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/Operator.h"
23 using namespace llvm;
24 
25 /// AreEquivalentAddressValues - Test if A and B will obviously have the same
26 /// value. This includes recognizing that %t0 and %t1 will have the same
27 /// value in code like this:
28 ///   %t0 = getelementptr \@a, 0, 3
29 ///   store i32 0, i32* %t0
30 ///   %t1 = getelementptr \@a, 0, 3
31 ///   %t2 = load i32* %t1
32 ///
33 static bool AreEquivalentAddressValues(const Value *A, const Value *B) {
34   // Test if the values are trivially equivalent.
35   if (A == B) return true;
36 
37   // Test if the values come from identical arithmetic instructions.
38   // Use isIdenticalToWhenDefined instead of isIdenticalTo because
39   // this function is only used when one address use dominates the
40   // other, which means that they'll always either have the same
41   // value or one of them will have an undefined value.
42   if (isa<BinaryOperator>(A) || isa<CastInst>(A) ||
43       isa<PHINode>(A) || isa<GetElementPtrInst>(A))
44     if (const Instruction *BI = dyn_cast<Instruction>(B))
45       if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
46         return true;
47 
48   // Otherwise they may not be equivalent.
49   return false;
50 }
51 
52 /// isSafeToLoadUnconditionally - Return true if we know that executing a load
53 /// from this value cannot trap.  If it is not obviously safe to load from the
54 /// specified pointer, we do a quick local scan of the basic block containing
55 /// ScanFrom, to determine if the address is already accessed.
56 bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
57                                        unsigned Align, const DataLayout *TD) {
58   int64_t ByteOffset = 0;
59   Value *Base = V;
60   if (TD)
61     Base = GetPointerBaseWithConstantOffset(V, ByteOffset, *TD);
62 
63   if (ByteOffset < 0) // out of bounds
64     return false;
65 
66   Type *BaseType = 0;
67   unsigned BaseAlign = 0;
68   if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
69     // An alloca is safe to load from as load as it is suitably aligned.
70     BaseType = AI->getAllocatedType();
71     BaseAlign = AI->getAlignment();
72   } else if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
73     // Global variables are safe to load from but their size cannot be
74     // guaranteed if they are overridden.
75     if (!GV->mayBeOverridden()) {
76       BaseType = GV->getType()->getElementType();
77       BaseAlign = GV->getAlignment();
78     }
79   }
80 
81   if (BaseType && BaseType->isSized()) {
82     if (TD && BaseAlign == 0)
83       BaseAlign = TD->getPrefTypeAlignment(BaseType);
84 
85     if (Align <= BaseAlign) {
86       if (!TD)
87         return true; // Loading directly from an alloca or global is OK.
88 
89       // Check if the load is within the bounds of the underlying object.
90       PointerType *AddrTy = cast<PointerType>(V->getType());
91       uint64_t LoadSize = TD->getTypeStoreSize(AddrTy->getElementType());
92       if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType) &&
93           (Align == 0 || (ByteOffset % Align) == 0))
94         return true;
95     }
96   }
97 
98   // Otherwise, be a little bit aggressive by scanning the local block where we
99   // want to check to see if the pointer is already being loaded or stored
100   // from/to.  If so, the previous load or store would have already trapped,
101   // so there is no harm doing an extra load (also, CSE will later eliminate
102   // the load entirely).
103   BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin();
104 
105   while (BBI != E) {
106     --BBI;
107 
108     // If we see a free or a call which may write to memory (i.e. which might do
109     // a free) the pointer could be marked invalid.
110     if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
111         !isa<DbgInfoIntrinsic>(BBI))
112       return false;
113 
114     if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
115       if (AreEquivalentAddressValues(LI->getOperand(0), V)) return true;
116     } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
117       if (AreEquivalentAddressValues(SI->getOperand(1), V)) return true;
118     }
119   }
120   return false;
121 }
122 
123 /// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the
124 /// instruction before ScanFrom) checking to see if we have the value at the
125 /// memory address *Ptr locally available within a small number of instructions.
126 /// If the value is available, return it.
127 ///
128 /// If not, return the iterator for the last validated instruction that the
129 /// value would be live through.  If we scanned the entire block and didn't find
130 /// something that invalidates *Ptr or provides it, ScanFrom would be left at
131 /// begin() and this returns null.  ScanFrom could also be left
132 ///
133 /// MaxInstsToScan specifies the maximum instructions to scan in the block.  If
134 /// it is set to 0, it will scan the whole block. You can also optionally
135 /// specify an alias analysis implementation, which makes this more precise.
136 ///
137 /// If TBAATag is non-null and a load or store is found, the TBAA tag from the
138 /// load or store is recorded there.  If there is no TBAA tag or if no access
139 /// is found, it is left unmodified.
140 Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
141                                       BasicBlock::iterator &ScanFrom,
142                                       unsigned MaxInstsToScan,
143                                       AliasAnalysis *AA,
144                                       MDNode **TBAATag) {
145   if (MaxInstsToScan == 0) MaxInstsToScan = ~0U;
146 
147   // If we're using alias analysis to disambiguate get the size of *Ptr.
148   uint64_t AccessSize = 0;
149   if (AA) {
150     Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
151     AccessSize = AA->getTypeStoreSize(AccessTy);
152   }
153 
154   while (ScanFrom != ScanBB->begin()) {
155     // We must ignore debug info directives when counting (otherwise they
156     // would affect codegen).
157     Instruction *Inst = --ScanFrom;
158     if (isa<DbgInfoIntrinsic>(Inst))
159       continue;
160 
161     // Restore ScanFrom to expected value in case next test succeeds
162     ScanFrom++;
163 
164     // Don't scan huge blocks.
165     if (MaxInstsToScan-- == 0) return 0;
166 
167     --ScanFrom;
168     // If this is a load of Ptr, the loaded value is available.
169     // (This is true even if the load is volatile or atomic, although
170     // those cases are unlikely.)
171     if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
172       if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) {
173         if (TBAATag) *TBAATag = LI->getMetadata(LLVMContext::MD_tbaa);
174         return LI;
175       }
176 
177     if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
178       // If this is a store through Ptr, the value is available!
179       // (This is true even if the store is volatile or atomic, although
180       // those cases are unlikely.)
181       if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) {
182         if (TBAATag) *TBAATag = SI->getMetadata(LLVMContext::MD_tbaa);
183         return SI->getOperand(0);
184       }
185 
186       // If Ptr is an alloca and this is a store to a different alloca, ignore
187       // the store.  This is a trivial form of alias analysis that is important
188       // for reg2mem'd code.
189       if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) &&
190           (isa<AllocaInst>(SI->getOperand(1)) ||
191            isa<GlobalVariable>(SI->getOperand(1))))
192         continue;
193 
194       // If we have alias analysis and it says the store won't modify the loaded
195       // value, ignore the store.
196       if (AA &&
197           (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
198         continue;
199 
200       // Otherwise the store that may or may not alias the pointer, bail out.
201       ++ScanFrom;
202       return 0;
203     }
204 
205     // If this is some other instruction that may clobber Ptr, bail out.
206     if (Inst->mayWriteToMemory()) {
207       // If alias analysis claims that it really won't modify the load,
208       // ignore it.
209       if (AA &&
210           (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
211         continue;
212 
213       // May modify the pointer, bail out.
214       ++ScanFrom;
215       return 0;
216     }
217   }
218 
219   // Got to the start of the block, we didn't find it, but are done for this
220   // block.
221   return 0;
222 }
223