBUG_TYPE "consthoist"
77
78STATISTIC(NumConstantsHoisted, "Number of constants hoisted");
79STATISTIC(NumConstantsRebased, "Number of constants rebased");
80
81static cl::opt<bool> ConstHoistWithBlockFrequency(
82 "consthoist-with-block-frequency", cl::init(Val: true), cl::Hidden,
83 cl::desc("Enable the use of the block frequency analysis to reduce the "
84 "chance to execute const materialization more frequently than "
85 "without hoisting."));
86
87static cl::opt<bool> ConstHoistGEP(
88 "consthoist-gep", cl::init(Val: false), cl::Hidden,
89 cl::desc("Try hoisting constant gep expressions"));
90
91static cl::opt<unsigned>
92MinNumOfDependentToRebase("consthoist-min-num-to-rebase",
93 cl::desc("Do not rebase if number of dependent constants of a Base is less "
94 "than this number."),
95 cl::init(Val: 0), cl::Hidden);
96
97namespace {
98
99/// The constant hoisting pass.
100class ConstantHoistingLegacyPass : public FunctionPass {
101public:
102 static char ID; // Pass identification, replacement for typeid
103
104 ConstantHoistingLegacyPass() : FunctionPass(ID) {
105 initializeConstantHoistingLegacyPassPass(*PassRegistry::getPassRegistry());
106 }
107
108 bool runOnFunction(Function &Fn) override;
109
110 StringRef getPassName() const override { return "Constant Hoisting"; }
111
112 void getAnalysisUsage(AnalysisUsage &AU) const override {
113 AU.setPreservesCFG();
114 if (ConstHoistWithBlockFrequency)
115 AU.addRequired<BlockFrequencyInfoWrapperPass>();
116 AU.addRequired<DominatorTreeWrapperPass>();
117 AU.addRequired<ProfileSummaryInfoWrapperPass>();
118 AU.addRequired<TargetTransformInfoWrapperPass>();
119 }
120
121private:
122 ConstantHoistingPass Impl;
123};
124
125} // end anonymous namespace
126
127char ConstantHoistingLegacyPass::ID = 0;
128
129INITIALIZE_PASS_BEGIN(ConstantHoistingLegacyPass, "consthoist",
130 "Constant Hoisting", false, false)
131INITIALIZE_PASS_DEPENDENCY(BlockFrequencyInfoWrapperPass)
132INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
133INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)
134INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
135INITIALIZE_PASS_END(ConstantHoistingLegacyPass, "consthoist",
136 "Constant Hoisting", false, false)
137
138FunctionPass *llvm::createConstantHoistingPass() {
139 return new ConstantHoistingLegacyPass();
140}
141
142/// Perform the constant hoisting optimization for the given function.
143bool ConstantHoistingLegacyPass::runOnFunction(Function &Fn) {
144 if (skipFunction(F: Fn))
145 return false;
146
147 LLVM_DEBUG(dbgs() << "********** Begin Constant Hoisting **********\n");
148 LLVM_DEBUG(dbgs() << "********** Function: " << Fn.getName() << '\n');
149
150 bool MadeChange =
151 Impl.runImpl(F&: Fn, TTI&: getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F: Fn),
152 DT&: getAnalysis<DominatorTreeWrapperPass>().getDomTree(),
153 BFI: ConstHoistWithBlockFrequency
154 ? &getAnalysis<BlockFrequencyInfoWrapperPass>().getBFI()
155 : nullptr,
156 Entry&: Fn.getEntryBlock(),
157 PSI: &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI());
158
159 LLVM_DEBUG(dbgs() << "********** End Constant Hoisting **********\n");
160
161 return MadeChange;
162}
163
164void ConstantHoistingPass::collectMatInsertPts(
165 const RebasedConstantListType &RebasedConstants,
166 SmallVectorImpl<BasicBlock::iterator> &MatInsertPts) const {
167 for (const RebasedConstantInfo &RCI : RebasedConstants)
168 for (const ConstantUser &U : RCI.Uses)
169 MatInsertPts.emplace_back(Args: findMatInsertPt(Inst: U.Inst, Idx: U.OpndIdx));
170}
171
172/// Find the constant materialization insertion point.
173BasicBlock::iterator ConstantHoistingPass::findMatInsertPt(Instruction *Inst,
174 unsigned Idx) const {
175 // If the operand is a cast instruction, then we have to materialize the
176 // constant before the cast instruction.
177 if (Idx != ~0U) {
178 Value *Opnd = Inst->getOperand(i: Idx);
179 if (auto CastInst = dyn_cast<Instruction>(Val: Opnd))
180 if (CastInst->isCast())
181 return CastInst->getIterator();
182 }
183
184 // The simple and common case. This also includes constant expressions.
185 if (!isa<PHINode>(Val: Inst) && !Inst->isEHPad())
186 return Inst->getIterator();
187
188 // We can't insert directly before a phi node or an eh pad. Insert before
189 // the terminator of the incoming or dominating block.
190 assert(Entry != Inst->getParent() && "PHI or landing pad in entry block!");
191 BasicBlock *InsertionBlock = nullptr;
192 if (Idx != ~0U && isa<PHINode>(Val: Inst)) {
193 InsertionBlock = cast<PHINode>(Val: Inst)->getIncomingBlock(i: Idx);
194 if (!InsertionBlock->isEHPad()) {
195 return InsertionBlock->getTerminator()->getIterator();
196 }
197 } else {
198 InsertionBlock = Inst->getParent();
199 }
200
201 // This must be an EH pad. Iterate over immediate dominators until we find a
202 // non-EH pad. We need to skip over catchswitch blocks, which are both EH pads
203 // and terminators.
204 auto *IDom = DT->getNode(BB: InsertionBlock)->getIDom();
205 while (IDom->getBlock()->isEHPad()) {
206 assert(Entry != IDom->getBlock() && "eh pad in entry block");
207 IDom = IDom->getIDom();
208 }
209
210 return IDom->getBlock()->getTerminator()->getIterator();
211}
212
213/// Given \p BBs as input, find another set of BBs which collectively
214/// dominates \p BBs and have the minimal sum of frequencies. Return the BB
215/// set found in \p BBs.
216static void findBestInsertionSet(DominatorTree &DT, BlockFrequencyInfo &BFI,
217 BasicBlock *Entry,
218 SetVector<BasicBlock *> &BBs) {
219 assert(!BBs.count(Entry) && "Assume Entry is not in BBs");
220 // Nodes on the current path to the root.
221 SmallPtrSet<BasicBlock *, 8> Path;
222 // Candidates includes any block 'BB' in set 'BBs' that is not strictly
223 // dominated by any other blocks in set 'BBs', and all nodes in the path
224 // in the dominator tree from Entry to 'BB'.
225 SmallPtrSet<BasicBlock *, 16> Candidates;
226 for (auto *BB : BBs) {
227 // Ignore unreachable basic blocks.
228 if (!DT.isReachableFromEntry(A: BB))
229 continue;
230 Path.clear();
231 // Walk up the dominator tree until Entry or another BB in BBs
232 // is reached. Insert the nodes on the way to the Path.
233 BasicBlock *Node = BB;
234 // The "Path" is a candidate path to be added into Candidates set.
235 bool isCandidate = false;
236 do {
237 Path.insert(Ptr: Node);
238 if (Node == Entry || Candidates.count(Ptr: Node)) {
239 isCandidate = true;
240 break;
241 }
242 assert(DT.getNode(Node)->getIDom() &&
243 "Entry doens't dominate current Node");
244 Node = DT.getNode(BB: Node)->getIDom()->getBlock();
245 } while (!BBs.count(key: Node));
246
247 // If isCandidate is false, Node is another Block in BBs dominating
248 // current 'BB'. Drop the nodes on the Path.
249 if (!isCandidate)
250 continue;
251
252 // Add nodes on the Path into Candidates.
253 Candidates.insert(I: Path.begin(), E: Path.end());
254 }
255
256 // Sort the nodes in Candidates in top-down order and save the nodes
257 // in Orders.
258 unsigned Idx = 0;
259 SmallVector<BasicBlock *, 16> Orders;
260 Orders.push_back(Elt: Entry);
261 while (Idx != Orders.size()) {
262 BasicBlock *Node = Orders[Idx++];
263 for (auto *ChildDomNode : DT.getNode(BB: Node)->children()) {
264 if (Candidates.count(Ptr: ChildDomNode->getBlock()))
265 Orders.push_back(Elt: ChildDomNode->getBlock());
266 }
267 }
268
269 // Visit Orders in bottom-up order.
270 using InsertPtsCostPair =
271 std::pair<SetVector<BasicBlock *>, BlockFrequency>;
272
273 // InsertPtsMap is a map from a BB to the best insertion points for the
274 // subtree of BB (subtree not including the BB itself).
275 DenseMap<BasicBlock *, InsertPtsCostPair> InsertPtsMap;
276 InsertPtsMap.reserve(NumEntries: Orders.size() + 1);
277 for (BasicBlock *Node : llvm::reverse(C&: Orders)) {
278 bool NodeInBBs = BBs.count(key: Node);
279 auto &InsertPts = InsertPtsMap[Node].first;
280 BlockFrequency &InsertPtsFreq = InsertPtsMap[Node].second;
281
282 // Return the optimal insert points in BBs.
283 if (Node == Entry) {
284 BBs.clear();
285 if (InsertPtsFreq > BFI.getBlockFreq(BB: Node) ||
286 (InsertPtsFreq == BFI.getBlockFreq(BB: Node) && InsertPts.size() > 1))
287 BBs.insert(X: Entry);
288 else
289 BBs.insert(Start: InsertPts.begin(), End: InsertPts.end());
290 break;
291 }
292
293 BasicBlock *Parent = DT.getNode(BB: Node)->getIDom()->getBlock();
294 // Initially, ParentInsertPts is empty and ParentPtsFreq is 0. Every child
295 // will update its parent's ParentInsertPts and ParentPtsFreq.
296 auto &ParentInsertPts = InsertPtsMap[Parent].first;
297 BlockFrequency &ParentPtsFreq = InsertPtsMap[Parent].second;
298 // Choose to insert in Node or in subtree of Node.
299 // Don't hoist to EHPad because we may not find a proper place to insert
300 // in EHPad.
301 // If the total frequency of InsertPts is the same as the frequency of the
302 // target Node, and InsertPts contains more than one nodes, choose hoisting
303 // to reduce code size.
304 if (NodeInBBs ||
305 (!Node->isEHPad() &&
306 (InsertPtsFreq > BFI.getBlockFreq(BB: Node) ||
307 (InsertPtsFreq == BFI.getBlockFreq(BB: Node) && InsertPts.size() > 1)))) {
308 ParentInsertPts.insert(X: Node);
309 ParentPtsFreq += BFI.getBlockFreq(BB: Node);
310 } else {
311 ParentInsertPts.insert(Start: InsertPts.begin(), End: InsertPts.end());
312 ParentPtsFreq += InsertPtsFreq;
313 }
314 }
315}
316
317/// Find an insertion point that dominates all uses.
318SetVector<BasicBlock::iterator>
319ConstantHoistingPass::findConstantInsertionPoint(
320 const ConstantInfo &ConstInfo,
321 const ArrayRef<BasicBlock::iterator> MatInsertPts) const {
322 assert(!ConstInfo.RebasedConstants.empty() && "Invalid constant info entry.");
323 // Collect all basic blocks.
324 SetVector<BasicBlock *> BBs;
325 SetVector<BasicBlock::iterator> InsertPts;
326
327 for (BasicBlock::iterator MatInsertPt : MatInsertPts)
328 BBs.insert(X: MatInsertPt->getParent());
329
330 if (BBs.count(key: Entry)) {
331 InsertPts.insert(X: Entry->begin());
332 return InsertPts;
333 }
334
335 if (BFI) {
336 findBestInsertionSet(DT&: *DT, BFI&: *BFI, Entry, BBs);
337 for (BasicBlock *BB : BBs)
338 InsertPts.insert(X: BB->getFirstInsertionPt());
339 return InsertPts;
340 }
341
342 while (BBs.size() >= 2) {
343 BasicBlock *BB, *BB1, *BB2;
344 BB1 = BBs.pop_back_val();
345 BB2 = BBs.pop_back_val();
346 BB = DT->findNearestCommonDominator(A: BB1, B: BB2);
347 if (BB == Entry) {
348 InsertPts.insert(X: Entry->begin());
349 return InsertPts;
350 }
351 BBs.insert(X: BB);
352 }
353 assert((BBs.size() == 1) && "Expected only one element.");
354 Instruction &FirstInst = (*BBs.begin())->front();
355 InsertPts.insert(X: findMatInsertPt(Inst: &FirstInst));
356 return InsertPts;
357}
358
359/// Record constant integer ConstInt for instruction Inst at operand
360/// index Idx.
361///
362/// The operand at index Idx is not necessarily the constant integer itself. It
363/// could also be a cast instruction or a constant expression that uses the
364/// constant integer.
365void ConstantHoistingPass::collectConstantCandidates(
366 ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx,
367 ConstantInt *ConstInt) {
368 if (ConstInt->getType()->isVectorTy())
369 return;
370
371 InstructionCost Cost;
372 // Ask the target about the cost of materializing the constant for the given
373 // instruction and operand index.
374 if (auto IntrInst = dyn_cast<IntrinsicInst>(Val: Inst))
375 Cost = TTI->getIntImmCostIntrin(IID: IntrInst->getIntrinsicID(), Idx,
376 Imm: ConstInt->getValue(), Ty: ConstInt->getType(),
377 CostKind: TargetTransformInfo::TCK_SizeAndLatency);
378 else
379 Cost = TTI->getIntImmCostInst(
380 Opc: Inst->getOpcode(), Idx, Imm: ConstInt->getValue(), Ty: ConstInt->getType(),
381 CostKind: TargetTransformInfo::TCK_SizeAndLatency, Inst);
382
383 // Ignore cheap integer constants.
384 if (Cost > TargetTransformInfo::TCC_Basic) {
385 ConstCandMapType::iterator Itr;
386 bool Inserted;
387 ConstPtrUnionType Cand = ConstInt;
388 std::tie(args&: Itr, args&: Inserted) = ConstCandMap.insert(KV: std::make_pair(x&: Cand, y: 0));
389 if (Inserted) {
390 ConstIntCandVec.push_back(x: ConstantCandidate(ConstInt));
391 Itr->second = ConstIntCandVec.size() - 1;
392 }
393 ConstIntCandVec[Itr->second].addUser(Inst, Idx, Cost: *Cost.getValue());
394 LLVM_DEBUG(if (isa<ConstantInt>(Inst->getOperand(Idx))) dbgs()
395 << "Collect constant " << *ConstInt << " from " << *Inst
396 << " with cost " << Cost << '\n';
397 else dbgs() << "Collect constant " << *ConstInt
398 << " indirectly from " << *Inst << " via "
399 << *Inst->getOperand(Idx) << " with cost " << Cost
400 << '\n';);
401 }
402}
403
404/// Record constant GEP expression for instruction Inst at operand index Idx.
405void ConstantHoistingPass::collectConstantCandidates(
406 ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx,
407 ConstantExpr *ConstExpr) {
408 // TODO: Handle vector GEPs
409 if (ConstExpr->getType()->isVectorTy())
410 return;
411
412 GlobalVariable *BaseGV = dyn_cast<GlobalVariable>(Val: ConstExpr->getOperand(i_nocapture: 0));
413 if (!BaseGV)
414 return;
415
416 // Get offset from the base GV.
417 PointerType *GVPtrTy = cast<PointerType>(Val: BaseGV->getType());
418 IntegerType *OffsetTy = DL->getIndexType(C&: *Ctx, AddressSpace: GVPtrTy->getAddressSpace());
419 APInt Offset(DL->getTypeSizeInBits(Ty: OffsetTy), /*val*/ 0, /*isSigned*/ true);
420 auto *GEPO = cast<GEPOperator>(Val: ConstExpr);
421
422 // TODO: If we have a mix of inbounds and non-inbounds GEPs, then basing a
423 // non-inbounds GEP on an inbounds GEP is potentially incorrect. Restrict to
424 // inbounds GEP for now -- alternatively, we could drop inbounds from the
425 // constant expression,
426 if (!GEPO->isInBounds())
427 return;
428
429 if (!GEPO->accumulateConstantOffset(DL: *DL, Offset))
430 return;
431
432 if (!Offset.isIntN(N: 32))
433 return;
434
435 // A constant GEP expression that has a GlobalVariable as base pointer is
436 // usually lowered to a load from constant pool. Such operation is unlikely
437 // to be cheaper than compute it by <Base + Offset>, which can be lowered to
438 // an ADD instruction or folded into Load/Store instruction.
439 InstructionCost Cost =
440 TTI->getIntImmCostInst(Opc: Instruction::Add, Idx: 1, Imm: Offset, Ty: OffsetTy,
441 CostKind: TargetTransformInfo::TCK_SizeAndLatency, Inst);
442 ConstCandVecType &ExprCandVec = ConstGEPCandMap[BaseGV];
443 ConstCandMapType::iterator Itr;
444 bool Inserted;
445 ConstPtrUnionType Cand = ConstExpr;
446 std::tie(args&: Itr, args&: Inserted) = ConstCandMap.insert(KV: std::make_pair(x&: Cand, y: 0));
447 if (Inserted) {
448 ExprCandVec.push_back(x: ConstantCandidate(
449 ConstantInt::get(Ty: Type::getInt32Ty(C&: *Ctx), V: Offset.getLimitedValue()),
450 ConstExpr));
451 Itr->second = ExprCandVec.size() - 1;
452 }
453 ExprCandVec[Itr->second].addUser(Inst, Idx, Cost: *Cost.getValue());
454}
455
456/// Check the operand for instruction Inst at index Idx.
457void ConstantHoistingPass::collectConstantCandidates(
458 ConstCandMapType &ConstCandMap, Instruction *Inst, unsigned Idx) {
459 Value *Opnd = Inst->getOperand(i: Idx);
460
461 // Visit constant integers.
462 if (auto ConstInt = dyn_cast<ConstantInt>(Val: Opnd)) {
463 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
464 return;
465 }
466
467 // Visit cast instructions that have constant integers.
468 if (auto CastInst = dyn_cast<Instruction>(Val: Opnd)) {
469 // Only visit cast instructions, which have been skipped. All other
470 // instructions should have already been visited.
471 if (!CastInst->isCast())
472 return;
473
474 if (auto *ConstInt = dyn_cast<ConstantInt>(Val: CastInst->getOperand(i: 0))) {
475 // Pretend the constant is directly used by the instruction and ignore
476 // the cast instruction.
477 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
478 return;
479 }
480 }
481
482 // Visit constant expressions that have constant integers.
483 if (auto ConstExpr = dyn_cast<ConstantExpr>(Val: Opnd)) {
484 // Handle constant gep expressions.
485 if (ConstHoistGEP && isa<GEPOperator>(Val: ConstExpr))
486 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstExpr);
487
488 // Only visit constant cast expressions.
489 if (!ConstExpr->isCast())
490 return;
491
492 if (auto ConstInt = dyn_cast<ConstantInt>(Val: ConstExpr->getOperand(i_nocapture: 0))) {
493 // Pretend the constant is directly used by the instruction and ignore
494 // the constant expression.
495 collectConstantCandidates(ConstCandMap, Inst, Idx, ConstInt);
496 return;
497 }
498 }
499}
500
501/// Scan the instruction for expensive integer constants and record them
502/// in the constant candidate vector.
503void ConstantHoistingPass::collectConstantCandidates(
504 ConstCandMapType &ConstCandMap, Instruction *Inst) {
505 // Skip all cast instructions. They are visited indirectly later on.
506 if (Inst->isCast())
507 return;
508
509 // Scan all operands.
510 for (unsigned Idx = 0, E = Inst->getNumOperands(); Idx != E; ++Idx) {
511 // The cost of materializing the constants (defined in
512 // `TargetTransformInfo::getIntImmCostInst`) for instructions which only
513 // take constant variables is lower than `TargetTransformInfo::TCC_Basic`.
514 // So it's safe for us to collect constant candidates from all
515 // IntrinsicInsts.
516 if (canReplaceOperandWithVariable(I: Inst, OpIdx: Idx)) {
517 collectConstantCandidates(ConstCandMap, Inst, Idx);
518 }
519 } // end of for all operands
520}
521
522/// Collect all integer constants in the function that cannot be folded
523/// into an instruction itself.
524void ConstantHoistingPass::collectConstantCandidates(Function &Fn) {
525 ConstCandMapType ConstCandMap;
526 for (BasicBlock &BB : Fn) {
527 // Ignore unreachable basic blocks.
528 if (!DT->isReachableFromEntry(A: &BB))
529 continue;
530 for (Instruction &Inst : BB)
531 if (!TTI->preferToKeepConstantsAttached(Inst, Fn))
532 collectConstantCandidates(ConstCandMap, Inst: &Inst);
533 }
534}
535
536// This helper function is necessary to deal with values that have different
537// bit widths (APInt Operator- does not like that). If the value cannot be
538// represented in uint64 we return an "empty" APInt. This is then interpreted
539// as the value is not in range.
540static std::optional<APInt> calculateOffsetDiff(const APInt &V1,
541 const APInt &V2) {
542 std::optional<APInt> Res;
543 unsigned BW = V1.getBitWidth() > V2.getBitWidth() ?
544 V1.getBitWidth() : V2.getBitWidth();
545 uint64_t LimVal1 = V1.getLimitedValue();
546 uint64_t LimVal2 = V2.getLimitedValue();
547
548 if (LimVal1 == ~0ULL || LimVal2 == ~0ULL)
549 return Res;
550
551 uint64_t Diff = LimVal1 - LimVal2;
552 return APInt(BW, Diff, true);
553}
554
555// From a list of constants, one needs to picked as the base and the other
556// constants will be transformed into an offset from that base constant. The
557// question is which we can pick best? For example, consider these constants
558// and their number of uses:
559//
560// Constants| 2 | 4 | 12 | 42 |
561// NumUses | 3 | 2 | 8 | 7 |
562//
563// Selecting constant 12 because it has the most uses will generate negative
564// offsets for constants 2 and 4 (i.e. -10 and -8 respectively). If negative
565// offsets lead to less optimal code generation, then there might be better
566// solutions. Suppose immediates in the range of 0..35 are most optimally
567// supported by the architecture, then selecting constant 2 is most optimal
568// because this will generate offsets: 0, 2, 10, 40. Offsets 0, 2 and 10 are in
569// range 0..35, and thus 3 + 2 + 8 = 13 uses are in range. Selecting 12 would
570// have only 8 uses in range, so choosing 2 as a base is more optimal. Thus, in
571// selecting the base constant the range of the offsets is a very important
572// factor too that we take into account here. This algorithm calculates a total
573// costs for selecting a constant as the base and substract the costs if
574// immediates are out of range. It has quadratic complexity, so we call this
575// function only when we're optimising for size and there are less than 100
576// constants, we fall back to the straightforward algorithm otherwise
577// which does not do all the offset calculations.
578unsigned
579ConstantHoistingPass::maximizeConstantsInRange(ConstCandVecType::iterator S,
580 ConstCandVecType::iterator E,
581 ConstCandVecType::iterator &MaxCostItr) {
582 unsigned NumUses = 0;
583
584 if (!OptForSize || std::distance(first: S,last: E) > 100) {
585 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
586 NumUses += ConstCand->Uses.size();
587 if (ConstCand->CumulativeCost > MaxCostItr->CumulativeCost)
588 MaxCostItr = ConstCand;
589 }
590 return NumUses;
591 }
592
593 LLVM_DEBUG(dbgs() << "== Maximize constants in range ==\n");
594 InstructionCost MaxCost = -1;
595 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
596 auto Value = ConstCand->ConstInt->getValue();
597 Type *Ty = ConstCand->ConstInt->getType();
598 InstructionCost Cost = 0;
599 NumUses += ConstCand->Uses.size();
600 LLVM_DEBUG(dbgs() << "= Constant: " << ConstCand->ConstInt->getValue()
601 << "\n");
602
603 for (auto User : ConstCand->Uses) {
604 unsigned Opcode = User.Inst->getOpcode();
605 unsigned OpndIdx = User.OpndIdx;
606 Cost += TTI->getIntImmCostInst(Opc: Opcode, Idx: OpndIdx, Imm: Value, Ty,
607 CostKind: TargetTransformInfo::TCK_SizeAndLatency);
608 LLVM_DEBUG(dbgs() << "Cost: " << Cost << "\n");
609
610 for (auto C2 = S; C2 != E; ++C2) {
611 std::optional<APInt> Diff = calculateOffsetDiff(
612 V1: C2->ConstInt->getValue(), V2: ConstCand->ConstInt->getValue());
613 if (Diff) {
614 const InstructionCost ImmCosts =
615 TTI->getIntImmCodeSizeCost(Opc: Opcode, Idx: OpndIdx, Imm: *Diff, Ty);
616 Cost -= ImmCosts;
617 LLVM_DEBUG(dbgs() << "Offset " << *Diff << " "
618 << "has penalty: " << ImmCosts << "\n"
619 << "Adjusted cost: " << Cost << "\n");
620 }
621 }
622 }
623 LLVM_DEBUG(dbgs() << "Cumulative cost: " << Cost << "\n");
624 if (Cost > MaxCost) {
625 MaxCost = Cost;
626 MaxCostItr = ConstCand;
627 LLVM_DEBUG(dbgs() << "New candidate: " << MaxCostItr->ConstInt->getValue()
628 << "\n");
629 }
630 }
631 return NumUses;
632}
633
634/// Find the base constant within the given range and rebase all other
635/// constants with respect to the base constant.
636void ConstantHoistingPass::findAndMakeBaseConstant(
637 ConstCandVecType::iterator S, ConstCandVecType::iterator E,
638 SmallVectorImpl<consthoist::ConstantInfo> &ConstInfoVec) {
639 auto MaxCostItr = S;
640 unsigned NumUses = maximizeConstantsInRange(S, E, MaxCostItr);
641
642 // Don't hoist constants that have only one use.
643 if (NumUses <= 1)
644 return;
645
646 ConstantInt *ConstInt = MaxCostItr->ConstInt;
647 ConstantExpr *ConstExpr = MaxCostItr->ConstExpr;
648 ConstantInfo ConstInfo;
649 ConstInfo.BaseInt = ConstInt;
650 ConstInfo.BaseExpr = ConstExpr;
651 Type *Ty = ConstInt->getType();
652
653 // Rebase the constants with respect to the base constant.
654 for (auto ConstCand = S; ConstCand != E; ++ConstCand) {
655 APInt Diff = ConstCand->ConstInt->getValue() - ConstInt->getValue();
656 Constant *Offset = Diff == 0 ? nullptr : ConstantInt::get(Ty, V: Diff);
657 Type *ConstTy =
658 ConstCand->ConstExpr ? ConstCand->ConstExpr->getType() : nullptr;
659 ConstInfo.RebasedConstants.push_back(
660 Elt: RebasedConstantInfo(std::move(ConstCand->Uses), Offset, ConstTy));
661 }
662 ConstInfoVec.push_back(Elt: std::move(ConstInfo));
663}
664
665/// Finds and combines constant candidates that can be easily
666/// rematerialized with an add from a common base constant.
667void ConstantHoistingPass::findBaseConstants(GlobalVariable *BaseGV) {
668 // If BaseGV is nullptr, find base among candidate constant integers;
669 // Otherwise find base among constant GEPs that share the same BaseGV.
670 ConstCandVecType &ConstCandVec = BaseGV ?
671 ConstGEPCandMap[BaseGV] : ConstIntCandVec;
672 ConstInfoVecType &ConstInfoVec = BaseGV ?
673 ConstGEPInfoMap[BaseGV] : ConstIntInfoVec;
674
675 // Sort the constants by value and type. This invalidates the mapping!
676 llvm::stable_sort(Range&: ConstCandVec, C: [](const ConstantCandidate &LHS,
677 const ConstantCandidate &RHS) {
678 if (LHS.ConstInt->getType() != RHS.ConstInt->getType())
679 return LHS.ConstInt->getBitWidth() < RHS.ConstInt->getBitWidth();
680 return LHS.ConstInt->getValue().ult(RHS: RHS.ConstInt->getValue());
681 });
682
683 // Simple linear scan through the sorted constant candidate vector for viable
684 // merge candidates.
685 auto MinValItr = ConstCandVec.begin();
686 for (auto CC = std::next(x: ConstCandVec.begin()), E = ConstCandVec.end();
687 CC != E; ++CC) {
688 if (MinValItr->ConstInt->getType() == CC->ConstInt->getType()) {
689 Type *MemUseValTy = nullptr;
690 for (auto &U : CC->Uses) {
691 auto *UI = U.Inst;
692 if (LoadInst *LI = dyn_cast<LoadInst>(Val: UI)) {
693 MemUseValTy = LI->getType();
694 break;
695 } else if (StoreInst *SI = dyn_cast<StoreInst>(Val: UI)) {
696 // Make sure the constant is used as pointer operand of the StoreInst.
697 if (SI->getPointerOperand() == SI->getOperand(i_nocapture: U.OpndIdx)) {
698 MemUseValTy = SI->getValueOperand()->getType();
699 break;
700 }
701 }
702 }
703
704 // Check if the constant is in range of an add with immediate.
705 APInt Diff = CC->ConstInt->getValue() - MinValItr->ConstInt->getValue();
706 if ((Diff.getBitWidth() <= 64) &&
707 TTI->isLegalAddImmediate(Imm: Diff.getSExtValue()) &&
708 // Check if Diff can be used as offset in addressing mode of the user
709 // memory instruction.
710 (!MemUseValTy || TTI->isLegalAddressingMode(Ty: MemUseValTy,
711 /*BaseGV*/nullptr, /*BaseOffset*/Diff.getSExtValue(),
712 /*HasBaseReg*/true, /*Scale*/0)))
713 continue;
714 }
715 // We either have now a different constant type or the constant is not in
716 // range of an add with immediate anymore.
717 findAndMakeBaseConstant(S: MinValItr, E: CC, ConstInfoVec);
718 // Start a new base constant search.
719 MinValItr = CC;
720 }
721 // Finalize the last base constant search.
722 findAndMakeBaseConstant(S: MinValItr, E: ConstCandVec.end(), ConstInfoVec);
723}
724
725/// Updates the operand at Idx in instruction Inst with the result of
726/// instruction Mat. If the instruction is a PHI node then special
727/// handling for duplicate values from the same incoming basic block is
728/// required.
729/// \return The update will always succeed, but the return value indicated if
730/// Mat was used for the update or not.
731static bool updateOperand(Instruction *Inst, unsigned Idx, Instruction *Mat) {
732 if (auto PHI = dyn_cast<PHINode>(Val: Inst)) {
733 // Check if any previous operand of the PHI node has the same incoming basic
734 // block. This is a very odd case that happens when the incoming basic block
735 // has a switch statement. In this case use the same value as the previous
736 // operand(s), otherwise we will fail verification due to different values.
737 // The values are actually the same, but the variable names are different
738 // and the verifier doesn't like that.
739 BasicBlock *IncomingBB = PHI->getIncomingBlock(i: Idx);
740 for (unsigned i = 0; i < Idx; ++i) {
741 if (PHI->getIncomingBlock(i) == IncomingBB) {
742 Value *IncomingVal = PHI->getIncomingValue(i);
743 Inst->setOperand(i: Idx, Val: IncomingVal);
744 return false;
745 }
746 }
747 }
748
749 Inst->setOperand(i: Idx, Val: Mat);
750 return true;
751}
752
753/// Emit materialization code for all rebased constants and update their
754/// users.
755void ConstantHoistingPass::emitBaseConstants(Instruction *Base,
756 UserAdjustment *Adj) {
757 Instruction *Mat = Base;
758
759 // The same offset can be dereferenced to different types in nested struct.
760 if (!Adj->Offset && Adj->Ty && Adj->Ty != Base->getType())
761 Adj->Offset = ConstantInt::get(Ty: Type::getInt32Ty(C&: *Ctx), V: 0);
762
763 if (Adj->Offset) {
764 if (Adj->Ty) {
765 // Constant being rebased is a ConstantExpr.
766 Mat = GetElementPtrInst::Create(PointeeType: Type::getInt8Ty(C&: *Ctx), Ptr: Base, IdxList: Adj->Offset,
767 NameStr: "mat_gep", InsertBefore: Adj->MatInsertPt);
768 // Hide it behind a bitcast.
769 Mat = new BitCastInst(Mat, Adj->Ty, "mat_bitcast",
770 Adj->MatInsertPt->getIterator());
771 } else
772 // Constant being rebased is a ConstantInt.
773 Mat =
774 BinaryOperator::Create(Op: Instruction::Add, S1: Base, S2: Adj->Offset,
775 Name: "const_mat", InsertBefore: Adj->MatInsertPt->getIterator());
776
777 LLVM_DEBUG(dbgs() << "Materialize constant (" << *Base->getOperand(0)
778 << " + " << *Adj->Offset << ") in BB "
779 << Mat->getParent()->getName() << '\n'
780 << *Mat << '\n');
781 Mat->setDebugLoc(Adj->User.Inst->getDebugLoc());
782 }
783 Value *Opnd = Adj->User.Inst->getOperand(i: Adj->User.OpndIdx);
784
785 // Visit constant integer.
786 if (isa<ConstantInt>(Val: Opnd)) {
787 LLVM_DEBUG(dbgs() << "Update: " << *Adj->User.Inst << '\n');
788 if (!updateOperand(Inst: Adj->User.Inst, Idx: Adj->User.OpndIdx, Mat) && Adj->Offset)
789 Mat->eraseFromParent();
790 LLVM_DEBUG(dbgs() << "To : " << *Adj->User.Inst << '\n');
791 return;
792 }
793
794 // Visit cast instruction.
795 if (auto CastInst = dyn_cast<Instruction>(Val: Opnd)) {
796 assert(CastInst->isCast() && "Expected an cast instruction!");
797 // Check if we already have visited this cast instruction before to avoid
798 // unnecessary cloning.
799 Instruction *&ClonedCastInst = ClonedCastMap[CastInst];
800 if (!ClonedCastInst) {
801 ClonedCastInst = CastInst->clone();
802 ClonedCastInst->setOperand(i: 0, Val: Mat);
803 ClonedCastInst->insertAfter(InsertPos: CastInst);
804 // Use the same debug location as the original cast instruction.
805 ClonedCastInst->setDebugLoc(CastInst->getDebugLoc());
806 LLVM_DEBUG(dbgs() << "Clone instruction: " << *CastInst << '\n'
807 << "To : " << *ClonedCastInst << '\n');
808 }
809
810 LLVM_DEBUG(dbgs() << "Update: " << *Adj->User.Inst << '\n');
811 updateOperand(Inst: Adj->User.Inst, Idx: Adj->User.OpndIdx, Mat: ClonedCastInst);
812 LLVM_DEBUG(dbgs() << "To : " << *Adj->User.Inst << '\n');
813 return;
814 }
815
816 // Visit constant expression.
817 if (auto ConstExpr = dyn_cast<ConstantExpr>(Val: Opnd)) {
818 if (isa<GEPOperator>(Val: ConstExpr)) {
819 // Operand is a ConstantGEP, replace it.
820 updateOperand(Inst: Adj->User.Inst, Idx: Adj->User.OpndIdx, Mat);
821 return;
822 }
823
824 // Aside from constant GEPs, only constant cast expressions are collected.
825 assert(ConstExpr->isCast() && "ConstExpr should be a cast");
826 Instruction *ConstExprInst = ConstExpr->getAsInstruction();
827 ConstExprInst->insertBefore(InsertPos: Adj->MatInsertPt);
828 ConstExprInst->setOperand(i: 0, Val: Mat);
829
830 // Use the same debug location as the instruction we are about to update.
831 ConstExprInst->setDebugLoc(Adj->User.Inst->getDebugLoc());
832
833 LLVM_DEBUG(dbgs() << "Create instruction: " << *ConstExprInst << '\n'
834 << "From : " << *ConstExpr << '\n');
835 LLVM_DEBUG(dbgs() << "Update: " << *Adj->User.Inst << '\n');
836 if (!updateOperand(Inst: Adj->User.Inst, Idx: Adj->User.OpndIdx, Mat: ConstExprInst)) {
837 ConstExprInst->eraseFromParent();
838 if (Adj->Offset)
839 Mat->eraseFromParent();
840 }
841 LLVM_DEBUG(dbgs() << "To : " << *Adj->User.Inst << '\n');
842 return;
843 }
844}
845
846/// Hoist and hide the base constant behind a bitcast and emit
847/// materialization code for derived constants.
848bool ConstantHoistingPass::emitBaseConstants(GlobalVariable *BaseGV) {
849 bool MadeChange = false;
850 SmallVectorImpl<consthoist::ConstantInfo> &ConstInfoVec =
851 BaseGV ? ConstGEPInfoMap[BaseGV] : ConstIntInfoVec;
852 for (const consthoist::ConstantInfo &ConstInfo : ConstInfoVec) {
853 SmallVector<BasicBlock::iterator, 4> MatInsertPts;
854 collectMatInsertPts(RebasedConstants: ConstInfo.RebasedConstants, MatInsertPts);
855 SetVector<BasicBlock::iterator> IPSet =
856 findConstantInsertionPoint(ConstInfo, MatInsertPts);
857 // We can have an empty set if the function contains unreachable blocks.
858 if (IPSet.empty())
859 continue;
860
861 unsigned UsesNum = 0;
862 unsigned ReBasesNum = 0;
863 unsigned NotRebasedNum = 0;
864 for (const BasicBlock::iterator &IP : IPSet) {
865 // First, collect constants depending on this IP of the base.
866 UsesNum = 0;
867 SmallVector<UserAdjustment, 4> ToBeRebased;
868 unsigned MatCtr = 0;
869 for (auto const &RCI : ConstInfo.RebasedConstants) {
870 UsesNum += RCI.Uses.size();
871 for (auto const &U : RCI.Uses) {
872 const BasicBlock::iterator &MatInsertPt = MatInsertPts[MatCtr++];
873 BasicBlock *OrigMatInsertBB = MatInsertPt->getParent();
874 // If Base constant is to be inserted in multiple places,
875 // generate rebase for U using the Base dominating U.
876 if (IPSet.size() == 1 ||
877 DT->dominates(A: IP->getParent(), B: OrigMatInsertBB))
878 ToBeRebased.emplace_back(Args: RCI.Offset, Args: RCI.Ty, Args: MatInsertPt, Args: U);
879 }
880 }
881
882 // If only few constants depend on this IP of base, skip rebasing,
883 // assuming the base and the rebased have the same materialization cost.
884 if (ToBeRebased.size() < MinNumOfDependentToRebase) {
885 NotRebasedNum += ToBeRebased.size();
886 continue;
887 }
888
889 // Emit an instance of the base at this IP.
890 Instruction *Base = nullptr;
891 // Hoist and hide the base constant behind a bitcast.
892 if (ConstInfo.BaseExpr) {
893 assert(BaseGV && "A base constant expression must have an base GV");
894 Type *Ty = ConstInfo.BaseExpr->getType();
895 Base = new BitCastInst(ConstInfo.BaseExpr, Ty, "const", IP);
896 } else {
897 IntegerType *Ty = ConstInfo.BaseInt->getIntegerType();
898 Base = new BitCastInst(ConstInfo.BaseInt, Ty, "const", IP);
899 }
900
901 Base->setDebugLoc(IP->getDebugLoc());
902
903 LLVM_DEBUG(dbgs() << "Hoist constant (" << *ConstInfo.BaseInt
904 << ") to BB " << IP->getParent()->getName() << '\n'
905 << *Base << '\n');
906
907 // Emit materialization code for rebased constants depending on this IP.
908 for (UserAdjustment &R : ToBeRebased) {
909 emitBaseConstants(Base, Adj: &R);
910 ReBasesNum++;
911 // Use the same debug location as the last user of the constant.
912 Base->setDebugLoc(DILocation::getMergedLocation(
913 LocA: Base->getDebugLoc(), LocB: R.User.Inst->getDebugLoc()));
914 }
915 assert(!Base->use_empty() && "The use list is empty!?");
916 assert(isa<Instruction>(Base->user_back()) &&
917 "All uses should be instructions.");
918 }
919 (void)UsesNum;
920 (void)ReBasesNum;
921 (void)NotRebasedNum;
922 // Expect all uses are rebased after rebase is done.
923 assert(UsesNum == (ReBasesNum + NotRebasedNum) &&
924 "Not all uses are rebased");
925
926 NumConstantsHoisted++;
927
928 // Base constant is also included in ConstInfo.RebasedConstants, so
929 // deduct 1 from ConstInfo.RebasedConstants.size().
930 NumConstantsRebased += ConstInfo.RebasedConstants.size() - 1;
931
932 MadeChange = true;
933 }
934 return MadeChange;
935}
936
937/// Check all cast instructions we made a copy of and remove them if they
938/// have no more users.
939void ConstantHoistingPass::deleteDeadCastInst() const {
940 for (auto const &I : ClonedCastMap)
941 if (I.first->use_empty())
942 I.first->eraseFromParent();
943}
944
945/// Optimize expensive integer constants in the given function.
946bool ConstantHoistingPass::runImpl(Function &Fn, TargetTransformInfo &TTI,
947 DominatorTree &DT, BlockFrequencyInfo *BFI,
948 BasicBlock &Entry, ProfileSummaryInfo *PSI) {
949 this->TTI = &TTI;
950 this->DT = &DT;
951 this->BFI = BFI;
952 this->DL = &Fn.getDataLayout();
953 this->Ctx = &Fn.getContext();
954 this->Entry = &Entry;
955 this->PSI = PSI;
956 this->OptForSize = Entry.getParent()->hasOptSize() ||
957 llvm::shouldOptimizeForSize(F: Entry.getParent(), PSI, BFI,
958 QueryType: PGSOQueryType::IRPass);
959
960 // Collect all constant candidates.
961 collectConstantCandidates(Fn);
962
963 // Combine constants that can be easily materialized with an add from a common
964 // base constant.
965 if (!ConstIntCandVec.empty())
966 findBaseConstants(BaseGV: nullptr);
967 for (const auto &MapEntry : ConstGEPCandMap)
968 if (!MapEntry.second.empty())
969 findBaseConstants(BaseGV: MapEntry.first);
970
971 // Finally hoist the base constant and emit materialization code for dependent
972 // constants.
973 bool MadeChange = false;
974 if (!ConstIntInfoVec.empty())
975 MadeChange = emitBaseConstants(BaseGV: nullptr);
976 for (const auto &MapEntry : ConstGEPInfoMap)
977 if (!MapEntry.second.empty())
978 MadeChange |= emitBaseConstants(BaseGV: MapEntry.first);
979
980
981 // Cleanup dead instructions.
982 deleteDeadCastInst();
983
984 cleanup();
985
986 return MadeChange;
987}
988
989PreservedAnalyses ConstantHoistingPass::run(Function &F,
990 FunctionAnalysisManager &AM) {
991 auto &DT = AM.getResult<DominatorTreeAnalysis>(IR&: F);
992 auto &TTI = AM.getResult<TargetIRAnalysis>(IR&: F);
993 auto BFI = ConstHoistWithBlockFrequency
994 ? &AM.getResult<BlockFrequencyAnalysis>(IR&: F)
995 : nullptr;
996 auto &MAMProxy = AM.getResult<ModuleAnalysisManagerFunctionProxy>(IR&: F);
997 auto *PSI = MAMProxy.getCachedResult<ProfileSummaryAnalysis>(IR&: *F.getParent());
998 if (!runImpl(Fn&: F, TTI, DT, BFI, Entry&: F.getEntryBlock(), PSI))
999 return PreservedAnalyses::all();
1000
1001 PreservedAnalyses PA;
1002 PA.preserveSet<CFGAnalyses>();
1003 return PA;
1004}
1005