[Dwarf-discuss] ISSUE: vector types. V2

Jakub Jelinek jakub@redhat.com
Thu Apr 6 12:03:27 GMT 2023

On Thu, Apr 06, 2023 at 07:52:32AM -0400, Ron Brender wrote:
> Various thoughts...
> > Not sure if supporting dimensions in the way which is done
> > for arrays is needed (I believe vector types are always one-dimensional
> > indexed from 0).
> "always"? There are many element by element operations on multidimensional
> arrays that might benefit
> from use of vector hardware. Why make that an orphaned option?

I'm not aware of any compiler having multi-dimensional vectors.

> > And we almost certainly want to allow vectors on DWARF expression stack;
> What is so special about vector types in this regard. What do you have
> against values of array or structure types being
> on the stack (the latter especially because they are often/mostly passed by
> value in many languages, even C)?
> I don't see the point of vector types being special.

I thought Ben has posted the details.
In memory they look the same as 0 based arrays, but they often have
different calling conventions (argument passing, returning), they support
various arithmetic operations on them, in C++ they support even comparisons
(though, unlike scalar comparisons, vector comparisons produce element
values 0 for false and -1/all ones for true), they don't promote to pointers.
So, for the debug info consumers it is significantly different behavior than arrays.
If a is int a[16];, then a + 2 is &a[0] + 2, while if a is int
__attribute__((vector_size (16 * sizeof (int)))), a + 2 would be either an
error if the language extension doesn't allow vector + scalar, or
{a[0]+2,a[1]+2,a[2]+2,...,a[15]+2} vector.


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