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+//===- Loads.cpp - Local load analysis ------------------------------------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines simple local analyses for load instructions.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/Loads.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/GlobalAlias.h"
+#include "llvm/IR/GlobalVariable.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/Operator.h"
+using namespace llvm;
+
+/// \brief Test if A and B will obviously have the same value.
+///
+/// This includes recognizing that %t0 and %t1 will have the same
+/// value in code like this:
+/// \code
+/// %t0 = getelementptr \@a, 0, 3
+/// store i32 0, i32* %t0
+/// %t1 = getelementptr \@a, 0, 3
+/// %t2 = load i32* %t1
+/// \endcode
+///
+static bool AreEquivalentAddressValues(const Value *A, const Value *B) {
+ // Test if the values are trivially equivalent.
+ if (A == B)
+ return true;
+
+ // Test if the values come from identical arithmetic instructions.
+ // Use isIdenticalToWhenDefined instead of isIdenticalTo because
+ // this function is only used when one address use dominates the
+ // other, which means that they'll always either have the same
+ // value or one of them will have an undefined value.
+ if (isa<BinaryOperator>(A) || isa<CastInst>(A) || isa<PHINode>(A) ||
+ isa<GetElementPtrInst>(A))
+ if (const Instruction *BI = dyn_cast<Instruction>(B))
+ if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI))
+ return true;
+
+ // Otherwise they may not be equivalent.
+ return false;
+}
+
+/// \brief Check if executing a load of this pointer value cannot trap.
+///
+/// If it is not obviously safe to load from the specified pointer, we do
+/// a quick local scan of the basic block containing \c ScanFrom, to determine
+/// if the address is already accessed.
+///
+/// This uses the pointee type to determine how many bytes need to be safe to
+/// load from the pointer.
+bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
+ unsigned Align) {
+ const DataLayout &DL = ScanFrom->getModule()->getDataLayout();
+
+ // Zero alignment means that the load has the ABI alignment for the target
+ if (Align == 0)
+ Align = DL.getABITypeAlignment(V->getType()->getPointerElementType());
+ assert(isPowerOf2_32(Align));
+
+ int64_t ByteOffset = 0;
+ Value *Base = V;
+ Base = GetPointerBaseWithConstantOffset(V, ByteOffset, DL);
+
+ if (ByteOffset < 0) // out of bounds
+ return false;
+
+ Type *BaseType = nullptr;
+ unsigned BaseAlign = 0;
+ if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
+ // An alloca is safe to load from as load as it is suitably aligned.
+ BaseType = AI->getAllocatedType();
+ BaseAlign = AI->getAlignment();
+ } else if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) {
+ // Global variables are not necessarily safe to load from if they are
+ // overridden. Their size may change or they may be weak and require a test
+ // to determine if they were in fact provided.
+ if (!GV->mayBeOverridden()) {
+ BaseType = GV->getType()->getElementType();
+ BaseAlign = GV->getAlignment();
+ }
+ }
+
+ PointerType *AddrTy = cast<PointerType>(V->getType());
+ uint64_t LoadSize = DL.getTypeStoreSize(AddrTy->getElementType());
+
+ // If we found a base allocated type from either an alloca or global variable,
+ // try to see if we are definitively within the allocated region. We need to
+ // know the size of the base type and the loaded type to do anything in this
+ // case.
+ if (BaseType && BaseType->isSized()) {
+ if (BaseAlign == 0)
+ BaseAlign = DL.getPrefTypeAlignment(BaseType);
+
+ if (Align <= BaseAlign) {
+ // Check if the load is within the bounds of the underlying object.
+ if (ByteOffset + LoadSize <= DL.getTypeAllocSize(BaseType) &&
+ ((ByteOffset % Align) == 0))
+ return true;
+ }
+ }
+
+ // Otherwise, be a little bit aggressive by scanning the local block where we
+ // want to check to see if the pointer is already being loaded or stored
+ // from/to. If so, the previous load or store would have already trapped,
+ // so there is no harm doing an extra load (also, CSE will later eliminate
+ // the load entirely).
+ BasicBlock::iterator BBI = ScanFrom->getIterator(),
+ E = ScanFrom->getParent()->begin();
+
+ // We can at least always strip pointer casts even though we can't use the
+ // base here.
+ V = V->stripPointerCasts();
+
+ while (BBI != E) {
+ --BBI;
+
+ // If we see a free or a call which may write to memory (i.e. which might do
+ // a free) the pointer could be marked invalid.
+ if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
+ !isa<DbgInfoIntrinsic>(BBI))
+ return false;
+
+ Value *AccessedPtr;
+ unsigned AccessedAlign;
+ if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
+ AccessedPtr = LI->getPointerOperand();
+ AccessedAlign = LI->getAlignment();
+ } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
+ AccessedPtr = SI->getPointerOperand();
+ AccessedAlign = SI->getAlignment();
+ } else
+ continue;
+
+ Type *AccessedTy = AccessedPtr->getType()->getPointerElementType();
+ if (AccessedAlign == 0)
+ AccessedAlign = DL.getABITypeAlignment(AccessedTy);
+ if (AccessedAlign < Align)
+ continue;
+
+ // Handle trivial cases.
+ if (AccessedPtr == V)
+ return true;
+
+ if (AreEquivalentAddressValues(AccessedPtr->stripPointerCasts(), V) &&
+ LoadSize <= DL.getTypeStoreSize(AccessedTy))
+ return true;
+ }
+ return false;
+}
+
+/// DefMaxInstsToScan - the default number of maximum instructions
+/// to scan in the block, used by FindAvailableLoadedValue().
+/// FindAvailableLoadedValue() was introduced in r60148, to improve jump
+/// threading in part by eliminating partially redundant loads.
+/// At that point, the value of MaxInstsToScan was already set to '6'
+/// without documented explanation.
+cl::opt<unsigned>
+llvm::DefMaxInstsToScan("available-load-scan-limit", cl::init(6), cl::Hidden,
+ cl::desc("Use this to specify the default maximum number of instructions "
+ "to scan backward from a given instruction, when searching for "
+ "available loaded value"));
+
+/// \brief Scan the ScanBB block backwards to see if we have the value at the
+/// memory address *Ptr locally available within a small number of instructions.
+///
+/// The scan starts from \c ScanFrom. \c MaxInstsToScan specifies the maximum
+/// instructions to scan in the block. If it is set to \c 0, it will scan the whole
+/// block.
+///
+/// If the value is available, this function returns it. If not, it returns the
+/// iterator for the last validated instruction that the value would be live
+/// through. If we scanned the entire block and didn't find something that
+/// invalidates \c *Ptr or provides it, \c ScanFrom is left at the last
+/// instruction processed and this returns null.
+///
+/// You can also optionally specify an alias analysis implementation, which
+/// makes this more precise.
+///
+/// If \c AATags is non-null and a load or store is found, the AA tags from the
+/// load or store are recorded there. If there are no AA tags or if no access is
+/// found, it is left unmodified.
+Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
+ BasicBlock::iterator &ScanFrom,
+ unsigned MaxInstsToScan,
+ AliasAnalysis *AA, AAMDNodes *AATags) {
+ if (MaxInstsToScan == 0)
+ MaxInstsToScan = ~0U;
+
+ Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
+
+ const DataLayout &DL = ScanBB->getModule()->getDataLayout();
+
+ // Try to get the store size for the type.
+ uint64_t AccessSize = DL.getTypeStoreSize(AccessTy);
+
+ Value *StrippedPtr = Ptr->stripPointerCasts();
+
+ while (ScanFrom != ScanBB->begin()) {
+ // We must ignore debug info directives when counting (otherwise they
+ // would affect codegen).
+ Instruction *Inst = &*--ScanFrom;
+ if (isa<DbgInfoIntrinsic>(Inst))
+ continue;
+
+ // Restore ScanFrom to expected value in case next test succeeds
+ ScanFrom++;
+
+ // Don't scan huge blocks.
+ if (MaxInstsToScan-- == 0)
+ return nullptr;
+
+ --ScanFrom;
+ // If this is a load of Ptr, the loaded value is available.
+ // (This is true even if the load is volatile or atomic, although
+ // those cases are unlikely.)
+ if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
+ if (AreEquivalentAddressValues(
+ LI->getPointerOperand()->stripPointerCasts(), StrippedPtr) &&
+ CastInst::isBitOrNoopPointerCastable(LI->getType(), AccessTy, DL)) {
+ if (AATags)
+ LI->getAAMetadata(*AATags);
+ return LI;
+ }
+
+ if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
+ Value *StorePtr = SI->getPointerOperand()->stripPointerCasts();
+ // If this is a store through Ptr, the value is available!
+ // (This is true even if the store is volatile or atomic, although
+ // those cases are unlikely.)
+ if (AreEquivalentAddressValues(StorePtr, StrippedPtr) &&
+ CastInst::isBitOrNoopPointerCastable(SI->getValueOperand()->getType(),
+ AccessTy, DL)) {
+ if (AATags)
+ SI->getAAMetadata(*AATags);
+ return SI->getOperand(0);
+ }
+
+ // If both StrippedPtr and StorePtr reach all the way to an alloca or
+ // global and they are different, ignore the store. This is a trivial form
+ // of alias analysis that is important for reg2mem'd code.
+ if ((isa<AllocaInst>(StrippedPtr) || isa<GlobalVariable>(StrippedPtr)) &&
+ (isa<AllocaInst>(StorePtr) || isa<GlobalVariable>(StorePtr)) &&
+ StrippedPtr != StorePtr)
+ continue;
+
+ // If we have alias analysis and it says the store won't modify the loaded
+ // value, ignore the store.
+ if (AA && (AA->getModRefInfo(SI, StrippedPtr, AccessSize) & MRI_Mod) == 0)
+ continue;
+
+ // Otherwise the store that may or may not alias the pointer, bail out.
+ ++ScanFrom;
+ return nullptr;
+ }
+
+ // If this is some other instruction that may clobber Ptr, bail out.
+ if (Inst->mayWriteToMemory()) {
+ // If alias analysis claims that it really won't modify the load,
+ // ignore it.
+ if (AA &&
+ (AA->getModRefInfo(Inst, StrippedPtr, AccessSize) & MRI_Mod) == 0)
+ continue;
+
+ // May modify the pointer, bail out.
+ ++ScanFrom;
+ return nullptr;
+ }
+ }
+
+ // Got to the start of the block, we didn't find it, but are done for this
+ // block.
+ return nullptr;
+}
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