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spirv: Make VertexIndex and VertexId both non-zero-based

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GLSL has gl_VertexID which is supposed to be non-zero-based.

SPIR-V has both VertexIndex and VertexId builtins whose meanings are
defined by the APIs.

Vulkan defines VertexIndex as being non-zero-based. In Vulkan VertexId
and InstanceId have no meaning and are pretty much just reserved for
OpenGL at this point.

GL_ARB_spirv removes VertexIndex and defines VertexId to be the same
as gl_VertexId (which is also non-zero-based).

Previously in Mesa it was treating VertexIndex as non-zero-based and
VertexId as zero-based, so it was breaking for GL. This behaviour was
apparently based on Khronos bug 14255. However that bug doesn’t seem
to have made a final decision for VertexId.

Assuming there really is no other definition for VertexId for Vulkan
it seems better to just make them both have the same value.

v2: update comment and commit descriptions, based on Jason Ekstrand
    explanation of the meaning/rationale behind all those builtins
    (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
This commit is contained in:
Neil Roberts 2018-04-19 17:17:41 +02:00 • committed by Alejandro Piñeiro
parent 624c00f1a6
commit be6f472b23
1 changed files with 7 additions and 7 deletions
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14
src/compiler/spirv/vtn_variables.c
View file

@ -1011,15 +1011,15 @@ vtn_get_builtin_location(struct vtn_builder *b,
case SpvBuiltInCullDistance:
*location = VARYING_SLOT_CULL_DIST0;
break;
case SpvBuiltInVertexIndex:
*location = SYSTEM_VALUE_VERTEX_ID;
set_mode_system_value(b, mode);
break;
case SpvBuiltInVertexId:
/* Vulkan defines VertexID to be zero-based and reserves the new
* builtin keyword VertexIndex to indicate the non-zero-based value.
case SpvBuiltInVertexIndex:
/* The Vulkan spec defines VertexIndex to be non-zero-based and doesn't
* allow VertexId. The ARB_gl_spirv spec defines VertexId to be the
* same as gl_VertexID, which is non-zero-based, and removes
* VertexIndex. Since they're both defined to be non-zero-based, we use
* SYSTEM_VALUE_VERTEX_ID for both.
*/
*location = SYSTEM_VALUE_VERTEX_ID_ZERO_BASE;
*location = SYSTEM_VALUE_VERTEX_ID;
set_mode_system_value(b, mode);
break;
case SpvBuiltInInstanceIndex: