nak: Add support for gl_PrimitiveShadingRateEXT

This aliases gl_ViewportIndex in the NVIDIA attribute space.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31585>

src/nouveau/compiler/nak.h

@@ -183,9 +183,12 @@ struct nak_shader_info {
    struct {
       bool writes_layer;
       bool writes_point_size;
+      bool writes_vprs_table_index;
       uint8_t clip_enable;
       uint8_t cull_enable;
 
+      uint8_t _pad[3];
+
       struct nak_xfb_info xfb;
    } vtg;
 

src/nouveau/compiler/nak/api.rs

@@ -290,6 +290,8 @@ impl ShaderBin {
                 ShaderIoInfo::Vtg(io) => nak_shader_info__bindgen_ty_2 {
                     writes_layer: io.attr_written(NAK_ATTR_RT_ARRAY_INDEX),
                     writes_point_size: io.attr_written(NAK_ATTR_POINT_SIZE),
+                    writes_vprs_table_index: io
+                        .attr_written(NAK_ATTR_VPRS_TABLE_INDEX),
                     clip_enable: io.clip_enable.try_into().unwrap(),
                     cull_enable: io.cull_enable.try_into().unwrap(),
                     xfb: if let Some(xfb) = &io.xfb {
@@ -297,6 +299,7 @@ impl ShaderBin {
                     } else {
                         unsafe { std::mem::zeroed() }
                     },
+                    _pad: Default::default(),
                 },
                 _ => unsafe { std::mem::zeroed() },
             },

src/nouveau/compiler/nak_nir.c

@@ -351,6 +351,9 @@ nak_varying_attr_addr(const struct nak_compiler *nak, gl_varying_slot slot)
       case VARYING_SLOT_POS:              return NAK_ATTR_POSITION;
       case VARYING_SLOT_CLIP_DIST0:       return NAK_ATTR_CLIP_CULL_DIST_0;
       case VARYING_SLOT_CLIP_DIST1:       return NAK_ATTR_CLIP_CULL_DIST_4;
+      case VARYING_SLOT_PRIMITIVE_SHADING_RATE:
+         return nak->sm >= 86 ? NAK_ATTR_VPRS_TABLE_INDEX
+                              : NAK_ATTR_VIEWPORT_INDEX;
       default: unreachable("Invalid varying slot");
       }
    }

src/nouveau/compiler/nak_private.h

@@ -37,6 +37,9 @@ enum ENUM_PACKED nak_attr {
    /* Patch attributes */
    NAK_ATTR_PATCH_START       = 0x020,
 
+   /* System values B? */
+   NAK_ATTR_VPRS_TABLE_INDEX  = 0x5c,
+
    /* System values B */
    NAK_ATTR_PRIMITIVE_ID      = 0x060,
    NAK_ATTR_RT_ARRAY_INDEX    = 0x064,