1 //===- Loads.cpp - Local load analysis ------------------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines simple local analyses for load instructions.
12 //===----------------------------------------------------------------------===//
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"
26 /// \brief Test if A and B will obviously have the same value.
28 /// This includes recognizing that %t0 and %t1 will have the same
29 /// value in code like this:
31 /// %t0 = getelementptr \@a, 0, 3
32 /// store i32 0, i32* %t0
33 /// %t1 = getelementptr \@a, 0, 3
34 /// %t2 = load i32* %t1
37 static bool AreEquivalentAddressValues(const Value *A, const Value *B) {
38 // Test if the values are trivially equivalent.
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))
53 // Otherwise they may not be equivalent.
57 /// \brief Check if executing a load of this pointer value cannot trap.
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.
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,
67 const DataLayout &DL = ScanFrom->getModule()->getDataLayout();
69 // Zero alignment means that the load has the ABI alignment for the target
71 Align = DL.getABITypeAlignment(V->getType()->getPointerElementType());
72 assert(isPowerOf2_32(Align));
74 int64_t ByteOffset = 0;
76 Base = GetPointerBaseWithConstantOffset(V, ByteOffset, DL);
78 if (ByteOffset < 0) // out of bounds
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();
97 PointerType *AddrTy = cast<PointerType>(V->getType());
98 uint64_t LoadSize = DL.getTypeStoreSize(AddrTy->getElementType());
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
104 if (BaseType && BaseType->isSized()) {
106 BaseAlign = DL.getPrefTypeAlignment(BaseType);
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))
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();
123 // We can at least always strip pointer casts even though we can't use the
125 V = V->stripPointerCasts();
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))
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();
147 Type *AccessedTy = AccessedPtr->getType()->getPointerElementType();
148 if (AccessedAlign == 0)
149 AccessedAlign = DL.getABITypeAlignment(AccessedTy);
150 if (AccessedAlign < Align)
153 // Handle trivial cases.
154 if (AccessedPtr == V)
157 if (AreEquivalentAddressValues(AccessedPtr->stripPointerCasts(), V) &&
158 LoadSize <= DL.getTypeStoreSize(AccessedTy))
164 /// \brief Scan the ScanBB block backwards to see if we have the value at the
165 /// memory address *Ptr locally available within a small number of instructions.
167 /// The scan starts from \c ScanFrom. \c MaxInstsToScan specifies the maximum
168 /// instructions to scan in the block. If it is set to \c 0, it will scan the whole
171 /// If the value is available, this function returns it. If not, it returns the
172 /// iterator for the last validated instruction that the value would be live
173 /// through. If we scanned the entire block and didn't find something that
174 /// invalidates \c *Ptr or provides it, \c ScanFrom is left at the last
175 /// instruction processed and this returns null.
177 /// You can also optionally specify an alias analysis implementation, which
178 /// makes this more precise.
180 /// If \c AATags is non-null and a load or store is found, the AA tags from the
181 /// load or store are recorded there. If there are no AA tags or if no access is
182 /// found, it is left unmodified.
183 Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
184 BasicBlock::iterator &ScanFrom,
185 unsigned MaxInstsToScan,
186 AliasAnalysis *AA, AAMDNodes *AATags) {
187 if (MaxInstsToScan == 0)
188 MaxInstsToScan = ~0U;
190 Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
192 const DataLayout &DL = ScanBB->getModule()->getDataLayout();
194 // Try to get the store size for the type.
195 uint64_t AccessSize = DL.getTypeStoreSize(AccessTy);
197 Value *StrippedPtr = Ptr->stripPointerCasts();
199 while (ScanFrom != ScanBB->begin()) {
200 // We must ignore debug info directives when counting (otherwise they
201 // would affect codegen).
202 Instruction *Inst = --ScanFrom;
203 if (isa<DbgInfoIntrinsic>(Inst))
206 // Restore ScanFrom to expected value in case next test succeeds
209 // Don't scan huge blocks.
210 if (MaxInstsToScan-- == 0)
214 // If this is a load of Ptr, the loaded value is available.
215 // (This is true even if the load is volatile or atomic, although
216 // those cases are unlikely.)
217 if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
218 if (AreEquivalentAddressValues(
219 LI->getPointerOperand()->stripPointerCasts(), StrippedPtr) &&
220 CastInst::isBitOrNoopPointerCastable(LI->getType(), AccessTy, DL)) {
222 LI->getAAMetadata(*AATags);
226 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
227 Value *StorePtr = SI->getPointerOperand()->stripPointerCasts();
228 // If this is a store through Ptr, the value is available!
229 // (This is true even if the store is volatile or atomic, although
230 // those cases are unlikely.)
231 if (AreEquivalentAddressValues(StorePtr, StrippedPtr) &&
232 CastInst::isBitOrNoopPointerCastable(SI->getValueOperand()->getType(),
235 SI->getAAMetadata(*AATags);
236 return SI->getOperand(0);
239 // If both StrippedPtr and StorePtr reach all the way to an alloca or
240 // global and they are different, ignore the store. This is a trivial form
241 // of alias analysis that is important for reg2mem'd code.
242 if ((isa<AllocaInst>(StrippedPtr) || isa<GlobalVariable>(StrippedPtr)) &&
243 (isa<AllocaInst>(StorePtr) || isa<GlobalVariable>(StorePtr)) &&
244 StrippedPtr != StorePtr)
247 // If we have alias analysis and it says the store won't modify the loaded
248 // value, ignore the store.
250 (AA->getModRefInfo(SI, StrippedPtr, AccessSize) &
251 AliasAnalysis::Mod) == 0)
254 // Otherwise the store that may or may not alias the pointer, bail out.
259 // If this is some other instruction that may clobber Ptr, bail out.
260 if (Inst->mayWriteToMemory()) {
261 // If alias analysis claims that it really won't modify the load,
264 (AA->getModRefInfo(Inst, StrippedPtr, AccessSize) &
265 AliasAnalysis::Mod) == 0)
268 // May modify the pointer, bail out.
274 // Got to the start of the block, we didn't find it, but are done for this