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, 14 insertions, 0 deletions

diff --git a/src/compiler/shader_enums.h b/src/compiler/shader_enums.h
index 2aedbb9b3fa..8a277a14f21 100644
--- a/src/compiler/shader_enums.h
+++ b/src/compiler/shader_enums.h
@@ -504,6 +504,20 @@ typedef enum
    SYSTEM_VALUE_BASE_VERTEX,
 
    /**
+    * Depending on the type of the draw call (indexed or non-indexed),
+    * is the value of \c basevertex passed to \c glDrawElementsBaseVertex and
+    * similar, or is the value of \c first passed to \c glDrawArrays and
+    * similar.
+    *
+    * \note
+    * It can be used to calculate the \c SYSTEM_VALUE_VERTEX_ID as
+    * \c SYSTEM_VALUE_VERTEX_ID_ZERO_BASE plus \c SYSTEM_VALUE_FIRST_VERTEX.
+    *
+    * \sa SYSTEM_VALUE_VERTEX_ID_ZERO_BASE, SYSTEM_VALUE_VERTEX_ID
+    */
+   SYSTEM_VALUE_FIRST_VERTEX,
+
+   /**
     * Value of \c baseinstance passed to instanced draw entry points
     *
     * \sa SYSTEM_VALUE_INSTANCE_ID