On Fri, 16 Feb 2018, Jakub Jelinek wrote:
> Hi!
>
> As the system.h comment says, it took us years to get value initialization
> right, and unfortunately we've started hitting it in GCC codebase now.
> There are various related PRs, 4.2.[0-3] are most broken in this regard,
> value initialization of non-PODs doesn't actually initialize there anything,
> but https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84405#c5 shows a testcase
> where any GCC < 4.3 just calls the default ctor for the
> *entries = value_type ();
> in hash-table.h even when value_type has no user provided ctor, and one
> pointer field and another field with user provided ctor. We want to
> zero initialize the pointer and default initialize the other field, but
> < 4.3 just default initializes the other field and pointer (m_key) remains
> uninitialized.
> The vec_default_construct case can hit another bug in < 4.4, if we have a
> similar class with multiple fields and at least one of them with user provided
> ctor, and at least one of them without user provided ctor, the new T ()
> default initializes it rather than value initializes it.
>
> The following patch provides sufficient workaround so that GCC 3.4.6 as a
> system compiler can bootstrap GCC (haven't tested other versions). It is
> not enough to get GCC 4.2.[0-3] usable as bootstrap compiler, either we add
> 2-3 further workarounds, or just reject GCC >= 4.2.0 and <= 4.2.3 in
> configure.
>
> Ok for trunk?
Ok. I'll see if fixing libcpp makes GCC 4.2.[0-3] usable as well.
Richard.
> 2018-02-16 Jakub Jelinek <ja...@redhat.com>
>
> PR bootstrap/84405
> * system.h (BROKEN_VALUE_INITIALIZATION): Define for GCC < 4.3.
> * vec.h (vec_default_construct): Use memset instead of placement new
> if BROKEN_VALUE_INITIALIZATION is defined.
> * hash-table.h (hash_table<Descriptor, Allocator>::empty_slow): Use
> memset instead of value initialization if BROKEN_VALUE_INITIALIZATION
> is defined.
>
> --- gcc/system.h.jj 2018-01-03 10:19:54.530533857 +0100
> +++ gcc/system.h 2018-02-15 21:56:37.168139415 +0100
> @@ -824,6 +824,12 @@ extern void fancy_abort (const char *, i
> /* Some compilers do not allow the use of unsigned char in bitfields. */
> #define BOOL_BITFIELD unsigned int
>
> +/* GCC older than 4.4 have broken C++ value initialization handling, see
> + PR11309, PR30111, PR33916, PR82939 and PR84405 for more details. */
> +#if GCC_VERSION > 0 && GCC_VERSION < 4004 && !defined(__clang__)
> +# define BROKEN_VALUE_INITIALIZATION
> +#endif
> +
> /* As the last action in this file, we poison the identifiers that
> shouldn't be used. Note, luckily gcc-3.0's token-based integrated
> preprocessor won't trip on poisoned identifiers that arrive from
> --- gcc/vec.h.jj 2018-01-03 10:19:54.606533869 +0100
> +++ gcc/vec.h 2018-02-15 22:00:36.386216876 +0100
> @@ -490,8 +490,12 @@ template <typename T>
> inline void
> vec_default_construct (T *dst, unsigned n)
> {
> +#ifndef BROKEN_VALUE_INITIALIZATION
> for ( ; n; ++dst, --n)
> ::new (static_cast<void*>(dst)) T ();
> +#else
> + memset (dst, '\0', sizeof (T) * n);
> +#endif
> }
>
> /* Copy-construct N elements in DST from *SRC. */
> --- gcc/hash-table.h.jj 2018-01-03 10:19:54.899533916 +0100
> +++ gcc/hash-table.h 2018-02-15 21:58:14.449170915 +0100
> @@ -804,8 +804,12 @@ hash_table<Descriptor, Allocator>::empty
> }
> else
> {
> +#ifndef BROKEN_VALUE_INITIALIZATION
> for ( ; size; ++entries, --size)
> *entries = value_type ();
> +#else
> + memset (entries, 0, size * sizeof (value_type));
> +#endif
> }
> m_n_deleted = 0;
> m_n_elements = 0;
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
21284 (AG Nuernberg)
