feat: add experimental tailcall vm backend

fix-vm/src/bytecode_reader.rs

@@ -12,6 +12,7 @@ pub(crate) struct BytecodeReader<'a> {
 }
 
 impl<'a> BytecodeReader<'a> {
+    #[cfg_attr(feature = "tailcall", allow(dead_code))]
     pub(crate) fn new(bytecode: &'a [u8], pc: usize) -> Self {
         Self {
             bytecode,
@@ -20,6 +21,16 @@ impl<'a> BytecodeReader<'a> {
         }
     }
 
+    #[inline(always)]
+    #[cfg_attr(not(feature = "tailcall"), allow(dead_code))]
+    pub(crate) fn from_after_op(bytecode: &'a [u8], inst_start_pc: usize) -> Self {
+        Self {
+            bytecode,
+            pc: inst_start_pc + 1,
+            inst_start_pc,
+        }
+    }
+
     #[inline(always)]
     #[cfg_attr(debug_assertions, track_caller)]
     fn read_array<const N: usize>(&mut self) -> [u8; N] {
@@ -31,6 +42,7 @@ impl<'a> BytecodeReader<'a> {
     }
 
     #[inline(always)]
+    #[cfg_attr(feature = "tailcall", allow(dead_code))]
     pub(crate) fn read_op(&mut self) -> fix_codegen::Op {
         use fix_codegen::Op;
         self.inst_start_pc = self.pc;
@@ -105,16 +117,17 @@ impl<'a> BytecodeReader<'a> {
     }
 
     #[inline(always)]
-    pub(crate) fn read_attr_key_data<C: crate::VmContext>(&mut self, ctx: &C) -> crate::AttrKeyData {
+    pub(crate) fn read_attr_key_data<C: crate::VmContext>(
+        &mut self,
+        ctx: &C,
+    ) -> crate::AttrKeyData {
         use fix_codegen::AttrKeyType;
         let tag = self.read_u8();
         let ty = AttrKeyType::try_from_primitive(tag)
             .unwrap_or_else(|err| panic!("unknown key tag: {:#04x}", err.number));
         match ty {
             AttrKeyType::Static => crate::AttrKeyData::Static(self.read_string_id()),
-            AttrKeyType::Dynamic => {
-                crate::AttrKeyData::Dynamic(self.read_operand_data(ctx))
-            }
+            AttrKeyType::Dynamic => crate::AttrKeyData::Dynamic(self.read_operand_data(ctx)),
         }
     }