Re: recent troubles with float vectors & bitwise ops

[tbp <tbptbp at gmail dot com>]

Paolo Bonzini wrote:
> I'm not sure that it is *so* useful for a user to have access to it,
> except for specialized cases:
As there's other means, it may not be that useful but for sure it's 
extremely convenient.

> 2) selection operations on vectors, kind of (v1 <= v2 ? v3 : v4).  These
> can be written for example like this:
>
>      cmpleps xmm1, xmm2   ; xmm1 = xmm1 <= xmm2 ? all-ones : 0
>      andnps  xmm4, xmm1   ; xmm4 = xmm1 <= xmm2 ? 0 : xmm4
>      andps   xmm1, xmm3   ; xmm1 = xmm1 <= xmm2 ? xmm3 : 0
>      orps    xmm1, xmm4   ; xmm1 = xmm1 <= xmm2 ? xmm3 : xmm4
I suppose you'll find such variant of a conditional move pattern in 
every piece of SSE code.

But you can't condense bitwise vs float usage to a few patterns because 
when writing SSE, the efficiency of those operations is taken for granted.


> If we have a good extension for vector arithmetic, we should aim at
> improving it consistently rather than extending it in unpredictable
> ways.  For example, another useful extension would be the ability to
> access vectors by item using x[n] (at least with constant expressions).
Yes, yes and yes.