size value of vector_size attribute

[Xi Ruoyao]

On 2019-12-16 08:16 -0500, Chris Elrod wrote:
> I'm not the asked, but I would strongly prefer m256 if code could be
> generated masking the unused lane for safe loads/stores, at least on
> architectures where this is efficient (eg, Skylake-X).
> This automatic masking would make writing SIMD code easier when you
> don't
> have powers of 2, by saving the effort of passing the bitmask to each
> operation (which is at least an option with intrimin.h, not sure
> about
> GCC's built-in).

I perfer m256 too.  I'm already using vector_size(4*sizeof(double)) for
some calculation in 3D euclid space (only 3 elements are really used).

> However, if the asker doesn't want this for SIMD code, but wants a
> convenient vector to index for scalar code, I'd recommend defining
> your own
> class. Indexing SIMD vectors is inefficient, and it may interfere
> with
> optimizations like SROA. But I could be wrong; my experience is
> mostly with
> Julia which uses LLVM. GCC may do better.

I want SIMD code and I don't need much indexing.  But just curious, why
indexing SIMD vectors is inefficient?
-- 
Xi Ruoyao <xry111@mengyan1223.wang>
School of Aerospace Science and Technology, Xidian University