LLVM  13.0.0git
AlignmentFromAssumptions.cpp
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1 //===----------------------- AlignmentFromAssumptions.cpp -----------------===//
2 // Set Load/Store Alignments From Assumptions
3 //
4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5 // See https://llvm.org/LICENSE.txt for license information.
6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements a ScalarEvolution-based transformation to set
11 // the alignments of load, stores and memory intrinsics based on the truth
12 // expressions of assume intrinsics. The primary motivation is to handle
13 // complex alignment assumptions that apply to vector loads and stores that
14 // appear after vectorization and unrolling.
15 //
16 //===----------------------------------------------------------------------===//
17 
18 #include "llvm/IR/Instructions.h"
19 #include "llvm/InitializePasses.h"
20 #define AA_NAME "alignment-from-assumptions"
21 #define DEBUG_TYPE AA_NAME
23 #include "llvm/ADT/SmallPtrSet.h"
24 #include "llvm/ADT/Statistic.h"
28 #include "llvm/Analysis/LoopInfo.h"
31 #include "llvm/IR/Constant.h"
32 #include "llvm/IR/Dominators.h"
33 #include "llvm/IR/Instruction.h"
34 #include "llvm/IR/IntrinsicInst.h"
35 #include "llvm/IR/Intrinsics.h"
36 #include "llvm/IR/Module.h"
37 #include "llvm/Support/Debug.h"
39 #include "llvm/Transforms/Scalar.h"
40 using namespace llvm;
41 
42 STATISTIC(NumLoadAlignChanged,
43  "Number of loads changed by alignment assumptions");
44 STATISTIC(NumStoreAlignChanged,
45  "Number of stores changed by alignment assumptions");
46 STATISTIC(NumMemIntAlignChanged,
47  "Number of memory intrinsics changed by alignment assumptions");
48 
49 namespace {
50 struct AlignmentFromAssumptions : public FunctionPass {
51  static char ID; // Pass identification, replacement for typeid
52  AlignmentFromAssumptions() : FunctionPass(ID) {
54  }
55 
56  bool runOnFunction(Function &F) override;
57 
58  void getAnalysisUsage(AnalysisUsage &AU) const override {
62 
63  AU.setPreservesCFG();
69  }
70 
72 };
73 }
74 
76 static const char aip_name[] = "Alignment from assumptions";
77 INITIALIZE_PASS_BEGIN(AlignmentFromAssumptions, AA_NAME,
78  aip_name, false, false)
82 INITIALIZE_PASS_END(AlignmentFromAssumptions, AA_NAME,
84 
86  return new AlignmentFromAssumptions();
87 }
88 
89 // Given an expression for the (constant) alignment, AlignSCEV, and an
90 // expression for the displacement between a pointer and the aligned address,
91 // DiffSCEV, compute the alignment of the displaced pointer if it can be reduced
92 // to a constant. Using SCEV to compute alignment handles the case where
93 // DiffSCEV is a recurrence with constant start such that the aligned offset
94 // is constant. e.g. {16,+,32} % 32 -> 16.
95 static MaybeAlign getNewAlignmentDiff(const SCEV *DiffSCEV,
96  const SCEV *AlignSCEV,
97  ScalarEvolution *SE) {
98  // DiffUnits = Diff % int64_t(Alignment)
99  const SCEV *DiffUnitsSCEV = SE->getURemExpr(DiffSCEV, AlignSCEV);
100 
101  LLVM_DEBUG(dbgs() << "\talignment relative to " << *AlignSCEV << " is "
102  << *DiffUnitsSCEV << " (diff: " << *DiffSCEV << ")\n");
103 
104  if (const SCEVConstant *ConstDUSCEV =
105  dyn_cast<SCEVConstant>(DiffUnitsSCEV)) {
106  int64_t DiffUnits = ConstDUSCEV->getValue()->getSExtValue();
107 
108  // If the displacement is an exact multiple of the alignment, then the
109  // displaced pointer has the same alignment as the aligned pointer, so
110  // return the alignment value.
111  if (!DiffUnits)
112  return cast<SCEVConstant>(AlignSCEV)->getValue()->getAlignValue();
113 
114  // If the displacement is not an exact multiple, but the remainder is a
115  // constant, then return this remainder (but only if it is a power of 2).
116  uint64_t DiffUnitsAbs = std::abs(DiffUnits);
117  if (isPowerOf2_64(DiffUnitsAbs))
118  return Align(DiffUnitsAbs);
119  }
120 
121  return None;
122 }
123 
124 // There is an address given by an offset OffSCEV from AASCEV which has an
125 // alignment AlignSCEV. Use that information, if possible, to compute a new
126 // alignment for Ptr.
127 static Align getNewAlignment(const SCEV *AASCEV, const SCEV *AlignSCEV,
128  const SCEV *OffSCEV, Value *Ptr,
129  ScalarEvolution *SE) {
130  const SCEV *PtrSCEV = SE->getSCEV(Ptr);
131  // On a platform with 32-bit allocas, but 64-bit flat/global pointer sizes
132  // (*cough* AMDGPU), the effective SCEV type of AASCEV and PtrSCEV
133  // may disagree. Trunc/extend so they agree.
134  PtrSCEV = SE->getTruncateOrZeroExtend(
135  PtrSCEV, SE->getEffectiveSCEVType(AASCEV->getType()));
136  const SCEV *DiffSCEV = SE->getMinusSCEV(PtrSCEV, AASCEV);
137 
138  // On 32-bit platforms, DiffSCEV might now have type i32 -- we've always
139  // sign-extended OffSCEV to i64, so make sure they agree again.
140  DiffSCEV = SE->getNoopOrSignExtend(DiffSCEV, OffSCEV->getType());
141 
142  // What we really want to know is the overall offset to the aligned
143  // address. This address is displaced by the provided offset.
144  DiffSCEV = SE->getAddExpr(DiffSCEV, OffSCEV);
145 
146  LLVM_DEBUG(dbgs() << "AFI: alignment of " << *Ptr << " relative to "
147  << *AlignSCEV << " and offset " << *OffSCEV
148  << " using diff " << *DiffSCEV << "\n");
149 
150  if (MaybeAlign NewAlignment = getNewAlignmentDiff(DiffSCEV, AlignSCEV, SE)) {
151  LLVM_DEBUG(dbgs() << "\tnew alignment: " << DebugStr(NewAlignment) << "\n");
152  return *NewAlignment;
153  }
154 
155  if (const SCEVAddRecExpr *DiffARSCEV = dyn_cast<SCEVAddRecExpr>(DiffSCEV)) {
156  // The relative offset to the alignment assumption did not yield a constant,
157  // but we should try harder: if we assume that a is 32-byte aligned, then in
158  // for (i = 0; i < 1024; i += 4) r += a[i]; not all of the loads from a are
159  // 32-byte aligned, but instead alternate between 32 and 16-byte alignment.
160  // As a result, the new alignment will not be a constant, but can still
161  // be improved over the default (of 4) to 16.
162 
163  const SCEV *DiffStartSCEV = DiffARSCEV->getStart();
164  const SCEV *DiffIncSCEV = DiffARSCEV->getStepRecurrence(*SE);
165 
166  LLVM_DEBUG(dbgs() << "\ttrying start/inc alignment using start "
167  << *DiffStartSCEV << " and inc " << *DiffIncSCEV << "\n");
168 
169  // Now compute the new alignment using the displacement to the value in the
170  // first iteration, and also the alignment using the per-iteration delta.
171  // If these are the same, then use that answer. Otherwise, use the smaller
172  // one, but only if it divides the larger one.
173  MaybeAlign NewAlignment = getNewAlignmentDiff(DiffStartSCEV, AlignSCEV, SE);
174  MaybeAlign NewIncAlignment =
175  getNewAlignmentDiff(DiffIncSCEV, AlignSCEV, SE);
176 
177  LLVM_DEBUG(dbgs() << "\tnew start alignment: " << DebugStr(NewAlignment)
178  << "\n");
179  LLVM_DEBUG(dbgs() << "\tnew inc alignment: " << DebugStr(NewIncAlignment)
180  << "\n");
181 
182  if (!NewAlignment || !NewIncAlignment)
183  return Align(1);
184 
185  const Align NewAlign = *NewAlignment;
186  const Align NewIncAlign = *NewIncAlignment;
187  if (NewAlign > NewIncAlign) {
188  LLVM_DEBUG(dbgs() << "\tnew start/inc alignment: "
189  << DebugStr(NewIncAlign) << "\n");
190  return NewIncAlign;
191  }
192  if (NewIncAlign > NewAlign) {
193  LLVM_DEBUG(dbgs() << "\tnew start/inc alignment: " << DebugStr(NewAlign)
194  << "\n");
195  return NewAlign;
196  }
197  assert(NewIncAlign == NewAlign);
198  LLVM_DEBUG(dbgs() << "\tnew start/inc alignment: " << DebugStr(NewAlign)
199  << "\n");
200  return NewAlign;
201  }
202 
203  return Align(1);
204 }
205 
207  unsigned Idx,
208  Value *&AAPtr,
209  const SCEV *&AlignSCEV,
210  const SCEV *&OffSCEV) {
211  Type *Int64Ty = Type::getInt64Ty(I->getContext());
212  OperandBundleUse AlignOB = I->getOperandBundleAt(Idx);
213  if (AlignOB.getTagName() != "align")
214  return false;
215  assert(AlignOB.Inputs.size() >= 2);
216  AAPtr = AlignOB.Inputs[0].get();
217  // TODO: Consider accumulating the offset to the base.
218  AAPtr = AAPtr->stripPointerCastsSameRepresentation();
219  AlignSCEV = SE->getSCEV(AlignOB.Inputs[1].get());
220  AlignSCEV = SE->getTruncateOrZeroExtend(AlignSCEV, Int64Ty);
221  if (AlignOB.Inputs.size() == 3)
222  OffSCEV = SE->getSCEV(AlignOB.Inputs[2].get());
223  else
224  OffSCEV = SE->getZero(Int64Ty);
225  OffSCEV = SE->getTruncateOrZeroExtend(OffSCEV, Int64Ty);
226  return true;
227 }
228 
230  unsigned Idx) {
231  Value *AAPtr;
232  const SCEV *AlignSCEV, *OffSCEV;
233  if (!extractAlignmentInfo(ACall, Idx, AAPtr, AlignSCEV, OffSCEV))
234  return false;
235 
236  // Skip ConstantPointerNull and UndefValue. Assumptions on these shouldn't
237  // affect other users.
238  if (isa<ConstantData>(AAPtr))
239  return false;
240 
241  const SCEV *AASCEV = SE->getSCEV(AAPtr);
242 
243  // Apply the assumption to all other users of the specified pointer.
246  for (User *J : AAPtr->users()) {
247  if (J == ACall)
248  continue;
249 
250  if (Instruction *K = dyn_cast<Instruction>(J))
251  WorkList.push_back(K);
252  }
253 
254  while (!WorkList.empty()) {
255  Instruction *J = WorkList.pop_back_val();
256  if (LoadInst *LI = dyn_cast<LoadInst>(J)) {
257  if (!isValidAssumeForContext(ACall, J, DT))
258  continue;
259  Align NewAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
260  LI->getPointerOperand(), SE);
261  if (NewAlignment > LI->getAlign()) {
262  LI->setAlignment(NewAlignment);
263  ++NumLoadAlignChanged;
264  }
265  } else if (StoreInst *SI = dyn_cast<StoreInst>(J)) {
266  if (!isValidAssumeForContext(ACall, J, DT))
267  continue;
268  Align NewAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
269  SI->getPointerOperand(), SE);
270  if (NewAlignment > SI->getAlign()) {
271  SI->setAlignment(NewAlignment);
272  ++NumStoreAlignChanged;
273  }
274  } else if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(J)) {
275  if (!isValidAssumeForContext(ACall, J, DT))
276  continue;
277  Align NewDestAlignment =
278  getNewAlignment(AASCEV, AlignSCEV, OffSCEV, MI->getDest(), SE);
279 
280  LLVM_DEBUG(dbgs() << "\tmem inst: " << DebugStr(NewDestAlignment)
281  << "\n";);
282  if (NewDestAlignment > *MI->getDestAlign()) {
283  MI->setDestAlignment(NewDestAlignment);
284  ++NumMemIntAlignChanged;
285  }
286 
287  // For memory transfers, there is also a source alignment that
288  // can be set.
289  if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(MI)) {
290  Align NewSrcAlignment =
291  getNewAlignment(AASCEV, AlignSCEV, OffSCEV, MTI->getSource(), SE);
292 
293  LLVM_DEBUG(dbgs() << "\tmem trans: " << DebugStr(NewSrcAlignment)
294  << "\n";);
295 
296  if (NewSrcAlignment > *MTI->getSourceAlign()) {
297  MTI->setSourceAlignment(NewSrcAlignment);
298  ++NumMemIntAlignChanged;
299  }
300  }
301  }
302 
303  // Now that we've updated that use of the pointer, look for other uses of
304  // the pointer to update.
305  Visited.insert(J);
306  for (User *UJ : J->users()) {
307  Instruction *K = cast<Instruction>(UJ);
308  if (!Visited.count(K))
309  WorkList.push_back(K);
310  }
311  }
312 
313  return true;
314 }
315 
317  if (skipFunction(F))
318  return false;
319 
320  auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
321  ScalarEvolution *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
322  DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
323 
324  return Impl.runImpl(F, AC, SE, DT);
325 }
326 
328  ScalarEvolution *SE_,
329  DominatorTree *DT_) {
330  SE = SE_;
331  DT = DT_;
332 
333  bool Changed = false;
334  for (auto &AssumeVH : AC.assumptions())
335  if (AssumeVH) {
336  CallInst *Call = cast<CallInst>(AssumeVH);
337  for (unsigned Idx = 0; Idx < Call->getNumOperandBundles(); Idx++)
338  Changed |= processAssumption(Call, Idx);
339  }
340 
341  return Changed;
342 }
343 
346 
350  if (!runImpl(F, AC, &SE, &DT))
351  return PreservedAnalyses::all();
352 
354  PA.preserveSet<CFGAnalyses>();
355  PA.preserve<AAManager>();
357  PA.preserve<GlobalsAA>();
358  return PA;
359 }
llvm::GlobalsAA
Analysis pass providing a never-invalidated alias analysis result.
Definition: GlobalsModRef.h:132
llvm::PreservedAnalyses
A set of analyses that are preserved following a run of a transformation pass.
Definition: PassManager.h:155
AssumptionCache.h
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A manager for alias analyses.
Definition: AliasAnalysis.h:1221
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Analysis pass that exposes the ScalarEvolution for a function.
Definition: ScalarEvolution.h:2076
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Definition: IRTranslator.cpp:100
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Definition: AllocatorList.h:23
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void initializeAlignmentFromAssumptionsPass(PassRegistry &)
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static MaybeAlign getNewAlignmentDiff(const SCEV *DiffSCEV, const SCEV *AlignSCEV, ScalarEvolution *SE)
Definition: AlignmentFromAssumptions.cpp:95
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Return a type with the same bitwidth as the given type and which represents how SCEV will treat the g...
Definition: ScalarEvolution.cpp:3763
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Definition: AlignmentFromAssumptions.cpp:229
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Definition: AlignmentFromAssumptions.h:38
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Definition: AlignmentFromAssumptions.h:29
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Definition: IntrinsicInst.h:869
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Definition: AlignmentFromAssumptions.h:37
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Definition: Alignment.h:360
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Definition: Type.h:46
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Definition: LoopInfo.h:1258
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const SCEV * getTruncateOrZeroExtend(const SCEV *V, Type *Ty, unsigned Depth=0)
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Definition: ScalarEvolution.cpp:4020
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Definition: AlignmentFromAssumptions.cpp:20
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Definition: Instruction.h:45
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Definition: AlignmentFromAssumptions.cpp:345
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Definition: Alignment.h:39
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Definition: None.h:23
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Definition: ScalarEvolution.cpp:379
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constexpr bool isPowerOf2_64(uint64_t Value)
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APFloat abs(APFloat X)
Returns the absolute value of the argument.
Definition: APFloat.h:1272
InitializePasses.h
llvm::Value
LLVM Value Representation.
Definition: Value.h:75
Debug.h
llvm::Value::users
iterator_range< user_iterator > users()
Definition: Value.h:434
llvm::SmallPtrSetImpl::insert
std::pair< iterator, bool > insert(PtrType Ptr)
Inserts Ptr if and only if there is no element in the container equal to Ptr.
Definition: SmallPtrSet.h:364
llvm::Intrinsic::ID
unsigned ID
Definition: TargetTransformInfo.h:38