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) {
184 } else if (NewAlignment > NewIncAlignment) {
185 if (NewAlignment % NewIncAlignment == 0) {
186 DEBUG(dbgs() << "\tnew start/inc alignment: " <<
187 NewIncAlignment << "\n");
188 return NewIncAlignment;
190 } else if (NewIncAlignment > NewAlignment) {
191 if (NewIncAlignment % NewAlignment == 0) {
192 DEBUG(dbgs() << "\tnew start/inc alignment: " <<
193 NewAlignment << "\n");
196 } else if (NewIncAlignment == NewAlignment) {
197 DEBUG(dbgs() << "\tnew start/inc alignment: " <<
198 NewAlignment << "\n");
206 bool AlignmentFromAssumptions::extractAlignmentInfo(CallInst *I,
207 Value *&AAPtr, const SCEV *&AlignSCEV,
208 const SCEV *&OffSCEV) {
209 // An alignment assume must be a statement about the least-significant
210 // bits of the pointer being zero, possibly with some offset.
211 ICmpInst *ICI = dyn_cast<ICmpInst>(I->getArgOperand(0));
215 // This must be an expression of the form: x & m == 0.
216 if (ICI->getPredicate() != ICmpInst::ICMP_EQ)
219 // Swap things around so that the RHS is 0.
220 Value *CmpLHS = ICI->getOperand(0);
221 Value *CmpRHS = ICI->getOperand(1);
222 const SCEV *CmpLHSSCEV = SE->getSCEV(CmpLHS);
223 const SCEV *CmpRHSSCEV = SE->getSCEV(CmpRHS);
224 if (CmpLHSSCEV->isZero())
225 std::swap(CmpLHS, CmpRHS);
226 else if (!CmpRHSSCEV->isZero())
229 BinaryOperator *CmpBO = dyn_cast<BinaryOperator>(CmpLHS);
230 if (!CmpBO || CmpBO->getOpcode() != Instruction::And)
233 // Swap things around so that the right operand of the and is a constant
234 // (the mask); we cannot deal with variable masks.
235 Value *AndLHS = CmpBO->getOperand(0);
236 Value *AndRHS = CmpBO->getOperand(1);
237 const SCEV *AndLHSSCEV = SE->getSCEV(AndLHS);
238 const SCEV *AndRHSSCEV = SE->getSCEV(AndRHS);
239 if (isa<SCEVConstant>(AndLHSSCEV)) {
240 std::swap(AndLHS, AndRHS);
241 std::swap(AndLHSSCEV, AndRHSSCEV);
244 const SCEVConstant *MaskSCEV = dyn_cast<SCEVConstant>(AndRHSSCEV);
248 // The mask must have some trailing ones (otherwise the condition is
249 // trivial and tells us nothing about the alignment of the left operand).
250 unsigned TrailingOnes =
251 MaskSCEV->getValue()->getValue().countTrailingOnes();
255 // Cap the alignment at the maximum with which LLVM can deal (and make sure
256 // we don't overflow the shift).
258 TrailingOnes = std::min(TrailingOnes,
259 unsigned(sizeof(unsigned) * CHAR_BIT - 1));
260 Alignment = std::min(1u << TrailingOnes, +Value::MaximumAlignment);
262 Type *Int64Ty = Type::getInt64Ty(I->getParent()->getParent()->getContext());
263 AlignSCEV = SE->getConstant(Int64Ty, Alignment);
265 // The LHS might be a ptrtoint instruction, or it might be the pointer
269 if (PtrToIntInst *PToI = dyn_cast<PtrToIntInst>(AndLHS)) {
270 AAPtr = PToI->getPointerOperand();
271 OffSCEV = SE->getConstant(Int64Ty, 0);
272 } else if (const SCEVAddExpr* AndLHSAddSCEV =
273 dyn_cast<SCEVAddExpr>(AndLHSSCEV)) {
274 // Try to find the ptrtoint; subtract it and the rest is the offset.
275 for (SCEVAddExpr::op_iterator J = AndLHSAddSCEV->op_begin(),
276 JE = AndLHSAddSCEV->op_end(); J != JE; ++J)
277 if (const SCEVUnknown *OpUnk = dyn_cast<SCEVUnknown>(*J))
278 if (PtrToIntInst *PToI = dyn_cast<PtrToIntInst>(OpUnk->getValue())) {
279 AAPtr = PToI->getPointerOperand();
280 OffSCEV = SE->getMinusSCEV(AndLHSAddSCEV, *J);
288 // Sign extend the offset to 64 bits (so that it is like all of the other
290 unsigned OffSCEVBits = OffSCEV->getType()->getPrimitiveSizeInBits();
291 if (OffSCEVBits < 64)
292 OffSCEV = SE->getSignExtendExpr(OffSCEV, Int64Ty);
293 else if (OffSCEVBits > 64)
296 AAPtr = AAPtr->stripPointerCasts();
300 bool AlignmentFromAssumptions::processAssumption(CallInst *ACall) {
302 const SCEV *AlignSCEV, *OffSCEV;
303 if (!extractAlignmentInfo(ACall, AAPtr, AlignSCEV, OffSCEV))
306 const SCEV *AASCEV = SE->getSCEV(AAPtr);
308 // Apply the assumption to all other users of the specified pointer.
309 SmallPtrSet<Instruction *, 32> Visited;
310 SmallVector<Instruction*, 16> WorkList;
311 for (User *J : AAPtr->users()) {
315 if (Instruction *K = dyn_cast<Instruction>(J))
316 if (isValidAssumeForContext(ACall, K, DL, DT))
317 WorkList.push_back(K);
320 while (!WorkList.empty()) {
321 Instruction *J = WorkList.pop_back_val();
323 if (LoadInst *LI = dyn_cast<LoadInst>(J)) {
324 unsigned NewAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
325 LI->getPointerOperand(), SE);
327 if (NewAlignment > LI->getAlignment()) {
328 LI->setAlignment(NewAlignment);
329 ++NumLoadAlignChanged;
331 } else if (StoreInst *SI = dyn_cast<StoreInst>(J)) {
332 unsigned NewAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
333 SI->getPointerOperand(), SE);
335 if (NewAlignment > SI->getAlignment()) {
336 SI->setAlignment(NewAlignment);
337 ++NumStoreAlignChanged;
339 } else if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(J)) {
340 unsigned NewDestAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
343 // For memory transfers, we need a common alignment for both the
344 // source and destination. If we have a new alignment for this
345 // instruction, but only for one operand, save it. If we reach the
346 // other operand through another assumption later, then we may
347 // change the alignment at that point.
348 if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(MI)) {
349 unsigned NewSrcAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
350 MTI->getSource(), SE);
352 DenseMap<MemTransferInst *, unsigned>::iterator DI =
353 NewDestAlignments.find(MTI);
354 unsigned AltDestAlignment = (DI == NewDestAlignments.end()) ?
357 DenseMap<MemTransferInst *, unsigned>::iterator SI =
358 NewSrcAlignments.find(MTI);
359 unsigned AltSrcAlignment = (SI == NewSrcAlignments.end()) ?
362 DEBUG(dbgs() << "\tmem trans: " << NewDestAlignment << " " <<
363 AltDestAlignment << " " << NewSrcAlignment <<
364 " " << AltSrcAlignment << "\n");
366 // Of these four alignments, pick the largest possible...
367 unsigned NewAlignment = 0;
368 if (NewDestAlignment <= std::max(NewSrcAlignment, AltSrcAlignment))
369 NewAlignment = std::max(NewAlignment, NewDestAlignment);
370 if (AltDestAlignment <= std::max(NewSrcAlignment, AltSrcAlignment))
371 NewAlignment = std::max(NewAlignment, AltDestAlignment);
372 if (NewSrcAlignment <= std::max(NewDestAlignment, AltDestAlignment))
373 NewAlignment = std::max(NewAlignment, NewSrcAlignment);
374 if (AltSrcAlignment <= std::max(NewDestAlignment, AltDestAlignment))
375 NewAlignment = std::max(NewAlignment, AltSrcAlignment);
377 if (NewAlignment > MI->getAlignment()) {
378 MI->setAlignment(ConstantInt::get(Type::getInt32Ty(
379 MI->getParent()->getContext()), NewAlignment));
380 ++NumMemIntAlignChanged;
383 NewDestAlignments.insert(std::make_pair(MTI, NewDestAlignment));
384 NewSrcAlignments.insert(std::make_pair(MTI, NewSrcAlignment));
385 } else if (NewDestAlignment > MI->getAlignment()) {
386 assert((!isa<MemIntrinsic>(MI) || isa<MemSetInst>(MI)) &&
387 "Unknown memory intrinsic");
389 MI->setAlignment(ConstantInt::get(Type::getInt32Ty(
390 MI->getParent()->getContext()), NewDestAlignment));
391 ++NumMemIntAlignChanged;
395 // Now that we've updated that use of the pointer, look for other uses of
396 // the pointer to update.
398 for (User *UJ : J->users()) {
399 Instruction *K = cast<Instruction>(UJ);
400 if (!Visited.count(K) && isValidAssumeForContext(ACall, K, DL, DT))
401 WorkList.push_back(K);
408 bool AlignmentFromAssumptions::runOnFunction(Function &F) {
409 bool Changed = false;
410 AT = &getAnalysis<AssumptionTracker>();
411 SE = &getAnalysis<ScalarEvolution>();
412 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
413 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
414 DL = DLP ? &DLP->getDataLayout() : nullptr;
416 NewDestAlignments.clear();
417 NewSrcAlignments.clear();
419 for (auto &I : AT->assumptions(&F))
420 Changed |= processAssumption(I);