[Mesa-dev] GLSL swizzle lowering

[Ian Romanick]

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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]);

This potentially replaces a big pile of instructions with three:

 1. Load the address register.
 2. Do the dot-product.
 3. Re-load the address register.

This means we'd also want to add support to ir_algebraic to convert
dot(v, vec3(0.0, 1.0, 0.0)) to v.y.

The down-side is that it uses constant slots.  Architectures that lack
the ability to do real vector indexing also tend to be starved for both
instructions and constant slots.  R500 may be an exception here, but
R300 and i915 are definitely in this category.  Are there cases where
this optimization could cause a shader to not fit in hardware limits
when it would have otherwise?
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