implement immutable variables codegen

Signed-off-by: xermicus <cyrill@parity.io>

crates/llvm-context/src/polkavm/context/mod.rs

@@ -162,6 +162,18 @@ where
         })
     }
 
+    fn link_immutable_data(&self, contract_path: &str) -> anyhow::Result<()> {
+        let size = self.solidity().immutables_size() as u32;
+        let exports = revive_runtime_api::immutable_data::module(self.llvm(), size);
+        self.module.link_in_module(exports).map_err(|error| {
+            anyhow::anyhow!(
+                "The contract `{}` immutable data module linking error: {}",
+                contract_path,
+                error
+            )
+        })
+    }
+
     /// Configure the PolkaVM minimum stack size.
     fn set_polkavm_stack_size(
         llvm: &'ctx inkwell::context::Context,
@@ -239,6 +251,7 @@ where
         let module_clone = self.module.clone();
 
         self.link_polkavm_exports(contract_path)?;
+        self.link_immutable_data(contract_path)?;
 
         let target_machine = TargetMachine::new(Target::PVM, self.optimizer.settings())?;
         target_machine.set_target_data(self.module());
@@ -381,6 +394,15 @@ where
         }
     }
 
+    /// Declare an external global.
+    pub fn declare_global<T>(&mut self, name: &str, r#type: T, address_space: AddressSpace)
+    where
+        T: BasicType<'ctx> + Clone + Copy,
+    {
+        let global = Global::declare(self, r#type, address_space, name);
+        self.globals.insert(name.to_owned(), global);
+    }
+
     /// Returns the LLVM intrinsics collection reference.
     pub fn intrinsics(&self) -> &Intrinsics<'ctx> {
         &self.intrinsics