[Mesa-dev] [PATCH 02/10] spirv: Make VertexIndex and VertexId both non-zero-based

Alejandro Piñeiro apinheiro at igalia.com
Thu Aug 9 13:43:44 UTC 2018

From: Neil Roberts <nroberts at igalia.com>

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. I can’t find
any mention of what VertexId is supposed to be.

GL_ARB_spirv removes VertexIndex and defines VertexId to be the same
as gl_VertexId (which is alo 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
 src/compiler/spirv/vtn_variables.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/src/compiler/spirv/vtn_variables.c b/src/compiler/spirv/vtn_variables.c
index 8dab86abd74..b92bda59828 100644
--- a/src/compiler/spirv/vtn_variables.c
+++ b/src/compiler/spirv/vtn_variables.c
@@ -1011,15 +1011,16 @@ vtn_get_builtin_location(struct vtn_builder *b,
    case SpvBuiltInCullDistance:
       *location = VARYING_SLOT_CULL_DIST0;
-   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:
+      /* For whatever reason, both of these are defined in the SPIR-V spec.
+       * The Vulkan spec defines VertexIndex to be non-zero-based and doesn’t
+       * mention VertexId. The ARB_gl_spirv spec defines VertexId to be the
+       * same as gl_VertexID, which is non-zero-based, and removes
+       * VertexIndex. Assuming there is no use for VertexId in Vulkan yet, we
+       * can just make them both be the same.
+      *location = SYSTEM_VALUE_VERTEX_ID;
       set_mode_system_value(b, mode);
    case SpvBuiltInInstanceIndex:

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