Re: [PATCH] Avoid default-initializing auto_vec<T, N> storage

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Feb 24, 2023 at 09:50:33AM +0000, Jonathan Wakely wrote:
> On Fri, 24 Feb 2023 at 09:49, Jakub Jelinek wrote:
> >
> > Assuming a compiler handles the T m_vecdata[1]; as flexible array member
> > like (which we need because standard C++ doesn't have flexible array members
> > nor [0] arrays), I wonder if we instead of the m_auto followed by m_data
> > trick couldn't make auto_vec have
> > alignas(vec<vl_embed>) unsigned char buf m_data[sizeof (vec<vl_embed>) + (N - 1) * sizeof (T)];
> > and do a placement new of vec<vl_embed> into that m_data during auto_vec
> > construction.  Isn't it then similar to how are flexible array members
> > normally used in C, where one uses malloc or alloca to allocate storage
> > for them and the storage can be larger than the structure itself and
> > flexible array member then can use storage after it?
> 
> You would still be accessing past the end of the
> vec<vl_embed>::m_vecdata array which is UB.

Pedantically sure, but because C++ doesn't have flexible array members,
people in the wild use the flexible array member like arrays for that
purpose.
If there was T m_vecdata[];, would it still be UB (with the flexible
array member extensions)?
We could use T m_vecdata[]; if the host compiler supports them and
T m_vecdata[1]; otherwise in the hope that the compiler handles it
similarly.  After all, I think lots of other real-world programs do the
same.

	Jakub