nir/i965: use two slots from inputs_read for dvec3/dvec4 vertex input attributes

So far, input_reads was a bitmap tracking which vertex input locations
were being used.

In OpenGL, an attribute bigger than a vec4 (like a dvec3 or dvec4)
consumes just one location, any other small attribute. So we mark the
proper bit in inputs_read, and also the same bit in double_inputs_read
if the attribute is a dvec3/dvec4.

But in Vulkan, this is slightly different: a dvec3/dvec4 attribute
consumes two locations, not just one. And hence two bits would be marked
in inputs_read for the same vertex input attribute.

To avoid handling two different situations in NIR, we just choose the
latest one: in OpenGL, when creating NIR from GLSL/IR, any dvec3/dvec4
vertex input attribute is marked with two bits in the inputs_read bitmap
(and also in the double_inputs_read), and following attributes are
adjusted accordingly.

As example, if in our GLSL/IR shader we have three attributes:

layout(location = 0) vec3  attr0;
layout(location = 1) dvec4 attr1;
layout(location = 2) dvec3 attr2;

then in our NIR shader we put attr0 in location 0, attr1 in locations 1
and 2, and attr2 in location 3 and 4.

Checking carefully, basically we are using slots rather than locations
in NIR.

When emitting the vertices, we do a inverse map to know the
corresponding location for each slot.

v2 (Jason):
- use two slots from inputs_read for dvec3/dvec4 NIR from GLSL/IR.

v3 (Jason):
- Fix commit log error.
- Use ladder ifs and fix braces.
- elements_double is divisible by 2, don't need DIV_ROUND_UP().
- Use if ladder instead of a switch.
- Add comment about hardware restriction in 64bit vertex attributes.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir_gather_info.c

@@ -53,11 +53,6 @@ set_io_mask(nir_shader *shader, nir_variable *var, int offset, int len)
          else
             shader->info->inputs_read |= bitfield;
 
-         /* double inputs read is only for vertex inputs */
-         if (shader->stage == MESA_SHADER_VERTEX &&
-             glsl_type_is_dual_slot(glsl_without_array(var->type)))
-            shader->info->double_inputs_read |= bitfield;
-
          if (shader->stage == MESA_SHADER_FRAGMENT) {
             shader->info->fs.uses_sample_qualifier |= var->data.sample;
          }
@@ -83,26 +78,21 @@ static void
 mark_whole_variable(nir_shader *shader, nir_variable *var)
 {
    const struct glsl_type *type = var->type;
-   bool is_vertex_input = false;
 
    if (nir_is_per_vertex_io(var, shader->stage)) {
       assert(glsl_type_is_array(type));
       type = glsl_get_array_element(type);
    }
 
-   if (shader->stage == MESA_SHADER_VERTEX &&
-       var->data.mode == nir_var_shader_in)
-      is_vertex_input = true;
-
    const unsigned slots =
       var->data.compact ? DIV_ROUND_UP(glsl_get_length(type), 4)
-                        : glsl_count_attribute_slots(type, is_vertex_input);
+                        : glsl_count_attribute_slots(type, false);
 
    set_io_mask(shader, var, 0, slots);
 }
 
 static unsigned
-get_io_offset(nir_deref_var *deref, bool is_vertex_input)
+get_io_offset(nir_deref_var *deref)
 {
    unsigned offset = 0;
 
@@ -117,7 +107,7 @@ get_io_offset(nir_deref_var *deref, bool is_vertex_input)
             return -1;
          }
 
-         offset += glsl_count_attribute_slots(tail->type, is_vertex_input) *
+         offset += glsl_count_attribute_slots(tail->type, false) *
             deref_array->base_offset;
       }
       /* TODO: we can get the offset for structs here see nir_lower_io() */
@@ -163,12 +153,7 @@ try_mask_partial_io(nir_shader *shader, nir_deref_var *deref)
       return false;
    }
 
-   bool is_vertex_input = false;
-   if (shader->stage == MESA_SHADER_VERTEX &&
-       var->data.mode == nir_var_shader_in)
-      is_vertex_input = true;
-
-   unsigned offset = get_io_offset(deref, is_vertex_input);
+   unsigned offset = get_io_offset(deref);
    if (offset == -1)
       return false;
 
@@ -184,8 +169,7 @@ try_mask_partial_io(nir_shader *shader, nir_deref_var *deref)
    }
 
    /* double element width for double types that takes two slots */
-   if (!is_vertex_input &&
-       glsl_type_is_dual_slot(glsl_without_array(type))) {
+   if (glsl_type_is_dual_slot(glsl_without_array(type))) {
       elem_width *= 2;
    }
 
@@ -220,13 +204,27 @@ gather_intrinsic_info(nir_intrinsic_instr *instr, nir_shader *shader)
    case nir_intrinsic_interp_var_at_sample:
    case nir_intrinsic_interp_var_at_offset:
    case nir_intrinsic_load_var:
-   case nir_intrinsic_store_var:
-      if (instr->variables[0]->var->data.mode == nir_var_shader_in ||
-          instr->variables[0]->var->data.mode == nir_var_shader_out) {
+   case nir_intrinsic_store_var: {
+      nir_variable *var = instr->variables[0]->var;
+
+      if (var->data.mode == nir_var_shader_in ||
+          var->data.mode == nir_var_shader_out) {
          if (!try_mask_partial_io(shader, instr->variables[0]))
-            mark_whole_variable(shader, instr->variables[0]->var);
+            mark_whole_variable(shader, var);
+
+         /* We need to track which input_reads bits correspond to a
+          * dvec3/dvec4 input attribute */
+         if (shader->stage == MESA_SHADER_VERTEX &&
+             var->data.mode == nir_var_shader_in &&
+             glsl_type_is_dual_slot(glsl_without_array(var->type))) {
+            for (uint i = 0; i < glsl_count_attribute_slots(var->type, false); i++) {
+               int idx = var->data.location + i;
+               shader->info->double_inputs_read |= BITFIELD64_BIT(idx);
+            }
+         }
       }
       break;
+   }
 
    case nir_intrinsic_load_draw_id:
    case nir_intrinsic_load_front_face: