RFC: C++ delayed folding merge

[Jason Merrill]

On 11/09/2015 04:08 AM, Richard Biener wrote:
> On Mon, 9 Nov 2015, Jason Merrill wrote:
>
>> I'm planning to merge the C++ delayed folding branch this week, but I need to
>> get approval of the back end changes (the first patch attached).  Most of
>> these are the introduction of non-folding variants of convert_to_*, but there
>> are a few others.
>>
>> One question: The branch changes 'convert' to not fold its result, and it's
>> not clear to me whether that's part of the expected behavior of a front end
>> 'convert' function or not.
>
> History.  convert is purely frontend (but shared, unfortunately between
> all frontends).  I would expect that FEs that do not do delayed folding
> expect convert to fold.
>
>> Also, I'm a bit uncertain about merging this at the end of stage 1, since it's
>> a large internal change with relatively small user impact; it just improves
>> handling of constant expression corner cases.  I'm inclined to go ahead with
>> it at this point, but I'm interested in contrary opinions.
>
> I welcome this change as it should allow cleaning up the FE-middle-end
> interface a bit more.  It should be possible to remove all
> NON_LVALUE_EXPR adding/removal from the middle-end folders.
>
> Looks like the backend patch included frontend parts but as far as I
> skimmed it only
>
> diff --git a/gcc/fold-const.c b/gcc/fold-const.c
> index 5e32901..d754a90 100644
> --- a/gcc/fold-const.c
> +++ b/gcc/fold-const.c
> @@ -2091,6 +2091,17 @@ fold_convert_const (enum tree_code code, tree type,
> tree arg1)
>         else if (TREE_CODE (arg1) == REAL_CST)
>          return fold_convert_const_fixed_from_real (type, arg1);
>       }
> +  else if (TREE_CODE (type) == VECTOR_TYPE)
> +    {
> +      if (TREE_CODE (arg1) == VECTOR_CST
> +         && TYPE_MAIN_VARIANT (type) == TYPE_MAIN_VARIANT (TREE_TYPE
> (arg1))
> +         && TYPE_VECTOR_SUBPARTS (type) == VECTOR_CST_NELTS (arg1))
> +       {
> +         tree r = copy_node (arg1);
> +         TREE_TYPE (arg1) = type;
> +         return r;
> +       }
> +    }
>
>
> looks suspicious.  The issue here is that the vector elements will
> have the wrong type after this simple handling.

I was aiming to just handle simple cv-qualifier changes; that's why the 
TYPE_MAIN_VARIANT comparison is there.

> If you fix that you can as well handle all kind of element type
> changes via recursing to fold_convert_const (that includes
> float to int / int to float changes).

But I'll try this.

> Not sure if we can even
> write a testcase for such conversions with the GCC vector extensions
> though.

Jason