1 //===----------------------- AlignmentFromAssumptions.cpp -----------------===//
2 // Set Load/Store Alignments From Assumptions
4 // The LLVM Compiler Infrastructure
6 // This file is distributed under the University of Illinois Open Source
7 // License. See LICENSE.TXT for details.
9 //===----------------------------------------------------------------------===//
11 // This file implements a ScalarEvolution-based transformation to set
12 // the alignments of load, stores and memory intrinsics based on the truth
13 // expressions of assume intrinsics. The primary motivation is to handle
14 // complex alignment assumptions that apply to vector loads and stores that
15 // appear after vectorization and unrolling.
17 //===----------------------------------------------------------------------===//
19 #define AA_NAME "alignment-from-assumptions"
20 #define DEBUG_TYPE AA_NAME
21 #include "llvm/Transforms/Scalar.h"
22 #include "llvm/ADT/SmallPtrSet.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/Analysis/AssumptionTracker.h"
25 #include "llvm/Analysis/LoopInfo.h"
26 #include "llvm/Analysis/ValueTracking.h"
27 #include "llvm/Analysis/ScalarEvolution.h"
28 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
29 #include "llvm/IR/Constant.h"
30 #include "llvm/IR/Dominators.h"
31 #include "llvm/IR/Instruction.h"
32 #include "llvm/IR/IntrinsicInst.h"
33 #include "llvm/IR/Intrinsics.h"
34 #include "llvm/IR/DataLayout.h"
35 #include "llvm/Support/Debug.h"
36 #include "llvm/Support/raw_ostream.h"
39 STATISTIC(NumLoadAlignChanged,
40 "Number of loads changed by alignment assumptions");
41 STATISTIC(NumStoreAlignChanged,
42 "Number of stores changed by alignment assumptions");
43 STATISTIC(NumMemIntAlignChanged,
44 "Number of memory intrinsics changed by alignment assumptions");
47 struct AlignmentFromAssumptions : public FunctionPass {
48 static char ID; // Pass identification, replacement for typeid
49 AlignmentFromAssumptions() : FunctionPass(ID) {
50 initializeAlignmentFromAssumptionsPass(*PassRegistry::getPassRegistry());
53 bool runOnFunction(Function &F);
55 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
56 AU.addRequired<AssumptionTracker>();
57 AU.addRequired<ScalarEvolution>();
58 AU.addRequired<DominatorTreeWrapperPass>();
61 AU.addPreserved<LoopInfo>();
62 AU.addPreserved<DominatorTreeWrapperPass>();
63 AU.addPreserved<ScalarEvolution>();
66 // For memory transfers, we need a common alignment for both the source and
67 // destination. If we have a new alignment for only one operand of a transfer
68 // instruction, save it in these maps. If we reach the other operand through
69 // another assumption later, then we may change the alignment at that point.
70 DenseMap<MemTransferInst *, unsigned> NewDestAlignments, NewSrcAlignments;
72 AssumptionTracker *AT;
77 bool extractAlignmentInfo(CallInst *I, Value *&AAPtr, const SCEV *&AlignSCEV,
78 const SCEV *&OffSCEV);
79 bool processAssumption(CallInst *I);
83 char AlignmentFromAssumptions::ID = 0;
84 static const char aip_name[] = "Alignment from assumptions";
85 INITIALIZE_PASS_BEGIN(AlignmentFromAssumptions, AA_NAME,
86 aip_name, false, false)
87 INITIALIZE_PASS_DEPENDENCY(AssumptionTracker)
88 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
89 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
90 INITIALIZE_PASS_END(AlignmentFromAssumptions, AA_NAME,
91 aip_name, false, false)
93 FunctionPass *llvm::createAlignmentFromAssumptionsPass() {
94 return new AlignmentFromAssumptions();
97 // Given an expression for the (constant) alignment, AlignSCEV, and an
98 // expression for the displacement between a pointer and the aligned address,
99 // DiffSCEV, compute the alignment of the displaced pointer if it can be reduced
100 // to a constant. Using SCEV to compute alignment handles the case where
101 // DiffSCEV is a recurrence with constant start such that the aligned offset
102 // is constant. e.g. {16,+,32} % 32 -> 16.
103 static unsigned getNewAlignmentDiff(const SCEV *DiffSCEV,
104 const SCEV *AlignSCEV,
105 ScalarEvolution *SE) {
106 // DiffUnits = Diff % int64_t(Alignment)
107 const SCEV *DiffAlignDiv = SE->getUDivExpr(DiffSCEV, AlignSCEV);
108 const SCEV *DiffAlign = SE->getMulExpr(DiffAlignDiv, AlignSCEV);
109 const SCEV *DiffUnitsSCEV = SE->getMinusSCEV(DiffAlign, DiffSCEV);
111 DEBUG(dbgs() << "\talignment relative to " << *AlignSCEV << " is " <<
112 *DiffUnitsSCEV << " (diff: " << *DiffSCEV << ")\n");
114 if (const SCEVConstant *ConstDUSCEV =
115 dyn_cast<SCEVConstant>(DiffUnitsSCEV)) {
116 int64_t DiffUnits = ConstDUSCEV->getValue()->getSExtValue();
118 // If the displacement is an exact multiple of the alignment, then the
119 // displaced pointer has the same alignment as the aligned pointer, so
120 // return the alignment value.
123 cast<SCEVConstant>(AlignSCEV)->getValue()->getSExtValue();
125 // If the displacement is not an exact multiple, but the remainder is a
126 // constant, then return this remainder (but only if it is a power of 2).
127 uint64_t DiffUnitsAbs = abs64(DiffUnits);
128 if (isPowerOf2_64(DiffUnitsAbs))
129 return (unsigned) DiffUnitsAbs;
135 // There is an address given by an offset OffSCEV from AASCEV which has an
136 // alignment AlignSCEV. Use that information, if possible, to compute a new
137 // alignment for Ptr.
138 static unsigned getNewAlignment(const SCEV *AASCEV, const SCEV *AlignSCEV,
139 const SCEV *OffSCEV, Value *Ptr,
140 ScalarEvolution *SE) {
141 const SCEV *PtrSCEV = SE->getSCEV(Ptr);
142 const SCEV *DiffSCEV = SE->getMinusSCEV(PtrSCEV, AASCEV);
144 // What we really want to know is the overall offset to the aligned
145 // address. This address is displaced by the provided offset.
146 DiffSCEV = SE->getMinusSCEV(DiffSCEV, OffSCEV);
148 DEBUG(dbgs() << "AFI: alignment of " << *Ptr << " relative to " <<
149 *AlignSCEV << " and offset " << *OffSCEV <<
150 " using diff " << *DiffSCEV << "\n");
152 unsigned NewAlignment = getNewAlignmentDiff(DiffSCEV, AlignSCEV, SE);
153 DEBUG(dbgs() << "\tnew alignment: " << NewAlignment << "\n");
157 } else if (const SCEVAddRecExpr *DiffARSCEV =
158 dyn_cast<SCEVAddRecExpr>(DiffSCEV)) {
159 // The relative offset to the alignment assumption did not yield a constant,
160 // but we should try harder: if we assume that a is 32-byte aligned, then in
161 // for (i = 0; i < 1024; i += 4) r += a[i]; not all of the loads from a are
162 // 32-byte aligned, but instead alternate between 32 and 16-byte alignment.
163 // As a result, the new alignment will not be a constant, but can still
164 // be improved over the default (of 4) to 16.
166 const SCEV *DiffStartSCEV = DiffARSCEV->getStart();
167 const SCEV *DiffIncSCEV = DiffARSCEV->getStepRecurrence(*SE);
169 DEBUG(dbgs() << "\ttrying start/inc alignment using start " <<
170 *DiffStartSCEV << " and inc " << *DiffIncSCEV << "\n");
172 // Now compute the new alignment using the displacement to the value in the
173 // first iteration, and also the alignment using the per-iteration delta.
174 // If these are the same, then use that answer. Otherwise, use the smaller
175 // one, but only if it divides the larger one.
176 NewAlignment = getNewAlignmentDiff(DiffStartSCEV, AlignSCEV, SE);
177 unsigned NewIncAlignment = getNewAlignmentDiff(DiffIncSCEV, AlignSCEV, SE);
179 DEBUG(dbgs() << "\tnew start alignment: " << NewAlignment << "\n");
180 DEBUG(dbgs() << "\tnew inc alignment: " << NewIncAlignment << "\n");
182 if (NewAlignment > NewIncAlignment) {
183 if (NewAlignment % NewIncAlignment == 0) {
184 DEBUG(dbgs() << "\tnew start/inc alignment: " <<
185 NewIncAlignment << "\n");
186 return NewIncAlignment;
188 } else if (NewIncAlignment > NewAlignment) {
189 if (NewIncAlignment % NewAlignment == 0) {
190 DEBUG(dbgs() << "\tnew start/inc alignment: " <<
191 NewAlignment << "\n");
194 } else if (NewIncAlignment == NewAlignment && NewIncAlignment) {
195 DEBUG(dbgs() << "\tnew start/inc alignment: " <<
196 NewAlignment << "\n");
204 bool AlignmentFromAssumptions::extractAlignmentInfo(CallInst *I,
205 Value *&AAPtr, const SCEV *&AlignSCEV,
206 const SCEV *&OffSCEV) {
207 // An alignment assume must be a statement about the least-significant
208 // bits of the pointer being zero, possibly with some offset.
209 ICmpInst *ICI = dyn_cast<ICmpInst>(I->getArgOperand(0));
213 // This must be an expression of the form: x & m == 0.
214 if (ICI->getPredicate() != ICmpInst::ICMP_EQ)
217 // Swap things around so that the RHS is 0.
218 Value *CmpLHS = ICI->getOperand(0);
219 Value *CmpRHS = ICI->getOperand(1);
220 const SCEV *CmpLHSSCEV = SE->getSCEV(CmpLHS);
221 const SCEV *CmpRHSSCEV = SE->getSCEV(CmpRHS);
222 if (CmpLHSSCEV->isZero())
223 std::swap(CmpLHS, CmpRHS);
224 else if (!CmpRHSSCEV->isZero())
227 BinaryOperator *CmpBO = dyn_cast<BinaryOperator>(CmpLHS);
228 if (!CmpBO || CmpBO->getOpcode() != Instruction::And)
231 // Swap things around so that the right operand of the and is a constant
232 // (the mask); we cannot deal with variable masks.
233 Value *AndLHS = CmpBO->getOperand(0);
234 Value *AndRHS = CmpBO->getOperand(1);
235 const SCEV *AndLHSSCEV = SE->getSCEV(AndLHS);
236 const SCEV *AndRHSSCEV = SE->getSCEV(AndRHS);
237 if (isa<SCEVConstant>(AndLHSSCEV)) {
238 std::swap(AndLHS, AndRHS);
239 std::swap(AndLHSSCEV, AndRHSSCEV);
242 const SCEVConstant *MaskSCEV = dyn_cast<SCEVConstant>(AndRHSSCEV);
246 // The mask must have some trailing ones (otherwise the condition is
247 // trivial and tells us nothing about the alignment of the left operand).
248 unsigned TrailingOnes =
249 MaskSCEV->getValue()->getValue().countTrailingOnes();
253 // Cap the alignment at the maximum with which LLVM can deal (and make sure
254 // we don't overflow the shift).
256 TrailingOnes = std::min(TrailingOnes,
257 unsigned(sizeof(unsigned) * CHAR_BIT - 1));
258 Alignment = std::min(1u << TrailingOnes, +Value::MaximumAlignment);
260 Type *Int64Ty = Type::getInt64Ty(I->getParent()->getParent()->getContext());
261 AlignSCEV = SE->getConstant(Int64Ty, Alignment);
263 // The LHS might be a ptrtoint instruction, or it might be the pointer
267 if (PtrToIntInst *PToI = dyn_cast<PtrToIntInst>(AndLHS)) {
268 AAPtr = PToI->getPointerOperand();
269 OffSCEV = SE->getConstant(Int64Ty, 0);
270 } else if (const SCEVAddExpr* AndLHSAddSCEV =
271 dyn_cast<SCEVAddExpr>(AndLHSSCEV)) {
272 // Try to find the ptrtoint; subtract it and the rest is the offset.
273 for (SCEVAddExpr::op_iterator J = AndLHSAddSCEV->op_begin(),
274 JE = AndLHSAddSCEV->op_end(); J != JE; ++J)
275 if (const SCEVUnknown *OpUnk = dyn_cast<SCEVUnknown>(*J))
276 if (PtrToIntInst *PToI = dyn_cast<PtrToIntInst>(OpUnk->getValue())) {
277 AAPtr = PToI->getPointerOperand();
278 OffSCEV = SE->getMinusSCEV(AndLHSAddSCEV, *J);
286 // Sign extend the offset to 64 bits (so that it is like all of the other
288 unsigned OffSCEVBits = OffSCEV->getType()->getPrimitiveSizeInBits();
289 if (OffSCEVBits < 64)
290 OffSCEV = SE->getSignExtendExpr(OffSCEV, Int64Ty);
291 else if (OffSCEVBits > 64)
294 AAPtr = AAPtr->stripPointerCasts();
298 bool AlignmentFromAssumptions::processAssumption(CallInst *ACall) {
300 const SCEV *AlignSCEV, *OffSCEV;
301 if (!extractAlignmentInfo(ACall, AAPtr, AlignSCEV, OffSCEV))
304 const SCEV *AASCEV = SE->getSCEV(AAPtr);
306 // Apply the assumption to all other users of the specified pointer.
307 SmallPtrSet<Instruction *, 32> Visited;
308 SmallVector<Instruction*, 16> WorkList;
309 for (User *J : AAPtr->users()) {
313 if (Instruction *K = dyn_cast<Instruction>(J))
314 if (isValidAssumeForContext(ACall, K, DL, DT))
315 WorkList.push_back(K);
318 while (!WorkList.empty()) {
319 Instruction *J = WorkList.pop_back_val();
321 if (LoadInst *LI = dyn_cast<LoadInst>(J)) {
322 unsigned NewAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
323 LI->getPointerOperand(), SE);
325 if (NewAlignment > LI->getAlignment()) {
326 LI->setAlignment(NewAlignment);
327 ++NumLoadAlignChanged;
329 } else if (StoreInst *SI = dyn_cast<StoreInst>(J)) {
330 unsigned NewAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
331 SI->getPointerOperand(), SE);
333 if (NewAlignment > SI->getAlignment()) {
334 SI->setAlignment(NewAlignment);
335 ++NumStoreAlignChanged;
337 } else if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(J)) {
338 unsigned NewDestAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
341 // For memory transfers, we need a common alignment for both the
342 // source and destination. If we have a new alignment for this
343 // instruction, but only for one operand, save it. If we reach the
344 // other operand through another assumption later, then we may
345 // change the alignment at that point.
346 if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(MI)) {
347 unsigned NewSrcAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
348 MTI->getSource(), SE);
350 DenseMap<MemTransferInst *, unsigned>::iterator DI =
351 NewDestAlignments.find(MTI);
352 unsigned AltDestAlignment = (DI == NewDestAlignments.end()) ?
355 DenseMap<MemTransferInst *, unsigned>::iterator SI =
356 NewSrcAlignments.find(MTI);
357 unsigned AltSrcAlignment = (SI == NewSrcAlignments.end()) ?
360 DEBUG(dbgs() << "\tmem trans: " << NewDestAlignment << " " <<
361 AltDestAlignment << " " << NewSrcAlignment <<
362 " " << AltSrcAlignment << "\n");
364 // Of these four alignments, pick the largest possible...
365 unsigned NewAlignment = 0;
366 if (NewDestAlignment <= std::max(NewSrcAlignment, AltSrcAlignment))
367 NewAlignment = std::max(NewAlignment, NewDestAlignment);
368 if (AltDestAlignment <= std::max(NewSrcAlignment, AltSrcAlignment))
369 NewAlignment = std::max(NewAlignment, AltDestAlignment);
370 if (NewSrcAlignment <= std::max(NewDestAlignment, AltDestAlignment))
371 NewAlignment = std::max(NewAlignment, NewSrcAlignment);
372 if (AltSrcAlignment <= std::max(NewDestAlignment, AltDestAlignment))
373 NewAlignment = std::max(NewAlignment, AltSrcAlignment);
375 if (NewAlignment > MI->getAlignment()) {
376 MI->setAlignment(ConstantInt::get(Type::getInt32Ty(
377 MI->getParent()->getContext()), NewAlignment));
378 ++NumMemIntAlignChanged;
381 NewDestAlignments.insert(std::make_pair(MTI, NewDestAlignment));
382 NewSrcAlignments.insert(std::make_pair(MTI, NewSrcAlignment));
383 } else if (NewDestAlignment > MI->getAlignment()) {
384 assert((!isa<MemIntrinsic>(MI) || isa<MemSetInst>(MI)) &&
385 "Unknown memory intrinsic");
387 MI->setAlignment(ConstantInt::get(Type::getInt32Ty(
388 MI->getParent()->getContext()), NewDestAlignment));
389 ++NumMemIntAlignChanged;
393 // Now that we've updated that use of the pointer, look for other uses of
394 // the pointer to update.
396 for (User *UJ : J->users()) {
397 Instruction *K = cast<Instruction>(UJ);
398 if (!Visited.count(K) && isValidAssumeForContext(ACall, K, DL, DT))
399 WorkList.push_back(K);
406 bool AlignmentFromAssumptions::runOnFunction(Function &F) {
407 bool Changed = false;
408 AT = &getAnalysis<AssumptionTracker>();
409 SE = &getAnalysis<ScalarEvolution>();
410 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
411 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
412 DL = DLP ? &DLP->getDataLayout() : nullptr;
414 NewDestAlignments.clear();
415 NewSrcAlignments.clear();
417 for (auto &I : AT->assumptions(&F))
418 Changed |= processAssumption(I);