Re: C++ delayed folding branch review

[Jason Merrill <jason@redhat.com>]

On 08/27/2015 06:39 AM, Kai Tietz wrote:
> 2015-08-27 4:56 GMT+02:00 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 gets introduced in convert.c about line 836 in function
> convert_to_integer_1 AFAIK.  There should be the alternative solution
> for this issue by disallowing for PLUS/MINUS/... expressions the
> sinking of the cast into the expression, if dofold is false, and type
> has same width as inner_type, and is of vector-kind.

Why would we be calling convert_to_integer for conversions between 
vector types?

>>>>> 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.
>
> (vec) (const vector) { ... } expression can't be folded.

It currently isn't folded, but why can't we change that?

> This cast to
> none-const variant happens due the 'constexpr v = v +
> <constant-value>' pattern in testcase.  v is still of type vec, even
> if function itself is constexpr.

I don't see that pattern in the testcase:

typedef long vec __attribute__((vector_size (2 * sizeof (long))));
constexpr vec v = { 3, 4 };
constexpr vec s = v + v;
constexpr vec w = __builtin_shuffle (v, v);

If we have v + constant-value, that's because we pulled out the constant 
value of one of the v's, which we ought to be doing for both of them.

>>>>>>> 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.
>
> No other operand isn't a constant-value.  See code-pattern in
> testcase.  It is of type 'vec', which isn't constant (well, 'v' is,
> but constexpr doesn't know about it).

What do you mean, "constexpr doesn't know about it"?

>>> 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.
>
> Sure, but the variable 'v' is the problem, not a constant-value itself.

>>>>> 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.
>
> Hmm, it isn't related to a view-convert.  So moving it into
> fold_unary_loc wouldn't solve here anything.  Issue is in constexpr
> code, not in folding itself.

What TREE_CODE does the conversion (vec) (const vector) { ... } use?

Jason