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"
24using namespace llvm;
25
26/// \brief Test if A and B will obviously have the same value.
27///
28/// This includes recognizing that %t0 and %t1 will have the same
29/// value in code like this:
30/// \code
31///   %t0 = getelementptr \@a, 0, 3
32///   store i32 0, i32* %t0
33///   %t1 = getelementptr \@a, 0, 3
34///   %t2 = load i32* %t1
35/// \endcode
36///
37static bool AreEquivalentAddressValues(const Value *A, const Value *B) {
38  // Test if the values are trivially equivalent.
39  if (A == B)
40    return true;
41
42  // Test if the values come from identical arithmetic instructions.
43  // Use isIdenticalToWhenDefined instead of isIdenticalTo because
44  // this function is only used when one address use dominates the
45  // other, which means that they'll always either have the same
46  // value or one of them will have an undefined value.
47  if (isa<BinaryOperator>(A) || isa<CastInst>(A) || isa<PHINode>(A) ||
48      isa<GetElementPtrInst>(A))
49    if (const Instruction *BI = dyn_cast<Instruction>(B))
50      if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
51        return true;
52
53  // Otherwise they may not be equivalent.
54  return false;
55}
56
57/// \brief Check if executing a load of this pointer value cannot trap.
58///
59/// If it is not obviously safe to load from the specified pointer, we do
60/// a quick local scan of the basic block containing \c ScanFrom, to determine
61/// if the address is already accessed.
62///
63/// This uses the pointee type to determine how many bytes need to be safe to
64/// load from the pointer.
65bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
66                                       unsigned Align) {
67  const DataLayout &DL = ScanFrom->getModule()->getDataLayout();
68
69  // Zero alignment means that the load has the ABI alignment for the target
70  if (Align == 0)
71    Align = DL.getABITypeAlignment(V->getType()->getPointerElementType());
72  assert(isPowerOf2_32(Align));
73
74  int64_t ByteOffset = 0;
75  Value *Base = V;
76  Base = GetPointerBaseWithConstantOffset(V, ByteOffset, DL);
77
78  if (ByteOffset < 0) // out of bounds
79    return false;
80
81  Type *BaseType = nullptr;
82  unsigned BaseAlign = 0;
83  if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
84    // An alloca is safe to load from as load as it is suitably aligned.
85    BaseType = AI->getAllocatedType();
86    BaseAlign = AI->getAlignment();
87  } else if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
88    // Global variables are not necessarily safe to load from if they are
89    // overridden. Their size may change or they may be weak and require a test
90    // to determine if they were in fact provided.
91    if (!GV->mayBeOverridden()) {
92      BaseType = GV->getType()->getElementType();
93      BaseAlign = GV->getAlignment();
94    }
95  }
96
97  PointerType *AddrTy = cast<PointerType>(V->getType());
98  uint64_t LoadSize = DL.getTypeStoreSize(AddrTy->getElementType());
99
100  // If we found a base allocated type from either an alloca or global variable,
101  // try to see if we are definitively within the allocated region. We need to
102  // know the size of the base type and the loaded type to do anything in this
103  // case.
104  if (BaseType && BaseType->isSized()) {
105    if (BaseAlign == 0)
106      BaseAlign = DL.getPrefTypeAlignment(BaseType);
107
108    if (Align <= BaseAlign) {
109      // Check if the load is within the bounds of the underlying object.
110      if (ByteOffset + LoadSize <= DL.getTypeAllocSize(BaseType) &&
111          ((ByteOffset % Align) == 0))
112        return true;
113    }
114  }
115
116  // Otherwise, be a little bit aggressive by scanning the local block where we
117  // want to check to see if the pointer is already being loaded or stored
118  // from/to.  If so, the previous load or store would have already trapped,
119  // so there is no harm doing an extra load (also, CSE will later eliminate
120  // the load entirely).
121  BasicBlock::iterator BBI = ScanFrom->getIterator(),
122                       E = ScanFrom->getParent()->begin();
123
124  // We can at least always strip pointer casts even though we can't use the
125  // base here.
126  V = V->stripPointerCasts();
127
128  while (BBI != E) {
129    --BBI;
130
131    // If we see a free or a call which may write to memory (i.e. which might do
132    // a free) the pointer could be marked invalid.
133    if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
134        !isa<DbgInfoIntrinsic>(BBI))
135      return false;
136
137    Value *AccessedPtr;
138    unsigned AccessedAlign;
139    if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
140      AccessedPtr = LI->getPointerOperand();
141      AccessedAlign = LI->getAlignment();
142    } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
143      AccessedPtr = SI->getPointerOperand();
144      AccessedAlign = SI->getAlignment();
145    } else
146      continue;
147
148    Type *AccessedTy = AccessedPtr->getType()->getPointerElementType();
149    if (AccessedAlign == 0)
150      AccessedAlign = DL.getABITypeAlignment(AccessedTy);
151    if (AccessedAlign < Align)
152      continue;
153
154    // Handle trivial cases.
155    if (AccessedPtr == V)
156      return true;
157
158    if (AreEquivalentAddressValues(AccessedPtr->stripPointerCasts(), V) &&
159        LoadSize <= DL.getTypeStoreSize(AccessedTy))
160      return true;
161  }
162  return false;
163}
164
165/// DefMaxInstsToScan - the default number of maximum instructions
166/// to scan in the block, used by FindAvailableLoadedValue().
167/// FindAvailableLoadedValue() was introduced in r60148, to improve jump
168/// threading in part by eliminating partially redundant loads.
169/// At that point, the value of MaxInstsToScan was already set to '6'
170/// without documented explanation.
171cl::opt<unsigned>
172llvm::DefMaxInstsToScan("available-load-scan-limit", cl::init(6), cl::Hidden,
173  cl::desc("Use this to specify the default maximum number of instructions "
174           "to scan backward from a given instruction, when searching for "
175           "available loaded value"));
176
177/// \brief Scan the ScanBB block backwards to see if we have the value at the
178/// memory address *Ptr locally available within a small number of instructions.
179///
180/// The scan starts from \c ScanFrom. \c MaxInstsToScan specifies the maximum
181/// instructions to scan in the block. If it is set to \c 0, it will scan the whole
182/// block.
183///
184/// If the value is available, this function returns it. If not, it returns the
185/// iterator for the last validated instruction that the value would be live
186/// through. If we scanned the entire block and didn't find something that
187/// invalidates \c *Ptr or provides it, \c ScanFrom is left at the last
188/// instruction processed and this returns null.
189///
190/// You can also optionally specify an alias analysis implementation, which
191/// makes this more precise.
192///
193/// If \c AATags is non-null and a load or store is found, the AA tags from the
194/// load or store are recorded there. If there are no AA tags or if no access is
195/// found, it is left unmodified.
196Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
197                                      BasicBlock::iterator &ScanFrom,
198                                      unsigned MaxInstsToScan,
199                                      AliasAnalysis *AA, AAMDNodes *AATags) {
200  if (MaxInstsToScan == 0)
201    MaxInstsToScan = ~0U;
202
203  Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
204
205  const DataLayout &DL = ScanBB->getModule()->getDataLayout();
206
207  // Try to get the store size for the type.
208  uint64_t AccessSize = DL.getTypeStoreSize(AccessTy);
209
210  Value *StrippedPtr = Ptr->stripPointerCasts();
211
212  while (ScanFrom != ScanBB->begin()) {
213    // We must ignore debug info directives when counting (otherwise they
214    // would affect codegen).
215    Instruction *Inst = &*--ScanFrom;
216    if (isa<DbgInfoIntrinsic>(Inst))
217      continue;
218
219    // Restore ScanFrom to expected value in case next test succeeds
220    ScanFrom++;
221
222    // Don't scan huge blocks.
223    if (MaxInstsToScan-- == 0)
224      return nullptr;
225
226    --ScanFrom;
227    // If this is a load of Ptr, the loaded value is available.
228    // (This is true even if the load is volatile or atomic, although
229    // those cases are unlikely.)
230    if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
231      if (AreEquivalentAddressValues(
232              LI->getPointerOperand()->stripPointerCasts(), StrippedPtr) &&
233          CastInst::isBitOrNoopPointerCastable(LI->getType(), AccessTy, DL)) {
234        if (AATags)
235          LI->getAAMetadata(*AATags);
236        return LI;
237      }
238
239    if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
240      Value *StorePtr = SI->getPointerOperand()->stripPointerCasts();
241      // If this is a store through Ptr, the value is available!
242      // (This is true even if the store is volatile or atomic, although
243      // those cases are unlikely.)
244      if (AreEquivalentAddressValues(StorePtr, StrippedPtr) &&
245          CastInst::isBitOrNoopPointerCastable(SI->getValueOperand()->getType(),
246                                               AccessTy, DL)) {
247        if (AATags)
248          SI->getAAMetadata(*AATags);
249        return SI->getOperand(0);
250      }
251
252      // If both StrippedPtr and StorePtr reach all the way to an alloca or
253      // global and they are different, ignore the store. This is a trivial form
254      // of alias analysis that is important for reg2mem'd code.
255      if ((isa<AllocaInst>(StrippedPtr) || isa<GlobalVariable>(StrippedPtr)) &&
256          (isa<AllocaInst>(StorePtr) || isa<GlobalVariable>(StorePtr)) &&
257          StrippedPtr != StorePtr)
258        continue;
259
260      // If we have alias analysis and it says the store won't modify the loaded
261      // value, ignore the store.
262      if (AA && (AA->getModRefInfo(SI, StrippedPtr, AccessSize) & MRI_Mod) == 0)
263        continue;
264
265      // Otherwise the store that may or may not alias the pointer, bail out.
266      ++ScanFrom;
267      return nullptr;
268    }
269
270    // If this is some other instruction that may clobber Ptr, bail out.
271    if (Inst->mayWriteToMemory()) {
272      // If alias analysis claims that it really won't modify the load,
273      // ignore it.
274      if (AA &&
275          (AA->getModRefInfo(Inst, StrippedPtr, AccessSize) & MRI_Mod) == 0)
276        continue;
277
278      // May modify the pointer, bail out.
279      ++ScanFrom;
280      return nullptr;
281    }
282  }
283
284  // Got to the start of the block, we didn't find it, but are done for this
285  // block.
286  return nullptr;
287}
288