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Marek Ol?ák wrote:
> On Tue, Aug 31, 2010 at 2:14 AM, Ian Romanick <i...@freedesktop.org
> <mailto:i...@freedesktop.org>> wrote:
>
> While I was trying to get one of the Humus demos working today, it
> occurred to me that we can possibly do better than
> ir_vec_index_to_cond_assign to lower variable indexing of vectors. In
> addition to using conditional assignment, we can also use a dot-product
> to pick a single element out of a vector. The variable index operation
> becomes:
>
> const vec4 gl_vec_selector[4] =
> vec4[4](vec4(1.0, 0.0, 0.0, 0.0),
> vec4(0.0, 1.0, 0.0, 0.0),
> vec4(0.0, 0.0, 1.0, 0.0),
> vec4(0.0, 0.0, 0.0, 1.0));
>
> ...
>
> float f = dot(v, gl_vec_selector[i]);
[snip]
> Neither r300 nor r500 supports the ARL opcode in fragment shaders (it's
Meh. I always forget about that asymmetry.
> a D3D10 feature), which kind of makes this optimization a no-go. I
> suggest using SEQ instead:
>
> bvec4 selector = equal(vec4(i), vec4(0,1,2,3));
> float f = dot(v, vec4(selector));
>
> which should end up being just SEQ followed by DP4.
SEQ isn't part of the ARB_fragment_program / ARB_vertex_program
instruction set either. Does R300 support that? I won't be surprised
if i915 doesn't. Of course, it doesn't support the ARL-based
optimization either.
SEQ is itself lowered to a sequence of instructions:
SGE t0, i.xxxx, { 0, 1, 2, 3};
SGE t1, i.xxxx, {-0, -1, -2, -3};
MUL selector, t0, t1;
DP4 result, v, selector;
So, that probably still would be better than the mess that gets
generated today.
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