reflect: further improve link i/o pass by ensuring that the output SPIR-V is valid according to Vulkan 1.0 semantics

The pass will now create and reassign types of downgraded outputs to a type with Private storage class, without affecting the type of other outputs.

Downgraded outputs also get removed from the entrypoint interface

librashader-reflect/src/front/spirv_passes/link_input_outputs.rs

@@ -108,8 +108,74 @@ impl<'a> LinkInputs<'a> {
             .filter_map(|i| self.outputs.get(i).cloned())
             .collect::<FxHashSet<spirv::Word>>();
 
-        let module = self.vert_builder.module_mut();
+        let mut pointer_types_to_downgrade = FxHashSet::default();
-        for global in module.types_global_values.iter_mut() {
+
+        // Map from Pointer type to pointee
+        let mut pointer_type_pointee = Vec::new();
+
+        // Map from StorageClass Output to StorageClass Private
+        let mut downgraded_pointer_types = FxHashMap::default();
+
+        // First collect all the pointer types that are needed for dead outputs.
+        for global in self.vert_builder.module_ref().types_global_values.iter() {
+            if global.class.opcode != spirv::Op::Variable
+                || global.operands[0] != Operand::StorageClass(StorageClass::Output)
+            {
+                continue;
+            }
+
+            if let Some(id) = global.result_id {
+                if !dead_outputs.contains(&id) {
+                    continue;
+                }
+
+                if let Some(result_type) = global.result_type {
+                    pointer_types_to_downgrade.insert(result_type);
+                }
+            }
+        }
+
+        // Collect all the pointee types of pointer types to downgrade
+        for global in self.vert_builder.module_ref().types_global_values.iter() {
+            if global.class.opcode != spirv::Op::TypePointer
+                || global.operands[0] != Operand::StorageClass(StorageClass::Output)
+            {
+                continue;
+            }
+
+            if let Some(id) = global.result_id {
+                if !pointer_types_to_downgrade.contains(&id) {
+                    continue;
+                }
+
+                let Some(pointee_type) = global.operands[1].id_ref_any() else {
+                    continue;
+                };
+
+                pointer_type_pointee.push((id, pointee_type));
+            }
+        }
+
+        // Create pointer types for everything we saw above with Private storage class.
+        // We don't have to deal with OpTypeForwardPointer, because PhysicalStorageBuffer
+        // is not valid in slang shaders, and we're only working with Vulkan inputs.
+        for (pointer_type, pointee_type) in pointer_type_pointee.into_iter() {
+            // Create a new private type
+            let private_pointer_type =
+                self.vert_builder
+                    .type_pointer(None, StorageClass::Private, pointee_type);
+
+            // Add it to the mapping
+            downgraded_pointer_types.insert(pointer_type, private_pointer_type);
+        }
+
+        // Downgrade the OpVariable storage class and reassign the types.
+        for global in self
+            .vert_builder
+            .module_mut()
+            .types_global_values
+            .iter_mut()
+        {
             if global.class.opcode != spirv::Op::Variable
                 || global.operands[0] != Operand::StorageClass(StorageClass::Output)
             {
@@ -123,10 +189,25 @@ impl<'a> LinkInputs<'a> {
 
                 // downgrade the OpVariable if it's in dead_outputs
                 global.operands[0] = Operand::StorageClass(StorageClass::Private);
+
+                // Get the result type. If there's no result type it's invalid anyways
+                // so it doesn't matter that we downgraded early (better downgraded than unmatched)
+                let Some(result_type) = &mut global.result_type else {
+                    continue;
+                };
+
+                let Some(new_type) = downgraded_pointer_types.get(&result_type) else {
+                    // We should have created one above.
+                    continue;
+                };
+
+                // Set the type of the OpVariable to the same type with Private storageclass.
+                *result_type = *new_type;
             }
         }
 
-        module.annotations.retain_mut(|op| {
+        // Strip decorations of downgraded variables.
+        self.vert_builder.module_mut().annotations.retain_mut(|op| {
             if op.class.opcode != Op::Decorate {
                 return true;
             }
@@ -142,6 +223,27 @@ impl<'a> LinkInputs<'a> {
             // If target is in dead outputs, then don't keep it.
             !dead_outputs.contains(&target)
         });
+
+        for entry_point in self.vert_builder.module_mut().entry_points.iter_mut() {
+            let mut index = 0;
+            entry_point.operands.retain(|s| {
+                // Skip the execution mode, entry point reference, and name.
+                if index < 3 {
+                    index += 1;
+                    return true;
+                }
+
+                index += 1;
+
+                // Ignore any non-IdRef
+                let Operand::IdRef(id_ref) = s else {
+                    return true;
+                };
+
+                // If the entry point contains a dead outputs, remove it from the interface.
+                !dead_outputs.contains(id_ref)
+            });
+        }
     }
 
     // Trim unused fragment shader inputs
@@ -218,4 +320,4 @@ impl<'a> LinkInputs<'a> {
                 !self.inputs_to_remove.contains_key(&id)
             });
     }
-}
+}