compiler_builtins/mem/

x86_64.rs

// On most modern Intel and AMD processors, "rep movsq" and "rep stosq" have
// been enhanced to perform better than an simple qword loop, making them ideal
// for implementing memcpy/memset. Note that "rep cmps" has received no such
// enhancement, so it is not used to implement memcmp.
//
// On certain recent Intel processors, "rep movsb" and "rep stosb" have been
// further enhanced to automatically select the best microarchitectural
// implementation based on length and alignment. See the following features from
// the "Intel® 64 and IA-32 Architectures Optimization Reference Manual":
//  - ERMSB - Enhanced REP MOVSB and STOSB (Ivy Bridge and later)
//  - FSRM - Fast Short REP MOV (Ice Lake and later)
//  - Fast Zero-Length MOVSB (On no current hardware)
//  - Fast Short STOSB (On no current hardware)
//
// To simplify things, we switch to using the byte-based variants if the "ermsb"
// feature is present at compile-time. We don't bother detecting other features.
// Note that ERMSB does not enhance the backwards (DF=1) "rep movsb".

use core::arch::asm;
use core::{intrinsics, mem};

#[inline(always)]
#[cfg(target_feature = "ermsb")]
pub unsafe fn copy_forward(dest: *mut u8, src: *const u8, count: usize) {
    // FIXME: Use the Intel syntax once we drop LLVM 9 support on rust-lang/rust.
    core::arch::asm!(
        "repe movsb (%rsi), (%rdi)",
        inout("rcx") count => _,
        inout("rdi") dest => _,
        inout("rsi") src => _,
        options(att_syntax, nostack, preserves_flags)
    );
}

#[inline(always)]
#[cfg(not(target_feature = "ermsb"))]
pub unsafe fn copy_forward(mut dest: *mut u8, mut src: *const u8, count: usize) {
    let (pre_byte_count, qword_count, byte_count) = rep_param(dest, count);
    // Separating the blocks gives the compiler more freedom to reorder instructions.
    asm!(
        "rep movsb",
        inout("ecx") pre_byte_count => _,
        inout("rdi") dest => dest,
        inout("rsi") src => src,
        options(att_syntax, nostack, preserves_flags)
    );
    asm!(
        "rep movsq",
        inout("rcx") qword_count => _,
        inout("rdi") dest => dest,
        inout("rsi") src => src,
        options(att_syntax, nostack, preserves_flags)
    );
    asm!(
        "rep movsb",
        inout("ecx") byte_count => _,
        inout("rdi") dest => _,
        inout("rsi") src => _,
        options(att_syntax, nostack, preserves_flags)
    );
}

#[inline(always)]
pub unsafe fn copy_backward(dest: *mut u8, src: *const u8, count: usize) {
    let (pre_byte_count, qword_count, byte_count) = rep_param(dest, count);
    // We can't separate this block due to std/cld
    asm!(
        "std",
        "rep movsb",
        "sub $7, %rsi",
        "sub $7, %rdi",
        "mov {qword_count}, %rcx",
        "rep movsq",
        "test {pre_byte_count:e}, {pre_byte_count:e}",
        "add $7, %rsi",
        "add $7, %rdi",
        "mov {pre_byte_count:e}, %ecx",
        "rep movsb",
        "cld",
        pre_byte_count = in(reg) pre_byte_count,
        qword_count = in(reg) qword_count,
        inout("ecx") byte_count => _,
        inout("rdi") dest.add(count - 1) => _,
        inout("rsi") src.add(count - 1) => _,
        // We modify flags, but we restore it afterwards
        options(att_syntax, nostack, preserves_flags)
    );
}

#[inline(always)]
#[cfg(target_feature = "ermsb")]
pub unsafe fn set_bytes(dest: *mut u8, c: u8, count: usize) {
    // FIXME: Use the Intel syntax once we drop LLVM 9 support on rust-lang/rust.
    core::arch::asm!(
        "repe stosb %al, (%rdi)",
        inout("rcx") count => _,
        inout("rdi") dest => _,
        inout("al") c => _,
        options(att_syntax, nostack, preserves_flags)
    )
}

#[inline(always)]
#[cfg(not(target_feature = "ermsb"))]
pub unsafe fn set_bytes(mut dest: *mut u8, c: u8, count: usize) {
    let c = c as u64 * 0x0101_0101_0101_0101;
    let (pre_byte_count, qword_count, byte_count) = rep_param(dest, count);
    // Separating the blocks gives the compiler more freedom to reorder instructions.
    asm!(
        "rep stosb",
        inout("ecx") pre_byte_count => _,
        inout("rdi") dest => dest,
        in("rax") c,
        options(att_syntax, nostack, preserves_flags)
    );
    asm!(
        "rep stosq",
        inout("rcx") qword_count => _,
        inout("rdi") dest => dest,
        in("rax") c,
        options(att_syntax, nostack, preserves_flags)
    );
    asm!(
        "rep stosb",
        inout("ecx") byte_count => _,
        inout("rdi") dest => _,
        in("rax") c,
        options(att_syntax, nostack, preserves_flags)
    );
}

#[inline(always)]
pub unsafe fn compare_bytes(a: *const u8, b: *const u8, n: usize) -> i32 {
    #[inline(always)]
    unsafe fn cmp<T, U, F>(mut a: *const T, mut b: *const T, n: usize, f: F) -> i32
    where
        T: Clone + Copy + Eq,
        U: Clone + Copy + Eq,
        F: FnOnce(*const U, *const U, usize) -> i32,
    {
        // Ensure T is not a ZST.
        const { assert!(mem::size_of::<T>() != 0) };

        let end = a.add(intrinsics::unchecked_div(n, mem::size_of::<T>()));
        while a != end {
            if a.read_unaligned() != b.read_unaligned() {
                return f(a.cast(), b.cast(), mem::size_of::<T>());
            }
            a = a.add(1);
            b = b.add(1);
        }
        f(
            a.cast(),
            b.cast(),
            intrinsics::unchecked_rem(n, mem::size_of::<T>()),
        )
    }
    let c1 = |mut a: *const u8, mut b: *const u8, n| {
        for _ in 0..n {
            if a.read() != b.read() {
                return i32::from(a.read()) - i32::from(b.read());
            }
            a = a.add(1);
            b = b.add(1);
        }
        0
    };
    let c2 = |a: *const u16, b, n| cmp(a, b, n, c1);
    let c4 = |a: *const u32, b, n| cmp(a, b, n, c2);
    let c8 = |a: *const u64, b, n| cmp(a, b, n, c4);
    let c16 = |a: *const u128, b, n| cmp(a, b, n, c8);
    c16(a.cast(), b.cast(), n)
}

// In order to process more than on byte simultaneously when executing strlen,
// two things must be considered:
// * An n byte read with an n-byte aligned address will never cross
//   a page boundary and will always succeed. Any smaller alignment
//   may result in a read that will cross a page boundary, which may
//   trigger an access violation.
// * Surface Rust considers any kind of out-of-bounds read as undefined
//   behaviour. To dodge this, memory access operations are written
//   using inline assembly.

#[cfg(target_feature = "sse2")]
#[inline(always)]
pub unsafe fn c_string_length(mut s: *const core::ffi::c_char) -> usize {
    use core::arch::x86_64::{__m128i, _mm_cmpeq_epi8, _mm_movemask_epi8, _mm_set1_epi8};

    let mut n = 0;

    // The use of _mm_movemask_epi8 and company allow for speedups,
    // but they aren't cheap by themselves. Thus, possibly small strings
    // are handled in simple loops.

    for _ in 0..4 {
        if *s == 0 {
            return n;
        }

        n += 1;
        s = s.add(1);
    }

    // Shave of the least significand bits to align the address to a 16
    // byte boundary. The shaved of bits are used to correct the first iteration.

    let align = s as usize & 15;
    let mut s = ((s as usize) - align) as *const __m128i;
    let zero = _mm_set1_epi8(0);

    let x = {
        let r;
        asm!(
            "movdqa ({addr}), {dest}",
            addr = in(reg) s,
            dest = out(xmm_reg) r,
            options(att_syntax, nostack),
        );
        r
    };
    let cmp = _mm_movemask_epi8(_mm_cmpeq_epi8(x, zero)) >> align;

    if cmp != 0 {
        return n + cmp.trailing_zeros() as usize;
    }

    n += 16 - align;
    s = s.add(1);

    loop {
        let x = {
            let r;
            asm!(
                "movdqa ({addr}), {dest}",
                addr = in(reg) s,
                dest = out(xmm_reg) r,
                options(att_syntax, nostack),
            );
            r
        };
        let cmp = _mm_movemask_epi8(_mm_cmpeq_epi8(x, zero)) as u32;
        if cmp == 0 {
            n += 16;
            s = s.add(1);
        } else {
            return n + cmp.trailing_zeros() as usize;
        }
    }
}

// Provided for scenarios like kernel development, where SSE might not
// be available.
#[cfg(not(target_feature = "sse2"))]
#[inline(always)]
pub unsafe fn c_string_length(mut s: *const core::ffi::c_char) -> usize {
    let mut n = 0;

    // Check bytes in steps of one until
    // either a zero byte is discovered or
    // pointer is aligned to an eight byte boundary.

    while s as usize & 7 != 0 {
        if *s == 0 {
            return n;
        }
        n += 1;
        s = s.add(1);
    }

    // Check bytes in steps of eight until a zero
    // byte is discovered.

    let mut s = s as *const u64;

    loop {
        let mut cs = {
            let r: u64;
            asm!(
                "mov ({addr}), {dest}",
                addr = in(reg) s,
                dest = out(reg) r,
                options(att_syntax, nostack),
            );
            r
        };
        // Detect if a word has a zero byte, taken from
        // https://graphics.stanford.edu/~seander/bithacks.html
        if (cs.wrapping_sub(0x0101010101010101) & !cs & 0x8080808080808080) != 0 {
            loop {
                if cs & 255 == 0 {
                    return n;
                } else {
                    cs >>= 8;
                    n += 1;
                }
            }
        } else {
            n += 8;
            s = s.add(1);
        }
    }
}

/// Determine optimal parameters for a `rep` instruction.
fn rep_param(dest: *mut u8, mut count: usize) -> (usize, usize, usize) {
    // Unaligned writes are still slow on modern processors, so align the destination address.
    let pre_byte_count = ((8 - (dest as usize & 0b111)) & 0b111).min(count);
    count -= pre_byte_count;
    let qword_count = count >> 3;
    let byte_count = count & 0b111;
    (pre_byte_count, qword_count, byte_count)
}