Generator::allocate_register used to scan the free pool to find the
lowest-numbered register and then Vec::remove it, making every
allocation O(n) in the size of the pool. When loading https://x.com/
on my Linux machine, we spent ~800ms in this function alone!
This logic only existed to match the C++ register allocation ordering
while transitioning from C++ to Rust in the LibJS compiler, so now
we can simply get rid of it and make it instant. :^)
So drop the "always hand out the lowest-numbered free register" policy
and use the pool as a plain LIFO stack. Pushing and popping the back
of the Vec are both O(1), and peak register usage is unchanged since
the policy only affects which specific register gets reused, not how
aggressively.
Teach the Rust bytecode generator to treat the synthetic entry
GetLexicalEnvironment as a removable prologue load.
We still model reg4 as the saved entry lexical environment during
codegen, but assemble() now deletes that load when no emitted
instruction refers to the saved environment register. This keeps the
semantics of unwinding and environment restoration intact while letting
empty functions and other simple bodies start at their first real
instruction.
Specialize only the fixed unary case in the bytecode generator and let
all other argument counts keep using the generic Call instruction. This
keeps the builtin bytecode simple while still covering the common fast
path.
The asm interpreter handles int32 inputs directly, applies the ToUint16
mask in-place, and reuses the VM's cached ASCII single-character
strings when the result is 7-bit representable. Non-ASCII single code
unit results stay on the dedicated builtin path via a small helper, and
the dedicated slow path still handles the generic cases.
