Re: [patch c++]: Fix PR/53904

[Richard Biener <richard dot guenther at gmail dot com>]

On Thu, Nov 20, 2014 at 8:48 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
> Hello,
>
> this issue fixes a type-overflow issue caused by trying to cast a UHWI
> via tree_to_shwi.
> As soon as value gets larger then SHWI_MAX, we get an error for it.
> So we need to cast it
> via tree_to_uhwi, and then casting it to the signed variant.

I think it's better to handle the degenerate case (no element) explicitely.
And I would think that sth like "nelts" should have a positive result,
thus why is 'max' not unsigned?  Also 'max' and using 'nelts' looks
like a mismatch?  max == nelts - 1.  Ah, because array_type_nelts
returns nelts - 1 ... how useful ;)

Still you want to special-case the array_type_nelts == -1 case.

Richard.

> ChangeLog
>
> 2014-11-20  Kai Tietz  <ktietz@redhat.com>
>
>     PR c++/63904
>     * constexpr.c (cxx_eval_vec_init_1): Avoid
>     type-overflow issue.
>
> 2014-11-20  Kai Tietz  <ktietz@redhat.com>
>
>     PR c++/63904
>     * g++.dg/cpp0x/pr63904.C: New.
>
>
> Regression tested for x86_64-unknown-linux-gnu.  Ok for apply?
>
> Regards,
> Kai
>
> Index: gcc/gcc/cp/constexpr.c
> ===================================================================
> --- gcc.orig/gcc/cp/constexpr.c
> +++ gcc/gcc/cp/constexpr.c
> @@ -2006,12 +2050,12 @@ cxx_eval_vec_init_1 (const constexpr_ctx
>               bool *non_constant_p, bool *overflow_p)
>  {
>    tree elttype = TREE_TYPE (atype);
> -  int max = tree_to_shwi (array_type_nelts (atype));
> +  HOST_WIDE_INT max = (HOST_WIDE_INT) tree_to_uhwi (array_type_nelts (atype));
>    verify_ctor_sanity (ctx, atype);
>    vec<constructor_elt, va_gc> **p = &CONSTRUCTOR_ELTS (ctx->ctor);
>    vec_alloc (*p, max + 1);
>    bool pre_init = false;
> -  int i;
> +  HOST_WIDE_INT i;
>
>    /* For the default constructor, build up a call to the default
>       constructor of the element type.  We only need to handle class types
> Index: gcc/gcc/testsuite/g++.dg/cpp0x/pr63904.C
> ===================================================================
> --- /dev/null
> +++ gcc/gcc/testsuite/g++.dg/cpp0x/pr63904.C
> @@ -0,0 +1,13 @@
> +// { dg-do compile { target c++11 } }
> +
> +template<int N>
> +struct foo {
> +    constexpr foo() : a() {}
> +    int a[N];
> +};
> +
> +int main() {
> +  foo< (foo<1>{}).a[0] > f;
> +  return 0;
> +}
> +