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]>

3 files changed, 6 insertions, 0 deletions

diff --git a/src/compiler/nir/nir.c b/src/compiler/nir/nir.c
index ef911540f37..ea28fbd1af5 100644
--- a/src/compiler/nir/nir.c
+++ b/src/compiler/nir/nir.c
@@ -1991,6 +1991,8 @@ nir_intrinsic_from_system_value(gl_system_value val)
       return nir_intrinsic_load_base_instance;
    case SYSTEM_VALUE_VERTEX_ID_ZERO_BASE:
       return nir_intrinsic_load_vertex_id_zero_base;
+   case SYSTEM_VALUE_FIRST_VERTEX:
+      return nir_intrinsic_load_first_vertex;
    case SYSTEM_VALUE_BASE_VERTEX:
       return nir_intrinsic_load_base_vertex;
    case SYSTEM_VALUE_INVOCATION_ID:
@@ -2066,6 +2068,8 @@ nir_system_value_from_intrinsic(nir_intrinsic_op intrin)
       return SYSTEM_VALUE_BASE_INSTANCE;
    case nir_intrinsic_load_vertex_id_zero_base:
       return SYSTEM_VALUE_VERTEX_ID_ZERO_BASE;
+   case nir_intrinsic_load_first_vertex:
+      return SYSTEM_VALUE_FIRST_VERTEX;
    case nir_intrinsic_load_base_vertex:
       return SYSTEM_VALUE_BASE_VERTEX;
    case nir_intrinsic_load_invocation_id:
diff --git a/src/compiler/nir/nir_gather_info.c b/src/compiler/nir/nir_gather_info.c
index eaf0ca68b0f..a6a699ab25f 100644
--- a/src/compiler/nir/nir_gather_info.c
+++ b/src/compiler/nir/nir_gather_info.c
@@ -265,6 +265,7 @@ gather_intrinsic_info(nir_intrinsic_instr *instr, nir_shader *shader)
    case nir_intrinsic_load_vertex_id:
    case nir_intrinsic_load_vertex_id_zero_base:
    case nir_intrinsic_load_base_vertex:
+   case nir_intrinsic_load_first_vertex:
    case nir_intrinsic_load_base_instance:
    case nir_intrinsic_load_instance_id:
    case nir_intrinsic_load_sample_id:
diff --git a/src/compiler/nir/nir_intrinsics.py b/src/compiler/nir/nir_intrinsics.py
index 1bc99552cd7..f26aaf35ee3 100644
--- a/src/compiler/nir/nir_intrinsics.py
+++ b/src/compiler/nir/nir_intrinsics.py
@@ -413,6 +413,7 @@ system_value("frag_coord", 4)
 system_value("front_face", 1)
 system_value("vertex_id", 1)
 system_value("vertex_id_zero_base", 1)
+system_value("first_vertex", 1)
 system_value("base_vertex", 1)
 system_value("instance_id", 1)
 system_value("base_instance", 1)