Re: C++ delayed folding branch review

[Jason Merrill <jason@redhat.com>]

On 08/24/2015 03:15 AM, Kai Tietz wrote:
> 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.

I don't see any casts in that testcase.  So the compiler is introducing 
introducing conversions back and forth between const and non-const, 
then?  I suppose it doesn't so much matter where they come from, they 
should be folded away regardless.

>>> 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).

I don't understand how this is an answer to my question.

>>>>> 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.

Isn't the other operand also constant?  In constexpr evaluation, either 
we're dealing with a bunch of constants, in which case we should be 
folding things fully, including conversions between const and non-const, 
or we don't care.

> 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.

We don't need to worry about that in constexpr evaluation, since we only 
care about constant operands.

>>> 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?

Folding const into a constant is hardly code modification.  But perhaps 
it should go into fold_unary_loc:VIEW_CONVERT_EXPR rather than into 
convert.c.

Jason