Implement Tail Call Optimisation

core/engine/src/bytecompiler/mod.rs

@@ -1510,6 +1510,31 @@ impl<'ctx> ByteCompiler<'ctx> {
         }
     }
 
+    fn optimize(bytecode: &mut Vec<u8>, flags: CodeBlockFlags) {
+        // only perform Tail Call Optimisation in strict mode
+        if flags.contains(CodeBlockFlags::STRICT) {
+            // if a sequence of Call, SetReturnValue, CheckReturn, Return is found
+            // replace Call with TailCall to allow frame elimination
+            for i in 0..bytecode.len() {
+                let opcode: Opcode = bytecode[i].into();
+                if matches!(opcode, Opcode::Call) {
+                    if bytecode.len() > i + 5 {
+                        let second = bytecode[i + 2].into();
+                        let third = bytecode[i + 3].into();
+                        let fourth = bytecode[i + 4].into();
+                        if let (Opcode::SetReturnValue, Opcode::CheckReturn, Opcode::Return) =
+                            (second, third, fourth)
+                        {
+                            bytecode[i] = Opcode::TailCall as u8;
+                        }
+                    } else {
+                        return;
+                    }
+                }
+            }
+        }
+    }
+
     /// Finish compiling code with the [`ByteCompiler`] and return the generated [`CodeBlock`].
     #[inline]
     #[must_use]
@@ -1521,6 +1546,8 @@ impl<'ctx> ByteCompiler<'ctx> {
         }
         self.r#return(false);
 
+        Self::optimize(&mut self.bytecode, self.code_block_flags);
+
         CodeBlock {
             name: self.function_name,
             length: self.length,

core/engine/src/vm/code_block.rs

@@ -444,6 +444,9 @@ impl CodeBlock {
             | Instruction::Call {
                 argument_count: value,
             }
+            | Instruction::TailCall {
+                argument_count: value,
+            }
             | Instruction::New {
                 argument_count: value,
             }
@@ -746,8 +749,7 @@ impl CodeBlock {
             | Instruction::Reserved54
             | Instruction::Reserved55
             | Instruction::Reserved56
-            | Instruction::Reserved57
+            | Instruction::Reserved57 => unreachable!("Reserved opcodes are unreachable"),
-            | Instruction::Reserved58 => unreachable!("Reserved opcodes are unrechable"),
         }
     }
 }

core/engine/src/vm/flowgraph/mod.rs

@@ -201,6 +201,7 @@ impl CodeBlock {
                 }
                 Instruction::CallEval { .. }
                 | Instruction::Call { .. }
+                | Instruction::TailCall { .. }
                 | Instruction::New { .. }
                 | Instruction::SuperCall { .. }
                 | Instruction::ConcatToString { .. }
@@ -515,8 +516,7 @@ impl CodeBlock {
                 | Instruction::Reserved54
                 | Instruction::Reserved55
                 | Instruction::Reserved56
-                | Instruction::Reserved57
+                | Instruction::Reserved57 => unreachable!("Reserved opcodes are unreachable"),
-                | Instruction::Reserved58 => unreachable!("Reserved opcodes are unrechable"),
             }
         }
 

core/engine/src/vm/opcode/call/mod.rs

@@ -1,5 +1,5 @@
 use crate::{
-    builtins::{promise::PromiseCapability, Promise},
+    builtins::{function::OrdinaryFunction, promise::PromiseCapability, Promise},
     error::JsNativeError,
     module::{ModuleKind, Referrer},
     object::FunctionObjectBuilder,
@@ -200,6 +200,82 @@ impl Operation for Call {
     }
 }
 
+/// `TailCall` implements the Opcode Operation for `Opcode::TailCall`
+///
+/// Operation:
+///  - Tail call a function
+#[derive(Debug, Clone, Copy)]
+pub(crate) struct TailCall;
+
+impl TailCall {
+    fn operation(context: &mut Context, argument_count: usize) -> JsResult<CompletionType> {
+        let at = context.vm.stack.len() - argument_count;
+        let func = &context.vm.stack[at - 1];
+
+        let Some(object) = func.as_object() else {
+            return Err(JsNativeError::typ()
+                .with_message("not a callable function")
+                .into());
+        };
+
+        let is_ordinary_function = object.is::<OrdinaryFunction>();
+        object.__call__(argument_count).resolve(context)?;
+
+        // only tail call for ordinary functions
+        // don't tail call on the main script
+        // TODO: the 3 needs to be reviewed
+        if is_ordinary_function && context.vm.frames.len() > 3 {
+            // check that caller is also ordinary function
+            let frames = &context.vm.frames;
+            let caller_frame = &frames[frames.len() - 2];
+            let caller_function = caller_frame
+                .function(&context.vm)
+                .expect("there must be a caller function");
+            if caller_function.is::<OrdinaryFunction>() {
+                // remove caller's CallFrame
+                let frames = &mut context.vm.frames;
+                let caller_frame = frames.swap_remove(frames.len() - 2);
+
+                // remove caller's prologue from stack
+                // this + func + arguments
+                let to_remove = 1 + 1 + caller_frame.argument_count as usize;
+                context
+                    .vm
+                    .stack
+                    .drain((caller_frame.fp as usize)..(caller_frame.fp as usize + to_remove));
+
+                // update invoked function's fp
+                let frames = &mut context.vm.frames;
+                let invoked_frame = frames.last_mut().expect("invoked frame must exist");
+                invoked_frame.set_exit_early(caller_frame.exit_early());
+                invoked_frame.fp -= to_remove as u32;
+            }
+        }
+        Ok(CompletionType::Normal)
+    }
+}
+
+impl Operation for TailCall {
+    const NAME: &'static str = "TailCall";
+    const INSTRUCTION: &'static str = "INST - TailCall";
+    const COST: u8 = 3;
+
+    fn execute(context: &mut Context) -> JsResult<CompletionType> {
+        let argument_count = context.vm.read::<u8>();
+        Self::operation(context, argument_count as usize)
+    }
+
+    fn execute_with_u16_operands(context: &mut Context) -> JsResult<CompletionType> {
+        let argument_count = context.vm.read::<u16>() as usize;
+        Self::operation(context, argument_count)
+    }
+
+    fn execute_with_u32_operands(context: &mut Context) -> JsResult<CompletionType> {
+        let argument_count = context.vm.read::<u32>();
+        Self::operation(context, argument_count as usize)
+    }
+}
+
 #[derive(Debug, Clone, Copy)]
 pub(crate) struct CallSpread;
 

core/engine/src/vm/opcode/mod.rs

@@ -1700,6 +1700,13 @@ generate_opcodes! {
     /// Stack: this, func, argument_1, ... argument_n **=>** result
     Call { argument_count: VaryingOperand },
 
+    /// Tail call a function.
+    ///
+    /// Operands: argument_count: `u32`
+    ///
+    /// Stack: this, func, argument_1, ... argument_n **=>**
+    TailCall { argument_count: VaryingOperand },
+
     /// Call a function where the arguments contain spreads.
     ///
     /// Operands:
@@ -2220,8 +2227,6 @@ generate_opcodes! {
     Reserved56 => Reserved,
     /// Reserved [`Opcode`].
     Reserved57 => Reserved,
-    /// Reserved [`Opcode`].
-    Reserved58 => Reserved,
 }
 
 /// Specific opcodes for bindings.