Re: [RFC] trailing_wide_ints with runtime variable lengths

[Aldy Hernandez <aldyh@redhat.com>]

On Fri, Jul 1, 2022 at 8:53 PM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Fri, Jul 01, 2022 at 07:43:28PM +0200, Aldy Hernandez wrote:
> > You can still say N=255 and things continue to work as they do now, since
> > m_len[] is still statically determined. The only difference is that before,
> > the size of the structure would be 2+1+255+sizeof(int) whereas now it would
> > be 1 more because of the byte I'm using for num_elements.
>
> So, what N do you want to use for SSA_NAME_RANGE_INFO?
> N=255 wouldn't be very space efficient especially if the common case is a
> single range or two.
> For such cases making e.g. m_len not an embedded array, but pointer to
> somewhere after the HOST_WIDE_INT array in the same allocation would be
> better.

As I mentioned in my original post, 12.  This means that I'm taking
the 4 bytes that are left over from the current padding plus 8
(64-bits).  My trailing wide int structure for SSA_NAME_RANGE_INFO
will be one word larger than what is currently there.  But we'll be
able to store up to 5 pairs plus one for the nonzero bits plus one for
future development (5*2 + 1 + 1 = 12), all without going over the 64
bit alignment.

This is a theoretical max, in reality as I mentioned, 99% of ranges
calculated in infinite precision by the ranger fit into 3-4 pairs.

Aldy