[Mesa-dev] [PATCH 7/8] gallium: remove TGSI opcode SCS

[Jose Fonseca]

On 20/08/17 01:49, Marek Olšák wrote:
> From: Marek Olšák <marek.olsak at amd.com>
> 
> use COS+SIN instead.

I don't know if any existing gallium driver leverages that, but it's a 
basic trigonometric principle that one can easily extract the sin from 
cos or vice-versa.  It requires some extra care for getting the right 
sign.  But the fact is that it should be considerably cheaper to comput 
both simultaneously than indepdently.

Unfortunately GLSL/SPIR-V doesn't allow to express that.  D3D9/D3D11 and 
Metal all do.  And from what I've seen from D3D9/D3D11 apps, 99% of the 
times the shader wants both SIN/COS at the same time.

If we want one opcode to rule them all, then a combined SIN+COS seems a 
better choice IMO.  On SM4 the sincos has two outputs: 
https://msdn.microsoft.com/en-us/library/windows/desktop/hh447234.aspx 
but they are both optional to use.  I don't know if there's a precedent 
for that.  I recall we had similar discussions about UMUL/UMUL_HI, and I 
suspect we chose not to go that route.


Don't GPUs allow to express the computation of both sin/cos with a 
single opcode?  If nothing else there would be a non-negligible impact 
of leveraging this in llvmpipe at some point.  On the other hand, is 
possible that LLVM common-subexpression elimination optimization passes 
already do that, so we gain nothing.


In short, not big deal either way, but I think it's worth give it a 2nd 
thought here.

Jose