LLVM API Documentation

Loads.cpp
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00001 //===- Loads.cpp - Local load analysis ------------------------------------===//
00002 //
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file is distributed under the University of Illinois Open Source
00006 // License. See LICENSE.TXT for details.
00007 //
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This file defines simple local analyses for load instructions.
00011 //
00012 //===----------------------------------------------------------------------===//
00013 
00014 #include "llvm/Analysis/Loads.h"
00015 #include "llvm/Analysis/AliasAnalysis.h"
00016 #include "llvm/Analysis/ValueTracking.h"
00017 #include "llvm/IR/DataLayout.h"
00018 #include "llvm/IR/GlobalAlias.h"
00019 #include "llvm/IR/GlobalVariable.h"
00020 #include "llvm/IR/IntrinsicInst.h"
00021 #include "llvm/IR/LLVMContext.h"
00022 #include "llvm/IR/Operator.h"
00023 using namespace llvm;
00024 
00025 /// AreEquivalentAddressValues - Test if A and B will obviously have the same
00026 /// value. This includes recognizing that %t0 and %t1 will have the same
00027 /// value in code like this:
00028 ///   %t0 = getelementptr \@a, 0, 3
00029 ///   store i32 0, i32* %t0
00030 ///   %t1 = getelementptr \@a, 0, 3
00031 ///   %t2 = load i32* %t1
00032 ///
00033 static bool AreEquivalentAddressValues(const Value *A, const Value *B) {
00034   // Test if the values are trivially equivalent.
00035   if (A == B) return true;
00036 
00037   // Test if the values come from identical arithmetic instructions.
00038   // Use isIdenticalToWhenDefined instead of isIdenticalTo because
00039   // this function is only used when one address use dominates the
00040   // other, which means that they'll always either have the same
00041   // value or one of them will have an undefined value.
00042   if (isa<BinaryOperator>(A) || isa<CastInst>(A) ||
00043       isa<PHINode>(A) || isa<GetElementPtrInst>(A))
00044     if (const Instruction *BI = dyn_cast<Instruction>(B))
00045       if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
00046         return true;
00047 
00048   // Otherwise they may not be equivalent.
00049   return false;
00050 }
00051 
00052 /// isSafeToLoadUnconditionally - Return true if we know that executing a load
00053 /// from this value cannot trap.  If it is not obviously safe to load from the
00054 /// specified pointer, we do a quick local scan of the basic block containing
00055 /// ScanFrom, to determine if the address is already accessed.
00056 bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
00057                                        unsigned Align, const DataLayout *TD) {
00058   int64_t ByteOffset = 0;
00059   Value *Base = V;
00060   Base = GetPointerBaseWithConstantOffset(V, ByteOffset, TD);
00061 
00062   if (ByteOffset < 0) // out of bounds
00063     return false;
00064 
00065   Type *BaseType = nullptr;
00066   unsigned BaseAlign = 0;
00067   if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
00068     // An alloca is safe to load from as load as it is suitably aligned.
00069     BaseType = AI->getAllocatedType();
00070     BaseAlign = AI->getAlignment();
00071   } else if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
00072     // Global variables are safe to load from but their size cannot be
00073     // guaranteed if they are overridden.
00074     if (!GV->mayBeOverridden()) {
00075       BaseType = GV->getType()->getElementType();
00076       BaseAlign = GV->getAlignment();
00077     }
00078   }
00079 
00080   if (BaseType && BaseType->isSized()) {
00081     if (TD && BaseAlign == 0)
00082       BaseAlign = TD->getPrefTypeAlignment(BaseType);
00083 
00084     if (Align <= BaseAlign) {
00085       if (!TD)
00086         return true; // Loading directly from an alloca or global is OK.
00087 
00088       // Check if the load is within the bounds of the underlying object.
00089       PointerType *AddrTy = cast<PointerType>(V->getType());
00090       uint64_t LoadSize = TD->getTypeStoreSize(AddrTy->getElementType());
00091       if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType) &&
00092           (Align == 0 || (ByteOffset % Align) == 0))
00093         return true;
00094     }
00095   }
00096 
00097   // Otherwise, be a little bit aggressive by scanning the local block where we
00098   // want to check to see if the pointer is already being loaded or stored
00099   // from/to.  If so, the previous load or store would have already trapped,
00100   // so there is no harm doing an extra load (also, CSE will later eliminate
00101   // the load entirely).
00102   BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin();
00103 
00104   while (BBI != E) {
00105     --BBI;
00106 
00107     // If we see a free or a call which may write to memory (i.e. which might do
00108     // a free) the pointer could be marked invalid.
00109     if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
00110         !isa<DbgInfoIntrinsic>(BBI))
00111       return false;
00112 
00113     if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
00114       if (AreEquivalentAddressValues(LI->getOperand(0), V)) return true;
00115     } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
00116       if (AreEquivalentAddressValues(SI->getOperand(1), V)) return true;
00117     }
00118   }
00119   return false;
00120 }
00121 
00122 /// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the
00123 /// instruction before ScanFrom) checking to see if we have the value at the
00124 /// memory address *Ptr locally available within a small number of instructions.
00125 /// If the value is available, return it.
00126 ///
00127 /// If not, return the iterator for the last validated instruction that the 
00128 /// value would be live through.  If we scanned the entire block and didn't find
00129 /// something that invalidates *Ptr or provides it, ScanFrom would be left at
00130 /// begin() and this returns null.  ScanFrom could also be left 
00131 ///
00132 /// MaxInstsToScan specifies the maximum instructions to scan in the block.  If
00133 /// it is set to 0, it will scan the whole block. You can also optionally
00134 /// specify an alias analysis implementation, which makes this more precise.
00135 ///
00136 /// If TBAATag is non-null and a load or store is found, the TBAA tag from the
00137 /// load or store is recorded there.  If there is no TBAA tag or if no access
00138 /// is found, it is left unmodified.
00139 Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
00140                                       BasicBlock::iterator &ScanFrom,
00141                                       unsigned MaxInstsToScan,
00142                                       AliasAnalysis *AA,
00143                                       MDNode **TBAATag) {
00144   if (MaxInstsToScan == 0) MaxInstsToScan = ~0U;
00145 
00146   // If we're using alias analysis to disambiguate get the size of *Ptr.
00147   uint64_t AccessSize = 0;
00148   if (AA) {
00149     Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
00150     AccessSize = AA->getTypeStoreSize(AccessTy);
00151   }
00152   
00153   while (ScanFrom != ScanBB->begin()) {
00154     // We must ignore debug info directives when counting (otherwise they
00155     // would affect codegen).
00156     Instruction *Inst = --ScanFrom;
00157     if (isa<DbgInfoIntrinsic>(Inst))
00158       continue;
00159 
00160     // Restore ScanFrom to expected value in case next test succeeds
00161     ScanFrom++;
00162    
00163     // Don't scan huge blocks.
00164     if (MaxInstsToScan-- == 0) return nullptr;
00165     
00166     --ScanFrom;
00167     // If this is a load of Ptr, the loaded value is available.
00168     // (This is true even if the load is volatile or atomic, although
00169     // those cases are unlikely.)
00170     if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
00171       if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) {
00172         if (TBAATag) *TBAATag = LI->getMetadata(LLVMContext::MD_tbaa);
00173         return LI;
00174       }
00175     
00176     if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
00177       // If this is a store through Ptr, the value is available!
00178       // (This is true even if the store is volatile or atomic, although
00179       // those cases are unlikely.)
00180       if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) {
00181         if (TBAATag) *TBAATag = SI->getMetadata(LLVMContext::MD_tbaa);
00182         return SI->getOperand(0);
00183       }
00184       
00185       // If Ptr is an alloca and this is a store to a different alloca, ignore
00186       // the store.  This is a trivial form of alias analysis that is important
00187       // for reg2mem'd code.
00188       if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) &&
00189           (isa<AllocaInst>(SI->getOperand(1)) ||
00190            isa<GlobalVariable>(SI->getOperand(1))))
00191         continue;
00192       
00193       // If we have alias analysis and it says the store won't modify the loaded
00194       // value, ignore the store.
00195       if (AA &&
00196           (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
00197         continue;
00198       
00199       // Otherwise the store that may or may not alias the pointer, bail out.
00200       ++ScanFrom;
00201       return nullptr;
00202     }
00203     
00204     // If this is some other instruction that may clobber Ptr, bail out.
00205     if (Inst->mayWriteToMemory()) {
00206       // If alias analysis claims that it really won't modify the load,
00207       // ignore it.
00208       if (AA &&
00209           (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
00210         continue;
00211       
00212       // May modify the pointer, bail out.
00213       ++ScanFrom;
00214       return nullptr;
00215     }
00216   }
00217   
00218   // Got to the start of the block, we didn't find it, but are done for this
00219   // block.
00220   return nullptr;
00221 }