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

[Bob Wilson]

On Sep 29, 2010, at 2:11 PM, Douglas Gregor 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.
>
> 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.

I don't know much of anything about OpenCL, Clang, or the history of  
this issue, but here's my $0.02: I can see a few possible versions  
of ?: with vectors.

1) bool ? vec1 : vec2
That is pretty unambiguous to me.

2) vector of bools ? vec1 : vec2
This too seems unambiguous.  It must be a vector select (if it's legal).

3) vector comparison ? vec1 : vec2
e.g. "vec1 < vec2 ? vec1 : vec2" would be a vector minimum
How does Clang handle comparisons of its extended vector types?

As Doug pointed out, interpretations (1) and (2) are not going to be  
confused unless you have conversions from vector types to bool.  All  
the other vector operations in LLVM (maybe not in Clang?) are defined  
to be the element-wise equivalents of the standard operations, so that  
would argue for allowing (2).

It seems to me that case (3) might help clarify this.  If Clang allows  
element-wise comparisons of its extended vector types, you ought to be  
able to replace the vector minimum example above with: "tmp = vec1 <  
vec2; min = tmp ? vec1 : vec2".  That would mean that case (2) should  
be allowed.  If Clang does not support element-wise vector  
comparisons, is there any particular reason why not?  If you want to  
do vector comparisons and selects with Clang's extended vectors, what  
is the alternative?  Builtins?