// A WORD OF CAUTION

//

// This entire file basically needs to be kept in sync with itself. It's not

// really possible to modify just one bit of this file without understanding

// all the other bits. Documentation tries to reference various bits here and

// there but try to make sure to read over everything before tweaking things!

use core::arch::naked_asm;

#[inline(never)] // FIXME(rust-lang/rust#148307)

pub(crate) unsafe extern "C" fn wasmtime_fiber_switch(top_of_stack: *mut u8) {

    unsafe { wasmtime_fiber_switch_(top_of_stack) }

}

#[unsafe(naked)]

unsafe extern "C" fn wasmtime_fiber_switch_(top_of_stack: *mut u8 /* rdi */) {

    naked_asm!(

        "

        // We're switching to arbitrary code somewhere else, so pessimistically

        // assume that all callee-save register are clobbered. This means we need

        // to save/restore all of them.

        //

        // Note that this order for saving is important since we use CFI directives

        // below to point to where all the saved registers are.

        push rbp

        push rbx

        push r12

        push r13

        push r14

        push r15

        // Load pointer that we're going to resume at and store where we're going

        // to get resumed from. This is in accordance with the diagram at the top

        // of unix.rs.

        mov rax, -0x10[rdi]

        mov -0x10[rdi], rsp

        // Swap stacks and restore all our callee-saved registers

        mov rsp, rax

        pop r15

        pop r14

        pop r13

        pop r12

        pop rbx

        pop rbp

        ret

    ",

    );

}

pub(crate) unsafe fn wasmtime_fiber_init(

    top_of_stack: *mut u8,

    entry_point: extern "C" fn(*mut u8, *mut u8) -> *mut u8,

    entry_arg0: *mut u8,

) {

    #[repr(C)]

    #[derive(Default)]

    struct InitialStack {

        r15: *mut u8,

        r14: *mut u8,

        r13: *mut u8,

        r12: *mut u8,

        rbx: *mut u8,

        rbp: *mut u8,

        return_address: *mut u8,

        // unix.rs reserved space

        last_sp: *mut u8,

        run_result: *mut u8,

    }

    unsafe {

        let initial_stack = top_of_stack.cast::<InitialStack>().sub(1);

        initial_stack.write(InitialStack {

            r13: wasmtime_fiber_switch_ as *mut u8,

            r12: entry_arg0,

            rbx: entry_point as *mut u8,

            rbp: top_of_stack,

            return_address: wasmtime_fiber_start as *mut u8,

            last_sp: initial_stack.cast(),

            ..InitialStack::default()

        });

    }

}

// This is a pretty special function that has no real signature. Its use is to

// be the "base" function of all fibers. This entrypoint is used in

// `wasmtime_fiber_init` to bootstrap the execution of a new fiber.

//

// We also use this function as a persistent frame on the stack to emit dwarf

// information to unwind into the caller. This allows us to unwind from the

// fiber's stack back to the main stack that the fiber was called from. We use

// special dwarf directives here to do so since this is a pretty nonstandard

// function.

//

// If you're curious a decent introduction to CFI things and unwinding is at

// https://www.imperialviolet.org/2017/01/18/cfi.html

#[unsafe(naked)]

unsafe extern "C" fn wasmtime_fiber_start() -> ! {

    naked_asm!(

        "

        // Use the `simple` directive on the startproc here which indicates that

        // some default settings for the platform are omitted, since this

        // function is so nonstandard

        .cfi_startproc simple

        .cfi_def_cfa_offset 0

        // This is where things get special, we're specifying a custom dwarf

        // expression for how to calculate the CFA. The goal here is that we

        // need to load the parent's stack pointer just before the call it made

        // into `wasmtime_fiber_switch`. Note that the CFA value changes over

        // time as well because a fiber may be resumed multiple times from

        // different points on the original stack. This means that our custom

        // CFA directive involves `DW_OP_deref`, which loads data from memory.

        //

        // The expression we're encoding here is that the CFA, the stack pointer

        // of whatever called into `wasmtime_fiber_start`, is:

        //

        //        *$rsp + 0x38

        //

        // $rsp is the stack pointer of `wasmtime_fiber_start` at the time the

        // next instruction after the `.cfi_escape` is executed. Our $rsp at the

        // start of this function is 16 bytes below the top of the stack (0xAff0

        // in the diagram in unix.rs). The $rsp to resume at is stored at that

        // location, so we dereference the stack pointer to load it.

        //

        // After dereferencing, though, we have the $rsp value for

        // `wasmtime_fiber_switch` itself. That's a weird function which sort of

        // and sort of doesn't exist on the stack.  We want to point to the

        // caller of `wasmtime_fiber_switch`, so to do that we need to skip the

        // stack space reserved by `wasmtime_fiber_switch`, which is the 6 saved

        // registers plus the return address of the caller's `call` instruction.

        // Hence we offset another 0x38 bytes.

        .cfi_escape 0x0f, /* DW_CFA_def_cfa_expression */ \

            4,            /* the byte length of this expression */ \

            0x57,         /* DW_OP_reg7 (rsp) */ \

            0x06,         /* DW_OP_deref */ \

            0x23, 0x38    /* DW_OP_plus_uconst 0x38 */

        // And now after we've indicated where our CFA is for our parent

        // function, we can define that where all of the saved registers are

        // located. This uses standard `.cfi` directives which indicate that

        // these registers are all stored relative to the CFA. Note that this

        // order is kept in sync with the above register spills in

        // `wasmtime_fiber_switch`.

        .cfi_rel_offset rip, -8

        .cfi_rel_offset rbp, -16

        .cfi_rel_offset rbx, -24

        .cfi_rel_offset r12, -32

        .cfi_rel_offset r13, -40

        .cfi_rel_offset r14, -48

        .cfi_rel_offset r15, -56

        // The body of this function is pretty similar. All our parameters are

        // already loaded into registers by the switch function. The

        // `wasmtime_fiber_init` routine arranged the various values to be

        // materialized into the registers used here. Our job is to then move

        // the values into the ABI-defined registers and call the entry-point.

        mov rdi, r12

        mov rsi, rbp

        call rbx      // entry_point

        // Once the entrypoint has returned the final switch for this fiber is

        // executed. The address of the routine is in `r13` and its argument is

        // the return value of the startup routine.

        mov rdi, rax

        call r13      // wasmtime_fiber_switch_

        // This should never be reached, but in case it accidentally does then

        // terminate immediately.

        ud2

        .cfi_endproc

        ",

    );

}