Re: C++ delayed folding branch review

[Kai Tietz <ktietz70@googlemail.com>]

Hello Jason,

after a longer delay the answer to your question.

2015-08-03 17:39 GMT+02:00 Jason Merrill <jason@redhat.com>:
> On 08/03/2015 05:42 AM, Kai Tietz wrote:
>>
>> 2015-08-03 5:49 GMT+02:00 Jason Merrill <jason@redhat.com>:
>>>
>>> On 07/31/2015 05:54 PM, Kai Tietz wrote:
>>>>
>>>>
>>>> The "STRIP_NOPS-requirement in 'reduced_constant_expression_p'" I could
>>>> remove, but for one case in constexpr.  Without folding we don't do
>>>> type-sinking/raising.
>>>
>>>
>>>
>>> Right.
>>>
>>>> So binary/unary operations might be containing cast, which were in the
>>>> past unexpected.
>>>
>>>
>>> Why aren't the casts folded away?
>>
>>
>> On such cast constructs, as for this vector-sample, we can't fold away
>
>
> Which testcase is this?

It is the g++.dg/ext/vector20.C testcase.  IIRC I mentioned this
testcase already earlier as reference, but I might be wrong here.

>> the cast chain.  The difference here to none-delayed-folding branch is
>> that the cast isn't moved out of the plus-expr.  What we see now is
>> (plus ((vec) (const vector ...) { .... }), ...).  Before we had (vec)
>> (plus (const vector ...) { ... }).
>
>
> How could a PLUS_EXPR be considered a reduced constant, regardless of where
> the cast is?

Of course it is just possible to sink out a cast from PLUS_EXPR, in
pretty few circumstance (eg. on constants if both types just differ in
const-attribute, if conversion is no view-convert).

>>>> On verify_constant we check by reduced_constant_expression_p, if value
>>>> is
>>>> a constant.  We don't handle here, that NOP_EXPRs are something we want
>>>> to
>>>> look through here, as it doesn't change anything if this is a constant,
>>>> or
>>>> not.
>>>
>>>
>>> NOPs around constants should have been folded away by the time we get
>>> there.
>>
>>
>> Not in this cases, as the we actually have here a switch from const to
>> none-const.  So there is an attribute-change, which we can't ignore in
>> general.
>
>
> I wasn't suggesting we ignore it, we should be able to change the type of
> the vector_cst.

Well, the vector_cst we can change type, but this wouldn't help
AFAICS.  As there is still one cast surviving within PLUS_EXPR for the
other operand.
So the way to solve it would be to move such conversion out of the
expression.  For integer-scalars we do this, and for some
floating-points too.  So it might be something we don't handle for
operations with vector-type.

>> But I agree that for constexpr's we could special case cast
>> from const to none-const (as required in expressions like const vec v
>> = v + 1).
>
>
> Right.  But really this should happen in convert.c, it shouldn't be specific
> to C++.

Hmm, maybe.  But isn't one of our different goals to move such
implicit code-modification to match.pd instead?

> Jason
>


Kai