[Mesa-dev] [PATCH 4/5] glsl: Vectorize multiple scalar assignments

[Matt Turner]

On Thu, Jan 9, 2014 at 11:28 AM, Ian Romanick <idr at freedesktop.org> wrote:
> On 01/08/2014 12:43 PM, Matt Turner wrote:
>> +/**
>> + * \file opt_vectorize.cpp
>> + *
>> + * Combines scalar assignments of the same expression (modulo swizzle) to
>> + * multiple channels of the same variable into a single vectorized expression
>> + * and assignment.
>> + *
>> + * Many generated shaders contain scalarized code. That is, they contain
>> + *
>> + * r1.x = log2(v0.x);
>> + * r1.y = log2(v0.y);
>> + * r1.z = log2(v0.z);
>> + *
>> + * rather than
>> + *
>> + * r1.xyz = log2(v0.xyz);
>> + *
>> + * We look for consecutive assignments of the same expression (modulo swizzle)
>> + * to each channel of the same variable.
>> + *
>> + * For instance, we want to convert these three scalar operations
>> + *
>> + * (assign (x) (var_ref r1) (expression float log2 (swiz x (var_ref v0))))
>> + * (assign (y) (var_ref r1) (expression float log2 (swiz y (var_ref v0))))
>> + * (assign (z) (var_ref r1) (expression float log2 (swiz z (var_ref v0))))
>> + *
>> + * into a single vector operation
>> + *
>> + * (assign (xyz) (var_ref r1) (expression vec3 log2 (swiz xyz (var_ref v0))))
>
> I think it's worth adding a note that this pass only attempts to combine
> assignments that are sequential.

That comment block already says that:

+ * We look for consecutive assignments of the same expression (modulo swizzle)
+ * to each channel of the same variable.

I'll change the first comment to use the word consecutive.

> The above example gets fully
> vectorized, but this sequence would not:
>
> (assign (x) (var_ref r1) (expression float log2 (swiz x (var_ref v0))))
> (assign (x) (var_ref r2) (expression float log2 (swiz y (var_ref v0))))
> (assign (y) (var_ref r1) (expression float log2 (swiz z (var_ref v0))))
> (assign (y) (var_ref r2) (expression float log2 (swiz w (var_ref v0))))
>
> I think this will also break on code like
>
> (assign (x) (var_ref r1) (expression float log2 (swiz w (var_ref r1))))
> (assign (y) (var_ref r1) (expression float log2 (swiz z (var_ref r1))))
> # r1.xy have different values now.
> (assign (z) (var_ref r1) (expression float log2 (swiz y (var_ref r1))))
> (assign (w) (var_ref r1) (expression float log2 (swiz x (var_ref r1))))
>
> Maybe just skip assignments where the LHS also appears in the RHS for
> now?  Or does the check write_mask_matches_swizzle take care of this?

It won't break because the code rejects expressions that contain
swizzles that don't match the LHS's write mask. See the call to
write_mask_matches_swizzle().

The good thing about this is that we can combine expressions that use
the LHS, like

(assign (x) (var_ref r1) (expression float log2 (swiz x (var_ref r1))))
(assign (y) (var_ref r1) (expression float log2 (swiz y (var_ref r1))))