[PATCH] Introducing initial UseAuto transform

[Dmitri Gribenko]

On Tue, Feb 12, 2013 at 9:48 PM, Vane, Edwin <edwin.vane at intel.com> wrote:
>>   It can be done easier, I think.  We know that all functions that return iterators have a return value type with correct sugar.  That is, `vector<T>::begin()` return value is always `vector<T>::iterator`.
>>
>> [REV] This isn't always true. When dealing with for-loops I found from dumping the AST that you don't get vector<T>::iterator but whatever vector<T>::iterator typedefs to which is quite long and complex with libstdc++. It was this realization that prompted the comparison of canonical types.
>
> I guess there is something not entirely correct in that test, because this is what I see in my copy of libstdc++ 4.7:
>
>       iterator
>       begin() _GLIBCXX_NOEXCEPT /*...*/
>
> [REV] I put this:
>
> llvm::errs() << D->getType().getAsString() << " = " << E->getType().getAsString() << "\n";
>
> right before the cananonical type equality test. Here's the output when running the iterator.cpp test:
>
> class MyIterator = class MyIterator
> class DummyIterator = class DummyIterator
> std::vector<int>::iterator = iterator
> int_iterator = std::vector<int>::iterator
> std::vector<int>::iterator = iterator
> std::vector<int>::reverse_iterator = reverse_iterator
> std::vector<int>::iterator = iterator
> std::vector<int>::iterator = class __gnu_cxx::__normal_iterator<int *, class std::vector<int, class std::allocator<int> > >
> int_iterator = iterator
> int_iterator = iterator
>
> Notice how the RHS type, even though not user-provided, varies quite a lot.

All of these look good (they should be processed by that algorithm
correctly), except for the 'class __gnu_cxx::...' -- it comes from a
'__begin' variable in the range-based for, which comes from 'auto'
deduction, which is implemented as argument type deduction.  Might be
fixable, though non-trivial.

> This is with libstdc++4.7. I'm not entirely sure this matters. From what you're suggesting about unsugaring user types it sounds like we do the canonical type test and then do an extra bit of work to make sure the user-provided type is no less sugared than the types allowed by the spec?

Yes.

> How do you propose we deal with custom iterators? The canonical type may then be the LHS type.

The user is free to do whatever crazy stuff they like with their
iterators, I don't think we should check anything there.

Given the apparent complexity of implementing the check for all cases,
I think we can leave it for later.

Dmitri

-- 
main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
(j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/