asahi: rework VBO lower for divisor=0

silly special case that we should handle.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29179>

src/asahi/lib/agx_linker.h

@@ -47,7 +47,7 @@ struct agx_velem_key {
    uint32_t divisor;
    uint16_t stride;
    uint8_t format;
-   uint8_t pad;
+   bool instanced;
 };
 
 struct agx_vs_prolog_key {

src/asahi/lib/agx_nir_lower_vbo.c

@@ -157,8 +157,12 @@ pass(struct nir_builder *b, nir_intrinsic_instr *intr, void *data)
     * the divisor for per-instance data. Divisor=0 specifies per-vertex data.
     */
    nir_def *el;
-   if (attrib.divisor) {
-      el = nir_udiv_imm(b, nir_load_instance_id(b), attrib.divisor);
+   if (attrib.instanced) {
+      if (attrib.divisor > 0)
+         el = nir_udiv_imm(b, nir_load_instance_id(b), attrib.divisor);
+      else
+         el = nir_imm_int(b, 0);
+
       el = nir_iadd(b, el, nir_load_base_instance(b));
 
       BITSET_SET(b->shader->info.system_values_read,

src/asahi/lib/agx_nir_lower_vbo.h

@@ -21,13 +21,16 @@ extern "C" {
  * be small so it can be embedded into a shader key.
  */
 struct agx_attribute {
+   /* If instanced, Zero means all get the same value (Vulkan semantics). */
    uint32_t divisor;
    uint32_t stride;
    uint16_t src_offset;
-   uint8_t buf;
 
    /* pipe_format, all vertex formats should be <= 255 */
    uint8_t format;
+
+   unsigned buf   : 7;
+   bool instanced : 1;
 };
 
 bool agx_nir_lower_vbo(nir_shader *shader, struct agx_attribute *attribs);

src/asahi/lib/agx_nir_prolog_epilog.c

@@ -65,6 +65,7 @@ lower_vbo(nir_shader *s, const struct agx_velem_key *key)
          .divisor = key[i].divisor,
          .stride = key[i].stride,
          .format = key[i].format,
+         .instanced = key[i].instanced,
       };
    }
 

src/gallium/drivers/asahi/agx_state.c

@@ -1447,6 +1447,7 @@ agx_create_vertex_elements(struct pipe_context *ctx, unsigned count,
          .stride = ve.src_stride,
          .format = ve.src_format,
          .divisor = ve.instance_divisor,
+         .instanced = ve.instance_divisor > 0,
       };
    }