spirv: Implement OpArrayLength for OpenGL

Uses same NIR intrinsic as glsl_to_nir.  Make it an option so it is
easy later to move Vulkan drivers incrementally to use it.

Fixes piglit test spec/arb_gl_spirv/execution/ssbo/unsized-array-length.

Closes: #3691
Fixes: 15e43907 ("iris: Enable ARB_gl_spirv and ARB_spirv_extensions")
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <!8136>

src/compiler/spirv/nir_spirv.h

@@ -69,6 +69,11 @@ struct spirv_to_nir_options {
    /* Create a nir library. */
    bool create_library;
 
+   /* Whether to use nir_intrinsic_deref_buffer_array_length intrinsic instead
+    * of nir_intrinsic_get_ssbo_size to lower OpArrayLength.
+    */
+   bool use_deref_buffer_array_length;
+
    /* Initial value for shader_info::float_controls_execution_mode,
     * indicates hardware requirements rather than shader author intent
     */

src/compiler/spirv/vtn_variables.c

@@ -2449,30 +2449,46 @@ vtn_handle_variables(struct vtn_builder *b, SpvOp opcode,
                   "OpArrayLength must reference the last memeber of the "
                   "structure and that must be an array");
 
-      const uint32_t offset = ptr->type->offsets[field];
-      const uint32_t stride = ptr->type->members[field]->stride;
-
-      if (!ptr->block_index) {
+      if (b->options->use_deref_buffer_array_length) {
          struct vtn_access_chain chain = {
-            .length = 0,
+            .length = 1,
+            .link = {
+               { .mode = vtn_access_mode_literal, .id = field },
+            }
          };
-         ptr = vtn_pointer_dereference(b, ptr, &chain);
-         vtn_assert(ptr->block_index);
-      }
+         struct vtn_pointer *array = vtn_pointer_dereference(b, ptr, &chain);
 
-      nir_ssa_def *buf_size = nir_get_ssbo_size(&b->nb, ptr->block_index);
+         nir_ssa_def *array_length =
+            nir_build_deref_buffer_array_length(&b->nb, 32,
+                                                vtn_pointer_to_ssa(b, array));
 
-      /* array_length = max(buffer_size - offset, 0) / stride */
-      nir_ssa_def *array_length =
-         nir_idiv(&b->nb,
-                  nir_imax(&b->nb,
-                           nir_isub(&b->nb,
-                                    buf_size,
-                                    nir_imm_int(&b->nb, offset)),
-                           nir_imm_int(&b->nb, 0u)),
-                  nir_imm_int(&b->nb, stride));
+         vtn_push_nir_ssa(b, w[2], array_length);
+      } else {
+         const uint32_t offset = ptr->type->offsets[field];
+         const uint32_t stride = ptr->type->members[field]->stride;
+
+         if (!ptr->block_index) {
+            struct vtn_access_chain chain = {
+               .length = 0,
+            };
+            ptr = vtn_pointer_dereference(b, ptr, &chain);
+            vtn_assert(ptr->block_index);
+         }
+
+         nir_ssa_def *buf_size = nir_get_ssbo_size(&b->nb, ptr->block_index);
 
-      vtn_push_nir_ssa(b, w[2], array_length);
+         /* array_length = max(buffer_size - offset, 0) / stride */
+         nir_ssa_def *array_length =
+            nir_idiv(&b->nb,
+                     nir_imax(&b->nb,
+                              nir_isub(&b->nb,
+                                       buf_size,
+                                       nir_imm_int(&b->nb, offset)),
+                              nir_imm_int(&b->nb, 0u)),
+                     nir_imm_int(&b->nb, stride));
+
+         vtn_push_nir_ssa(b, w[2], array_length);
+      }
       break;
    }
 

src/mesa/main/glspirv.c

@@ -244,6 +244,7 @@ _mesa_spirv_to_nir(struct gl_context *ctx,
    const struct spirv_to_nir_options spirv_options = {
       .environment = NIR_SPIRV_OPENGL,
       .frag_coord_is_sysval = ctx->Const.GLSLFragCoordIsSysVal,
+      .use_deref_buffer_array_length = true,
       .caps = ctx->Const.SpirVCapabilities,
       .ubo_addr_format = nir_address_format_32bit_index_offset,
       .ssbo_addr_format = nir_address_format_32bit_index_offset,