Re: [PATCH] Fix bootstrap with GCC 3.4 (and likely anything < 4.4 except 4.2.[0-3]) (PR bootstrap/84405)

[Richard Biener <rguenther at suse dot de>]

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  <jakub@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 <rguenther@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)