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Re: C++ delayed folding branch review

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
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