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/Operator.h"
25 /// \brief Test if A and B will obviously have the same value.
27 /// This includes recognizing that %t0 and %t1 will have the same
28 /// value in code like this:
30 /// %t0 = getelementptr \@a, 0, 3
31 /// store i32 0, i32* %t0
32 /// %t1 = getelementptr \@a, 0, 3
33 /// %t2 = load i32* %t1
36 static bool AreEquivalentAddressValues(const Value *A, const Value *B) {
37 // Test if the values are trivially equivalent.
38 if (A == B) return true;
40 // Test if the values come from identical arithmetic instructions.
41 // Use isIdenticalToWhenDefined instead of isIdenticalTo because
42 // this function is only used when one address use dominates the
43 // other, which means that they'll always either have the same
44 // value or one of them will have an undefined value.
45 if (isa<BinaryOperator>(A) || isa<CastInst>(A) ||
46 isa<PHINode>(A) || isa<GetElementPtrInst>(A))
47 if (const Instruction *BI = dyn_cast<Instruction>(B))
48 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
51 // Otherwise they may not be equivalent.
55 /// isSafeToLoadUnconditionally - Return true if we know that executing a load
56 /// from this value cannot trap. If it is not obviously safe to load from the
57 /// specified pointer, we do a quick local scan of the basic block containing
58 /// ScanFrom, to determine if the address is already accessed.
59 bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
60 unsigned Align, const DataLayout *TD) {
61 int64_t ByteOffset = 0;
63 Base = GetPointerBaseWithConstantOffset(V, ByteOffset, TD);
65 if (ByteOffset < 0) // out of bounds
68 Type *BaseType = nullptr;
69 unsigned BaseAlign = 0;
70 if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
71 // An alloca is safe to load from as load as it is suitably aligned.
72 BaseType = AI->getAllocatedType();
73 BaseAlign = AI->getAlignment();
74 } else if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
75 // Global variables are safe to load from but their size cannot be
76 // guaranteed if they are overridden.
77 if (!GV->mayBeOverridden()) {
78 BaseType = GV->getType()->getElementType();
79 BaseAlign = GV->getAlignment();
83 if (BaseType && BaseType->isSized()) {
84 if (TD && BaseAlign == 0)
85 BaseAlign = TD->getPrefTypeAlignment(BaseType);
87 if (Align <= BaseAlign) {
89 return true; // Loading directly from an alloca or global is OK.
91 // Check if the load is within the bounds of the underlying object.
92 PointerType *AddrTy = cast<PointerType>(V->getType());
93 uint64_t LoadSize = TD->getTypeStoreSize(AddrTy->getElementType());
94 if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType) &&
95 (Align == 0 || (ByteOffset % Align) == 0))
100 // Otherwise, be a little bit aggressive by scanning the local block where we
101 // want to check to see if the pointer is already being loaded or stored
102 // from/to. If so, the previous load or store would have already trapped,
103 // so there is no harm doing an extra load (also, CSE will later eliminate
104 // the load entirely).
105 BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin();
110 // If we see a free or a call which may write to memory (i.e. which might do
111 // a free) the pointer could be marked invalid.
112 if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
113 !isa<DbgInfoIntrinsic>(BBI))
116 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
117 if (AreEquivalentAddressValues(LI->getOperand(0), V)) return true;
118 } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
119 if (AreEquivalentAddressValues(SI->getOperand(1), V)) return true;
125 /// \brief Scan the ScanBB block backwards to see if we have the value at the
126 /// memory address *Ptr locally available within a small number of instructions.
128 /// The scan starts from \c ScanFrom. \c MaxInstsToScan specifies the maximum
129 /// instructions to scan in the block. If it is set to \c 0, it will scan the whole
132 /// If the value is available, this function returns it. If not, it returns the
133 /// iterator for the last validated instruction that the value would be live
134 /// through. If we scanned the entire block and didn't find something that
135 /// invalidates \c *Ptr or provides it, \c ScanFrom is left at the last
136 /// instruction processed and this returns null.
138 /// You can also optionally specify an alias analysis implementation, which
139 /// makes this more precise.
141 /// If \c AATags is non-null and a load or store is found, the AA tags from the
142 /// load or store are recorded there. If there are no AA tags or if no access is
143 /// found, it is left unmodified.
144 Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
145 BasicBlock::iterator &ScanFrom,
146 unsigned MaxInstsToScan,
147 AliasAnalysis *AA, AAMDNodes *AATags) {
148 if (MaxInstsToScan == 0)
149 MaxInstsToScan = ~0U;
151 // If we're using alias analysis to disambiguate get the size of *Ptr.
152 uint64_t AccessSize = 0;
154 Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
155 AccessSize = AA->getTypeStoreSize(AccessTy);
158 while (ScanFrom != ScanBB->begin()) {
159 // We must ignore debug info directives when counting (otherwise they
160 // would affect codegen).
161 Instruction *Inst = --ScanFrom;
162 if (isa<DbgInfoIntrinsic>(Inst))
165 // Restore ScanFrom to expected value in case next test succeeds
168 // Don't scan huge blocks.
169 if (MaxInstsToScan-- == 0)
173 // If this is a load of Ptr, the loaded value is available.
174 // (This is true even if the load is volatile or atomic, although
175 // those cases are unlikely.)
176 if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
177 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) {
179 LI->getAAMetadata(*AATags);
183 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
184 // If this is a store through Ptr, the value is available!
185 // (This is true even if the store is volatile or atomic, although
186 // those cases are unlikely.)
187 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) {
189 SI->getAAMetadata(*AATags);
190 return SI->getOperand(0);
193 // If Ptr is an alloca and this is a store to a different alloca, ignore
194 // the store. This is a trivial form of alias analysis that is important
195 // for reg2mem'd code.
196 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) &&
197 (isa<AllocaInst>(SI->getOperand(1)) ||
198 isa<GlobalVariable>(SI->getOperand(1))))
201 // If we have alias analysis and it says the store won't modify the loaded
202 // value, ignore the store.
204 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
207 // Otherwise the store that may or may not alias the pointer, bail out.
212 // If this is some other instruction that may clobber Ptr, bail out.
213 if (Inst->mayWriteToMemory()) {
214 // If alias analysis claims that it really won't modify the load,
217 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
220 // May modify the pointer, bail out.
226 // Got to the start of the block, we didn't find it, but are done for this