Add a simple boilerplate.

src/function_body.rs

@@ -2,6 +2,73 @@ use backend::*;
 use error::Error;
 use wasmparser::{FunctionBody, Operator};
 
+/// Type of a control frame.
+#[derive(Debug, Copy, Clone, PartialEq)]
+enum ControlFrameKind {
+    /// A regular block frame.
+    ///
+    /// Can be used for an implicit function block.
+    Block { end_label: Label },
+    /// Loop frame (branching to the beginning of block).
+    Loop { header: Label },
+    /// True-subblock of if expression.
+    IfTrue {
+        /// If jump happens inside the if-true block then control will
+        /// land on this label.
+        end_label: Label,
+
+        /// If the condition of the `if` statement is unsatisfied, control
+        /// will land on this label. This label might point to `else` block if it
+        /// exists. Otherwise it equal to `end_label`.
+        if_not: Label,
+    },
+    /// False-subblock of if expression.
+    IfFalse { end_label: Label },
+}
+
+impl ControlFrameKind {
+    /// Returns a label which should be used as a branch destination.
+    fn br_destination(&self) -> Label {
+        match *self {
+            ControlFrameKind::Block { end_label } => end_label,
+            ControlFrameKind::Loop { header } => header,
+            ControlFrameKind::IfTrue { end_label, .. } => end_label,
+            ControlFrameKind::IfFalse { end_label } => end_label,
+        }
+    }
+
+    /// Returns `true` if this block of a loop kind.
+    fn is_loop(&self) -> bool {
+        match *self {
+            ControlFrameKind::Loop { .. } => true,
+            _ => false,
+        }
+    }
+}
+
+struct ControlFrame {
+    kind: ControlFrameKind,
+    /// Boolean which signals whether value stack became polymorphic. Value stack starts in non-polymorphic state and
+	/// becomes polymorphic only after an instruction that never passes control further is executed,
+	/// i.e. `unreachable`, `br` (but not `br_if`!), etc.
+    stack_polymorphic: bool,
+    // TODO: type, stack height, etc
+}
+
+impl ControlFrame {
+    pub fn new(kind: ControlFrameKind) -> ControlFrame {
+        ControlFrame {
+            kind,
+            stack_polymorphic: false,
+        }
+    }
+
+    /// Marks this control frame as reached stack-polymorphic state.
+    pub fn mark_stack_polymorphic(&mut self) {
+        self.stack_polymorphic = true;
+    }
+}
+
 pub fn translate(session: &mut CodeGenSession, body: &FunctionBody) -> Result<(), Error> {
     let locals = body.get_locals_reader()?;
 
@@ -24,24 +91,60 @@ pub fn translate(session: &mut CodeGenSession, body: &FunctionBody) -> Result<()
         copy_incoming_arg(&mut ctx, arg_pos);
     }
 
+    let mut control_frames = Vec::new();
+
+    // Upon entering the function implicit frame for function body is pushed. It has the same
+    // result type as the function itself. Branching to it is equivalent to returning from the function.
+    let epilogue_label = create_label(&mut ctx);
+    control_frames.push(ControlFrame::new(
+        ControlFrameKind::Block {
+            end_label: epilogue_label,
+        },
+    ));
+
     for op in operators {
         match op? {
+            Operator::Unreachable => {
+                control_frames
+                    .last_mut()
+                    .expect("control stack is never empty")
+                    .mark_stack_polymorphic();
+                trap(&mut ctx);
+            }
+            Operator::If { ty } => {
+                let end_label = create_label(&mut ctx);
+                let if_not = create_label(&mut ctx);
+                control_frames.push(ControlFrame::new(
+                    ControlFrameKind::IfTrue {
+                        end_label,
+                        if_not,
+                    },
+                ));
+
+                // TODO: Generate code that pops a value and executes the if_true part if the value
+                // is not equal to zero.
+            }
+            Operator::End => {
+                let control_frame = control_frames.pop().expect("control stack is never empty");
+                if !control_frame.kind.is_loop() {
+                    // Branches to a control frame with block type directs control flow to the header of the loop 
+                    // and we don't need to resolve it here. Branching to other control frames always lead
+                    // control flow to the corresponding `end`.
+                    define_label(&mut ctx, control_frame.kind.br_destination());
+                }
+            }
             Operator::I32Add => {
                 add_i32(&mut ctx);
             }
             Operator::GetLocal { local_index } => {
                 get_local_i32(&mut ctx, local_index);
             }
-            Operator::End => {
-                // TODO: This is super naive and makes a lot of unfounded assumptions 
-                // but will do for the start.
-                prepare_return_value(&mut ctx);
-            }
             _ => {
-                unsupported_opcode(&mut ctx);
+                trap(&mut ctx);
             }
         }
     }
+    prepare_return_value(&mut ctx);
     epilogue(&mut ctx);
 
     Ok(())