⚡ Bolt: Optimize string allocation in BytecodeDisassembler

.jules/bolt.md

@@ -17,3 +17,6 @@
 ## 2025-02-12 - String allocation optimization
 **Learning:** In heavily used loops, especially for rendering or stringifying tasks, `push_str(&format!(...))` allocates temporary strings unnecessarily. Replacing it with `write!` directly onto the mutable `String` buffer (using `std::fmt::Write`) skips the intermediate allocations and improves performance significantly.
 **Action:** Use `std::fmt::Write` and the `write!` macro instead of `push_str(&format!(...))` when generating strings in loops to prevent temporary allocations and improve performance.
+## 2024-08-01 - Bytecode Disassembler String Allocation Optimization
+**Learning:** Formatting directly into a string buffer inside a tight loop with `write!(buffer, ...)` avoids unnecessary string heap allocations compared to `buffer.push_str(&format!(...))`.
+**Action:** Always prefer formatting directly into the target String buffer when concatenating strings in loops in performance-sensitive paths like debuggers or disassemblers.

compiler/bytecode/src/disassembler.rs

@@ -28,45 +28,46 @@ impl BytecodeDisassembler {
             func.name, func.param_count, func.local_count, func.max_stack_size
         );
 
+        // Bolt performance optimization: Avoid unnecessary string allocation for inst.op.
+        // `inst.op` implements `Debug`, so we can format it directly into the final string
+        // instead of pre-allocating intermediate Strings with `format!`.
         for (idx, inst) in func.chunk.instructions.iter().enumerate() {
-            let op_str = format!("{:?}", inst.op);
-            let mut line = format!("{:04}  {:<15}", idx, op_str);
+            let _ = write!(output, "{:04}  {:<15?}", idx, inst.op);
 
             for op in &inst.operands {
                 match op {
                     Operand::ConstantIndex(c_idx) => {
                         if let Some(lit) = func.chunk.constants.get(*c_idx) {
-                            let _ = write!(line, "  #{:<3} ({:?})", c_idx, lit);
+                            let _ = write!(output, "  #{:<3} ({:?})", c_idx, lit);
                         } else {
-                            let _ = write!(line, "  #{:<3} (invalid)", c_idx);
+                            let _ = write!(output, "  #{:<3} (invalid)", c_idx);
                         }
                     }
                     Operand::LocalIndex(l_idx) => {
                         if let Some(name) = func.debug_symbols.local_names.get(l_idx) {
-                            let _ = write!(line, "  local_{:<2} ({})", l_idx, name);
+                            let _ = write!(output, "  local_{:<2} ({})", l_idx, name);
                         } else {
-                            let _ = write!(line, "  local_{}", l_idx);
+                            let _ = write!(output, "  local_{}", l_idx);
                         }
                     }
                     Operand::JumpOffset(offset) => {
                         let target = (idx as i32) + offset;
-                        let _ = write!(line, "  offset_{:<3} (target: {:04})", offset, target);
+                        let _ = write!(output, "  offset_{:<3} (target: {:04})", offset, target);
                     }
                     Operand::Count(n) => {
-                        let _ = write!(line, "  count_{}", n);
+                        let _ = write!(output, "  count_{}", n);
                     }
                     Operand::GlobalIndex(g_idx) => {
-                        let _ = write!(line, "  global_{}", g_idx);
+                        let _ = write!(output, "  global_{}", g_idx);
                     }
                     Operand::Register(r_idx) => {
-                        let _ = write!(line, "  reg_{}", r_idx);
+                        let _ = write!(output, "  reg_{}", r_idx);
                     }
                     Operand::FunctionIndex(f_idx) => {
-                        let _ = write!(line, "  func_{}", f_idx);
+                        let _ = write!(output, "  func_{}", f_idx);
                     }
                 }
             }
-            output.push_str(&line);
             output.push('\n');
         }
         output