arm.rs

// 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!
//
// Also at this time this file is heavily based off the x86_64 file, so you'll
// probably want to read that one as well.

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 /* r0 */) {
    naked_asm!(
        "
        // Save callee-saved registers
        push {{r4-r11,lr}}

        // Swap stacks, recording our current stack pointer
        ldr r4, [r0, #-0x08]
        str sp, [r0, #-0x08]
        mov sp, r4

        // Restore and return
        pop {{r4-r11,lr}}
        bx lr
        ",
    );
}

pub(crate) unsafe fn wasmtime_fiber_init(
    top_of_stack: *mut u8,
    entry_point: extern "C" fn(*mut u8, *mut u8),
    entry_arg0: *mut u8,
) {
    #[repr(C)]
    #[derive(Default)]
    struct InitialStack {
        r4: *mut u8,
        r5: *mut u8,
        r6: *mut u8,
        r7: *mut u8,
        r8: *mut u8,
        r9: *mut u8,
        r10: *mut u8,
        r11: *mut u8,
        lr: *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 {
            r9: entry_arg0,
            r10: entry_point as *mut u8,
            r11: top_of_stack,
            lr: wasmtime_fiber_start as *mut u8,
            last_sp: initial_stack.cast(),
            ..InitialStack::default()
        });
    }
}

#[unsafe(naked)]
unsafe extern "C" fn wasmtime_fiber_start() -> ! {
    naked_asm!(
        "
        .cfi_startproc simple
        .cfi_def_cfa_offset 0
        // See the x86_64 file for more commentary on what these CFI directives
        // are doing. Like over there note that the relative offsets to
        // registers here match the frame layout in `wasmtime_fiber_switch`.
        //
        // TODO: this is only lightly tested. This gets backtraces in gdb but
        // not at runtime. Perhaps the libgcc at runtime was too old? Doesn't
        // support something here? Unclear. Will need investigation if someone
        // ends up needing this and it still doesn't work.
        .cfi_escape 0x0f,    /* DW_CFA_def_cfa_expression */ \
            5,               /* the byte length of this expression */ \
            0x7d, 0x00,      /* DW_OP_breg14(%sp) + 0 */ \
            0x06,            /* DW_OP_deref */ \
            0x23, 0x24	 /* DW_OP_plus_uconst 0x24 */

        .cfi_rel_offset lr, -0x04
        .cfi_rel_offset r11, -0x08
        .cfi_rel_offset r10, -0x0c
        .cfi_rel_offset r9, -0x10
        .cfi_rel_offset r8, -0x14
        .cfi_rel_offset r7, -0x18
        .cfi_rel_offset r6, -0x1c
        .cfi_rel_offset r5, -0x20
        .cfi_rel_offset r4, -0x24

        mov r1, r11
        mov r0, r9
        blx r10
        .cfi_endproc
        ",
    );
}