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