LibJS: Add hand-written assembly bytecode interpreter - #8299
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Replace individual bool bitfields in Object (m_is_extensible, m_has_parameter_map, m_has_magical_length_property, etc.) with a single u8 m_flags field and Flag:: constants. This consolidates 8 scattered bitfields into one byte with explicit bit positions, making them easy to access from generated assembly code at a known offset. It also converts the virtual is_function() and is_ecmascript_function_object() methods to flag-based checks, avoiding virtual dispatch for these hot queries. ProxyObject now explicitly clears the IsFunction flag in its constructor when wrapping a non-callable target, instead of relying on a virtual is_function() override.
Move Interpreter::get() and set() from the .cpp file into the header as inline methods. Make handle_exception(), perform_call(), perform_call_impl(), and the HandleExceptionResponse enum public so they can be called by the upcoming assembly interpreter's C++ glue code. Also add set_running_execution_context() for the same reason.
Move the Bytecode.def parser, field type info, and layout computation out of Rust/build.rs into a standalone BytecodeDef crate. This allows both the Rust bytecode codegen (build.rs) and the upcoming AsmIntGen tool to share a single source of truth for instruction field offsets and sizes. The AsmIntGen directory is excluded from the workspace since it has its own Cargo.toml and is built separately by CMake.
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AsmIntGen is a Rust tool that compiles a custom assembly DSL into native x86_64 or aarch64 assembly (.S files). It reads struct field offsets from a generated constants file and instruction layouts from Bytecode.def (via the BytecodeDef crate) to emit platform-specific code for each bytecode handler. The DSL provides a portable instruction set with register aliases, field access syntax, labels, conditionals, and calls. Each backend (codegen_x86_64.rs, codegen_aarch64.rs) translates this into the appropriate platform assembly with correct calling conventions (SysV AMD64, AAPCS64).
Add a new interpreter that executes bytecode via generated assembly, written in a custom DSL (asmint.asm) that AsmIntGen compiles to native x86_64 or aarch64 code. The interpreter keeps the bytecode program counter and register file pointer in machine registers for fast access, dispatching opcodes through a jump table. Hot paths (arithmetic, comparisons, property access on simple objects) are handled entirely in assembly, with cold/complex operations calling into C++ helper functions defined in AsmInterpreter.cpp. A small build-time tool (gen_asm_offsets) uses offsetof() to emit struct field offsets as constants consumed by the DSL, ensuring the assembly stays in sync with C++ struct layouts. The interpreter is enabled by default on platforms that support it. The C++ interpreter can be selected via LIBJS_USE_CPP_INTERPRETER=1. Currently supported platforms: - Linux/x86_64 - Linux/aarch64 - macOS/x86_64 - macOS/aarch64
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This PR adds a new bytecode interpreter for LibJS that executes via generated assembly instead of the C++ indirect-threaded computed goto interpreter. The bytecode PC and register file pointer live in machine registers, opcodes dispatch through a jump table, and hot paths (arithmetic, comparisons, property access on simple objects) stay entirely in assembly. Cold and complex operations call into C++ helpers.
The assembly is written in a custom DSL (asmint.asm) that a Rust tool (AsmIntGen) compiles into native x86_64 or aarch64 assembly. Instruction layouts come from Bytecode.def and struct field offsets from a small build-time C++ tool, so the assembly stays in sync with C++ struct layouts automatically.
Supported on Linux and macOS, both x86_64 and aarch64. Enabled by default where supported. Set LIBJS_USE_CPP_INTERPRETER=1 to fall back.
Benchmarks look great. There's more work to do on squeezing the generated code for performance ofc :)
macOS (M3 MacBook Pro):
Linux (AMD Ryzen 7 PRO 8700GE)