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

[Jonathan Wakely <jwakely@redhat.com>]

On Fri, 24 Feb 2023 at 11:52, Jakub Jelinek <jakub@redhat.com> wrote:
>
> 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) ?

Yes. If alignas(T) is less than the natural alignment then this will
be an error. We want it to be the larger of  the two alignments, so we
need to specify both.

>
> > @@ -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
>