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 using 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 ///
37 static 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.
65 bool 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, E = ScanFrom->getParent()->begin();
122 
123   // We can at least always strip pointer casts even though we can't use the
124   // base here.
125   V = V->stripPointerCasts();
126 
127   while (BBI != E) {
128     --BBI;
129 
130     // If we see a free or a call which may write to memory (i.e. which might do
131     // a free) the pointer could be marked invalid.
132     if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
133         !isa<DbgInfoIntrinsic>(BBI))
134       return false;
135 
136     Value *AccessedPtr;
137     unsigned AccessedAlign;
138     if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
139       AccessedPtr = LI->getPointerOperand();
140       AccessedAlign = LI->getAlignment();
141     } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
142       AccessedPtr = SI->getPointerOperand();
143       AccessedAlign = SI->getAlignment();
144     } else
145       continue;
146 
147     Type *AccessedTy = AccessedPtr->getType()->getPointerElementType();
148     if (AccessedAlign == 0)
149       AccessedAlign = DL.getABITypeAlignment(AccessedTy);
150     if (AccessedAlign < Align)
151       continue;
152 
153     // Handle trivial cases.
154     if (AccessedPtr == V)
155       return true;
156 
157     if (AreEquivalentAddressValues(AccessedPtr->stripPointerCasts(), V) &&
158         LoadSize <= DL.getTypeStoreSize(AccessedTy))
159       return true;
160   }
161   return false;
162 }
163 
164 /// DefMaxInstsToScan - the default number of maximum instructions
165 /// to scan in the block, used by FindAvailableLoadedValue().
166 /// FindAvailableLoadedValue() was introduced in r60148, to improve jump
167 /// threading in part by eliminating partially redundant loads.
168 /// At that point, the value of MaxInstsToScan was already set to '6'
169 /// without documented explanation.
170 cl::opt<unsigned>
171 llvm::DefMaxInstsToScan("available-load-scan-limit", cl::init(6), cl::Hidden,
172   cl::desc("Use this to specify the default maximum number of instructions "
173            "to scan backward from a given instruction, when searching for "
174            "available loaded value"));
175 
176 /// \brief Scan the ScanBB block backwards to see if we have the value at the
177 /// memory address *Ptr locally available within a small number of instructions.
178 ///
179 /// The scan starts from \c ScanFrom. \c MaxInstsToScan specifies the maximum
180 /// instructions to scan in the block. If it is set to \c 0, it will scan the whole
181 /// block.
182 ///
183 /// If the value is available, this function returns it. If not, it returns the
184 /// iterator for the last validated instruction that the value would be live
185 /// through. If we scanned the entire block and didn't find something that
186 /// invalidates \c *Ptr or provides it, \c ScanFrom is left at the last
187 /// instruction processed and this returns null.
188 ///
189 /// You can also optionally specify an alias analysis implementation, which
190 /// makes this more precise.
191 ///
192 /// If \c AATags is non-null and a load or store is found, the AA tags from the
193 /// load or store are recorded there. If there are no AA tags or if no access is
194 /// found, it is left unmodified.
195 Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
196                                       BasicBlock::iterator &ScanFrom,
197                                       unsigned MaxInstsToScan,
198                                       AliasAnalysis *AA, AAMDNodes *AATags) {
199   if (MaxInstsToScan == 0)
200     MaxInstsToScan = ~0U;
201 
202   Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
203 
204   const DataLayout &DL = ScanBB->getModule()->getDataLayout();
205 
206   // Try to get the store size for the type.
207   uint64_t AccessSize = DL.getTypeStoreSize(AccessTy);
208 
209   Value *StrippedPtr = Ptr->stripPointerCasts();
210 
211   while (ScanFrom != ScanBB->begin()) {
212     // We must ignore debug info directives when counting (otherwise they
213     // would affect codegen).
214     Instruction *Inst = --ScanFrom;
215     if (isa<DbgInfoIntrinsic>(Inst))
216       continue;
217 
218     // Restore ScanFrom to expected value in case next test succeeds
219     ScanFrom++;
220 
221     // Don't scan huge blocks.
222     if (MaxInstsToScan-- == 0)
223       return nullptr;
224 
225     --ScanFrom;
226     // If this is a load of Ptr, the loaded value is available.
227     // (This is true even if the load is volatile or atomic, although
228     // those cases are unlikely.)
229     if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
230       if (AreEquivalentAddressValues(
231               LI->getPointerOperand()->stripPointerCasts(), StrippedPtr) &&
232           CastInst::isBitOrNoopPointerCastable(LI->getType(), AccessTy, DL)) {
233         if (AATags)
234           LI->getAAMetadata(*AATags);
235         return LI;
236       }
237 
238     if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
239       Value *StorePtr = SI->getPointerOperand()->stripPointerCasts();
240       // If this is a store through Ptr, the value is available!
241       // (This is true even if the store is volatile or atomic, although
242       // those cases are unlikely.)
243       if (AreEquivalentAddressValues(StorePtr, StrippedPtr) &&
244           CastInst::isBitOrNoopPointerCastable(SI->getValueOperand()->getType(),
245                                                AccessTy, DL)) {
246         if (AATags)
247           SI->getAAMetadata(*AATags);
248         return SI->getOperand(0);
249       }
250 
251       // If both StrippedPtr and StorePtr reach all the way to an alloca or
252       // global and they are different, ignore the store. This is a trivial form
253       // of alias analysis that is important for reg2mem'd code.
254       if ((isa<AllocaInst>(StrippedPtr) || isa<GlobalVariable>(StrippedPtr)) &&
255           (isa<AllocaInst>(StorePtr) || isa<GlobalVariable>(StorePtr)) &&
256           StrippedPtr != StorePtr)
257         continue;
258 
259       // If we have alias analysis and it says the store won't modify the loaded
260       // value, ignore the store.
261       if (AA && (AA->getModRefInfo(SI, StrippedPtr, AccessSize) & MRI_Mod) == 0)
262         continue;
263 
264       // Otherwise the store that may or may not alias the pointer, bail out.
265       ++ScanFrom;
266       return nullptr;
267     }
268 
269     // If this is some other instruction that may clobber Ptr, bail out.
270     if (Inst->mayWriteToMemory()) {
271       // If alias analysis claims that it really won't modify the load,
272       // ignore it.
273       if (AA &&
274           (AA->getModRefInfo(Inst, StrippedPtr, AccessSize) & MRI_Mod) == 0)
275         continue;
276 
277       // May modify the pointer, bail out.
278       ++ScanFrom;
279       return nullptr;
280     }
281   }
282 
283   // Got to the start of the block, we didn't find it, but are done for this
284   // block.
285   return nullptr;
286 }
287