Re: Missing elements in VECTOR_CST

[Mark Mitchell <mark at codesourcery dot com>]

Andrew Pinski wrote:
> The documention on VECTOR_CST is not clear if we can have missing
> elements in that the remaining elements are zero.  Right we produce such
> VECTOR_CST for things like:
> #define vector __attribute__((vector_size(16) ))
> vector int a = {1, 2};
>
> But is that valid?  We currently produce a VECTOR_CST with just two
> elements instead of 4.  Should we always have the same number of
> elements in a VECTOR_CST as there are elements in the vector type?

I think it is reasonable for front-ends to elide initializers and to 
follow the usual C semantics that elided initializers are (a) zero, if 
the constant is appearing as an initializer for static storage, or (b) 
unspecified, "random" values elsewhere.

Requiring explicit zeros is just a way to take up memory.  We clearly 
wouldn't want to do it for big arrays, and I think we might as well 
treat vectors the same way, since we already need logic to handle the 
implicit zeros in the back end for arrays and structures.

The counter-argument is that front ends should be explicit.  However, 
explicit doesn't need to mean verbose; as long as we specify the 
semantics I give above, eliding the elements is still perfectly clear.

> This is why PR 29091 is failing currently.  output_constant assumes
> VECTOR_CST have the correct number of elements but the C front-end via
> digest_init creates a VECTOR_CST with only 2 elements.

Thus, I think that output_constant should be changed to add the 
additional zeros.

--
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713