Re: [C++ Patch] Add maybe_constant_folded_value

[Paolo Carlini <paolo dot carlini at oracle dot com>]

Hi,

On 11/14/2014 03:35 AM, Jason Merrill wrote:
> On 11/13/2014 04:31 PM, Paolo Carlini wrote:
> > > I think this should be replaced with fold_ if
> > > (processing_template_decl) in build_enumerator.
> > Ok. The value can be NULL_TREE, thus in a straightforward change (per
> > the below) I have to check for that, otherwise we crash in
> > maybe_constant_value. Either that or just check for NULL_TREE at the
> > beginning of maybe_constant_value itself, I guess.
>
> The current fold_ already checks for NULL_TREE; I think we want to 
> preserve that behavior.
Ok, I added it to the new fold_.
> > This is the relatively most tricky change: we have a regression for
> > init/array11.C, because the gcc_assert at the end of
> > maybe_constant_value (called by maybe_constant_init) triggers:
> >
> >    gcc_assert (r == t
> >            || CONVERT_EXPR_P (t)
> >            || (TREE_CONSTANT (t) && !TREE_CONSTANT (r))
> >            || !cp_tree_equal (r, t));
> >
> > we have VIEW_CONVERT_EXPRs, neither is constant, r != t and
> > cp_tree_equal is true. Wild guess: are VIEW_CONVERT_EXPRs also Ok?
>
> Yes.
Ok.

> What did you think about avoiding the duplicate 
> instantiation_dependent_expression_p and potential_constant_expression 
> checks?
Frankly at some point I forgot that, sorry.

Today I figured out the below: the new fold_non_dependent_expr is much 
bigger but definitely calls instantiation_dependent_expression_p and 
potential_constant_expression at most once and should be always 
logically equivalent to instantiate_non_dependent_expr_sfinae followed 
by maybe_constant_value, even in the special cases of those 
TREE_OVERFLOW_Ps. Note: among various other simplifications, I tried 
removing the early return via the conditional:

      if (type_unknown_p (t)
          || BRACE_ENCLOSED_INITIALIZER_P (t))

but it's actually used by eg, g++.dg/cpp0x/constexpr-initlist5.C.

I'm attaching what passed testing on x86_64-linux.

Thanks!
Paolo.

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