agx: plumb vertex_id_zero_base

to accelerate SW VS.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Mary Guillemard <mary.guillemard@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34661>

src/asahi/compiler/README.md

@@ -15,9 +15,12 @@ The following section describes the ABI used by non-monolithic programs.
 Registers have the following layout at the beginning of the vertex shader
 (written by the vertex prolog):
 
-* `r0-r4` and `r7` undefined. This avoids preloading into the nesting counter or
+* `r0-r3` and `r7` undefined. This avoids preloading into the nesting counter or
   having unaligned values. The prolog is free to use these registers as
   temporaries.
+* `r4` is the zero-based vertex ID if the vertex shader is running as a hardware
+  compute shader, useful to avoid a redundant special register read in the main
+  shader. Undefined in hardware vertex shaders.
 * `r5-r6` retain their usual meanings, even if the vertex shader is running as a
   hardware compute shader. This allows software index fetch code to run in the
   prolog without contaminating the main shader key.

src/asahi/compiler/agx_compile.c

@@ -99,6 +99,13 @@ agx_tess_coord_y(agx_builder *b)
    return agx_cached_preload(b->shader, 6, AGX_SIZE_32);
 }
 
+static agx_index
+agx_vertex_id_zero_base(agx_builder *b)
+{
+   return agx_cached_preload(b->shader, AGX_ABI_VIN_VERTEX_ID_ZERO_BASE,
+                             AGX_SIZE_32);
+}
+
 static agx_index
 agx_vertex_id(agx_builder *b)
 {
@@ -1417,6 +1424,10 @@ agx_emit_intrinsic(agx_builder *b, nir_intrinsic_instr *instr)
                          agx_get_sr_coverage(b, 32, AGX_SR_IS_ACTIVE_THREAD),
                          agx_zero(), AGX_ICOND_UEQ, false);
 
+   case nir_intrinsic_load_vertex_id_zero_base:
+      assert(stage == MESA_SHADER_COMPUTE && "only for SW VS");
+      return agx_mov_to(b, dst, agx_abs(agx_vertex_id_zero_base(b)));
+
    case nir_intrinsic_load_vertex_id:
       /* We don't assert the HW stage since we use this same ABI with SW VS */
       return agx_mov_to(b, dst, agx_abs(agx_vertex_id(b)));

src/asahi/lib/agx_abi.h

@@ -8,9 +8,10 @@
 
 /* See compiler/README.md for the ABI */
 
-#define AGX_ABI_VIN_ATTRIB(i)   (2 * (8 + i))
-#define AGX_ABI_VIN_VERTEX_ID   (2 * 5)
-#define AGX_ABI_VIN_INSTANCE_ID (2 * 6)
+#define AGX_ABI_VIN_ATTRIB(i)           (2 * (8 + i))
+#define AGX_ABI_VIN_VERTEX_ID_ZERO_BASE (2 * 4)
+#define AGX_ABI_VIN_VERTEX_ID           (2 * 5)
+#define AGX_ABI_VIN_INSTANCE_ID         (2 * 6)
 
 #define AGX_ABI_FIN_SAMPLE_MASK (2)
 

src/asahi/lib/agx_nir_prolog_epilog.c

@@ -193,6 +193,11 @@ agx_nir_vs_prolog(nir_builder *b, const void *key_)
       nir_export_agx(b, nir_channel(b, vec, c), .base = AGX_ABI_VIN_ATTRIB(i));
    }
 
+   if (!key->hw) {
+      nir_export_agx(b, nir_channel(b, nir_load_global_invocation_id(b, 32), 0),
+                     .base = AGX_ABI_VIN_VERTEX_ID_ZERO_BASE);
+   }
+
    nir_export_agx(b, nir_load_vertex_id(b), .base = AGX_ABI_VIN_VERTEX_ID);
    nir_export_agx(b, nir_load_instance_id(b), .base = AGX_ABI_VIN_INSTANCE_ID);