On Tue, Oct 06, 2020 at 01:41:54PM -0400, Andrew MacLeod wrote:
> On 10/6/20 1:32 PM, Jakub Jelinek via Gcc-patches wrote:
> > On Tue, Oct 06, 2020 at 10:42:12AM -0600, Martin Sebor wrote:
> > > The manual documents the [0] extension and mentions but discourages
> > > using [1]. Nothing is said about other sizes and the warnings such
> > > as -Warray-bounds have been increasingly complaining about accesses
> > > past the declared constant bound (it won't complain about past-
> > > the-end accesses to a mem[1], but will about those to mem[2]).
> > >
> > > It would be nice if existing GCC code could eventually be converted
> > > to avoid relying on the [1] hack. I would hope we would avoid making
> > > use of it in new code (and certainly avoid extending its uses to other
> > > sizes).
> > I don't see how we could, because [0] is an extension and GCC needs to
> > support host compilers that don't support it, and similarly [] is an
> > extension in C++ and can't be relied on.
> > Changing say rtl or gimple structs from the way we define them currently
> > to say templates dependent on the size of the embedded arrays is I'm
> > afraid not really possible.
> >
> > Jakub
> >
> Aldy is currently testing this.. I presume this is what you had in mind?
Yes.
I'm not actually sure if it is best to use struct irange (and why struct
irange rather than just irange?) for the r variable, whether it shouldn't
be unsigned char * or whatever is best suitable type for placement new.
> diff --git a/gcc/value-range.h b/gcc/value-range.h
> index 94b48e55e77..c86301fe885 100644
> --- a/gcc/value-range.h
> +++ b/gcc/value-range.h
> @@ -668,13 +668,13 @@ irange_allocator::allocate (unsigned num_pairs)
> if (num_pairs < 2)
> num_pairs = 2;
>
> - struct newir {
> - irange range;
> - tree mem[1];
> - };
> - size_t nbytes = (sizeof (newir) + sizeof (tree) * 2 * (num_pairs - 1));
> - struct newir *r = (newir *) obstack_alloc (&m_obstack, nbytes);
> - return new (r) irange (r->mem, num_pairs);
> + // Allocate the irange and required memory for the vector.
> + struct irange *r = (irange *) obstack_alloc (&m_obstack, sizeof (irange));
> +
> + size_t nbytes = sizeof (tree) * 2 * num_pairs;
> + tree *mem = (tree *) obstack_alloc (&m_obstack, nbytes);
> +
> + return new (r) irange (mem, num_pairs);
> }
>
> inline irange *
Jakub
