[cfe-commits] r147026 - in /cfe/trunk: lib/AST/Expr.cpp test/Sema/static-init.c

[Eli Friedman]

On Wed, Dec 21, 2011 at 9:27 PM, Richard Smith <richard at metafoo.co.uk> wrote:
> On Thu, December 22, 2011 04:48, Eli Friedman wrote:
>> On Wed, Dec 21, 2011 at 2:51 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
>>
>>> On Tue, Dec 20, 2011 at 7:42 PM, Richard Smith <richard at metafoo.co.uk>
>>> wrote:
>>>
>>>>> Err, great.  That will be a bit annoying to write.  It would make all
>>>>> the constant stuff a bit less complicated if Evaluate could handle all
>>>>> scalar expressions...
>>>>
>>>> I agree, and we're very close. The cases we can't evaluate are:
>>>>
>>>>
>>>>  - difference of two address labels (would need a new APValue kind)
>>>>  - calls to trivial constructors (which are not constexpr)
>>>>  - vector bitcasts
>>>>  - cast to union
>>>>
>>>
>>> I think there might be a few other cases you're missing (one or two
>>> involving compound literals, some cases involving string literals, maybe a
>>> couple others), but yes, it's close.
>>
>> Here's a more-or-less complete list of what is missing on trunk to get
>> through the clang regression tests with isConstantInitializer forwarding to
>> Evaluate:
>> - StringLiteral and ObjCEncodeExpr outside of an InitListExpr
>> - InitListExprs of scalar type (int, float, etc.)
>> - CXXTemporaryObjectExpr
>
> I'd be very interested in any specific examples where these don't work --
> those would probably be holes in our constexpr support. Note that InitListExpr
> for scalars is supported by ExprEvaluatorBase, CXXTemporaryObjectExpr derives
> from CXXConstructExpr so is supported by the code which handles that, and
> ObjCEncodeExpr is supported by LValueExprEvaluator.

For CXXTemporaryObjectExpr: we give an error for the following. I
can't think of any obvious reason why it isn't legal:

struct A { int x; };
constexpr A x = A();

For InitLists, it looks like there's some code in
ExprEvaluatorBase::VisitInitListExpr that we only enable in C++11 mode
which isn't obviously C++11-specific; am I missing something?  Also,
clang supports an extension involving _Complex that it would be nice
to have constexpr support for:

class B { _Complex float x = {1,2}; };
constexpr B y = B();

For StringLiterals, just trying to run 'char a[2] = "a";' through
Evaluate causes issues, but it's hard to come up with a testcase that
doesn't involve a patch.  On a side note, I'm not sure if it's really
related, but for StringLiterals, the following crashes:

class C { char a[4] = "foo"; };
constexpr C a = C();
constexpr int b = a.a[0];

For ObjCEncodeExpr, the following breaks; it looks like there's a FIXME, though.

constexpr char a[2] = @encode(int);
cosntexpr char b = a[0];

>> - RecordExprEvaluator::ValueInitialization
>
> For initializer-lists which don't initialize all members? OK.

Yes.

-Eli