Store call frames in Vec instead of singly-linked list (#2164)

This storage method should be more cache friendly since we store in contiguous memory, besides that it should make #2157 a bit easier. Will work on that next after this gets merged :)

It changes the following:
- Remove the unneeded `prev` field in `CallFrame`
- Preallocate some space for future calls (16 slots)

boa_engine/src/builtins/console/mod.rs

@@ -294,16 +294,14 @@ impl Console {
 
     fn get_stack_trace(context: &mut Context) -> Vec<String> {
         let mut stack_trace: Vec<String> = vec![];
-        let mut prev_frame = context.vm.frame.as_ref();
 
-        while let Some(frame) = prev_frame {
+        for frame in context.vm.frames.iter().rev() {
             stack_trace.push(
                 context
                     .interner()
                     .resolve_expect(frame.code.name)
                     .to_owned(),
             );
-            prev_frame = frame.prev.as_ref();
         }
 
         stack_trace

boa_engine/src/builtins/generator/mod.rs

@@ -263,7 +263,7 @@ impl Generator {
 
         let result = context.run();
 
-        generator_context.call_frame = *context
+        generator_context.call_frame = context
             .vm
             .pop_frame()
             .expect("generator call frame must exist");
@@ -384,7 +384,7 @@ impl Generator {
                 context.run()
             }
         };
-        generator_context.call_frame = *context
+        generator_context.call_frame = context
             .vm
             .pop_frame()
             .expect("generator call frame must exist");

boa_engine/src/context/mod.rs

@@ -714,7 +714,6 @@ impl Context {
         let _timer = Profiler::global().start_event("Execution", "Main");
 
         self.vm.push_frame(CallFrame {
-            prev: None,
             code: code_block,
             pc: 0,
             catch: Vec::new(),
@@ -833,7 +832,7 @@ impl ContextBuilder {
             console: Console::default(),
             intrinsics: Intrinsics::default(),
             vm: Vm {
-                frame: None,
+                frames: Vec::with_capacity(16),
                 stack: Vec::with_capacity(1024),
                 trace: false,
                 stack_size_limit: 1024,

boa_engine/src/vm/call_frame.rs

@@ -7,7 +7,6 @@ use boa_gc::{Finalize, Gc, Trace};
 
 #[derive(Clone, Debug, Finalize, Trace)]
 pub struct CallFrame {
-    pub(crate) prev: Option<Box<Self>>,
     pub(crate) code: Gc<CodeBlock>,
     pub(crate) pc: usize,
     #[unsafe_ignore_trace]

boa_engine/src/vm/code_block.rs

@@ -723,7 +723,6 @@ impl JsObject {
                 let has_expressions = code.params.has_expressions();
 
                 context.vm.push_frame(CallFrame {
-                    prev: None,
                     code,
                     pc: 0,
                     catch: Vec::new(),
@@ -955,7 +954,6 @@ impl JsObject {
                 let param_count = code.params.parameters.len();
 
                 let call_frame = CallFrame {
-                    prev: None,
                     code,
                     pc: 0,
                     catch: Vec::new(),
@@ -999,7 +997,7 @@ impl JsObject {
                         state: GeneratorState::SuspendedStart,
                         context: Some(Gc::new(Cell::new(GeneratorContext {
                             environments,
-                            call_frame: *call_frame,
+                            call_frame,
                             stack,
                         }))),
                     }),
@@ -1189,7 +1187,6 @@ impl JsObject {
                 let param_count = code.params.parameters.len();
 
                 context.vm.push_frame(CallFrame {
-                    prev: None,
                     code,
                     pc: 0,
                     catch: Vec::new(),

boa_engine/src/vm/mod.rs

@@ -39,7 +39,7 @@ mod tests;
 /// Virtual Machine.
 #[derive(Debug)]
 pub struct Vm {
-    pub(crate) frame: Option<Box<CallFrame>>,
+    pub(crate) frames: Vec<CallFrame>,
     pub(crate) stack: Vec<JsValue>,
     pub(crate) trace: bool,
     pub(crate) stack_size_limit: usize,
@@ -81,7 +81,7 @@ impl Vm {
     /// If there is no frame, then this will panic.
     #[inline]
     pub(crate) fn frame(&self) -> &CallFrame {
-        self.frame.as_ref().expect("no frame found")
+        self.frames.last().expect("no frame found")
     }
 
     /// Retrieves the VM frame mutably
@@ -91,21 +91,17 @@ impl Vm {
     /// If there is no frame, then this will panic.
     #[inline]
     pub(crate) fn frame_mut(&mut self) -> &mut CallFrame {
-        self.frame.as_mut().expect("no frame found")
+        self.frames.last_mut().expect("no frame found")
     }
 
     #[inline]
-    pub(crate) fn push_frame(&mut self, mut frame: CallFrame) {
-        let prev = self.frame.take();
-        frame.prev = prev;
-        self.frame = Some(Box::new(frame));
+    pub(crate) fn push_frame(&mut self, frame: CallFrame) {
+        self.frames.push(frame);
     }
 
     #[inline]
-    pub(crate) fn pop_frame(&mut self) -> Option<Box<CallFrame>> {
-        let mut current = self.frame.take()?;
-        self.frame = current.prev.take();
-        Some(current)
+    pub(crate) fn pop_frame(&mut self) -> Option<CallFrame> {
+        self.frames.pop()
     }
 }
 
@@ -2392,7 +2388,7 @@ impl Context {
         let _timer = Profiler::global().start_event("run", "vm");
 
         if self.vm.trace {
-            let msg = if self.vm.frame().prev.is_some() {
+            let msg = if self.vm.frames.get(self.vm.frames.len() - 2).is_some() {
                 " Call Frame "
             } else {
                 " VM Start "