perf(context): pass actor sp through registers, not TLS

switch_to_actor takes the target sp in rdi and returns the actor's
next saved sp in rax (handed over by switch_to_scheduler's shim).
Deletes the ACTOR_SP thread-local and halves the helper calls per
one-way switch (2 -> 1); the scheduler loop also drops its
set_actor_sp/get_actor_sp TLS round-trips. SCHEDULER_SP stays: a
yielding actor at arbitrary call depth has no argument channel back.

asm before/after in outputs/history.md session 1-2. Tests: 354 pass.

src/context.rs

@@ -4,16 +4,20 @@
 //! actor running on its own mmap'd stack. The compiler cannot do this; the
 //! whole point of `#[unsafe(naked)]` is that we control every instruction.
 //!
-//! `SCHEDULER_SP` and `ACTOR_SP` are thread-locals holding each side's saved
-//! stack pointer. `init_actor_stack` builds the initial stack so that the
-//! first `switch_to_actor` lands inside the entry function with `rsp % 16 == 8`
-//! (the x86-64 ABI requirement at function entry).
+//! The actor's stack pointer travels in registers: `switch_to_actor` takes
+//! the target sp as its argument and returns the sp the actor saved when it
+//! next yielded (handed back in `rax` by `switch_to_scheduler`'s shim). Only
+//! the *scheduler* sp lives in a thread-local — the yielding actor sits at
+//! arbitrary call depth with no argument channel back to the scheduler, so
+//! TLS is the one place it can find the way home. `init_actor_stack` builds
+//! the initial stack so that the first `switch_to_actor` lands inside the
+//! entry function with `rsp % 16 == 8` (the x86-64 ABI requirement at
+//! function entry).
 
 use std::cell::Cell;
 
 thread_local! {
     static SCHEDULER_SP: Cell<usize> = const { Cell::new(0) };
-    static ACTOR_SP:     Cell<usize> = const { Cell::new(0) };
 }
 
 fn get_scheduler_sp() -> usize {
@@ -22,12 +26,6 @@ fn get_scheduler_sp() -> usize {
 fn set_scheduler_sp(v: usize) {
     SCHEDULER_SP.with(|c| c.set(v))
 }
-pub fn get_actor_sp() -> usize {
-    ACTOR_SP.with(|c| c.get())
-}
-pub fn set_actor_sp(v: usize) {
-    ACTOR_SP.with(|c| c.set(v))
-}
 
 // ---------------------------------------------------------------------------
 // Initial stack layout
@@ -78,12 +76,24 @@ pub fn init_actor_stack(top: *mut u8, entry: extern "C-unwind" fn()) -> usize {
 // ---------------------------------------------------------------------------
 // Context switch shims
 //
-// Each shim:
-//   1. Pushes the six callee-saved integer registers.
-//   2. Snaps rsp into rdi and calls the Rust helper that stores it.
-//   3. Calls the Rust helper that returns the *other* side's saved rsp.
-//   4. Moves that into rsp.
-//   5. Pops the six registers and rets.
+// switch_to_actor_asm (rdi = target actor sp, returns rax = the sp the actor
+// saved when it next yielded):
+//   1. Pushes the six callee-saved integer registers (scheduler side).
+//   2. Stashes the target sp in rbx — free scratch: the register's live
+//      value is on the stack we just pushed to, and the pops below load the
+//      *other* side's values anyway — then snaps rsp into rdi and calls the
+//      Rust helper that stores it in SCHEDULER_SP.
+//   3. Installs the target sp and pops the actor's registers; `ret` lands
+//      where the actor yielded (or in `entry` on first resume).
+//
+// switch_to_scheduler_asm (no args; its "return value" materialises on the
+// OTHER stack, as switch_to_actor's rax):
+//   1. Pushes the six callee-saved integer registers (actor side).
+//   2. Stashes its own rsp in rbx (same free-scratch argument), asks the
+//      Rust helper for SCHEDULER_SP.
+//   3. Installs the scheduler sp, moves the saved actor sp into rax, pops
+//      the scheduler's registers and rets — completing the scheduler's
+//      `switch_to_actor(sp)` call with the actor's new sp as its result.
 //
 // XMM registers are NOT saved here. We rely on every yield happening through
 // a Rust call site, which means the compiler has spilled any live XMM state
@@ -94,31 +104,32 @@ pub fn init_actor_stack(top: *mut u8, entry: extern "C-unwind" fn()) -> usize {
 // ---------------------------------------------------------------------------
 
 #[unsafe(naked)]
-unsafe extern "C" fn switch_to_actor_asm() {
+unsafe extern "C" fn switch_to_actor_asm(actor_sp: usize) -> usize {
     core::arch::naked_asm!(
         "push rbx", "push rbp", "push r12", "push r13", "push r14", "push r15",
+        "mov rbx, rdi",
         "mov rdi, rsp",
         "call {set_sched_sp}",
-        "call {get_actor_sp}",
-        "mov rsp, rax",
+        "mov rsp, rbx",
         "pop r15", "pop r14", "pop r13", "pop r12", "pop rbp", "pop rbx",
         "ret",
         set_sched_sp = sym set_scheduler_sp,
-        get_actor_sp = sym get_actor_sp,
     );
 }
 
-/// Resume the actor whose sp is in `ACTOR_SP`. Returns when the actor yields.
+/// Resume the actor whose saved stack pointer is `actor_sp`. Returns when the
+/// actor yields, with the stack pointer the actor saved as it did — store it
+/// back into the slot for the next resume.
 ///
 /// # Safety
 ///
-/// The caller must be running on a scheduler thread with a valid actor stack
-/// pointer installed in `ACTOR_SP` — either by `init_actor_stack` (first
-/// resume) or by a prior `switch_to_scheduler` (subsequent resumes). Resuming
-/// with an unset or stale `ACTOR_SP` transfers control to an arbitrary address.
-/// Must not be called from within an actor (only the scheduler side may resume).
-pub unsafe fn switch_to_actor() {
-    unsafe { switch_to_actor_asm() };
+/// `actor_sp` must be a valid saved actor stack pointer — either from
+/// `init_actor_stack` (first resume) or the value a prior `switch_to_actor`
+/// returned for this actor (subsequent resumes). Resuming with a stale or
+/// forged sp transfers control to an arbitrary address. Must not be called
+/// from within an actor (only the scheduler side may resume).
+pub unsafe fn switch_to_actor(actor_sp: usize) -> usize {
+    unsafe { switch_to_actor_asm(actor_sp) }
 }
 
 /// Yield from the running actor back to its scheduler thread. Returns when the
@@ -135,13 +146,12 @@ pub unsafe fn switch_to_actor() {
 pub unsafe extern "C" fn switch_to_scheduler() {
     core::arch::naked_asm!(
         "push rbx", "push rbp", "push r12", "push r13", "push r14", "push r15",
-        "mov rdi, rsp",
-        "call {set_actor_sp}",
+        "mov rbx, rsp",
         "call {get_sched_sp}",
         "mov rsp, rax",
+        "mov rax, rbx",
         "pop r15", "pop r14", "pop r13", "pop r12", "pop rbp", "pop rbx",
         "ret",
-        set_actor_sp = sym set_actor_sp,
         get_sched_sp = sym get_scheduler_sp,
     );
 }

src/runtime.rs

@@ -116,7 +116,7 @@ use crate::actor::{
     take_last_outcome, Actor, Outcome,
 };
 use crate::channel::Sender;
-use crate::context::{get_actor_sp, set_actor_sp, switch_to_actor};
+use crate::context::switch_to_actor;
 use crate::io::IoThread;
 use crate::monitor::{Down, DownReason, MonitorId};
 use crate::pid::Pid;
@@ -2228,7 +2228,6 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
             .current_pid_index
             .store(pid.index(), Ordering::Relaxed);
 
-        set_actor_sp(sp);
         set_current_pid(pid);
         crate::preempt::set_current_stop(stop_flag);
         crate::preempt::set_current_slot(slot as *const Slot);
@@ -2254,7 +2253,7 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
         crate::causal::on_resume(slot);
 
         crate::te!(crate::trace::Event::Resume(pid));
-        unsafe { switch_to_actor() };
+        let saved_sp = unsafe { switch_to_actor(sp) };
 
         PREEMPTION_ENABLED.with(|c| c.set(false));
         // RFC 016 Chunk 2: charge the cycles this resume consumed to the actor
@@ -2268,7 +2267,6 @@ fn schedule_loop(inner: &Arc<RuntimeInner>, slot_idx: usize) {
         crate::preempt::clear_current_slot();
 
         let intent = YIELD_INTENT.with(|c| c.get());
-        let saved_sp = get_actor_sp();
         slot.sp.store(saved_sp, Ordering::Relaxed);
         // RFC 019 §2: sampled high-water — one branch + at most one store
         // into the line the store above just dirtied. Relaxed and advisory;

tests/context.rs

@@ -1,9 +1,7 @@
 //! Low-level context-switch tests. These poke `init_actor_stack` and the
 //! naked asm shims directly — no scheduler involved.
 
-use smarm::context::{
-    get_actor_sp, init_actor_stack, set_actor_sp, switch_to_actor, switch_to_scheduler,
-};
+use smarm::context::{init_actor_stack, switch_to_actor, switch_to_scheduler};
 use smarm::stack::Stack;
 use std::cell::Cell;
 
@@ -31,8 +29,7 @@ fn actor_runs_and_returns_to_scheduler() {
     reset_log();
     let stack = Stack::new(64 * 1024, 4096).unwrap();
     let sp = init_actor_stack(stack.top(), actor_simple);
-    set_actor_sp(sp);
-    unsafe { switch_to_actor() };
+    let _ = unsafe { switch_to_actor(sp) };
     assert_eq!(get_log(), 0x1);
 }
 
@@ -48,12 +45,11 @@ fn actor_yields_and_resumes() {
     reset_log();
     let stack = Stack::new(64 * 1024, 4096).unwrap();
     let sp = init_actor_stack(stack.top(), actor_two_steps);
-    set_actor_sp(sp);
 
-    unsafe { switch_to_actor() };
+    let sp = unsafe { switch_to_actor(sp) };
     assert_eq!(get_log(), 0x1, "after first resume");
 
-    unsafe { switch_to_actor() };
+    let _ = unsafe { switch_to_actor(sp) };
     assert_eq!(get_log(), 0x1 | 0x2, "after second resume");
 }
 
@@ -96,10 +92,9 @@ extern "C-unwind" fn actor_reg_check() {
 fn callee_saved_registers_survive_yield() {
     let stack = Stack::new(64 * 1024, 4096).unwrap();
     let sp = init_actor_stack(stack.top(), actor_reg_check);
-    set_actor_sp(sp);
     unsafe {
-        switch_to_actor();
-        switch_to_actor();
+        let sp = switch_to_actor(sp);
+        let _ = switch_to_actor(sp);
     }
     assert_eq!(
         REG_BEFORE.get().copied().unwrap(),
@@ -138,18 +133,11 @@ fn two_actors_dont_corrupt_each_other() {
     let sp_a = init_actor_stack(stack_a.top(), actor_a);
     let sp_b = init_actor_stack(stack_b.top(), actor_b);
 
-    set_actor_sp(sp_a);
-    unsafe { switch_to_actor() };
-    let sp_a = get_actor_sp();
+    let sp_a = unsafe { switch_to_actor(sp_a) };
+    let sp_b = unsafe { switch_to_actor(sp_b) };
 
-    set_actor_sp(sp_b);
-    unsafe { switch_to_actor() };
-    let sp_b = get_actor_sp();
-
-    set_actor_sp(sp_a);
-    unsafe { switch_to_actor() };
-    set_actor_sp(sp_b);
-    unsafe { switch_to_actor() };
+    let _ = unsafe { switch_to_actor(sp_a) };
+    let _ = unsafe { switch_to_actor(sp_b) };
 
     assert_eq!(A_VAL.with(|c| c.get()), 0xA00D);
     assert_eq!(B_VAL.with(|c| c.get()), 0xB00D);