Move FindAvailableLoadedValue isSafeToLoadUnconditionally out of

lib/Transforms/Utils and into lib/Analysis so that Analysis passes
can use them.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@104949 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 235 insertions, 0 deletions

diff --git a/lib/Analysis/Loads.cpp b/lib/Analysis/Loads.cpp
new file mode 100644
index 0000000000..2ba1d86cdb
--- /dev/null
+++ b/lib/Analysis/Loads.cpp
@@ -0,0 +1,235 @@
+//===- 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/Target/TargetData.h"
+#include "llvm/GlobalAlias.h"
+#include "llvm/GlobalVariable.h"
+#include "llvm/IntrinsicInst.h"
+using namespace llvm;
+
+/// AreEquivalentAddressValues - 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:
+///   %t0 = getelementptr \@a, 0, 3
+///   store i32 0, i32* %t0
+///   %t1 = getelementptr \@a, 0, 3
+///   %t2 = load i32* %t1
+///
+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;
+}
+
+/// getUnderlyingObjectWithOffset - Strip off up to MaxLookup GEPs and
+/// bitcasts to get back to the underlying object being addressed, keeping
+/// track of the offset in bytes from the GEPs relative to the result.
+/// This is closely related to Value::getUnderlyingObject but is located
+/// here to avoid making VMCore depend on TargetData.
+static Value *getUnderlyingObjectWithOffset(Value *V, const TargetData *TD,
+                                            uint64_t &ByteOffset,
+                                            unsigned MaxLookup = 6) {
+  if (!V->getType()->isPointerTy())
+    return V;
+  for (unsigned Count = 0; MaxLookup == 0 || Count < MaxLookup; ++Count) {
+    if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
+      if (!GEP->hasAllConstantIndices())
+        return V;
+      SmallVector<Value*, 8> Indices(GEP->op_begin() + 1, GEP->op_end());
+      ByteOffset += TD->getIndexedOffset(GEP->getPointerOperandType(),
+                                         &Indices[0], Indices.size());
+      V = GEP->getPointerOperand();
+    } else if (Operator::getOpcode(V) == Instruction::BitCast) {
+      V = cast<Operator>(V)->getOperand(0);
+    } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
+      if (GA->mayBeOverridden())
+        return V;
+      V = GA->getAliasee();
+    } else {
+      return V;
+    }
+    assert(V->getType()->isPointerTy() && "Unexpected operand type!");
+  }
+  return V;
+}
+
+/// isSafeToLoadUnconditionally - Return true if we know that executing a load
+/// from this 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
+/// ScanFrom, to determine if the address is already accessed.
+bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
+                                       unsigned Align, const TargetData *TD) {
+  uint64_t ByteOffset = 0;
+  Value *Base = V;
+  if (TD)
+    Base = getUnderlyingObjectWithOffset(V, TD, ByteOffset);
+
+  const Type *BaseType = 0;
+  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 GlobalValue *GV = dyn_cast<GlobalValue>(Base)) {
+    // Global variables are safe to load from but their size cannot be
+    // guaranteed if they are overridden.
+    if (!isa<GlobalAlias>(GV) && !GV->mayBeOverridden()) {
+      BaseType = GV->getType()->getElementType();
+      BaseAlign = GV->getAlignment();
+    }
+  }
+
+  if (BaseType && BaseType->isSized()) {
+    if (TD && BaseAlign == 0)
+      BaseAlign = TD->getPrefTypeAlignment(BaseType);
+
+    if (Align <= BaseAlign) {
+      if (!TD)
+        return true; // Loading directly from an alloca or global is OK.
+
+      // Check if the load is within the bounds of the underlying object.
+      const PointerType *AddrTy = cast<PointerType>(V->getType());
+      uint64_t LoadSize = TD->getTypeStoreSize(AddrTy->getElementType());
+      if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType) &&
+          (Align == 0 || (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, E = ScanFrom->getParent()->begin();
+
+  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;
+
+    if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
+      if (AreEquivalentAddressValues(LI->getOperand(0), V)) return true;
+    } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
+      if (AreEquivalentAddressValues(SI->getOperand(1), V)) return true;
+    }
+  }
+  return false;
+}
+
+/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the
+/// instruction before ScanFrom) checking to see if we have the value at the
+/// memory address *Ptr locally available within a small number of instructions.
+/// If the value is available, return it.
+///
+/// If not, return 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 *Ptr or provides it, ScanFrom would be left at
+/// begin() and this returns null.  ScanFrom could also be left 
+///
+/// MaxInstsToScan specifies the maximum instructions to scan in the block.  If
+/// it is set to 0, it will scan the whole block. You can also optionally
+/// specify an alias analysis implementation, which makes this more precise.
+Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
+                                      BasicBlock::iterator &ScanFrom,
+                                      unsigned MaxInstsToScan,
+                                      AliasAnalysis *AA) {
+  if (MaxInstsToScan == 0) MaxInstsToScan = ~0U;
+
+  // If we're using alias analysis to disambiguate get the size of *Ptr.
+  unsigned AccessSize = 0;
+  if (AA) {
+    const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
+    AccessSize = AA->getTypeStoreSize(AccessTy);
+  }
+  
+  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 0;
+    
+    --ScanFrom;
+    // If this is a load of Ptr, the loaded value is available.
+    if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
+      if (AreEquivalentAddressValues(LI->getOperand(0), Ptr))
+        return LI;
+    
+    if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
+      // If this is a store through Ptr, the value is available!
+      if (AreEquivalentAddressValues(SI->getOperand(1), Ptr))
+        return SI->getOperand(0);
+      
+      // If Ptr is an alloca and this is a store to a different alloca, ignore
+      // the store.  This is a trivial form of alias analysis that is important
+      // for reg2mem'd code.
+      if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) &&
+          (isa<AllocaInst>(SI->getOperand(1)) ||
+           isa<GlobalVariable>(SI->getOperand(1))))
+        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, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
+        continue;
+      
+      // Otherwise the store that may or may not alias the pointer, bail out.
+      ++ScanFrom;
+      return 0;
+    }
+    
+    // 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, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
+        continue;
+      
+      // May modify the pointer, bail out.
+      ++ScanFrom;
+      return 0;
+    }
+  }
+  
+  // Got to the start of the block, we didn't find it, but are done for this
+  // block.
+  return 0;
+}