On Wed, Apr 29, 2020 at 11:43 PM Marek Polacek via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Here we trip on the TYPE_USER_ALIGN (t) assert in strip_typedefs: it
> gets "const d[0]" with TYPE_USER_ALIGN=0 but the result built by
> build_cplus_array_type is "const char[0]" with TYPE_USER_ALIGN=1.
>
> When we strip_typedefs the element of the array "const d", we see it's
> a typedef_variant_p, so we look at its DECL_ORIGINAL_TYPE, which is
> char, but we need to add the const qualifier, so we call
> cp_build_qualified_type -> build_qualified_type
> where get_qualified_type checks to see if we already have such a type
> by walking the variants list, which in this case is:
>
> char -> c -> const char -> const char -> d -> const d
>
> Because check_base_type only checks TYPE_ALIGN and not TYPE_USER_ALIGN,
> we choose the first const char, which has TYPE_USER_ALIGN set. If the
> element type of an array has TYPE_USER_ALIGN, the array type gets it too.
>
> So we can make check_base_type stricter. I was afraid that it might make
> us reuse types less often, but measuring showed that we build the same
> amount of types with and without the patch, while bootstrapping.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/9/8?
Hmm, so while I wonder whether a mismatch in TYPE_USER_ALIGN would
affect program semantics we should make sure to handle this flag consistently.
Thus the patch is OK for trunk and branches after a while.
Note I question that we build new types during diagnostic printing.
Thanks,
Richard.
> PR c++/94775
> * tree.c (check_base_type): Return true only if TYPE_USER_ALIGN match.
> (check_aligned_type): Check if TYPE_USER_ALIGN match.
>
> * g++.dg/warn/Warray-bounds-10.C: New test.
> ---
> gcc/testsuite/g++.dg/warn/Warray-bounds-10.C | 40 ++++++++++++++++++++
> gcc/tree.c | 4 +-
> 2 files changed, 43 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.dg/warn/Warray-bounds-10.C
>
> diff --git a/gcc/testsuite/g++.dg/warn/Warray-bounds-10.C
> b/gcc/testsuite/g++.dg/warn/Warray-bounds-10.C
> new file mode 100644
> index 00000000000..0a18f637e0e
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Warray-bounds-10.C
> @@ -0,0 +1,40 @@
> +// PR c++/94775
> +// { dg-do compile { target c++14 } }
> +// { dg-options "-O2 -Warray-bounds" }
> +
> +template <typename> using a = int;
> +template <bool, typename, typename> using b = int;
> +typedef char d;
> +template <long> using e = int;
> +template <int f, int q> struct h { using i = b<q, a<e<f>>, e<f>>; };
> +template <long f, bool g> using j = typename h<f, g>::i;
> +long ab, k, aj;
> +const d l[]{};
> +class m {
> +public:
> + m(int);
> +};
> +class n {
> + void ad() const;
> + template <class ae> void o(long) const {
> + using c __attribute__((aligned(1))) = const ae;
> + }
> + long p;
> + template <class, class>
> + auto s(unsigned long, unsigned long, unsigned long, unsigned long) const;
> + template <bool = false> auto q(unsigned long, unsigned long) const;
> +};
> +template <class, class>
> +auto n::s(unsigned long, unsigned long, unsigned long, unsigned long t)
> const {
> + o<d>(p);
> + return t;
> +}
> +template <bool g> auto n::q(unsigned long p1, unsigned long p2) const {
> + using r = j<4, false>;
> + using ai = j<4, g>;
> + return s<ai, r>(ab, k, p1, p2);
> +}
> +void n::ad() const {
> + long f(l[aj]); // { dg-warning "outside array bounds" }
> + m(q(8, f));
> +}
> diff --git a/gcc/tree.c b/gcc/tree.c
> index e451401822c..341766c51e5 100644
> --- a/gcc/tree.c
> +++ b/gcc/tree.c
> @@ -6493,7 +6493,8 @@ check_base_type (const_tree cand, const_tree base)
> TYPE_ATTRIBUTES (base)))
> return false;
> /* Check alignment. */
> - if (TYPE_ALIGN (cand) == TYPE_ALIGN (base))
> + if (TYPE_ALIGN (cand) == TYPE_ALIGN (base)
> + && TYPE_USER_ALIGN (cand) == TYPE_USER_ALIGN (base))
> return true;
> /* Atomic types increase minimal alignment. We must to do so as well
> or we get duplicated canonical types. See PR88686. */
> @@ -6528,6 +6529,7 @@ check_aligned_type (const_tree cand, const_tree base,
> unsigned int align)
> && TYPE_CONTEXT (cand) == TYPE_CONTEXT (base)
> /* Check alignment. */
> && TYPE_ALIGN (cand) == align
> + && TYPE_USER_ALIGN (cand) == TYPE_USER_ALIGN (base)
> && attribute_list_equal (TYPE_ATTRIBUTES (cand),
> TYPE_ATTRIBUTES (base))
> && check_lang_type (cand, base));
>
> base-commit: 8f1591763fd50b143af0dc1770741f326a97583a
> --
> Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA
>
