[Mesa-dev] [PATCH v3 2/8] compiler: Add SYSTEM_VALUE_FIRST_VERTEX and instrinsics

[Antia Puentes]

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).

Reviewed-by: Neil Roberts <nroberts at igalia.com>
Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/compiler/nir/nir.c             |  4 ++++
 src/compiler/nir/nir_gather_info.c |  1 +
 src/compiler/nir/nir_intrinsics.h  |  1 +
 src/compiler/shader_enums.c        |  1 +
 src/compiler/shader_enums.h        | 14 ++++++++++++++
 5 files changed, 21 insertions(+)

diff --git a/src/compiler/nir/nir.c b/src/compiler/nir/nir.c
index bdd8960403c..e69c2accbbf 100644
--- a/src/compiler/nir/nir.c
+++ b/src/compiler/nir/nir.c
@@ -1919,6 +1919,8 @@ 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:
@@ -1990,6 +1992,8 @@ 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 946939657ec..555ae77b1d3 100644
--- a/src/compiler/nir/nir_gather_info.c
+++ b/src/compiler/nir/nir_gather_info.c
@@ -247,6 +247,7 @@ 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.h b/src/compiler/nir/nir_intrinsics.h
index ede29277876..7d3421f0e30 100644
--- a/src/compiler/nir/nir_intrinsics.h
+++ b/src/compiler/nir/nir_intrinsics.h
@@ -333,6 +333,7 @@ SYSTEM_VALUE(frag_coord, 4, 0, xx, xx, xx)
 SYSTEM_VALUE(front_face, 1, 0, xx, xx, xx)
 SYSTEM_VALUE(vertex_id, 1, 0, xx, xx, xx)
 SYSTEM_VALUE(vertex_id_zero_base, 1, 0, xx, xx, xx)
+SYSTEM_VALUE(first_vertex, 1, 0, xx, xx, xx)
 SYSTEM_VALUE(base_vertex, 1, 0, xx, xx, xx)
 SYSTEM_VALUE(instance_id, 1, 0, xx, xx, xx)
 SYSTEM_VALUE(base_instance, 1, 0, xx, xx, xx)
diff --git a/src/compiler/shader_enums.c b/src/compiler/shader_enums.c
index 2179c475abd..5e123f29f37 100644
--- a/src/compiler/shader_enums.c
+++ b/src/compiler/shader_enums.c
@@ -214,6 +214,7 @@ 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),
diff --git a/src/compiler/shader_enums.h b/src/compiler/shader_enums.h
index ffe551ab20f..9f71194c146 100644
--- a/src/compiler/shader_enums.h
+++ b/src/compiler/shader_enums.h
@@ -472,6 +472,20 @@ 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
     *
-- 
2.14.1