implement `?return` expression

1 files changed, 33 insertions, 0 deletions

diff --git a/doc/codegen.md b/doc/codegen.md
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+# Code Generation
+
+## Data Representation
+
+Every type has a "handle". If a type is a simple primitive type such as i32 or
+f64, the handle is "by value", meaning that we pass around the value itself when
+we refer to a value of that type.
+
+If a type is a container, error union, maybe type, slice, or array, then its
+handle is a pointer, and everywhere we refer to a value of this type we refer to
+a pointer.
+
+Parameters and return values are always passed as handles.
+
+Error union types are represented as:
+
+    struct {
+        error: u32,
+        payload: T,
+    }
+
+Maybe types are represented as:
+
+    struct {
+        payload: T,
+        is_non_null: u1,
+    }
+
+## Data Optimizations
+
+Maybe pointer types are special: the 0x0 pointer value is used to represent a
+null pointer. Thus, instead of the struct above, maybe pointer types are
+represented as a `usize` in codegen and the handle is by value.