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// This module implements Identifier, a short-optimized string allowed to
// contain only the ASCII characters hyphen, dot, 0-9, A-Z, a-z.
//
// As of mid-2021, the distribution of pre-release lengths on crates.io is:
//
// length count length count length count
// 0 355929 11 81 24 2
// 1 208 12 48 25 6
// 2 236 13 55 26 10
// 3 1909 14 25 27 4
// 4 1284 15 15 28 1
// 5 1742 16 35 30 1
// 6 3440 17 9 31 5
// 7 5624 18 6 32 1
// 8 1321 19 12 36 2
// 9 179 20 2 37 379
// 10 65 23 11
//
// and the distribution of build metadata lengths is:
//
// length count length count length count
// 0 364445 8 7725 18 1
// 1 72 9 16 19 1
// 2 7 10 85 20 1
// 3 28 11 17 22 4
// 4 9 12 10 26 1
// 5 68 13 9 27 1
// 6 73 14 10 40 5
// 7 53 15 6
//
// Therefore it really behooves us to be able to use the entire 8 bytes of a
// pointer for inline storage. For both pre-release and build metadata there are
// vastly more strings with length exactly 8 bytes than the sum over all lengths
// longer than 8 bytes.
//
// To differentiate the inline representation from the heap allocated long
// representation, we'll allocate heap pointers with 2-byte alignment so that
// they are guaranteed to have an unset least significant bit. Then in the repr
// we store for pointers, we rotate a 1 into the most significant bit of the
// most significant byte, which is never set for an ASCII byte.
//
// Inline repr:
//
// 0xxxxxxx 0xxxxxxx 0xxxxxxx 0xxxxxxx 0xxxxxxx 0xxxxxxx 0xxxxxxx 0xxxxxxx
//
// Heap allocated repr:
//
// 1ppppppp pppppppp pppppppp pppppppp pppppppp pppppppp pppppppp pppppppp 0
// ^ most significant bit least significant bit of orig ptr, rotated out ^
//
// Since the most significant bit doubles as a sign bit for the similarly sized
// signed integer type, the CPU has an efficient instruction for inspecting it,
// meaning we can differentiate between an inline repr and a heap allocated repr
// in one instruction. Effectively an inline repr always looks like a positive
// i64 while a heap allocated repr always looks like a negative i64.
//
// For the inline repr, we store \0 padding on the end of the stored characters,
// and thus the string length is readily determined efficiently by a cttz (count
// trailing zeros) or bsf (bit scan forward) instruction.
//
// For the heap allocated repr, the length is encoded as a base-128 varint at
// the head of the allocation.
//
// Empty strings are stored as an all-1 bit pattern, corresponding to -1i64.
// Consequently the all-0 bit pattern is never a legal representation in any
// repr, leaving it available as a niche for downstream code. For example this
// allows size_of::<Version>() == size_of::<Option<Version>>().
use crate::alloc::alloc::{alloc, dealloc, handle_alloc_error, Layout};
use core::isize;
use core::mem;
use core::num::{NonZeroU64, NonZeroUsize};
use core::ptr::{self, NonNull};
use core::slice;
use core::str;
use core::usize;
const PTR_BYTES: usize = mem::size_of::<NonNull<u8>>();
// If pointers are already 8 bytes or bigger, then 0. If pointers are smaller
// than 8 bytes, then Identifier will contain a byte array to raise its size up
// to 8 bytes total.
const TAIL_BYTES: usize = 8 * (PTR_BYTES < 8) as usize - PTR_BYTES * (PTR_BYTES < 8) as usize;
#[repr(C, align(8))]
pub(crate) struct Identifier {
head: NonNull<u8>,
tail: [u8; TAIL_BYTES],
}
impl Identifier {
pub(crate) const fn empty() -> Self {
// This is a separate constant because unsafe function calls are not
// allowed in a const fn body, only in a const, until later rustc than
// what we support.
const HEAD: NonNull<u8> = unsafe { NonNull::new_unchecked(!0 as *mut u8) };
// `mov rax, -1`
Identifier {
head: HEAD,
tail: [!0; TAIL_BYTES],
}
}
// SAFETY: string must be ASCII and not contain \0 bytes.
pub(crate) unsafe fn new_unchecked(string: &str) -> Self {
let len = string.len();
debug_assert!(len <= isize::MAX as usize);
match len as u64 {
0 => Self::empty(),
1..=8 => {
let mut bytes = [0u8; mem::size_of::<Identifier>()];
// SAFETY: string is big enough to read len bytes, bytes is big
// enough to write len bytes, and they do not overlap.
unsafe { ptr::copy_nonoverlapping(string.as_ptr(), bytes.as_mut_ptr(), len) };
// SAFETY: the head field is nonzero because the input string
// was at least 1 byte of ASCII and did not contain \0.
unsafe { mem::transmute::<[u8; mem::size_of::<Identifier>()], Identifier>(bytes) }
}
9..=0xff_ffff_ffff_ffff => {
// SAFETY: len is in a range that does not contain 0.
let size = bytes_for_varint(unsafe { NonZeroUsize::new_unchecked(len) }) + len;
let align = 2;
// On 32-bit and 16-bit architecture, check for size overflowing
// isize::MAX. Making an allocation request bigger than this to
// the allocator is considered UB. All allocations (including
// static ones) are limited to isize::MAX so we're guaranteed
// len <= isize::MAX, and we know bytes_for_varint(len) <= 5
// because 128**5 > isize::MAX, which means the only problem
// that can arise is when isize::MAX - 5 <= len <= isize::MAX.
// This is pretty much guaranteed to be malicious input so we
// don't need to care about returning a good error message.
if mem::size_of::<usize>() < 8 {
let max_alloc = usize::MAX / 2 - align;
assert!(size <= max_alloc);
}
// SAFETY: align is not zero, align is a power of two, and
// rounding size up to align does not overflow isize::MAX.
let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
// SAFETY: layout's size is nonzero.
let ptr = unsafe { alloc(layout) };
if ptr.is_null() {
handle_alloc_error(layout);
}
let mut write = ptr;
let mut varint_remaining = len;
while varint_remaining > 0 {
// SAFETY: size is bytes_for_varint(len) bytes + len bytes.
// This is writing the first bytes_for_varint(len) bytes.
unsafe { ptr::write(write, varint_remaining as u8 | 0x80) };
varint_remaining >>= 7;
// SAFETY: still in bounds of the same allocation.
write = unsafe { write.add(1) };
}
// SAFETY: size is bytes_for_varint(len) bytes + len bytes. This
// is writing to the last len bytes.
unsafe { ptr::copy_nonoverlapping(string.as_ptr(), write, len) };
Identifier {
head: ptr_to_repr(ptr),
tail: [0; TAIL_BYTES],
}
}
0x100_0000_0000_0000..=0xffff_ffff_ffff_ffff => {
unreachable!("please refrain from storing >64 petabytes of text in semver version");
}
#[cfg(no_exhaustive_int_match)] // rustc <1.33
_ => unreachable!(),
}
}
pub(crate) fn is_empty(&self) -> bool {
// `cmp rdi, -1` -- basically: `repr as i64 == -1`
let empty = Self::empty();
let is_empty = self.head == empty.head && self.tail == empty.tail;
// The empty representation does nothing on Drop. We can't let this one
// drop normally because `impl Drop for Identifier` calls is_empty; that
// would be an infinite recursion.
mem::forget(empty);
is_empty
}
fn is_inline(&self) -> bool {
// `test rdi, rdi` -- basically: `repr as i64 >= 0`
self.head.as_ptr() as usize >> (PTR_BYTES * 8 - 1) == 0
}
fn is_empty_or_inline(&self) -> bool {
// `cmp rdi, -2` -- basically: `repr as i64 > -2`
self.is_empty() || self.is_inline()
}
pub(crate) fn as_str(&self) -> &str {
if self.is_empty() {
""
} else if self.is_inline() {
// SAFETY: repr is in the inline representation.
unsafe { inline_as_str(self) }
} else {
// SAFETY: repr is in the heap allocated representation.
unsafe { ptr_as_str(&self.head) }
}
}
}
impl Clone for Identifier {
fn clone(&self) -> Self {
if self.is_empty_or_inline() {
Identifier {
head: self.head,
tail: self.tail,
}
} else {
let ptr = repr_to_ptr(self.head);
// SAFETY: ptr is one of our own heap allocations.
let len = unsafe { decode_len(ptr) };
let size = bytes_for_varint(len) + len.get();
let align = 2;
// SAFETY: align is not zero, align is a power of two, and rounding
// size up to align does not overflow isize::MAX. This is just
// duplicating a previous allocation where all of these guarantees
// were already made.
let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
// SAFETY: layout's size is nonzero.
let clone = unsafe { alloc(layout) };
if clone.is_null() {
handle_alloc_error(layout);
}
// SAFETY: new allocation cannot overlap the previous one (this was
// not a realloc). The argument ptrs are readable/writeable
// respectively for size bytes.
unsafe { ptr::copy_nonoverlapping(ptr, clone, size) }
Identifier {
head: ptr_to_repr(clone),
tail: [0; TAIL_BYTES],
}
}
}
}
impl Drop for Identifier {
fn drop(&mut self) {
if self.is_empty_or_inline() {
return;
}
let ptr = repr_to_ptr_mut(self.head);
// SAFETY: ptr is one of our own heap allocations.
let len = unsafe { decode_len(ptr) };
let size = bytes_for_varint(len) + len.get();
let align = 2;
// SAFETY: align is not zero, align is a power of two, and rounding
// size up to align does not overflow usize::MAX. These guarantees were
// made when originally allocating this memory.
let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
// SAFETY: ptr was previously allocated by the same allocator with the
// same layout.
unsafe { dealloc(ptr, layout) }
}
}
impl PartialEq for Identifier {
fn eq(&self, rhs: &Self) -> bool {
if self.is_empty_or_inline() {
// Fast path (most common)
self.head == rhs.head && self.tail == rhs.tail
} else if rhs.is_empty_or_inline() {
false
} else {
// SAFETY: both reprs are in the heap allocated representation.
unsafe { ptr_as_str(&self.head) == ptr_as_str(&rhs.head) }
}
}
}
unsafe impl Send for Identifier {}
unsafe impl Sync for Identifier {}
// We use heap pointers that are 2-byte aligned, meaning they have an
// insignificant 0 in the least significant bit. We take advantage of that
// unneeded bit to rotate a 1 into the most significant bit to make the repr
// distinguishable from ASCII bytes.
fn ptr_to_repr(original: *mut u8) -> NonNull<u8> {
// `mov eax, 1`
// `shld rax, rdi, 63`
let modified = (original as usize | 1).rotate_right(1);
// `original + (modified - original)`, but being mindful of provenance.
let diff = modified.wrapping_sub(original as usize);
let modified = original.wrapping_add(diff);
// SAFETY: the most significant bit of repr is known to be set, so the value
// is not zero.
unsafe { NonNull::new_unchecked(modified) }
}
// Shift out the 1 previously placed into the most significant bit of the least
// significant byte. Shift in a low 0 bit to reconstruct the original 2-byte
// aligned pointer.
fn repr_to_ptr(modified: NonNull<u8>) -> *const u8 {
// `lea rax, [rdi + rdi]`
let modified = modified.as_ptr();
let original = (modified as usize) << 1;
// `modified + (original - modified)`, but being mindful of provenance.
let diff = original.wrapping_sub(modified as usize);
modified.wrapping_add(diff)
}
fn repr_to_ptr_mut(repr: NonNull<u8>) -> *mut u8 {
repr_to_ptr(repr) as *mut u8
}
// Compute the length of the inline string, assuming the argument is in short
// string representation. Short strings are stored as 1 to 8 nonzero ASCII
// bytes, followed by \0 padding for the remaining bytes.
//
// SAFETY: the identifier must indeed be in the inline representation.
unsafe fn inline_len(repr: &Identifier) -> NonZeroUsize {
// SAFETY: Identifier's layout is align(8) and at least size 8. We're doing
// an aligned read of the first 8 bytes from it. The bytes are not all zero
// because inline strings are at least 1 byte long and cannot contain \0.
let repr = unsafe { ptr::read(repr as *const Identifier as *const NonZeroU64) };
// Rustc >=1.53 has intrinsics for counting zeros on a non-zeroable integer.
// On many architectures these are more efficient than counting on ordinary
// zeroable integers (bsf vs cttz). On rustc <1.53 without those intrinsics,
// we count zeros in the u64 rather than the NonZeroU64.
#[cfg(no_nonzero_bitscan)]
let repr = repr.get();
#[cfg(target_endian = "little")]
let zero_bits_on_string_end = repr.leading_zeros();
#[cfg(target_endian = "big")]
let zero_bits_on_string_end = repr.trailing_zeros();
let nonzero_bytes = 8 - zero_bits_on_string_end as usize / 8;
// SAFETY: repr is nonzero, so it has at most 63 zero bits on either end,
// thus at least one nonzero byte.
unsafe { NonZeroUsize::new_unchecked(nonzero_bytes) }
}
// SAFETY: repr must be in the inline representation, i.e. at least 1 and at
// most 8 nonzero ASCII bytes padded on the end with \0 bytes.
unsafe fn inline_as_str(repr: &Identifier) -> &str {
let ptr = repr as *const Identifier as *const u8;
let len = unsafe { inline_len(repr) }.get();
// SAFETY: we are viewing the nonzero ASCII prefix of the inline repr's
// contents as a slice of bytes. Input/output lifetimes are correctly
// associated.
let slice = unsafe { slice::from_raw_parts(ptr, len) };
// SAFETY: the string contents are known to be only ASCII bytes, which are
// always valid UTF-8.
unsafe { str::from_utf8_unchecked(slice) }
}
// Decode varint. Varints consist of between one and eight base-128 digits, each
// of which is stored in a byte with most significant bit set. Adjacent to the
// varint in memory there is guaranteed to be at least 9 ASCII bytes, each of
// which has an unset most significant bit.
//
// SAFETY: ptr must be one of our own heap allocations, with the varint header
// already written.
unsafe fn decode_len(ptr: *const u8) -> NonZeroUsize {
// SAFETY: There is at least one byte of varint followed by at least 9 bytes
// of string content, which is at least 10 bytes total for the allocation,
// so reading the first two is no problem.
let [first, second] = unsafe { ptr::read(ptr as *const [u8; 2]) };
if second < 0x80 {
// SAFETY: the length of this heap allocated string has been encoded as
// one base-128 digit, so the length is at least 9 and at most 127. It
// cannot be zero.
unsafe { NonZeroUsize::new_unchecked((first & 0x7f) as usize) }
} else {
return unsafe { decode_len_cold(ptr) };
// Identifiers 128 bytes or longer. This is not exercised by any crate
// version currently published to crates.io.
#[cold]
#[inline(never)]
unsafe fn decode_len_cold(mut ptr: *const u8) -> NonZeroUsize {
let mut len = 0;
let mut shift = 0;
loop {
// SAFETY: varint continues while there are bytes having the
// most significant bit set, i.e. until we start hitting the
// ASCII string content with msb unset.
let byte = unsafe { *ptr };
if byte < 0x80 {
// SAFETY: the string length is known to be 128 bytes or
// longer.
return unsafe { NonZeroUsize::new_unchecked(len) };
}
// SAFETY: still in bounds of the same allocation.
ptr = unsafe { ptr.add(1) };
len += ((byte & 0x7f) as usize) << shift;
shift += 7;
}
}
}
}
// SAFETY: repr must be in the heap allocated representation, with varint header
// and string contents already written.
unsafe fn ptr_as_str(repr: &NonNull<u8>) -> &str {
let ptr = repr_to_ptr(*repr);
let len = unsafe { decode_len(ptr) };
let header = bytes_for_varint(len);
let slice = unsafe { slice::from_raw_parts(ptr.add(header), len.get()) };
// SAFETY: all identifier contents are ASCII bytes, which are always valid
// UTF-8.
unsafe { str::from_utf8_unchecked(slice) }
}
// Number of base-128 digits required for the varint representation of a length.
fn bytes_for_varint(len: NonZeroUsize) -> usize {
#[cfg(no_nonzero_bitscan)] // rustc <1.53
let len = len.get();
let usize_bits = mem::size_of::<usize>() * 8;
let len_bits = usize_bits - len.leading_zeros() as usize;
(len_bits + 6) / 7
}