OK.
On Wed, Mar 1, 2017 at 10:16 AM, Marek Polacek <pola...@redhat.com> wrote:
> On Tue, Feb 28, 2017 at 01:12:38PM -1000, Jason Merrill wrote:
>> On Tue, Feb 28, 2017 at 10:10 AM, Marek Polacek <pola...@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 <pola...@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.
>
> Got it. I think the following should be the real fix. I ran g++ dg.exp
> with some logging to see how often the new check triggers, and it only
> triggered in the two new tests, so I'm fairly happy with that.
>
> Bootstrapped/regtested on x86_64-linux, ok for trunk and 6?
>
> 2017-03-01 Marek Polacek <pola...@redhat.com>
>
> PR c++/79687
> * init.c (constant_value_1): Break if the variable has a dynamic
> initializer.
>
> * g++.dg/expr/ptrmem8.C: New test.
> * g++.dg/expr/ptrmem9.C: New test.
>
> diff --git gcc/cp/init.c gcc/cp/init.c
> index 7ded37e..12e6bf4 100644
> --- gcc/cp/init.c
> +++ gcc/cp/init.c
> @@ -2193,6 +2193,13 @@ constant_value_1 (tree decl, bool strict_p, bool
> return_aggregate_cst_ok_p)
> if (TREE_CODE (init) == CONSTRUCTOR
> && !DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (decl))
> break;
> + /* If the variable has a dynamic initializer, don't use its
> + DECL_INITIAL which doesn't reflect the real value. */
> + if (VAR_P (decl)
> + && TREE_STATIC (decl)
> + && !DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P (decl)
> + && DECL_NONTRIVIALLY_INITIALIZED_P (decl))
> + break;
> decl = unshare_expr (init);
> }
> return decl;
> diff --git gcc/testsuite/g++.dg/expr/ptrmem8.C
> gcc/testsuite/g++.dg/expr/ptrmem8.C
> index e69de29..c5a766a 100644
> --- gcc/testsuite/g++.dg/expr/ptrmem8.C
> +++ gcc/testsuite/g++.dg/expr/ptrmem8.C
> @@ -0,0 +1,15 @@
> +// PR c++/79687
> +// { dg-do run }
> +
> +struct A
> +{
> + char c;
> +};
> +
> +int main()
> +{
> + char A::* p = &A::c;
> + static char A::* const q = p;
> + A a;
> + return &(a.*q) - &a.c;
> +}
> diff --git gcc/testsuite/g++.dg/expr/ptrmem9.C
> gcc/testsuite/g++.dg/expr/ptrmem9.C
> index e69de29..32ce777 100644
> --- gcc/testsuite/g++.dg/expr/ptrmem9.C
> +++ gcc/testsuite/g++.dg/expr/ptrmem9.C
> @@ -0,0 +1,19 @@
> +// PR c++/79687
> +// { dg-do run }
> +
> +struct A
> +{
> + char c;
> +};
> +
> +int main()
> +{
> + static char A::* p1 = &A::c;
> + char A::* const q1 = p1;
> +
> + char A::* p2 = &A::c;
> + static char A::* const q2 = p2;
> +
> + A a;
> + return (&(a.*q1) - &a.c) || (&(a.*q2) - &a.c);
> +}
>
> Marek
