Re: C++ PATCH to fix wrong-code with pointer-to-data-members (PR c++/79687)

[Jason Merrill <jason at redhat dot com>]

On Tue, Feb 28, 2017 at 10:10 AM, Marek Polacek <polacek@redhat.com> wrote:
> On Fri, Feb 24, 2017 at 11:11:05AM -0800, Jason Merrill wrote:
>> On Fri, Feb 24, 2017 at 8:22 AM, Marek Polacek <polacek@redhat.com> wrote:
>> > I had an interesting time tracking down some of the problems with this code.
>> > Hopefully I've sussed out now how this stuff works.
>> >
>> > We've got
>> >
>> > struct A { char c; };
>> > char A::*p = &A::c;
>> > static char A::*const q = p;
>> > and then
>> > &(a.*q) - &a.c
>> > which should evaluate to 0.  Here "p" will be 0, that's the offset from the
>> > start of the struct to "c".  "q" is const-qualified and static and initialized
>> > with "p", so we get to cp_fold_maybe_rvalue -> decl_constant_value ->
>> > constant_value_1.  Now, NULL pointer-to-data-members are represented by -1, so
>> > that a null pointer is distinguishable from an offset of the first member of a
>> > struct (0).  So constant_value_1 looks at the DECL_INITIAL of "q", which is -1,
>> > a constant, we fold "q" to -1, and sadness ensues.  I believe the -1 value is
>> > only an internal representation and shouldn't be used like that.
>>
>> Since q is initialized from p, it shouldn't have a DECL_INITIAL of -1;
>> that sounds like the bug.
>
> The DECL_INITIAL of -1 comes from cp_finish_decl:
>  7038              The memory occupied by any object of static storage
>  7039              duration is zero-initialized at program startup before
>  7040              any other initialization takes place.
>  7041
>  7042              We cannot create an appropriate initializer until after
>  7043              the type of DECL is finalized.  If DECL_INITIAL is set,
>  7044              then the DECL is statically initialized, and any
>  7045              necessary zero-initialization has already been performed.  */
>  7046           if (TREE_STATIC (decl) && !DECL_INITIAL (decl))
>  7047             DECL_INITIAL (decl) = build_zero_init (TREE_TYPE (decl),
>  7048                                                    /*nelts=*/NULL_TREE,
>  7049                                                    /*static_storage_p=*/true);

Ah, that makes sense.  We do want to do constant-initialization with
-1 before we do dynamic initialization with p.

So we need to detect in constant_value_1 that the variable has a
dynamic initializer and therefore return the variable rather than -1.
DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P seems useful, perhaps in
combination with DECL_NONTRIVIALLY_INITIALIZED.

Jason