xref: /freebsd/contrib/llvm-project/llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp (revision 0d8fe2373503aeac48492f28073049a8bfa4feb5)
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
22 #include "llvm/Transforms/Scalar/AlignmentFromAssumptions.h"
23 #include "llvm/ADT/SmallPtrSet.h"
24 #include "llvm/ADT/Statistic.h"
25 #include "llvm/Analysis/AliasAnalysis.h"
26 #include "llvm/Analysis/AssumptionCache.h"
27 #include "llvm/Analysis/GlobalsModRef.h"
28 #include "llvm/Analysis/LoopInfo.h"
29 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
30 #include "llvm/Analysis/ValueTracking.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"
38 #include "llvm/Support/raw_ostream.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) {
53     initializeAlignmentFromAssumptionsPass(*PassRegistry::getPassRegistry());
54   }
55 
56   bool runOnFunction(Function &F) override;
57 
58   void getAnalysisUsage(AnalysisUsage &AU) const override {
59     AU.addRequired<AssumptionCacheTracker>();
60     AU.addRequired<ScalarEvolutionWrapperPass>();
61     AU.addRequired<DominatorTreeWrapperPass>();
62 
63     AU.setPreservesCFG();
64     AU.addPreserved<AAResultsWrapperPass>();
65     AU.addPreserved<GlobalsAAWrapperPass>();
66     AU.addPreserved<LoopInfoWrapperPass>();
67     AU.addPreserved<DominatorTreeWrapperPass>();
68     AU.addPreserved<ScalarEvolutionWrapperPass>();
69   }
70 
71   AlignmentFromAssumptionsPass Impl;
72 };
73 }
74 
75 char AlignmentFromAssumptions::ID = 0;
76 static const char aip_name[] = "Alignment from assumptions";
77 INITIALIZE_PASS_BEGIN(AlignmentFromAssumptions, AA_NAME,
78                       aip_name, false, false)
79 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
80 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
81 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
82 INITIALIZE_PASS_END(AlignmentFromAssumptions, AA_NAME,
83                     aip_name, false, false)
84 
85 FunctionPass *llvm::createAlignmentFromAssumptionsPass() {
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->getMinusSCEV(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 
206 bool AlignmentFromAssumptionsPass::extractAlignmentInfo(CallInst *I,
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 
229 bool AlignmentFromAssumptionsPass::processAssumption(CallInst *ACall,
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.
244   SmallPtrSet<Instruction *, 32> Visited;
245   SmallVector<Instruction*, 16> WorkList;
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 
316 bool AlignmentFromAssumptions::runOnFunction(Function &F) {
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 
327 bool AlignmentFromAssumptionsPass::runImpl(Function &F, AssumptionCache &AC,
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 
344 PreservedAnalyses
345 AlignmentFromAssumptionsPass::run(Function &F, FunctionAnalysisManager &AM) {
346 
347   AssumptionCache &AC = AM.getResult<AssumptionAnalysis>(F);
348   ScalarEvolution &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
349   DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
350   if (!runImpl(F, AC, &SE, &DT))
351     return PreservedAnalyses::all();
352 
353   PreservedAnalyses PA;
354   PA.preserveSet<CFGAnalyses>();
355   PA.preserve<AAManager>();
356   PA.preserve<ScalarEvolutionAnalysis>();
357   PA.preserve<GlobalsAA>();
358   return PA;
359 }
360