rust/library/compiler-builtins/src/x86.rs

#![allow(unused_imports)]

use core::intrinsics;

// NOTE These functions are implemented using assembly because they using a custom
// calling convention which can't be implemented using a normal Rust function

// NOTE These functions are never mangled as they are not tested against compiler-rt
// and mangling ___chkstk would break the `jmp ___chkstk` instruction in __alloca

#[cfg(all(
    windows,
    target_env = "gnu",
    not(feature = "no-asm"),
    not(feature = "mangled-names")
))]
#[naked]
#[no_mangle]
pub unsafe extern "C" fn ___chkstk_ms() {
    core::arch::asm!(
        "push   %ecx",
        "push   %eax",
        "cmp    $0x1000,%eax",
        "lea    12(%esp),%ecx",
        "jb     1f",
        "2:",
        "sub    $0x1000,%ecx",
        "test   %ecx,(%ecx)",
        "sub    $0x1000,%eax",
        "cmp    $0x1000,%eax",
        "ja     2b",
        "1:",
        "sub    %eax,%ecx",
        "test   %ecx,(%ecx)",
        "pop    %eax",
        "pop    %ecx",
        "ret",
        options(noreturn, att_syntax)
    );
}

// FIXME: __alloca should be an alias to __chkstk
#[cfg(all(
    windows,
    target_env = "gnu",
    not(feature = "no-asm"),
    not(feature = "mangled-names")
))]
#[naked]
#[no_mangle]
pub unsafe extern "C" fn __alloca() {
    core::arch::asm!(
        "jmp ___chkstk", // Jump to ___chkstk since fallthrough may be unreliable"
        options(noreturn, att_syntax)
    );
}

#[cfg(all(
    windows,
    target_env = "gnu",
    not(feature = "no-asm"),
    not(feature = "mangled-names")
))]
#[naked]
#[no_mangle]
pub unsafe extern "C" fn ___chkstk() {
    core::arch::asm!(
        "push   %ecx",
        "cmp    $0x1000,%eax",
        "lea    8(%esp),%ecx", // esp before calling this routine -> ecx
        "jb     1f",
        "2:",
        "sub    $0x1000,%ecx",
        "test   %ecx,(%ecx)",
        "sub    $0x1000,%eax",
        "cmp    $0x1000,%eax",
        "ja     2b",
        "1:",
        "sub    %eax,%ecx",
        "test   %ecx,(%ecx)",
        "lea    4(%esp),%eax",  // load pointer to the return address into eax
        "mov    %ecx,%esp",     // install the new top of stack pointer into esp
        "mov    -4(%eax),%ecx", // restore ecx
        "push   (%eax)",        // push return address onto the stack
        "sub    %esp,%eax",     // restore the original value in eax
        "ret",
        options(noreturn, att_syntax)
    );
}