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