[cfe-dev] On which option should vector operator ? : depend?

[Daniel Dunbar]

On Wed, Sep 29, 2010 at 2:11 PM, Douglas Gregor <dgregor at apple.com> wrote:
>
> On Sep 29, 2010, at 12:35 PM, Daniel Dunbar wrote:
>
> On Wed, Sep 29, 2010 at 11:23 AM, Douglas Gregor <dgregor at apple.com> wrote:
>
> On Sep 29, 2010, at 11:08 AM, Sebastian Redl wrote:
>
>
> On Sep 29, 2010, at 11:01 AM, Daniel Dunbar wrote:
>
> On Wed, Sep 29, 2010 at 10:41 AM, Sebastian Redl
>
> <sebastian.redl at getdesigned.at> wrote:
>
> It seems weird to me too, but I think the question here is not whether to
> have it - IIUC OpenCL demands it - but whether it makes sense to make it
> work conditionally only for some language flags. And IMO, if you have ?:
> mean vector select in one dialect, you might as well make it work in all of
> them.
>
> Why? OpenCL is a different language in many ways.
>
> AltiVec vector support isn't.
>
> For AltiVec support, we have precedent to follow: what does GCC do? We need
> to do the same.
>
> Now, Clang's ext_vector_type is a specific vector type that Clang added for
> OpenCL. It seems reasonable that it would have OpenCL's semantics,
> regardless of dialect. For example, we permit the OpenCL "vec.xyzw" syntax
> for ext_vector_type vectors in all dialects.
>
> Yes, but that syntax has no (sensical) precedent in C, so programmers
> aren't likely to confuse it with "what C would normally do", at least
> in code that compiles.
>
> I think that's a wonderful argument against OpenCL's abuse of the ? :
> notation to mean vector selection. But, OpenCL made that decision, so our
> only decision is whether this syntax applies to Clang's extended vectors
> (the purpose of which is to support OpenCL) or to Clang when in OpenCL mode.
> We have precedent for the format.

I don't see Clang's extended vectors as being "to support OpenCL". I
see them as a sane syntax for working with vectors in C family code,
which is certainly a reasonable feature. There are many more users of
C family languages that might want good vector support than their are
OpenCL users (since the former is presumably a near superset).

> Note that there's no conversion from a vector type to bool, so
> vec? vec1 : vec2
> is currently ill-formed. So, while having that expression turn into a select
> might confuse a C programmer, at least we're not changing the semantics of
> existing, well-formed code.

Yes.

As it turns out, I only feel strongly enough about this to push back
three times, so now I no longer have an opinion. :)

 - Daniel

> - Doug