feat: Access references to globals through a custom GOT

This commit introduces the association of GOT indices to the LLVM index,
which then allows us to utilise that when generating references to
variables to check if a given variable has an entry in the GOT. If so,
we obtain its index, and generate the necessary LLVM IR to access the
address contained within the GOT rather than accessing the variable
directly.

Author: lewis-revill

src/codegen/generators/expression_generator.rs

@@ -282,6 +282,42 @@ impl<'ink, 'b> ExpressionCodeGenerator<'ink, 'b> {
         self.generate_expression_value(const_expression)
     }
 
+    /// Generate an access to the appropriate GOT entry to achieve an access to the given base
+    /// lvalue.
+    pub fn generate_got_access(
+        &self,
+        context: &AstNode,
+        llvm_type: &BasicTypeEnum<'ink>,
+    ) -> Result<Option<PointerValue<'ink>>, Diagnostic> {
+        match self.annotations.get(context) {
+            Some(StatementAnnotation::Variable { qualified_name, .. }) => {
+                // We will generate a GEP, which has as its base address the magic constant which
+                // will eventually be replaced by the location of the GOT.
+                let base = self
+                    .llvm
+                    .context
+                    .i64_type()
+                    .const_int(0xdeadbeef00000000, false)
+                    .const_to_pointer(llvm_type
+                        .ptr_type(AddressSpace::default())
+                        .ptr_type(AddressSpace::default()));
+
+                self.llvm_index
+                    .find_got_index(qualified_name)
+                    .map(|idx| {
+                        let ptr = self.llvm.load_array_element(
+                            base,
+                            &[self.llvm.context.i32_type().const_int(idx, false)],
+                            "",
+                        )?;
+                        Ok(self.llvm.load_pointer(&ptr, "").into_pointer_value())
+                    })
+                    .transpose()
+            }
+            _ => Ok(None),
+        }
+    }
+
     /// generates a binary expression (e.g. a + b, x AND y, etc.) and returns the resulting `BasicValueEnum`
     /// - `left` the AstStatement left of the operator
     /// - `right` the AstStatement right of the operator
@@ -1218,13 +1254,17 @@ impl<'ink, 'b> ExpressionCodeGenerator<'ink, 'b> {
             }
         }
 
+        let ctx_type =
+            self.annotations.get_type_or_void(context, self.index).get_type_information();
+
         // no context ... so just something like 'x'
         match self.annotations.get(context) {
             Some(StatementAnnotation::Variable { qualified_name, .. })
-            | Some(StatementAnnotation::Program { qualified_name, .. }) => self
-                .llvm_index
-                .find_loaded_associated_variable_value(qualified_name)
-                .ok_or_else(|| Diagnostic::unresolved_reference(name, offset.clone())),
+            | Some(StatementAnnotation::Program { qualified_name, .. }) =>
+                self.generate_got_access(context, &self.llvm_index.get_associated_type(ctx_type.get_name())?)?.map_or(self
+                    .llvm_index
+                    .find_loaded_associated_variable_value(qualified_name)
+                    .ok_or_else(|| Diagnostic::unresolved_reference(name, offset.clone())), Ok),
             _ => Err(Diagnostic::unresolved_reference(name, offset.clone())),
         }
     }

src/codegen/generators/variable_generator.rs

@@ -149,6 +149,7 @@ impl<'ctx, 'b> VariableGenerator<'ctx, 'b> {
             for (name, _) in &globals {
                 if let Some(idx) = got_entries.get(&name.to_string()) {
                     new_got_entries.insert(name.to_string(), *idx);
+                    index.associate_got_index(name, *idx);
                     new_got.insert(*idx, name.to_string());
                 } else {
                     new_globals.push(name.to_string());
@@ -162,6 +163,7 @@ impl<'ctx, 'b> VariableGenerator<'ctx, 'b> {
                     idx += 1;
                 }
                 new_got_entries.insert(name.to_string(), idx);
+                index.associate_got_index(name, idx);
                 new_got.insert(idx, name.to_string());
             }
 

src/codegen/llvm_index.rs

@@ -14,6 +14,7 @@ pub struct LlvmTypedIndex<'ink> {
     type_associations: FxHashMap<String, BasicTypeEnum<'ink>>,
     pou_type_associations: FxHashMap<String, BasicTypeEnum<'ink>>,
     global_values: FxHashMap<String, GlobalValue<'ink>>,
+    got_indices: FxHashMap<String, u64>,
     initial_value_associations: FxHashMap<String, BasicValueEnum<'ink>>,
     loaded_variable_associations: FxHashMap<String, PointerValue<'ink>>,
     implementations: FxHashMap<String, FunctionValue<'ink>>,
@@ -29,6 +30,7 @@ impl<'ink> LlvmTypedIndex<'ink> {
             type_associations: FxHashMap::default(),
             pou_type_associations: FxHashMap::default(),
             global_values: FxHashMap::default(),
+            got_indices: FxHashMap::default(),
             initial_value_associations: FxHashMap::default(),
             loaded_variable_associations: FxHashMap::default(),
             implementations: FxHashMap::default(),
@@ -51,6 +53,9 @@ impl<'ink> LlvmTypedIndex<'ink> {
         for (name, value) in other.global_values.drain() {
             self.global_values.insert(name, value);
         }
+        for (name, index) in other.got_indices.drain() {
+            self.got_indices.insert(name, index);
+        }
         for (name, assocication) in other.initial_value_associations.drain() {
             self.initial_value_associations.insert(name, assocication);
         }
@@ -110,6 +115,13 @@ impl<'ink> LlvmTypedIndex<'ink> {
             .or_else(|| self.parent_index.and_then(|it| it.find_global_value(name)))
     }
 
+    pub fn find_got_index(&self, name: &str) -> Option<u64> {
+        self.got_indices
+            .get(&name.to_lowercase())
+            .copied()
+            .or_else(|| self.parent_index.and_then(|it| it.find_got_index(name)))
+    }
+
     pub fn find_associated_type(&self, type_name: &str) -> Option<BasicTypeEnum<'ink>> {
         self.type_associations
             .get(&type_name.to_lowercase())
@@ -153,6 +165,15 @@ impl<'ink> LlvmTypedIndex<'ink> {
         Ok(())
     }
 
+    pub fn associate_got_index(
+        &mut self,
+        variable_name: &str,
+        index: u64,
+    ) -> Result<(), Diagnostic> {
+        self.got_indices.insert(variable_name.to_lowercase(), index);
+        Ok(())
+    }
+
     pub fn associate_implementation(
         &mut self,
         callable_name: &str,