Add a new helper method to Value to strip in-bounds constant offsets of

pointers, but accumulate the offset into an APInt in the process of
stripping it.

This is a pretty handy thing to have, such as when trying to determine
if two pointers are at some constant relative offset. I'll be committing
a patch shortly to use it for exactly that purpose.

llvm-svn: 189000

llvm/include/llvm/IR/Value.h

@@ -22,26 +22,29 @@
 
 namespace llvm {
 
-class Constant;
+class APInt;
 class Argument;
-class Instruction;
+class AssemblyAnnotationWriter;
 class BasicBlock;
-class GlobalValue;
+class Constant;
+class DataLayout;
 class Function;
-class GlobalVariable;
 class GlobalAlias;
+class GlobalValue;
+class GlobalVariable;
 class InlineAsm;
+class Instruction;
+class LLVMContext;
+class MDNode;
+class StringRef;
+class Twine;
+class Type;
+class ValueHandleBase;
 class ValueSymbolTable;
+class raw_ostream;
+
 template<typename ValueTy> class StringMapEntry;
 typedef StringMapEntry<Value*> ValueName;
-class raw_ostream;
-class AssemblyAnnotationWriter;
-class ValueHandleBase;
-class LLVMContext;
-class Twine;
-class MDNode;
-class Type;
-class StringRef;
 
 //===----------------------------------------------------------------------===//
 //                                 Value Class
@@ -287,6 +290,22 @@ public:
     return const_cast<Value*>(this)->stripInBoundsConstantOffsets();
   }
 
+  /// \brief Strips like \c stripInBoundsConstantOffsets but also accumulates
+  /// the constant offset stripped.
+  ///
+  /// Stores the resulting constant offset stripped into the APInt provided.
+  /// The provided APInt will be extended or truncated as needed to be the
+  /// correct bitwidth for an offset of this pointer type.
+  ///
+  /// If this is called on a non-pointer value, it returns 'this'.
+  Value *stripAndAccumulateInBoundsConstantOffsets(const DataLayout &DL,
+                                                   APInt &Offset);
+  const Value *stripAndAccumulateInBoundsConstantOffsets(const DataLayout &DL,
+                                                         APInt &Offset) const {
+    return const_cast<Value *>(this)
+        ->stripAndAccumulateInBoundsConstantOffsets(DL, Offset);
+  }
+
   /// \brief Strips off unneeded pointer casts and any in-bounds offsets from
   /// the specified value, returning the original pointer value.
   ///

llvm/lib/IR/Value.cpp

@@ -393,6 +393,40 @@ Value *Value::stripInBoundsConstantOffsets() {
   return stripPointerCastsAndOffsets<PSK_InBoundsConstantIndices>(this);
 }
 
+Value *Value::stripAndAccumulateInBoundsConstantOffsets(const DataLayout &DL,
+                                                        APInt &Offset) {
+  if (!getType()->isPointerTy())
+    return this;
+
+  assert(Offset.getBitWidth() == DL.getPointerSizeInBits(cast<PointerType>(
+                                     getType())->getAddressSpace()) &&
+         "The offset must have exactly as many bits as our pointer.");
+
+  // Even though we don't look through PHI nodes, we could be called on an
+  // instruction in an unreachable block, which may be on a cycle.
+  SmallPtrSet<Value *, 4> Visited;
+  Visited.insert(this);
+  Value *V = this;
+  do {
+    if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
+      if (!GEP->isInBounds())
+        return V;
+      if (!GEP->accumulateConstantOffset(DL, Offset))
+        return V;
+      V = GEP->getPointerOperand();
+    } else if (Operator::getOpcode(V) == Instruction::BitCast) {
+      V = cast<Operator>(V)->getOperand(0);
+    } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
+      V = GA->getAliasee();
+    } else {
+      return V;
+    }
+    assert(V->getType()->isPointerTy() && "Unexpected operand type!");
+  } while (Visited.insert(V));
+
+  return V;
+}
+
 Value *Value::stripInBoundsOffsets() {
   return stripPointerCastsAndOffsets<PSK_InBounds>(this);
 }