llvm-context: lazy handling of function arguments and immutable data (#282)

- Lazily load function arguments so that they can be passed as pointers.
- Lazily call the immutable store function to avoid storing zero sized
immutable data.

---------

Signed-off-by: Cyrill Leutwiler <bigcyrill@hotmail.com>

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

@@ -4,61 +4,98 @@
 #[derive(Debug, Clone)]
 pub struct Argument<'ctx> {
     /// The actual LLVM operand.
-    pub value: inkwell::values::BasicValueEnum<'ctx>,
+    pub value: Value<'ctx>,
     /// The original AST value. Used mostly for string literals.
     pub original: Option<String>,
     /// The preserved constant value, if available.
     pub constant: Option<num::BigUint>,
 }
 
+/// The function argument can be either a pointer or a integer value.
+/// This disambiguation allows for lazy loading of variables.
+#[derive(Clone, Debug)]
+pub enum Value<'ctx> {
+    Register(inkwell::values::BasicValueEnum<'ctx>),
+    Pointer {
+        pointer: crate::polkavm::context::Pointer<'ctx>,
+        id: String,
+    },
+}
+
 impl<'ctx> Argument<'ctx> {
-    /// The calldata offset argument index.
-    pub const ARGUMENT_INDEX_CALLDATA_OFFSET: usize = 0;
-
-    /// The calldata length argument index.
-    pub const ARGUMENT_INDEX_CALLDATA_LENGTH: usize = 1;
-
-    /// A shortcut constructor.
-    pub fn new(value: inkwell::values::BasicValueEnum<'ctx>) -> Self {
+    /// A shortcut constructor for register arguments.
+    pub fn value(value: inkwell::values::BasicValueEnum<'ctx>) -> Self {
         Self {
-            value,
+            value: Value::Register(value),
             original: None,
             constant: None,
         }
     }
 
-    /// A shortcut constructor.
-    pub fn new_with_original(
-        value: inkwell::values::BasicValueEnum<'ctx>,
-        original: String,
-    ) -> Self {
+    /// A shortcut constructor for stack arguments.
+    pub fn pointer(pointer: crate::polkavm::context::Pointer<'ctx>, id: String) -> Self {
         Self {
-            value,
-            original: Some(original),
+            value: Value::Pointer { pointer, id },
+            original: None,
             constant: None,
         }
     }
 
-    /// A shortcut constructor.
-    pub fn new_with_constant(
-        value: inkwell::values::BasicValueEnum<'ctx>,
-        constant: num::BigUint,
-    ) -> Self {
-        Self {
-            value,
-            original: None,
-            constant: Some(constant),
-        }
+    /// Set the original decleratation value.
+    pub fn with_original(mut self, original: String) -> Self {
+        self.original = Some(original);
+        self
+    }
+
+    /// Set the constant value.
+    pub fn with_constant(mut self, constant: num::BigUint) -> Self {
+        self.constant = Some(constant);
+        self
     }
 
     /// Returns the inner LLVM value.
-    pub fn to_llvm(&self) -> inkwell::values::BasicValueEnum<'ctx> {
-        self.value
+    ///
+    /// Panics if `self` is a pointer argument.
+    pub fn _to_llvm_value(&self) -> inkwell::values::BasicValueEnum<'ctx> {
+        match &self.value {
+            Value::Register(value) => *value,
+            Value::Pointer { .. } => unreachable!("invalid register value access"),
+        }
+    }
+
+    /// Access the underlying value.
+    ///
+    /// Will emit a stack load if `self` is a pointer argument.
+    pub fn access<D: crate::polkavm::Dependency + Clone>(
+        &self,
+        context: &crate::polkavm::context::Context<'ctx, D>,
+    ) -> anyhow::Result<inkwell::values::BasicValueEnum<'ctx>> {
+        match &self.value {
+            Value::Register(value) => Ok(*value),
+            Value::Pointer { pointer, id } => context.build_load(*pointer, id),
+        }
+    }
+
+    /// Access the underlying value.
+    ///
+    /// Will emit a stack load if `self` is a pointer argument.
+    pub fn as_pointer<D: crate::polkavm::Dependency + Clone>(
+        &self,
+        context: &crate::polkavm::context::Context<'ctx, D>,
+    ) -> anyhow::Result<crate::polkavm::context::Pointer<'ctx>> {
+        match &self.value {
+            Value::Register(value) => {
+                let pointer = context.build_alloca_at_entry(context.word_type(), "pvm_arg");
+                context.build_store(pointer, *value)?;
+                Ok(pointer)
+            }
+            Value::Pointer { pointer, .. } => Ok(*pointer),
+        }
     }
 }
 
 impl<'ctx> From<inkwell::values::BasicValueEnum<'ctx>> for Argument<'ctx> {
     fn from(value: inkwell::values::BasicValueEnum<'ctx>) -> Self {
-        Self::new(value)
+        Self::value(value)
     }
 }