[Mesa-dev] [PATCH 3/6] glsl: Generalize compute_packing_order for varying structs.

[Paul Berry]

On 23 January 2013 07:38, Ian Romanick <idr at freedesktop.org> wrote:

> On 01/21/2013 05:16 PM, Paul Berry wrote:
>
>> This patch paves the way for allowing varying structs by generalizing
>> varying_matches::compute_**packing_order to handle any type of varying.
>> Previously, we packed in the order (vec4, vec2, float, vec3), with
>> matrices being packed according to the size of their columns.  Now, we
>> pack everything according to its number of components mod 4, in the
>> order (0, 2, 1, 3).
>>
>> There is no behavioural change for vectors.  Matrices are now packed
>> slightly more sensibly (e.g. a mat2 is now packed with vec4's, which
>> guarantees that it will occupy a single varying slot, whereas it
>> previously was packed with vec2's, which meant that it might get split
>> across two adjacent slots).
>> ---
>>   src/glsl/link_varyings.cpp | 5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/src/glsl/link_varyings.cpp b/src/glsl/link_varyings.cpp
>> index 5a3240b..d8f501c 100644
>> --- a/src/glsl/link_varyings.cpp
>> +++ b/src/glsl/link_varyings.cpp
>> @@ -804,16 +804,15 @@ varying_matches::compute_**packing_order(ir_variable
>> *var)
>>   {
>>      const glsl_type *element_type = var->type;
>>
>> -   /* FINISHME: Support for "varying" records in GLSL 1.50. */
>>      while (element_type->base_type == GLSL_TYPE_ARRAY) {
>>         element_type = element_type->fields.array;
>>      }
>>
>> -   switch (element_type->vector_**elements) {
>> +   switch (element_type->component_**slots() % 4) {
>>
>
> Will this produce the same result for all matrix types as well?  I think a
> mat2x3 would be 3 in the old scheme, but 2 in the new scheme.  Right?


Yes, a number of matrix types will get new packing orders.  Here's the
complete list:

- mat2x2 gets assigned PACKING_ORDER_VEC4 instead of PACKING_ORDER_VEC2.
As I argue above, this is slightly better, because it guarantees that the
matrix occupies a single varying slot.

- mat2x3 gets assigned PACKING_ORDER_VEC2 instead of PACKING_ORDER_VEC3.
This is kind of a wash.  Previously, mat2x3 had a 25% chance of having
neither of its columns double parked, a 50% chance of having exactly one of
its columns double parked, and a 25% chance of having both of its columns
double parked.  Now it always has exactly one of its columns double parked.

- mat3x3 gets assigned PACKING_ORDER_SCALAR instead of PACKING_ORDER_VEC3.
This doesn't affect much, since in both cases there is no guarantee of how
the matrix will be aligned.

- mat4x2 gets assigned PACKING_ORDER_VEC4 instead of PACKING_ORDER_VEC2.  I
think this is slightly better for the same reason as in mat2x2.

- mat4x3 gets assigned PACKING_ORDER_VEC4 instead of PACKING_ORDER_VEC3.  I
think this is slightly better for the same reason as in mat2x2.

Do you think I should include these details in the commit message?


>
>
>       case 1: return PACKING_ORDER_SCALAR;
>>      case 2: return PACKING_ORDER_VEC2;
>>      case 3: return PACKING_ORDER_VEC3;
>> -   case 4: return PACKING_ORDER_VEC4;
>> +   case 0: return PACKING_ORDER_VEC4;
>>      default:
>>         assert(!"Unexpected value of vector_elements");
>>         return PACKING_ORDER_VEC4;
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freedesktop.org/archives/mesa-dev/attachments/20130123/0d997421/attachment.html>