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 /// \brief Check if executing a load of this pointer value cannot trap.
58 /// If it is not obviously safe to load from the specified pointer, we do
59 /// a quick local scan of the basic block containing \c ScanFrom, to determine
60 /// if the address is already accessed.
62 /// This uses the pointee type to determine how many bytes need to be safe to
63 /// load from the pointer.
64 bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
65 unsigned Align, const DataLayout *TD) {
66 int64_t ByteOffset = 0;
68 Base = GetPointerBaseWithConstantOffset(V, ByteOffset, TD);
70 if (ByteOffset < 0) // out of bounds
73 Type *BaseType = nullptr;
74 unsigned BaseAlign = 0;
75 if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
76 // An alloca is safe to load from as load as it is suitably aligned.
77 BaseType = AI->getAllocatedType();
78 BaseAlign = AI->getAlignment();
79 } else if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
80 // Global variables are safe to load from but their size cannot be
81 // guaranteed if they are overridden.
82 if (!GV->mayBeOverridden()) {
83 BaseType = GV->getType()->getElementType();
84 BaseAlign = GV->getAlignment();
88 if (BaseType && BaseType->isSized()) {
89 if (TD && BaseAlign == 0)
90 BaseAlign = TD->getPrefTypeAlignment(BaseType);
92 if (Align <= BaseAlign) {
94 return true; // Loading directly from an alloca or global is OK.
96 // Check if the load is within the bounds of the underlying object.
97 PointerType *AddrTy = cast<PointerType>(V->getType());
98 uint64_t LoadSize = TD->getTypeStoreSize(AddrTy->getElementType());
99 if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType) &&
100 (Align == 0 || (ByteOffset % Align) == 0))
105 // Otherwise, be a little bit aggressive by scanning the local block where we
106 // want to check to see if the pointer is already being loaded or stored
107 // from/to. If so, the previous load or store would have already trapped,
108 // so there is no harm doing an extra load (also, CSE will later eliminate
109 // the load entirely).
110 BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin();
115 // If we see a free or a call which may write to memory (i.e. which might do
116 // a free) the pointer could be marked invalid.
117 if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
118 !isa<DbgInfoIntrinsic>(BBI))
121 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
122 if (AreEquivalentAddressValues(LI->getOperand(0), V))
124 } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
125 if (AreEquivalentAddressValues(SI->getOperand(1), V))
132 /// \brief Scan the ScanBB block backwards to see if we have the value at the
133 /// memory address *Ptr locally available within a small number of instructions.
135 /// The scan starts from \c ScanFrom. \c MaxInstsToScan specifies the maximum
136 /// instructions to scan in the block. If it is set to \c 0, it will scan the whole
139 /// If the value is available, this function returns it. If not, it returns the
140 /// iterator for the last validated instruction that the value would be live
141 /// through. If we scanned the entire block and didn't find something that
142 /// invalidates \c *Ptr or provides it, \c ScanFrom is left at the last
143 /// instruction processed and this returns null.
145 /// You can also optionally specify an alias analysis implementation, which
146 /// makes this more precise.
148 /// If \c AATags is non-null and a load or store is found, the AA tags from the
149 /// load or store are recorded there. If there are no AA tags or if no access is
150 /// found, it is left unmodified.
151 Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
152 BasicBlock::iterator &ScanFrom,
153 unsigned MaxInstsToScan,
154 AliasAnalysis *AA, AAMDNodes *AATags) {
155 if (MaxInstsToScan == 0)
156 MaxInstsToScan = ~0U;
158 // If we're using alias analysis to disambiguate get the size of *Ptr.
159 uint64_t AccessSize = 0;
161 Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
162 AccessSize = AA->getTypeStoreSize(AccessTy);
165 while (ScanFrom != ScanBB->begin()) {
166 // We must ignore debug info directives when counting (otherwise they
167 // would affect codegen).
168 Instruction *Inst = --ScanFrom;
169 if (isa<DbgInfoIntrinsic>(Inst))
172 // Restore ScanFrom to expected value in case next test succeeds
175 // Don't scan huge blocks.
176 if (MaxInstsToScan-- == 0)
180 // If this is a load of Ptr, the loaded value is available.
181 // (This is true even if the load is volatile or atomic, although
182 // those cases are unlikely.)
183 if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
184 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) {
186 LI->getAAMetadata(*AATags);
190 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
191 // If this is a store through Ptr, the value is available!
192 // (This is true even if the store is volatile or atomic, although
193 // those cases are unlikely.)
194 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) {
196 SI->getAAMetadata(*AATags);
197 return SI->getOperand(0);
200 // If Ptr is an alloca and this is a store to a different alloca, ignore
201 // the store. This is a trivial form of alias analysis that is important
202 // for reg2mem'd code.
203 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) &&
204 (isa<AllocaInst>(SI->getOperand(1)) ||
205 isa<GlobalVariable>(SI->getOperand(1))))
208 // If we have alias analysis and it says the store won't modify the loaded
209 // value, ignore the store.
211 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
214 // Otherwise the store that may or may not alias the pointer, bail out.
219 // If this is some other instruction that may clobber Ptr, bail out.
220 if (Inst->mayWriteToMemory()) {
221 // If alias analysis claims that it really won't modify the load,
224 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
227 // May modify the pointer, bail out.
233 // Got to the start of the block, we didn't find it, but are done for this