Add the alloc_size attribute to clang, attempt 2.

This is a recommit of r290149, which was reverted in r290169 due to msan
failures. msan was failing because we were calling
`isMostDerivedAnUnsizedArray` on an invalid designator, which caused us
to read uninitialized memory. To fix this, the logic of the caller of
said function was simplified, and we now have a `!Invalid` assert in
`isMostDerivedAnUnsizedArray`, so we can catch this particular bug more
easily in the future.

Fingers crossed that this patch sticks this time. :)

Original commit message:

This patch does three things:
- Gives us the alloc_size attribute in clang, which lets us infer the
  number of bytes handed back to us by malloc/realloc/calloc/any user
  functions that act in a similar manner.
- Teaches our constexpr evaluator that evaluating some `const` variables
  is OK sometimes. This is why we have a change in
  test/SemaCXX/constant-expression-cxx11.cpp and other seemingly
  unrelated tests. Richard Smith okay'ed this idea some time ago in
  person.
- Uniques some Blocks in CodeGen, which was reviewed separately at
  D26410. Lack of uniquing only really shows up as a problem when
  combined with our new eagerness in the face of const.

llvm-svn: 290297

clang/include/clang/Basic/Attr.td

@@ -780,6 +780,15 @@ def EmptyBases : InheritableAttr, TargetSpecificAttr<TargetMicrosoftCXXABI> {
   let Documentation = [EmptyBasesDocs];
 }
 
+def AllocSize : InheritableAttr {
+  let Spellings = [GCC<"alloc_size">];
+  let Subjects = SubjectList<[HasFunctionProto], WarnDiag,
+                             "ExpectedFunctionWithProtoType">;
+  let Args = [IntArgument<"ElemSizeParam">, IntArgument<"NumElemsParam", 1>];
+  let TemplateDependent = 1;
+  let Documentation = [AllocSizeDocs];
+}
+
 def EnableIf : InheritableAttr {
   let Spellings = [GNU<"enable_if">];
   let Subjects = SubjectList<[Function]>;

clang/include/clang/Basic/AttrDocs.td

@@ -206,6 +206,44 @@ to enforce the provided alignment assumption.
   }];
 }
 
+def AllocSizeDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``alloc_size`` attribute can be placed on functions that return pointers in
+order to hint to the compiler how many bytes of memory will be available at the
+returned poiner. ``alloc_size`` takes one or two arguments.
+
+- ``alloc_size(N)`` implies that argument number N equals the number of
+  available bytes at the returned pointer.
+- ``alloc_size(N, M)`` implies that the product of argument number N and
+  argument number M equals the number of available bytes at the returned
+  pointer.
+
+Argument numbers are 1-based.
+
+An example of how to use ``alloc_size``
+
+.. code-block:: c
+
+  void *my_malloc(int a) __attribute__((alloc_size(1)));
+  void *my_calloc(int a, int b) __attribute__((alloc_size(1, 2)));
+
+  int main() {
+    void *const p = my_malloc(100);
+    assert(__builtin_object_size(p, 0) == 100);
+    void *const a = my_calloc(20, 5);
+    assert(__builtin_object_size(a, 0) == 100);
+  }
+
+.. Note:: This attribute works differently in clang than it does in GCC.
+  Specifically, clang will only trace ``const`` pointers (as above); we give up
+  on pointers that are not marked as ``const``. In the vast majority of cases,
+  this is unimportant, because LLVM has support for the ``alloc_size``
+  attribute. However, this may cause mildly unintuitive behavior when used with
+  other attributes, such as ``enable_if``.
+  }];
+}
+
 def EnableIfDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -2299,6 +2299,9 @@ def warn_attribute_pointers_only : Warning<
   "%0 attribute only applies to%select{| constant}1 pointer arguments">,
   InGroup<IgnoredAttributes>;
 def err_attribute_pointers_only : Error<warn_attribute_pointers_only.Text>;
+def err_attribute_integers_only : Error<
+  "%0 attribute argument may only refer to a function parameter of integer "
+  "type">;
 def warn_attribute_return_pointers_only : Warning<
   "%0 attribute only applies to return values that are pointers">,
   InGroup<IgnoredAttributes>;