On Fri, Feb 16, 2018 at 02:14:09PM -0700, Martin Sebor wrote:
> On 02/16/2018 01:44 PM, Jakub Jelinek wrote:
> > On Fri, Feb 16, 2018 at 09:25:30PM +0100, Richard Biener wrote:
> > > But the broken compilers will overwrite the memset data with possibly
> > > uninitialized fields of a TARGET_EXPR.
> >
> > Perhaps for hash-table.h we could use:
> > #ifndef BROKEN_VALUE_INITIALIZATION
> > for ( ; size; ++entries, --size)
> > *entries = value_type ();
> > #else
> > union U { char c[sizeof (value_type)]; value_type v; } u;
>
> In C++ 98 a type with a constructor cannot be a member of a union
> so the above wouldn't be valid but if I understand what you're
> trying to do you don't need the member. All you need is a buffer.
> Something like this?
>
> char __attribute__ ((aligned (__alignof__ (value_type))))
> buf [sizeof (value_type)] = "";
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.
--- 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
