Create reserve_padded_u32, nice for readability

compiler/gen_wasm/src/backend.rs

@@ -43,14 +43,11 @@ pub struct WasmBackend<'a> {
 
 impl<'a> WasmBackend<'a> {
     pub fn new(env: &'a Env<'a>, proc_symbols: Vec<'a, Symbol>) -> Self {
-        // Code section is prefixed with the number of Wasm functions
-        // For now, this is the same as the number of IR procedures (until we start inlining!)
         let mut code_section_bytes = std::vec::Vec::with_capacity(4096);
 
-        // Reserve space for code section header: inner byte length and number of functions
+        // Code section header
-        // Padded to the maximum 5 bytes each, so we can update later without moving everything
+        code_section_bytes.reserve_padded_u32(); // byte length, to be written at the end
-        code_section_bytes.resize(10, 0);
+        code_section_bytes.encode_padded_u32(proc_symbols.len() as u32); // modified later in unit tests
-        code_section_bytes.overwrite_padded_u32(5, proc_symbols.len() as u32); // gets modified in unit tests
 
         WasmBackend {
             env,

compiler/gen_wasm/src/serialize.rs

@@ -79,6 +79,7 @@ pub trait SerialBuffer {
     encode_sleb128!(encode_i32, i32);
     encode_sleb128!(encode_i64, i64);
 
+    fn reserve_padded_u32(&mut self) -> usize;
     fn encode_padded_u32(&mut self, value: u32) -> usize;
     fn overwrite_padded_u32(&mut self, index: usize, value: u32);
 
@@ -120,6 +121,11 @@ impl SerialBuffer for std::vec::Vec<u8> {
     fn size(&self) -> usize {
         self.len()
     }
+    fn reserve_padded_u32(&mut self) -> usize {
+        let index = self.len();
+        self.resize(index + 5, 0xff);
+        index
+    }
     fn encode_padded_u32(&mut self, value: u32) -> usize {
         let index = self.len();
         let new_len = index + 5;
@@ -142,6 +148,11 @@ impl<'a> SerialBuffer for Vec<'a, u8> {
     fn size(&self) -> usize {
         self.len()
     }
+    fn reserve_padded_u32(&mut self) -> usize {
+        let index = self.len();
+        self.resize(index + 5, 0xff);
+        index
+    }
     fn encode_padded_u32(&mut self, value: u32) -> usize {
         let index = self.len();
         let new_len = index + 5;