Line data Source code
1 : //===- Loads.cpp - Local load analysis ------------------------------------===//
2 : //
3 : // The LLVM Compiler Infrastructure
4 : //
5 : // This file is distributed under the University of Illinois Open Source
6 : // License. See LICENSE.TXT for details.
7 : //
8 : //===----------------------------------------------------------------------===//
9 : //
10 : // This file defines simple local analyses for load instructions.
11 : //
12 : //===----------------------------------------------------------------------===//
13 :
14 : #include "llvm/Analysis/Loads.h"
15 : #include "llvm/Analysis/AliasAnalysis.h"
16 : #include "llvm/Analysis/ValueTracking.h"
17 : #include "llvm/IR/DataLayout.h"
18 : #include "llvm/IR/GlobalAlias.h"
19 : #include "llvm/IR/GlobalVariable.h"
20 : #include "llvm/IR/IntrinsicInst.h"
21 : #include "llvm/IR/LLVMContext.h"
22 : #include "llvm/IR/Module.h"
23 : #include "llvm/IR/Operator.h"
24 : #include "llvm/IR/Statepoint.h"
25 :
26 : using namespace llvm;
27 :
28 2451109 : static bool isAligned(const Value *Base, const APInt &Offset, unsigned Align,
29 : const DataLayout &DL) {
30 2451109 : APInt BaseAlign(Offset.getBitWidth(), Base->getPointerAlignment(DL));
31 :
32 2451109 : if (!BaseAlign) {
33 10649 : Type *Ty = Base->getType()->getPointerElementType();
34 10649 : if (!Ty->isSized())
35 : return false;
36 10647 : BaseAlign = DL.getABITypeAlignment(Ty);
37 : }
38 :
39 2451107 : APInt Alignment(Offset.getBitWidth(), Align);
40 :
41 : assert(Alignment.isPowerOf2() && "must be a power of 2!");
42 12254147 : return BaseAlign.uge(Alignment) && !(Offset & (Alignment-1));
43 : }
44 :
45 2451109 : static bool isAligned(const Value *Base, unsigned Align, const DataLayout &DL) {
46 2451109 : Type *Ty = Base->getType();
47 : assert(Ty->isSized() && "must be sized");
48 2451109 : APInt Offset(DL.getTypeStoreSizeInBits(Ty), 0);
49 2451109 : return isAligned(Base, Offset, Align, DL);
50 : }
51 :
52 : /// Test if V is always a pointer to allocated and suitably aligned memory for
53 : /// a simple load or store.
54 4072455 : static bool isDereferenceableAndAlignedPointer(
55 : const Value *V, unsigned Align, const APInt &Size, const DataLayout &DL,
56 : const Instruction *CtxI, const DominatorTree *DT,
57 : SmallPtrSetImpl<const Value *> &Visited) {
58 : // Already visited? Bail out, we've likely hit unreachable code.
59 4072455 : if (!Visited.insert(V).second)
60 : return false;
61 :
62 : // Note that it is not safe to speculate into a malloc'd region because
63 : // malloc may return null.
64 :
65 : // bitcast instructions are no-ops as far as dereferenceability is concerned.
66 : if (const BitCastOperator *BC = dyn_cast<BitCastOperator>(V))
67 841796 : return isDereferenceableAndAlignedPointer(BC->getOperand(0), Align, Size,
68 420898 : DL, CtxI, DT, Visited);
69 :
70 3651555 : bool CheckForNonNull = false;
71 : APInt KnownDerefBytes(Size.getBitWidth(),
72 3651555 : V->getPointerDereferenceableBytes(DL, CheckForNonNull));
73 3651555 : if (KnownDerefBytes.getBoolValue()) {
74 2451260 : if (KnownDerefBytes.uge(Size))
75 2451121 : if (!CheckForNonNull || isKnownNonZero(V, DL, 0, nullptr, CtxI, DT))
76 2451109 : return isAligned(V, Align, DL);
77 : }
78 :
79 : // For GEPs, determine if the indexing lands within the allocated object.
80 : if (const GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
81 : const Value *Base = GEP->getPointerOperand();
82 :
83 873139 : APInt Offset(DL.getIndexTypeSizeInBits(GEP->getType()), 0);
84 1717419 : if (!GEP->accumulateConstantOffset(DL, Offset) || Offset.isNegative() ||
85 4250259 : !Offset.urem(APInt(Offset.getBitWidth(), Align)).isMinValue())
86 : return false;
87 :
88 : // If the base pointer is dereferenceable for Offset+Size bytes, then the
89 : // GEP (== Base + Offset) is dereferenceable for Size bytes. If the base
90 : // pointer is aligned to Align bytes, and the Offset is divisible by Align
91 : // then the GEP (== Base + Offset == k_0 * Align + k_1 * Align) is also
92 : // aligned to Align bytes.
93 :
94 : // Offset and Size may have different bit widths if we have visited an
95 : // addrspacecast, so we can't do arithmetic directly on the APInt values.
96 844259 : return isDereferenceableAndAlignedPointer(
97 1688518 : Base, Align, Offset + Size.sextOrTrunc(Offset.getBitWidth()),
98 : DL, CtxI, DT, Visited);
99 : }
100 :
101 : // For gc.relocate, look through relocations
102 : if (const GCRelocateInst *RelocateInst = dyn_cast<GCRelocateInst>(V))
103 5 : return isDereferenceableAndAlignedPointer(
104 10 : RelocateInst->getDerivedPtr(), Align, Size, DL, CtxI, DT, Visited);
105 :
106 : if (const AddrSpaceCastInst *ASC = dyn_cast<AddrSpaceCastInst>(V))
107 178 : return isDereferenceableAndAlignedPointer(ASC->getOperand(0), Align, Size,
108 178 : DL, CtxI, DT, Visited);
109 :
110 327124 : if (auto CS = ImmutableCallSite(V))
111 16084 : if (auto *RP = getArgumentAliasingToReturnedPointer(CS))
112 5 : return isDereferenceableAndAlignedPointer(RP, Align, Size, DL, CtxI, DT,
113 : Visited);
114 :
115 : // If we don't know, assume the worst.
116 : return false;
117 : }
118 :
119 105184 : bool llvm::isDereferenceableAndAlignedPointer(const Value *V, unsigned Align,
120 : const APInt &Size,
121 : const DataLayout &DL,
122 : const Instruction *CtxI,
123 : const DominatorTree *DT) {
124 : SmallPtrSet<const Value *, 32> Visited;
125 105184 : return ::isDereferenceableAndAlignedPointer(V, Align, Size, DL, CtxI, DT,
126 105184 : Visited);
127 : }
128 :
129 2701929 : bool llvm::isDereferenceableAndAlignedPointer(const Value *V, unsigned Align,
130 : const DataLayout &DL,
131 : const Instruction *CtxI,
132 : const DominatorTree *DT) {
133 : // When dereferenceability information is provided by a dereferenceable
134 : // attribute, we know exactly how many bytes are dereferenceable. If we can
135 : // determine the exact offset to the attributed variable, we can use that
136 : // information here.
137 2701929 : Type *VTy = V->getType();
138 2701929 : Type *Ty = VTy->getPointerElementType();
139 :
140 : // Require ABI alignment for loads without alignment specification
141 2701929 : if (Align == 0)
142 505 : Align = DL.getABITypeAlignment(Ty);
143 :
144 2701929 : if (!Ty->isSized())
145 : return false;
146 :
147 : SmallPtrSet<const Value *, 32> Visited;
148 2701926 : return ::isDereferenceableAndAlignedPointer(
149 5403852 : V, Align, APInt(DL.getIndexTypeSizeInBits(VTy), DL.getTypeStoreSize(Ty)), DL,
150 : CtxI, DT, Visited);
151 : }
152 :
153 2369648 : bool llvm::isDereferenceablePointer(const Value *V, const DataLayout &DL,
154 : const Instruction *CtxI,
155 : const DominatorTree *DT) {
156 2369648 : return isDereferenceableAndAlignedPointer(V, 1, DL, CtxI, DT);
157 : }
158 :
159 : /// Test if A and B will obviously have the same value.
160 : ///
161 : /// This includes recognizing that %t0 and %t1 will have the same
162 : /// value in code like this:
163 : /// \code
164 : /// %t0 = getelementptr \@a, 0, 3
165 : /// store i32 0, i32* %t0
166 : /// %t1 = getelementptr \@a, 0, 3
167 : /// %t2 = load i32* %t1
168 : /// \endcode
169 : ///
170 10551040 : static bool AreEquivalentAddressValues(const Value *A, const Value *B) {
171 : // Test if the values are trivially equivalent.
172 10551040 : if (A == B)
173 : return true;
174 :
175 : // Test if the values come from identical arithmetic instructions.
176 : // Use isIdenticalToWhenDefined instead of isIdenticalTo because
177 : // this function is only used when one address use dominates the
178 : // other, which means that they'll always either have the same
179 : // value or one of them will have an undefined value.
180 : if (isa<BinaryOperator>(A) || isa<CastInst>(A) || isa<PHINode>(A) ||
181 : isa<GetElementPtrInst>(A))
182 : if (const Instruction *BI = dyn_cast<Instruction>(B))
183 164082 : if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
184 312 : return true;
185 :
186 : // Otherwise they may not be equivalent.
187 : return false;
188 : }
189 :
190 : /// Check if executing a load of this pointer value cannot trap.
191 : ///
192 : /// If DT and ScanFrom are specified this method performs context-sensitive
193 : /// analysis and returns true if it is safe to load immediately before ScanFrom.
194 : ///
195 : /// If it is not obviously safe to load from the specified pointer, we do
196 : /// a quick local scan of the basic block containing \c ScanFrom, to determine
197 : /// if the address is already accessed.
198 : ///
199 : /// This uses the pointee type to determine how many bytes need to be safe to
200 : /// load from the pointer.
201 2316 : bool llvm::isSafeToLoadUnconditionally(Value *V, unsigned Align,
202 : const DataLayout &DL,
203 : Instruction *ScanFrom,
204 : const DominatorTree *DT) {
205 : // Zero alignment means that the load has the ABI alignment for the target
206 2316 : if (Align == 0)
207 256 : Align = DL.getABITypeAlignment(V->getType()->getPointerElementType());
208 : assert(isPowerOf2_32(Align));
209 :
210 : // If DT is not specified we can't make context-sensitive query
211 2316 : const Instruction* CtxI = DT ? ScanFrom : nullptr;
212 2316 : if (isDereferenceableAndAlignedPointer(V, Align, DL, CtxI, DT))
213 : return true;
214 :
215 1594 : int64_t ByteOffset = 0;
216 : Value *Base = V;
217 1594 : Base = GetPointerBaseWithConstantOffset(V, ByteOffset, DL);
218 :
219 1594 : if (ByteOffset < 0) // out of bounds
220 : return false;
221 :
222 : Type *BaseType = nullptr;
223 : unsigned BaseAlign = 0;
224 : if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
225 : // An alloca is safe to load from as load as it is suitably aligned.
226 2 : BaseType = AI->getAllocatedType();
227 : BaseAlign = AI->getAlignment();
228 : } else if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
229 : // Global variables are not necessarily safe to load from if they are
230 : // interposed arbitrarily. Their size may change or they may be weak and
231 : // require a test to determine if they were in fact provided.
232 : if (!GV->isInterposable()) {
233 1 : BaseType = GV->getType()->getElementType();
234 : BaseAlign = GV->getAlignment();
235 : }
236 : }
237 :
238 1594 : PointerType *AddrTy = cast<PointerType>(V->getType());
239 1594 : uint64_t LoadSize = DL.getTypeStoreSize(AddrTy->getElementType());
240 :
241 : // If we found a base allocated type from either an alloca or global variable,
242 : // try to see if we are definitively within the allocated region. We need to
243 : // know the size of the base type and the loaded type to do anything in this
244 : // case.
245 1594 : if (BaseType && BaseType->isSized()) {
246 3 : if (BaseAlign == 0)
247 0 : BaseAlign = DL.getPrefTypeAlignment(BaseType);
248 :
249 3 : if (Align <= BaseAlign) {
250 : // Check if the load is within the bounds of the underlying object.
251 0 : if (ByteOffset + LoadSize <= DL.getTypeAllocSize(BaseType) &&
252 0 : ((ByteOffset % Align) == 0))
253 : return true;
254 : }
255 : }
256 :
257 1594 : if (!ScanFrom)
258 : return false;
259 :
260 : // Otherwise, be a little bit aggressive by scanning the local block where we
261 : // want to check to see if the pointer is already being loaded or stored
262 : // from/to. If so, the previous load or store would have already trapped,
263 : // so there is no harm doing an extra load (also, CSE will later eliminate
264 : // the load entirely).
265 300 : BasicBlock::iterator BBI = ScanFrom->getIterator(),
266 300 : E = ScanFrom->getParent()->begin();
267 :
268 : // We can at least always strip pointer casts even though we can't use the
269 : // base here.
270 : V = V->stripPointerCasts();
271 :
272 3168 : while (BBI != E) {
273 : --BBI;
274 :
275 : // If we see a free or a call which may write to memory (i.e. which might do
276 : // a free) the pointer could be marked invalid.
277 2899 : if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
278 : !isa<DbgInfoIntrinsic>(BBI))
279 : return false;
280 :
281 : Value *AccessedPtr;
282 : unsigned AccessedAlign;
283 : if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
284 : AccessedPtr = LI->getPointerOperand();
285 : AccessedAlign = LI->getAlignment();
286 : } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
287 : AccessedPtr = SI->getPointerOperand();
288 : AccessedAlign = SI->getAlignment();
289 : } else
290 : continue;
291 :
292 386 : Type *AccessedTy = AccessedPtr->getType()->getPointerElementType();
293 386 : if (AccessedAlign == 0)
294 9 : AccessedAlign = DL.getABITypeAlignment(AccessedTy);
295 386 : if (AccessedAlign < Align)
296 : continue;
297 :
298 : // Handle trivial cases.
299 383 : if (AccessedPtr == V)
300 : return true;
301 :
302 360 : if (AreEquivalentAddressValues(AccessedPtr->stripPointerCasts(), V) &&
303 : LoadSize <= DL.getTypeStoreSize(AccessedTy))
304 : return true;
305 : }
306 : return false;
307 : }
308 :
309 : /// DefMaxInstsToScan - the default number of maximum instructions
310 : /// to scan in the block, used by FindAvailableLoadedValue().
311 : /// FindAvailableLoadedValue() was introduced in r60148, to improve jump
312 : /// threading in part by eliminating partially redundant loads.
313 : /// At that point, the value of MaxInstsToScan was already set to '6'
314 : /// without documented explanation.
315 : cl::opt<unsigned>
316 : llvm::DefMaxInstsToScan("available-load-scan-limit", cl::init(6), cl::Hidden,
317 : cl::desc("Use this to specify the default maximum number of instructions "
318 : "to scan backward from a given instruction, when searching for "
319 : "available loaded value"));
320 :
321 5891455 : Value *llvm::FindAvailableLoadedValue(LoadInst *Load,
322 : BasicBlock *ScanBB,
323 : BasicBlock::iterator &ScanFrom,
324 : unsigned MaxInstsToScan,
325 : AliasAnalysis *AA, bool *IsLoad,
326 : unsigned *NumScanedInst) {
327 : // Don't CSE load that is volatile or anything stronger than unordered.
328 : if (!Load->isUnordered())
329 : return nullptr;
330 :
331 5850510 : return FindAvailablePtrLoadStore(
332 5850510 : Load->getPointerOperand(), Load->getType(), Load->isAtomic(), ScanBB,
333 5850510 : ScanFrom, MaxInstsToScan, AA, IsLoad, NumScanedInst);
334 : }
335 :
336 5855287 : Value *llvm::FindAvailablePtrLoadStore(Value *Ptr, Type *AccessTy,
337 : bool AtLeastAtomic, BasicBlock *ScanBB,
338 : BasicBlock::iterator &ScanFrom,
339 : unsigned MaxInstsToScan,
340 : AliasAnalysis *AA, bool *IsLoadCSE,
341 : unsigned *NumScanedInst) {
342 5855287 : if (MaxInstsToScan == 0)
343 : MaxInstsToScan = ~0U;
344 :
345 5855287 : const DataLayout &DL = ScanBB->getModule()->getDataLayout();
346 :
347 : // Try to get the store size for the type.
348 : uint64_t AccessSize = DL.getTypeStoreSize(AccessTy);
349 :
350 : Value *StrippedPtr = Ptr->stripPointerCasts();
351 :
352 26141185 : while (ScanFrom != ScanBB->begin()) {
353 : // We must ignore debug info directives when counting (otherwise they
354 : // would affect codegen).
355 : Instruction *Inst = &*--ScanFrom;
356 : if (isa<DbgInfoIntrinsic>(Inst))
357 : continue;
358 :
359 : // Restore ScanFrom to expected value in case next test succeeds
360 : ScanFrom++;
361 :
362 21969761 : if (NumScanedInst)
363 15938 : ++(*NumScanedInst);
364 :
365 : // Don't scan huge blocks.
366 21969761 : if (MaxInstsToScan-- == 0)
367 : return nullptr;
368 :
369 : --ScanFrom;
370 : // If this is a load of Ptr, the loaded value is available.
371 : // (This is true even if the load is volatile or atomic, although
372 : // those cases are unlikely.)
373 : if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
374 4933457 : if (AreEquivalentAddressValues(
375 4935191 : LI->getPointerOperand()->stripPointerCasts(), StrippedPtr) &&
376 1734 : CastInst::isBitOrNoopPointerCastable(LI->getType(), AccessTy, DL)) {
377 :
378 : // We can value forward from an atomic to a non-atomic, but not the
379 : // other way around.
380 1730 : if (LI->isAtomic() < AtLeastAtomic)
381 : return nullptr;
382 :
383 1730 : if (IsLoadCSE)
384 1730 : *IsLoadCSE = true;
385 1730 : return LI;
386 : }
387 :
388 : if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
389 : Value *StorePtr = SI->getPointerOperand()->stripPointerCasts();
390 : // If this is a store through Ptr, the value is available!
391 : // (This is true even if the store is volatile or atomic, although
392 : // those cases are unlikely.)
393 5618960 : if (AreEquivalentAddressValues(StorePtr, StrippedPtr) &&
394 1735 : CastInst::isBitOrNoopPointerCastable(SI->getValueOperand()->getType(),
395 : AccessTy, DL)) {
396 :
397 : // We can value forward from an atomic to a non-atomic, but not the
398 : // other way around.
399 1672 : if (SI->isAtomic() < AtLeastAtomic)
400 : return nullptr;
401 :
402 1672 : if (IsLoadCSE)
403 1671 : *IsLoadCSE = false;
404 : return SI->getOperand(0);
405 : }
406 :
407 : // If both StrippedPtr and StorePtr reach all the way to an alloca or
408 : // global and they are different, ignore the store. This is a trivial form
409 : // of alias analysis that is important for reg2mem'd code.
410 5283363 : if ((isa<AllocaInst>(StrippedPtr) || isa<GlobalVariable>(StrippedPtr)) &&
411 2620773 : (isa<AllocaInst>(StorePtr) || isa<GlobalVariable>(StorePtr)) &&
412 : StrippedPtr != StorePtr)
413 : continue;
414 :
415 : // If we have alias analysis and it says the store won't modify the loaded
416 : // value, ignore the store.
417 8967626 : if (AA && !isModSet(AA->getModRefInfo(SI, StrippedPtr, AccessSize)))
418 : continue;
419 :
420 : // Otherwise the store that may or may not alias the pointer, bail out.
421 : ++ScanFrom;
422 356249 : return nullptr;
423 : }
424 :
425 : // If this is some other instruction that may clobber Ptr, bail out.
426 14303334 : if (Inst->mayWriteToMemory()) {
427 : // If alias analysis claims that it really won't modify the load,
428 : // ignore it.
429 783210 : if (AA && !isModSet(AA->getModRefInfo(Inst, StrippedPtr, AccessSize)))
430 578568 : continue;
431 :
432 : // May modify the pointer, bail out.
433 : ++ScanFrom;
434 204642 : return nullptr;
435 : }
436 : }
437 :
438 : // Got to the start of the block, we didn't find it, but are done for this
439 : // block.
440 : return nullptr;
441 : }
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