feat: allow VAR_EXTERNAL declarations (#1324)

* feat: allow `VAR_EXTERNAL` declarations
Adds a new variable block type for VAR_EXTERNAL to the compiler. These blocks do not have any functionality for now and the compiler will emit a warning when such blocks are declared.

compiler/plc_ast/src/ast.rs

@@ -360,6 +360,7 @@ pub enum VariableBlockType {
     Output,
     Global,
     InOut,
+    External,
 }
 
 impl Display for VariableBlockType {
@@ -371,6 +372,7 @@ impl Display for VariableBlockType {
             VariableBlockType::Output => write!(f, "Output"),
             VariableBlockType::Global => write!(f, "Global"),
             VariableBlockType::InOut => write!(f, "InOut"),
+            VariableBlockType::External => write!(f, "External"),
         }
     }
 }

compiler/plc_diagnostics/src/diagnostics/diagnostics_registry.rs

@@ -207,6 +207,7 @@ lazy_static! {
         E103,   Error,      include_str!("./error_codes/E103.md"),  // Immutable Hardware Binding
         E104,   Error,      include_str!("./error_codes/E104.md"),  // Config Variable With Incomplete Address
         E105,   Error,      include_str!("./error_codes/E105.md"),  // CONSTANT keyword in POU
+        E106,   Warning,    include_str!("./error_codes/E106.md"),  // VAR_EXTERNAL have no effect
         E107,   Error,      include_str!("./error_codes/E107.md"),  // Missing configuration for template variable
         E108,   Error,      include_str!("./error_codes/E108.md"),  // Template variable is configured multiple times
     );

compiler/plc_diagnostics/src/diagnostics/error_codes/E106.md

@@ -0,0 +1,23 @@
+# VAR_EXTERNAL blocks have no effect
+
+Variables declared in a `VAR_EXTERNAL` block are currently ignored and the referenced globals will be used instead.
+
+Example:
+```
+VAR_GLOBAL
+    myArray : ARRAY [0..10] OF INT;
+    myString: STRING;
+END_VAR
+
+FUNCTION main
+VAR_EXTERNAL CONSTANT
+    myArray : ARRAY [0..10] OF INT;
+END_VAR
+    myArray[5] := 42;
+    myString := 'Hello, world!';
+END_FUNCTION
+```
+
+In this example, even though `arr` is declared as `VAR_EXTERNAL CONSTANT`, the `CONSTANT` constraint will be ignored and
+the global `myArray` will be mutated. The global `myString` can be read from and written to from within `main` even though it
+is not declared in a `VAR_EXTERNAL` block.

src/codegen/generators/data_type_generator.rs

@@ -177,7 +177,7 @@ impl<'ink, 'b> DataTypeGenerator<'ink, 'b> {
         if let DataTypeInformation::Struct { source, members, .. } = information {
             let members = members
                 .iter()
-                .filter(|it| !it.is_temp() && !it.is_return())
+                .filter(|it| !(it.is_temp() || it.is_return() || it.is_var_external()))
                 .map(|m| self.types_index.get_associated_type(m.get_type_name()))
                 .collect::<Result<Vec<BasicTypeEnum>, Diagnostic>>()?;
 

src/codegen/generators/pou_generator.rs

@@ -596,7 +596,7 @@ impl<'ink, 'cg> PouGenerator<'ink, 'cg> {
         }
 
         // handle all parameters (without return!)
-        for m in members.iter().filter(|it| !it.is_return()) {
+        for m in members.iter().filter(|it| !(it.is_return() || it.is_var_external())) {
             let parameter_name = m.get_name();
 
             let (name, variable) = if m.is_parameter() {
@@ -713,7 +713,7 @@ impl<'ink, 'cg> PouGenerator<'ink, 'cg> {
         //Generate reference to parameter
         // cannot use index from members because return and temp variables may not be considered for index in build_struct_gep
         let mut var_count = 0;
-        for m in members.iter() {
+        for m in members.iter().filter(|it| !it.is_var_external()) {
             let parameter_name = m.get_name();
 
             let (name, variable) = if m.is_temp() || m.is_return() {

src/codegen/tests/code_gen_tests.rs

@@ -3516,3 +3516,127 @@ fn array_of_struct_as_member_of_another_struct_and_variable_declaration_is_initi
 
     insta::assert_snapshot!(res);
 }
+
+#[test]
+// XXX: this behaviour might change in future, for now `VAR_EXTERNAL` variables are ignored
+fn variables_in_var_external_block_are_not_generated() {
+    let res = codegen(
+        "
+        VAR_GLOBAL 
+            arr: ARRAY [0..100] OF INT; 
+        END_VAR
+
+        FUNCTION foo
+        VAR_EXTERNAL
+            arr : ARRAY [0..100] OF INT;
+        END_VAR
+        END_FUNCTION
+
+        FUNCTION_BLOCK bar
+        VAR_EXTERNAL CONSTANT
+            arr : ARRAY [0..100] OF INT;
+        END_VAR
+        END_FUNCTION_BLOCK
+
+        PROGRAM baz
+        VAR_EXTERNAL CONSTANT
+            arr : ARRAY [0..100] OF INT;
+        END_VAR
+        END_PROGRAM
+
+        CLASS qux
+        VAR_EXTERNAL
+            arr : ARRAY [0..100] OF INT;
+        END_VAR
+        END_CLASS
+        ",
+    );
+
+    insta::assert_snapshot!(res, @r###"
+    ; ModuleID = '<internal>'
+    source_filename = "<internal>"
+
+    %bar = type {}
+    %baz = type {}
+    %qux = type {}
+
+    @arr = global [101 x i16] zeroinitializer
+    @__bar__init = unnamed_addr constant %bar zeroinitializer
+    @baz_instance = global %baz zeroinitializer
+    @__qux__init = unnamed_addr constant %qux zeroinitializer
+
+    define void @foo() {
+    entry:
+      ret void
+    }
+
+    define void @bar(%bar* %0) {
+    entry:
+      ret void
+    }
+
+    define void @baz(%baz* %0) {
+    entry:
+      ret void
+    }
+
+    define void @qux(%qux* %0) {
+    entry:
+      ret void
+    }
+    ; ModuleID = '__initializers'
+    source_filename = "__initializers"
+
+    %baz = type {}
+    %bar = type {}
+    %qux = type {}
+
+    @baz_instance = external global %baz
+    @__bar__init = external global %bar
+    @__qux__init = external global %qux
+
+    define void @__init_baz(%baz* %0) {
+    entry:
+      %self = alloca %baz*, align 8
+      store %baz* %0, %baz** %self, align 8
+      ret void
+    }
+
+    declare void @baz(%baz*)
+
+    define void @__init_bar(%bar* %0) {
+    entry:
+      %self = alloca %bar*, align 8
+      store %bar* %0, %bar** %self, align 8
+      ret void
+    }
+
+    declare void @bar(%bar*)
+
+    define void @__init_qux(%qux* %0) {
+    entry:
+      %self = alloca %qux*, align 8
+      store %qux* %0, %qux** %self, align 8
+      ret void
+    }
+
+    declare void @qux(%qux*)
+    ; ModuleID = '__init___testproject'
+    source_filename = "__init___testproject"
+
+    %baz = type {}
+
+    @baz_instance = external global %baz
+    @llvm.global_ctors = appending global [1 x { i32, void ()*, i8* }] [{ i32, void ()*, i8* } { i32 0, void ()* @__init___testproject, i8* null }]
+
+    define void @__init___testproject() {
+    entry:
+      call void @__init_baz(%baz* @baz_instance)
+      ret void
+    }
+
+    declare void @__init_baz(%baz*)
+
+    declare void @baz(%baz*)
+    "###);
+}

src/codegen/tests/initialization_test/complex_initializers.rs

@@ -1296,3 +1296,75 @@ fn var_config_aliased_variables_initialized() {
     declare void @FB(%FB*)
     "###);
 }
+
+#[test]
+fn var_external_blocks_are_ignored_in_init_functions() {
+    let res = codegen(
+        r"
+    VAR_GLOBAL
+        s: STRING;
+        refString AT s : STRING;
+    END_VAR
+
+    FUNCTION_BLOCK foo
+    VAR_EXTERNAL
+        refString : STRING;
+    END_VAR
+    END_FUNCTION
+
+    FUNCTION bar
+    VAR_EXTERNAL
+        refString : STRING;
+    END_VAR
+    END_FUNCTION
+        ",
+    );
+
+    insta::assert_snapshot!(res, @r###"
+    ; ModuleID = '<internal>'
+    source_filename = "<internal>"
+
+    %foo = type {}
+
+    @s = global [81 x i8] zeroinitializer
+    @refString = global [81 x i8]* null
+    @__foo__init = unnamed_addr constant %foo zeroinitializer
+
+    define void @foo(%foo* %0) {
+    entry:
+      ret void
+    }
+
+    define void @bar() {
+    entry:
+      ret void
+    }
+    ; ModuleID = '__initializers'
+    source_filename = "__initializers"
+
+    %foo = type {}
+
+    @__foo__init = external global %foo
+
+    define void @__init_foo(%foo* %0) {
+    entry:
+      %self = alloca %foo*, align 8
+      store %foo* %0, %foo** %self, align 8
+      ret void
+    }
+
+    declare void @foo(%foo*)
+    ; ModuleID = '__init___testproject'
+    source_filename = "__init___testproject"
+
+    @s = external global [81 x i8]
+    @refString = external global [81 x i8]*
+    @llvm.global_ctors = appending global [1 x { i32, void ()*, i8* }] [{ i32, void ()*, i8* } { i32 0, void ()* @__init___testproject, i8* null }]
+
+    define void @__init___testproject() {
+    entry:
+      store [81 x i8]* @s, [81 x i8]** @refString, align 8
+      ret void
+    }
+    "###)
+}

src/index.rs

@@ -70,6 +70,8 @@ pub struct VariableIndexEntry {
     pub argument_type: ArgumentType,
     /// true if this variable is a compile-time-constant
     is_constant: bool,
+    // true if this variable is in a 'VAR_EXTERNAL' block
+    is_var_external: bool,
     /// the variable's datatype
     pub data_type_name: String,
     /// the index of the member-variable in it's container (e.g. struct). defautls to 0 (Single variables)
@@ -130,6 +132,7 @@ pub struct MemberInfo<'b> {
     variable_type_name: &'b str,
     binding: Option<HardwareBinding>,
     is_constant: bool,
+    is_var_external: bool,
     varargs: Option<VarArgs>,
 }
 
@@ -148,6 +151,7 @@ impl VariableIndexEntry {
             initial_value: None,
             argument_type,
             is_constant: false,
+            is_var_external: false,
             data_type_name: data_type_name.to_string(),
             location_in_parent,
             linkage: LinkageType::Internal,
@@ -169,6 +173,7 @@ impl VariableIndexEntry {
             initial_value: None,
             argument_type: ArgumentType::ByVal(VariableType::Global),
             is_constant: false,
+            is_var_external: false,
             data_type_name: data_type_name.to_string(),
             location_in_parent: 0,
             linkage: LinkageType::Internal,
@@ -203,6 +208,11 @@ impl VariableIndexEntry {
         self
     }
 
+    pub fn set_var_external(mut self, var_external: bool) -> Self {
+        self.is_var_external = var_external;
+        self
+    }
+
     /// Creates a new VariableIndexEntry from the current entry with a new container and type
     /// This is used to create new entries from previously generic entries
     pub fn into_typed(&self, container: &str, new_type: &str) -> Self {
@@ -253,6 +263,10 @@ impl VariableIndexEntry {
         self.linkage == LinkageType::External
     }
 
+    pub fn is_var_external(&self) -> bool {
+        self.is_var_external
+    }
+
     pub fn get_declaration_type(&self) -> ArgumentType {
         self.argument_type
     }
@@ -350,6 +364,7 @@ pub enum VariableType {
     InOut,
     Global,
     Return,
+    External,
 }
 
 impl VariableType {
@@ -368,6 +383,7 @@ impl std::fmt::Display for VariableType {
             VariableType::InOut => write!(f, "InOut"),
             VariableType::Global => write!(f, "Global"),
             VariableType::Return => write!(f, "Return"),
+            VariableType::External => write!(f, "External"),
         }
     }
 }
@@ -1138,13 +1154,18 @@ impl Index {
     /// Searches for variable name in the given container, if not found, attempts to search for it in super classes
     pub fn find_member(&self, container_name: &str, variable_name: &str) -> Option<&VariableIndexEntry> {
         // Find pou in index
-        self.find_local_member(container_name, variable_name).or_else(|| {
-            if let Some(class) = self.find_pou(container_name).and_then(|it| it.get_super_class()) {
-                self.find_member(class, variable_name)
-            } else {
-                None
-            }
-        })
+        self.find_local_member(container_name, variable_name)
+            .or_else(|| {
+                if let Some(class) = self.find_pou(container_name).and_then(|it| it.get_super_class()) {
+                    self.find_member(class, variable_name)
+                } else {
+                    None
+                }
+            })
+            .filter(|it| {
+                // VAR_EXTERNAL variables are not local members
+                !it.is_var_external()
+            })
     }
 
     /// Searches for method names in the given container, if not found, attempts to search for it in super class
@@ -1518,6 +1539,7 @@ impl Index {
         .set_initial_value(initial_value)
         .set_hardware_binding(member_info.binding)
         .set_varargs(member_info.varargs)
+        .set_var_external(member_info.is_var_external)
     }
 
     pub fn register_enum_variant(