Re: [PATCH 2/2] Avoid default-initializing auto_vec<T, N> storage, fix vec<vl_embed>

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Feb 24, 2023 at 12:44:44PM +0100, Richard Biener wrote:
> --- a/gcc/vec.h
> +++ b/gcc/vec.h
> @@ -586,8 +586,8 @@ public:
>    unsigned allocated (void) const { return m_vecpfx.m_alloc; }
>    unsigned length (void) const { return m_vecpfx.m_num; }
>    bool is_empty (void) const { return m_vecpfx.m_num == 0; }
> -  T *address (void) { return m_vecdata; }
> -  const T *address (void) const { return m_vecdata; }
> +  T *address (void) { return reinterpret_cast <T *> (this + 1); }
> +  const T *address (void) const { return reinterpret_cast <const T *> (this + 1); }

This is now too long.

>    T *begin () { return address (); }
>    const T *begin () const { return address (); }
>    T *end () { return address () + length (); }
> @@ -631,8 +631,7 @@ public:
>  
>    /* FIXME - These fields should be private, but we need to cater to
>  	     compilers that have stricter notions of PODness for types.  */
> -  vec_prefix m_vecpfx;
> -  T m_vecdata[1];
> +  alignas (T) vec_prefix m_vecpfx;

The comment needs adjustment and down't we need
alignas (T) alignas (vec_prefix) ?

> @@ -1588,7 +1587,7 @@ public:
>  
>  private:
>    vec<T, va_heap, vl_embed> m_auto;
> -  T m_data[MAX (N - 1, 1)];
> +  alignas(T) unsigned char m_data[sizeof (T) * N];
>  };

I still believe you don't need alignas(T) here (and space before (T) ).
Also, I think it needs to be MAX (N, 2) instead of N, because auto_vec
ctors use MAX (N, 2).  We could also change all those to MAX (N, 1)
now, but it can't be N because m_data[sizeof (T) * 0] is invalid in
standard C.

Anyway, I wonder if you get the -Werror=stringop-overflow= errors during
bootstrap that I got with my version or not.

	Jakub