panvk: Use LD_VAR_BUF[_IMM] when possible

If we determine that the amount of varyings will fit within the 8-bit
offset of LD_VAR_BUF[_IMM], instruct the compiler to use it for varyings
and skip setting up Attribute Descriptors.

This should save a bit of memory and overhead in reading varyings.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32969>

src/panfrost/compiler/bifrost_compile.c

@@ -594,9 +594,7 @@ bi_emit_load_vary(bi_builder *b, nir_intrinsic_instr *instr)
    bool immediate = bi_is_imm_var_desc_handle(b, instr, &imm_index);
    unsigned base = nir_intrinsic_base(instr);
 
-   /* LD_VAR_BUF[_IMM] takes an 8-bit offset, limiting its use to 64 or less
+   /* Only use LD_VAR_BUF[_IMM] if explicitly told by the driver
-    * varying components, assuming F32.
-    * Therefore, only use LD_VAR_BUF[_IMM] if explicitly told by the driver
     * through a compiler input value, falling back to LD_VAR[_IMM] +
     * Attribute Descriptors otherwise. */
    bool use_ld_var_buf =

src/panfrost/vulkan/csf/panvk_vX_cmd_draw.c

@@ -239,7 +239,10 @@ prepare_fs_driver_set(struct panvk_cmd_buffer *cmdbuf)
    const struct panvk_shader *fs = cmdbuf->state.gfx.fs.shader;
    const struct panvk_descriptor_state *desc_state =
       &cmdbuf->state.gfx.desc_state;
-   uint32_t num_varying_attr_descs = fs->desc_info.max_varying_loads;
+   /* If the shader is using LD_VAR_BUF[_IMM], we do not have to set up
+    * Attribute Descriptors for varying loads. */
+   uint32_t num_varying_attr_descs =
+      panvk_use_ld_var_buf(fs) ? 0 : fs->desc_info.max_varying_loads;
    uint32_t desc_count =
       fs->desc_info.dyn_bufs.count + num_varying_attr_descs + 1;
    struct panfrost_ptr driver_set = panvk_cmd_alloc_dev_mem(
@@ -249,6 +252,7 @@ prepare_fs_driver_set(struct panvk_cmd_buffer *cmdbuf)
    if (desc_count && !driver_set.gpu)
       return VK_ERROR_OUT_OF_DEVICE_MEMORY;
 
+   if (num_varying_attr_descs > 0)
       emit_varying_descs(cmdbuf, (struct mali_attribute_packed *)(&descs[0]));
 
    /* Dummy sampler always comes right after the varyings. */

src/panfrost/vulkan/panvk_shader.h

@@ -353,6 +353,18 @@ struct panvk_internal_shader {
 #endif
 };
 
+#if PAN_ARCH >= 9
+static inline bool
+panvk_use_ld_var_buf(const struct panvk_shader *shader)
+{
+   /* LD_VAR_BUF[_IMM] takes an 8-bit offset, limiting its use to 16 or less
+    * varyings, assuming highp vec4. */
+   if (shader->desc_info.max_varying_loads <= 16)
+      return true;
+   return false;
+}
+#endif
+
 VK_DEFINE_NONDISP_HANDLE_CASTS(panvk_internal_shader, vk.base, VkShaderEXT,
                                VK_OBJECT_TYPE_SHADER_EXT)
 

src/panfrost/vulkan/panvk_vX_nir_lower_descriptors.c

@@ -1258,7 +1258,13 @@ panvk_per_arch(nir_lower_descriptors)(
       goto out;
 
 #if PAN_ARCH >= 9
-   ctx.desc_info.num_varying_attr_descs = shader->desc_info.max_varying_loads;
+   ctx.desc_info.num_varying_attr_descs = 0;
+   /* We require Attribute Descriptors if we cannot use LD_VAR_BUF[_IMM] for
+    * varyings. */
+   if (shader->info.stage == MESA_SHADER_FRAGMENT &&
+       !panvk_use_ld_var_buf(shader))
+      ctx.desc_info.num_varying_attr_descs =
+         shader->desc_info.max_varying_loads;
 #endif
    create_copy_table(nir, &ctx);
    upload_shader_desc_info(dev, shader, &ctx.desc_info);

src/panfrost/vulkan/panvk_vX_shader.c

@@ -1053,10 +1053,6 @@ panvk_compile_shader(struct panvk_device *dev,
       .gpu_id = phys_dev->kmod.props.gpu_prod_id,
       .no_ubo_to_push = true,
       .view_mask = (state && state->rp) ? state->rp->view_mask : 0,
-#if PAN_ARCH >= 9
-      /* LD_VAR_BUF does not support maxVertexOutputComponents (128) */
-      .valhall.use_ld_var_buf = false,
-#endif
    };
 
    if (info->stage == MESA_SHADER_FRAGMENT && state != NULL &&
@@ -1066,6 +1062,12 @@ panvk_compile_shader(struct panvk_device *dev,
    panvk_lower_nir(dev, nir, info->set_layout_count, info->set_layouts,
                    info->robustness, noperspective_varyings, &inputs, shader);
 
+#if PAN_ARCH >= 9
+   if (info->stage == MESA_SHADER_FRAGMENT)
+      /* Use LD_VAR_BUF[_IMM] for varyings if possible. */
+      inputs.valhall.use_ld_var_buf = panvk_use_ld_var_buf(shader);
+#endif
+
    result = panvk_compile_nir(dev, nir, info->flags, &inputs, shader);
 
    /* We need to update info.push.count because it's used to initialize the