compiler: Add SYSTEM_VALUE_FIRST_VERTEX and instrinsics

This VS system value will contain the value passed as <basevertex> for
indexed draw calls or the value passed as <first> for non-indexed draw
calls. It can be used to calculate the gl_VertexID as
SYSTEM_VALUE_VERTEX_ID_ZERO_BASE plus SYSTEM_VALUE_FIRST_VERTEX.

From the OpenGL 4.6 spec, 10.4 "Drawing Commands Using Vertex Arrays":

-  Page 352:
"The index of any element transferred to the GL by DrawArraysOneInstance
is referred to as its vertex ID, and may be read by a vertex shader as
gl_VertexID.  The vertex ID of the ith element transferred is first +
i."

- Page 355:
"The index of any element transferred to the GL by
DrawElementsOneInstance is referred to as its vertex ID, and may be read
by a vertex shader as gl_VertexID.  The vertex ID of the ith element
transferred is the sum of basevertex and the value stored in the
currently bound element array buffer at offset indices + i."

Currently the gl_VertexID calculation uses SYSTEM_VALUE_BASE_VERTEX but
this will have to change when the value of gl_BaseVertex is
fixed. Currently its value is broken for non-indexed draw calls because
it must be zero but we are setting it to <first>.

v2: use SYSTEM_VALUE_FIRST_VERTEX as name for the value, instead of
SYSTEM_VALUE_BASE_VERTEX_ID (Kenneth).

v3 (idr): Rebase on Rob Clark converting nir_intrinsics.h to be
generated.  Reformat commit message to 72 columns.

Reviewed-by: Neil Roberts <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/compiler/shader_enums.c b/src/compiler/shader_enums.c
index d0ff11b41e2..ebee076b43c 100644
--- a/src/compiler/shader_enums.c
+++ b/src/compiler/shader_enums.c
@@ -216,6 +216,7 @@ gl_system_value_name(gl_system_value sysval)
      ENUM(SYSTEM_VALUE_INSTANCE_ID),
      ENUM(SYSTEM_VALUE_INSTANCE_INDEX),
      ENUM(SYSTEM_VALUE_VERTEX_ID_ZERO_BASE),
+     ENUM(SYSTEM_VALUE_FIRST_VERTEX),
      ENUM(SYSTEM_VALUE_BASE_VERTEX),
      ENUM(SYSTEM_VALUE_BASE_INSTANCE),
      ENUM(SYSTEM_VALUE_DRAW_ID),