nir: Implement ir_unop_get_buffer_size

This is how backends provide the buffer size required to compute
the size of unsized arrays in the previous patch

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>

src/glsl/nir/glsl_to_nir.cpp

@@ -1322,6 +1322,16 @@ nir_visitor::visit(ir_expression *ir)
          unreachable("not reached");
       }
       break;
+   case ir_unop_get_buffer_size: {
+      nir_intrinsic_instr *load = nir_intrinsic_instr_create(
+         this->shader,
+         nir_intrinsic_get_buffer_size);
+      load->num_components = ir->type->vector_elements;
+      load->src[0] = evaluate_rvalue(ir->operands[0]);
+      add_instr(&load->instr, ir->type->vector_elements);
+      return;
+   }
+
    case ir_binop_add:
    case ir_binop_sub:
    case ir_binop_mul:

src/glsl/nir/nir_intrinsics.h

@@ -61,6 +61,13 @@ INTRINSIC(interp_var_at_sample, 1, ARR(1), true, 0, 1, 0,
 INTRINSIC(interp_var_at_offset, 1, ARR(2), true, 0, 1, 0,
           NIR_INTRINSIC_CAN_ELIMINATE | NIR_INTRINSIC_CAN_REORDER)
 
+/*
+ * Ask the driver for the size of a given buffer. It takes the buffer index
+ * as source.
+ */
+INTRINSIC(get_buffer_size, 1, ARR(1), true, 1, 0, 0,
+          NIR_INTRINSIC_CAN_ELIMINATE | NIR_INTRINSIC_CAN_REORDER)
+
 /*
  * a barrier is an intrinsic with no inputs/outputs but which can't be moved
  * around/optimized in general