Replace the check_is_double pattern that loaded the full 64-bit
CANON_NAN_BITS constant (10-byte movabs on x86_64) and masked the
entire value, with a cheaper approach: extract the upper 16-bit tag
and check if (tag & NAN_BASE_TAG) == NAN_BASE_TAG.
This saves instructions at every double-check site. Additionally,
add a check_tag_is_double macro for call sites where the tag has
already been extracted into a register, avoiding redundant
extract_tag operations. This is used in 11 call sites across
coerce_to_doubles, strict_equality_core, numeric_compare, Div,
UnaryPlus, UnaryMinus, and ToInt32.
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
