[cfe-dev] Matrix Support in Clang

[Florian Hahn via cfe-dev]

> On Apr 2, 2020, at 21:49, John McCall <rjmccall at apple.com> wrote:
> On 2 Apr 2020, at 16:17, Florian Hahn wrote:
> 
>> > On Apr 2, 2020, at 17:32, John McCall <rjmccall at apple.com> wrote:
>>> 
>>> On 1 Apr 2020, at 13:15, Florian Hahn wrote:
>>> I agree that ideally we would not allow mis-matched binary operations to avoid surprises. It looks like the existing vector type does not perform implicit conversion for binary operations with 2 vector operands (unless the there is a type mis-match and the data size matches, then the LHS type is chosen). For binary ops with vector and scalar operands, the scalar operand is converted to the vector element type. So short4 + short4 -> short4, short4 + int -> short4, short4 + int4 -> invalid.
>>> 
>>> Given that precedence, I think we should opt for not providing conversions for binary operators with two matrix operands. For the matrix and scalar versions, we could convert to the element type automatically for convenience. But at that point, the gain is probably quite small and it would be simpler to don’t do conversions in any case. Does that sound reasonable?
>>> 
>>> I think converting scalars is a necessary convenience in C given that e.g. literals are normally of type int, but yes, I think it’s fine to not implicitly convert matrices as long as you have a way to convert them explicitly.
>>> 
>>> Converting them explicitly should be covered by the standard conversion rules, right?
>>> 
>>> Do you want matrices to be implicitly convertible in non-operator contexts? Like, if you assign a float4x4 to an int4x4, should that implicitly convert the elements or be ill-formed?
>>> 
>>> 
>> I think the current formulation allows for implicit conversions for matrixes.
>> 
>> But given the recent changes in the arithmetic context, it might be better to only allow explicit conversions. Having implicit conversions for non-operator contexts and not in operator contexts seems a bit inconsistent. I think we should keep the conversion rules based on the element types, but limit them to explicit conversions. Does that make sense?\
> 
> That makes sense to me.  If it’s actually a usability issue in practice, we can revisit, but I suspect that if we start strict we’ll avoid that.
> 
> 

I’ve updated the Standard Conversion section in https://reviews.llvm.org/D76612 with the following:

  The standard conversions are extended as follows, with the exception that for matrix types, standard conversions are not permissible for assignments.

Not sure if that’s the best way to put it, but hopefully the intention is clear.

Cheers,
Florian
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