[PATCH] Perform calculation of array size in chars instead of bits to prevent an assertion.

[Richard Trieu]

For large arrays, the size calculation in bits may overflow a uint64_t.  If only the size in chars is important, perform the calculation in chars to prevent this overflow which triggers an assertion.  This fixes PR15216.

http://llvm-reviews.chandlerc.com/D781

Files:
  test/Sema/offsetof.c
  include/clang/AST/ASTContext.h
  lib/AST/ASTContext.cpp

Index: test/Sema/offsetof.c
===================================================================
--- test/Sema/offsetof.c
+++ test/Sema/offsetof.c
@@ -69,3 +69,15 @@
 int test5() {
   return __builtin_offsetof(Array, array[*(int*)0]); // expected-warning{{indirection of non-volatile null pointer}} expected-note{{__builtin_trap}}
 }
+
+// PR15216
+// Don't crash on when taking computing the offset of structs with large arrays.
+const unsigned long Size = (1l << 62);
+
+struct Chunk {
+  char padding[Size];
+  char data;
+};
+
+int test6 = __builtin_offsetof(struct Chunk, data); 
+
Index: include/clang/AST/ASTContext.h
===================================================================
--- include/clang/AST/ASTContext.h
+++ include/clang/AST/ASTContext.h
@@ -1591,6 +1591,9 @@
   std::pair<CharUnits, CharUnits> getTypeInfoInChars(const Type *T) const;
   std::pair<CharUnits, CharUnits> getTypeInfoInChars(QualType T) const;
 
+  std::pair<CharUnits, CharUnits> getConstantArrayInfoInChars(
+      const ConstantArrayType* CAT) const;
+
   /// \brief Return the "preferred" alignment of the specified type \p T for
   /// the current target, in bits.
   ///
Index: lib/AST/ASTContext.cpp
===================================================================
--- lib/AST/ASTContext.cpp
+++ lib/AST/ASTContext.cpp
@@ -1345,11 +1345,28 @@
 
 std::pair<CharUnits, CharUnits>
 ASTContext::getTypeInfoInChars(const Type *T) const {
+  if (const ConstantArrayType *CAT = dyn_cast<ConstantArrayType>(T))
+    return getConstantArrayInfoInChars(CAT);
   std::pair<uint64_t, unsigned> Info = getTypeInfo(T);
   return std::make_pair(toCharUnitsFromBits(Info.first),
                         toCharUnitsFromBits(Info.second));
 }
 
+/// getConstantArrayInfoInChars - Performing the computation in CharUnits
+/// instead of in bits prevents overflowing the uint64_t for some large arrays.
+std::pair<CharUnits, CharUnits>
+ASTContext::getConstantArrayInfoInChars(const ConstantArrayType *CAT) const {
+  std::pair<uint64_t, unsigned> EltInfo = getTypeInfo(CAT->getElementType());
+  uint64_t Size = CAT->getSize().getZExtValue();
+  assert((Size == 0 || EltInfo.first/getCharWidth() <= (uint64_t)(-1)/Size) &&
+         "Overflow in array type char size evaluation");
+  uint64_t Width = EltInfo.first / getCharWidth() * Size;
+  unsigned Align = EltInfo.second / getCharWidth();
+  Width = llvm::RoundUpToAlignment(Width, Align);
+  return std::make_pair(CharUnits::fromQuantity(Width),
+                        CharUnits::fromQuantity(Align));
+}
+
 std::pair<CharUnits, CharUnits>
 ASTContext::getTypeInfoInChars(QualType T) const {
   return getTypeInfoInChars(T.getTypePtr());
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