[Mesa-dev] [PATCH v3] glsl/linker: location aliasing requires types to have the same width

[Iago Toral Quiroga]

Regarding location aliasing requirements, the OpenGL spec says:

  "Further, when location aliasing, the aliases sharing the location
   must have the same underlying numerical type  (floating-point or
   integer)."

Khronos has further clarified that this also requires the underlying
types to have the same width, so we can't put a float and a double
in the same location slot for example. Future versions of the spec will
be corrected to make this clear.

This patch amends our implementation to account for this restriction.

In the process of doing this, I also noticed that we would attempt
to check aliasing requirements for record variables (including the test
for the numerical type) which is not allowed, instead, we should be
producing a linker error as soon as we see any attempt to do location
aliasing on non-numerical variables. For the particular case of structs,
we were producing a linker error in this case, but only because we
assumed that struct fields use all components in each location, so
any attempt to alias locations consumed by struct fields would produce
a link error due to component aliasing, which is not accurate of the
actual problem. This patch would make it produce an error for attempting
to alias a non-numerical variable instead, which is always accurate.

v2:
  - Do not assert if we see invalid numerical types. These come
    straight from shader code, so we should produce linker errors if
    shaders attempt to do location aliasing on variables that are not
    numerical such as records.
  - While we are at it, improve error reporting for the case of
    numerical type mismatch to include the shader stage.

v3:
  - Allow location aliasing of images and samplers. If we get these
    it means bindless support is active and they should be handled
    as 64-bit integers (Ilia)
  - Make sure we produce link errors for any non-numerical type
    for which we attempt location aliasing, not just structs.
---
 src/compiler/glsl/link_varyings.cpp | 64 ++++++++++++++++++++++++++-----------
 1 file changed, 46 insertions(+), 18 deletions(-)

diff --git a/src/compiler/glsl/link_varyings.cpp b/src/compiler/glsl/link_varyings.cpp
index 1a9894baab..e0d757eaaf 100644
--- a/src/compiler/glsl/link_varyings.cpp
+++ b/src/compiler/glsl/link_varyings.cpp
@@ -405,15 +405,15 @@ compute_variable_location_slot(ir_variable *var, gl_shader_stage stage)
 
 struct explicit_location_info {
    ir_variable *var;
-   unsigned numerical_type;
+   int numerical_type;
    unsigned interpolation;
    bool centroid;
    bool sample;
    bool patch;
 };
 
-static inline unsigned
-get_numerical_type(const glsl_type *type)
+static inline int
+get_numerical_sized_type(const glsl_type *type)
 {
    /* From the OpenGL 4.6 spec, section 4.4.1 Input Layout Qualifiers, Page 68,
     * (Location aliasing):
@@ -421,10 +421,25 @@ get_numerical_type(const glsl_type *type)
     *    "Further, when location aliasing, the aliases sharing the location
     *     must have the same underlying numerical type  (floating-point or
     *     integer)
+    *
+    * Khronos has further clarified that this also requires the underlying
+    * types to have the same width, so we can't put a float and a double
+    * in the same location slot for example. Future versions of the spec will
+    * be corrected to make this clear.
+    *
+    * Notice that we allow location aliasing for bindless image/samplers too
+    * since these are defined as 64-bit integers.
     */
-   if (type->is_float() || type->is_double())
+   if (type->is_float())
       return GLSL_TYPE_FLOAT;
-   return GLSL_TYPE_INT;
+   else if (type->is_integer())
+      return GLSL_TYPE_INT;
+   else if (type->is_double())
+      return GLSL_TYPE_DOUBLE;
+   else if (type->is_integer_64() || type->is_sampler() || type->is_image())
+      return GLSL_TYPE_INT64;
+
+   return -1; /* Not a numerical type */
 }
 
 static bool
@@ -442,14 +457,17 @@ check_location_aliasing(struct explicit_location_info explicit_locations[][4],
                         gl_shader_stage stage)
 {
    unsigned last_comp;
-   if (type->without_array()->is_record()) {
-      /* The component qualifier can't be used on structs so just treat
-       * all component slots as used.
+   const glsl_type *type_without_array = type->without_array();
+   int numerical_type = get_numerical_sized_type(type_without_array);
+   if (numerical_type == -1) {
+      /* The component qualifier can't be used on non-numerical types so just
+       * treat all component slots as used. This will also make it so that
+       * any location aliasing attempt on non-numerical types is detected.
        */
       last_comp = 4;
    } else {
-      unsigned dmul = type->without_array()->is_64bit() ? 2 : 1;
-      last_comp = component + type->without_array()->vector_elements * dmul;
+      unsigned dmul = type_without_array->is_64bit() ? 2 : 1;
+      last_comp = component + type_without_array->vector_elements * dmul;
    }
 
    while (location < location_limit) {
@@ -459,8 +477,18 @@ check_location_aliasing(struct explicit_location_info explicit_locations[][4],
             &explicit_locations[location][comp];
 
          if (info->var) {
-            /* Component aliasing is not alloed */
-            if (comp >= component && comp < last_comp) {
+            if (numerical_type == -1 || info->numerical_type == -1) {
+               /* Location aliasing is only allowed for numerical variables */
+               linker_error(prog,
+                            "%s shader has location aliasing for "
+                            "non-numerical variable '%s'. Location %u "
+                            "component %u\n",
+                            _mesa_shader_stage_to_string(stage),
+                            numerical_type == -1 ? var->name : info->var->name,
+                            location, comp);
+               return false;
+            } else if (comp >= component && comp < last_comp) {
+               /* Component aliasing is not allowed */
                linker_error(prog,
                             "%s shader has multiple outputs explicitly "
                             "assigned to location %d and component %d\n",
@@ -471,12 +499,12 @@ check_location_aliasing(struct explicit_location_info explicit_locations[][4],
                /* For all other used components we need to have matching
                 * types, interpolation and auxiliary storage
                 */
-               if (info->numerical_type !=
-                   get_numerical_type(type->without_array())) {
+               if (info->numerical_type != numerical_type) {
                   linker_error(prog,
-                               "Varyings sharing the same location must "
-                               "have the same underlying numerical type. "
-                               "Location %u component %u\n",
+                               "%s shader has varyings sharing the same "
+                               "location that don't have the same underlying "
+                               "numerical type. Location %u component %u\n",
+                               _mesa_shader_stage_to_string(stage),
                                location, comp);
                   return false;
                }
@@ -502,7 +530,7 @@ check_location_aliasing(struct explicit_location_info explicit_locations[][4],
             }
          } else if (comp >= component && comp < last_comp) {
             info->var = var;
-            info->numerical_type = get_numerical_type(type->without_array());
+            info->numerical_type = numerical_type;
             info->interpolation = interpolation;
             info->centroid = centroid;
             info->sample = sample;
-- 
2.11.0