Hi!
On Sat, Feb 17, 2018 at 10:08:48AM +0100, Jakub Jelinek wrote:
> Note, this doesn't work, as G++ <= 4.2 rejects:
> template <typename value_type>
> struct S
> {
> char __attribute__ ((__aligned__ (__alignof__ (value_type)))) buf[sizeof
> (value_type)];
> };
>
> S<int> s;
>
> with:
> error: requested alignment is not a constant
>
> The following works though, and has been successfully bootstrapped/regtested
> on x86_64-linux and i686-linux. It will fail if we use hash_table on
> overaligned value_type type with alignment > 64, but that is not completely
> unlikely on any target I'm aware of. It uses __alignof__ which should be
> supported by the system GCC's we support for which
> BROKEN_VALUE_INITIALIZATION is true (i.e. >= 3.4 and <= 4.3), non-GCC
> compilers shouldn't get here due to GCC_VERSION != 0 unless they pretend to
> be GCC compatible.
>
> Bootstrapped with system gcc34/g++34 on x86_64-linux, ok for trunk?
>
> 2018-02-17 Jakub Jelinek <ja...@redhat.com>
>
> PR bootstrap/84405
> * vec.h (vec_default_construct): For BROKEN_VALUE_INITIALIZATION use
> memset and value initialization afterwards.
> * hash-table.h (hash_table<Descriptor, Allocator>::empty_slow): For
> BROKEN_VALUE_INITIALIZATION use placement new into a previously
> cleared appropriately aligned buffer.
I'd like to ping this patch, or if it isn't acceptable, at least the vec.h
part of it which isn't really more complex than what we have right now.
> --- gcc/vec.h.jj 2018-02-16 19:39:03.637058918 +0100
> +++ gcc/vec.h 2018-02-16 23:51:55.389582056 +0100
> @@ -490,12 +490,11 @@ 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
> +#ifdef BROKEN_VALUE_INITIALIZATION
> memset (dst, '\0', sizeof (T) * n);
> #endif
> + for ( ; n; ++dst, --n)
> + ::new (static_cast<void*>(dst)) T ();
> }
>
> /* Copy-construct N elements in DST from *SRC. */
> --- gcc/hash-table.h.jj 2018-02-16 19:39:03.660058934 +0100
> +++ gcc/hash-table.h 2018-02-17 00:01:24.207723925 +0100
> @@ -808,7 +808,18 @@ hash_table<Descriptor, Allocator>::empty
> for ( ; size; ++entries, --size)
> *entries = value_type ();
> #else
> - memset (entries, 0, size * sizeof (value_type));
> + /* To workaround older GCC versions which don't handle value
> + initialization right, use a placement new into previously
> + cleared buffer. */
> + char buf[2 * sizeof (value_type) + 64];
> + gcc_assert (__alignof__ (value_type) <= sizeof (value_type) + 64);
> + char *q = (buf + sizeof (value_type) + 64
> + - ((uintptr_t) buf % __alignof__ (value_type)));
> + memset (q, '\0', sizeof (value_type));
> + value_type *p = ::new (q) value_type ();
> + for ( ; size; ++entries, --size)
> + *entries = *p;
> + p->~value_type ();
> #endif
> }
> m_n_deleted = 0;
Jakub
