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+/// A trait for describing vector operations used by vectorized searchers.
+///
+/// The trait is highly constrained to low level vector operations needed.
+/// In general, it was invented mostly to be generic over x86's __m128i and
+/// __m256i types. At time of writing, it also supports wasm and aarch64
+/// 128-bit vector types as well.
+///
+/// # Safety
+///
+/// All methods are not safe since they are intended to be implemented using
+/// vendor intrinsics, which are also not safe. Callers must ensure that the
+/// appropriate target features are enabled in the calling function, and that
+/// the current CPU supports them. All implementations should avoid marking the
+/// routines with #[target_feature] and instead mark them as #[inline(always)]
+/// to ensure they get appropriately inlined. (inline(always) cannot be used
+/// with target_feature.)
+pub(crate) trait Vector: Copy + core::fmt::Debug {
+ /// The number of bits in the vector.
+ const BITS: usize;
+ /// The number of bytes in the vector. That is, this is the size of the
+ /// vector in memory.
+ const BYTES: usize;
+ /// The bits that must be zero in order for a `*const u8` pointer to be
+ /// correctly aligned to read vector values.
+ const ALIGN: usize;
+
+ /// The type of the value returned by `Vector::movemask`.
+ ///
+ /// This supports abstracting over the specific representation used in
+ /// order to accommodate different representations in different ISAs.
+ type Mask: MoveMask;
+
+ /// Create a vector with 8-bit lanes with the given byte repeated into each
+ /// lane.
+ unsafe fn splat(byte: u8) -> Self;
+
+ /// Read a vector-size number of bytes from the given pointer. The pointer
+ /// must be aligned to the size of the vector.
+ ///
+ /// # Safety
+ ///
+ /// Callers must guarantee that at least `BYTES` bytes are readable from
+ /// `data` and that `data` is aligned to a `BYTES` boundary.
+ unsafe fn load_aligned(data: *const u8) -> Self;
+
+ /// Read a vector-size number of bytes from the given pointer. The pointer
+ /// does not need to be aligned.
+ ///
+ /// # Safety
+ ///
+ /// Callers must guarantee that at least `BYTES` bytes are readable from
+ /// `data`.
+ unsafe fn load_unaligned(data: *const u8) -> Self;
+
+ /// _mm_movemask_epi8 or _mm256_movemask_epi8
+ unsafe fn movemask(self) -> Self::Mask;
+ /// _mm_cmpeq_epi8 or _mm256_cmpeq_epi8
+ unsafe fn cmpeq(self, vector2: Self) -> Self;
+ /// _mm_and_si128 or _mm256_and_si256
+ unsafe fn and(self, vector2: Self) -> Self;
+ /// _mm_or or _mm256_or_si256
+ unsafe fn or(self, vector2: Self) -> Self;
+ /// Returns true if and only if `Self::movemask` would return a mask that
+ /// contains at least one non-zero bit.
+ unsafe fn movemask_will_have_non_zero(self) -> bool {
+ self.movemask().has_non_zero()
+ }
+}
+
+/// A trait that abstracts over a vector-to-scalar operation called
+/// "move mask."
+///
+/// On x86-64, this is `_mm_movemask_epi8` for SSE2 and `_mm256_movemask_epi8`
+/// for AVX2. It takes a vector of `u8` lanes and returns a scalar where the
+/// `i`th bit is set if and only if the most significant bit in the `i`th lane
+/// of the vector is set. The simd128 ISA for wasm32 also supports this
+/// exact same operation natively.
+///
+/// ... But aarch64 doesn't. So we have to fake it with more instructions and
+/// a slightly different representation. We could do extra work to unify the
+/// representations, but then would require additional costs in the hot path
+/// for `memchr` and `packedpair`. So instead, we abstraction over the specific
+/// representation with this trait an ddefine the operations we actually need.
+pub(crate) trait MoveMask: Copy + core::fmt::Debug {
+ /// Return a mask that is all zeros except for the least significant `n`
+ /// lanes in a corresponding vector.
+ fn all_zeros_except_least_significant(n: usize) -> Self;
+
+ /// Returns true if and only if this mask has a a non-zero bit anywhere.
+ fn has_non_zero(self) -> bool;
+
+ /// Returns the number of bits set to 1 in this mask.
+ fn count_ones(self) -> usize;
+
+ /// Does a bitwise `and` operation between `self` and `other`.
+ fn and(self, other: Self) -> Self;
+
+ /// Does a bitwise `or` operation between `self` and `other`.
+ fn or(self, other: Self) -> Self;
+
+ /// Returns a mask that is equivalent to `self` but with the least
+ /// significant 1-bit set to 0.
+ fn clear_least_significant_bit(self) -> Self;
+
+ /// Returns the offset of the first non-zero lane this mask represents.
+ fn first_offset(self) -> usize;
+
+ /// Returns the offset of the last non-zero lane this mask represents.
+ fn last_offset(self) -> usize;
+}
+
+/// This is a "sensible" movemask implementation where each bit represents
+/// whether the most significant bit is set in each corresponding lane of a
+/// vector. This is used on x86-64 and wasm, but such a mask is more expensive
+/// to get on aarch64 so we use something a little different.
+///
+/// We call this "sensible" because this is what we get using native sse/avx
+/// movemask instructions. But neon has no such native equivalent.
+#[derive(Clone, Copy, Debug)]
+pub(crate) struct SensibleMoveMask(u32);
+
+impl SensibleMoveMask {
+ /// Get the mask in a form suitable for computing offsets.
+ ///
+ /// Basically, this normalizes to little endian. On big endian, this swaps
+ /// the bytes.
+ #[inline(always)]
+ fn get_for_offset(self) -> u32 {
+ #[cfg(target_endian = "big")]
+ {
+ self.0.swap_bytes()
+ }
+ #[cfg(target_endian = "little")]
+ {
+ self.0
+ }
+ }
+}
+
+impl MoveMask for SensibleMoveMask {
+ #[inline(always)]
+ fn all_zeros_except_least_significant(n: usize) -> SensibleMoveMask {
+ debug_assert!(n < 32);
+ SensibleMoveMask(!((1 << n) - 1))
+ }
+
+ #[inline(always)]
+ fn has_non_zero(self) -> bool {
+ self.0 != 0
+ }
+
+ #[inline(always)]
+ fn count_ones(self) -> usize {
+ self.0.count_ones() as usize
+ }
+
+ #[inline(always)]
+ fn and(self, other: SensibleMoveMask) -> SensibleMoveMask {
+ SensibleMoveMask(self.0 & other.0)
+ }
+
+ #[inline(always)]
+ fn or(self, other: SensibleMoveMask) -> SensibleMoveMask {
+ SensibleMoveMask(self.0 | other.0)
+ }
+
+ #[inline(always)]
+ fn clear_least_significant_bit(self) -> SensibleMoveMask {
+ SensibleMoveMask(self.0 & (self.0 - 1))
+ }
+
+ #[inline(always)]
+ fn first_offset(self) -> usize {
+ // We are dealing with little endian here (and if we aren't, we swap
+ // the bytes so we are in practice), where the most significant byte
+ // is at a higher address. That means the least significant bit that
+ // is set corresponds to the position of our first matching byte.
+ // That position corresponds to the number of zeros after the least
+ // significant bit.
+ self.get_for_offset().trailing_zeros() as usize
+ }
+
+ #[inline(always)]
+ fn last_offset(self) -> usize {
+ // We are dealing with little endian here (and if we aren't, we swap
+ // the bytes so we are in practice), where the most significant byte is
+ // at a higher address. That means the most significant bit that is set
+ // corresponds to the position of our last matching byte. The position
+ // from the end of the mask is therefore the number of leading zeros
+ // in a 32 bit integer, and the position from the start of the mask is
+ // therefore 32 - (leading zeros) - 1.
+ 32 - self.get_for_offset().leading_zeros() as usize - 1
+ }
+}
+
+#[cfg(target_arch = "x86_64")]
+mod x86sse2 {
+ use core::arch::x86_64::*;
+
+ use super::{SensibleMoveMask, Vector};
+
+ impl Vector for __m128i {
+ const BITS: usize = 128;
+ const BYTES: usize = 16;
+ const ALIGN: usize = Self::BYTES - 1;
+
+ type Mask = SensibleMoveMask;
+
+ #[inline(always)]
+ unsafe fn splat(byte: u8) -> __m128i {
+ _mm_set1_epi8(byte as i8)
+ }
+
+ #[inline(always)]
+ unsafe fn load_aligned(data: *const u8) -> __m128i {
+ _mm_load_si128(data as *const __m128i)
+ }
+
+ #[inline(always)]
+ unsafe fn load_unaligned(data: *const u8) -> __m128i {
+ _mm_loadu_si128(data as *const __m128i)
+ }
+
+ #[inline(always)]
+ unsafe fn movemask(self) -> SensibleMoveMask {
+ SensibleMoveMask(_mm_movemask_epi8(self) as u32)
+ }
+
+ #[inline(always)]
+ unsafe fn cmpeq(self, vector2: Self) -> __m128i {
+ _mm_cmpeq_epi8(self, vector2)
+ }
+
+ #[inline(always)]
+ unsafe fn and(self, vector2: Self) -> __m128i {
+ _mm_and_si128(self, vector2)
+ }
+
+ #[inline(always)]
+ unsafe fn or(self, vector2: Self) -> __m128i {
+ _mm_or_si128(self, vector2)
+ }
+ }
+}
+
+#[cfg(target_arch = "x86_64")]
+mod x86avx2 {
+ use core::arch::x86_64::*;
+
+ use super::{SensibleMoveMask, Vector};
+
+ impl Vector for __m256i {
+ const BITS: usize = 256;
+ const BYTES: usize = 32;
+ const ALIGN: usize = Self::BYTES - 1;
+
+ type Mask = SensibleMoveMask;
+
+ #[inline(always)]
+ unsafe fn splat(byte: u8) -> __m256i {
+ _mm256_set1_epi8(byte as i8)
+ }
+
+ #[inline(always)]
+ unsafe fn load_aligned(data: *const u8) -> __m256i {
+ _mm256_load_si256(data as *const __m256i)
+ }
+
+ #[inline(always)]
+ unsafe fn load_unaligned(data: *const u8) -> __m256i {
+ _mm256_loadu_si256(data as *const __m256i)
+ }
+
+ #[inline(always)]
+ unsafe fn movemask(self) -> SensibleMoveMask {
+ SensibleMoveMask(_mm256_movemask_epi8(self) as u32)
+ }
+
+ #[inline(always)]
+ unsafe fn cmpeq(self, vector2: Self) -> __m256i {
+ _mm256_cmpeq_epi8(self, vector2)
+ }
+
+ #[inline(always)]
+ unsafe fn and(self, vector2: Self) -> __m256i {
+ _mm256_and_si256(self, vector2)
+ }
+
+ #[inline(always)]
+ unsafe fn or(self, vector2: Self) -> __m256i {
+ _mm256_or_si256(self, vector2)
+ }
+ }
+}
+
+#[cfg(target_arch = "aarch64")]
+mod aarch64neon {
+ use core::arch::aarch64::*;
+
+ use super::{MoveMask, Vector};
+
+ impl Vector for uint8x16_t {
+ const BITS: usize = 128;
+ const BYTES: usize = 16;
+ const ALIGN: usize = Self::BYTES - 1;
+
+ type Mask = NeonMoveMask;
+
+ #[inline(always)]
+ unsafe fn splat(byte: u8) -> uint8x16_t {
+ vdupq_n_u8(byte)
+ }
+
+ #[inline(always)]
+ unsafe fn load_aligned(data: *const u8) -> uint8x16_t {
+ // I've tried `data.cast::<uint8x16_t>().read()` instead, but
+ // couldn't observe any benchmark differences.
+ Self::load_unaligned(data)
+ }
+
+ #[inline(always)]
+ unsafe fn load_unaligned(data: *const u8) -> uint8x16_t {
+ vld1q_u8(data)
+ }
+
+ #[inline(always)]
+ unsafe fn movemask(self) -> NeonMoveMask {
+ let asu16s = vreinterpretq_u16_u8(self);
+ let mask = vshrn_n_u16(asu16s, 4);
+ let asu64 = vreinterpret_u64_u8(mask);
+ let scalar64 = vget_lane_u64(asu64, 0);
+ NeonMoveMask(scalar64 & 0x8888888888888888)
+ }
+
+ #[inline(always)]
+ unsafe fn cmpeq(self, vector2: Self) -> uint8x16_t {
+ vceqq_u8(self, vector2)
+ }
+
+ #[inline(always)]
+ unsafe fn and(self, vector2: Self) -> uint8x16_t {
+ vandq_u8(self, vector2)
+ }
+
+ #[inline(always)]
+ unsafe fn or(self, vector2: Self) -> uint8x16_t {
+ vorrq_u8(self, vector2)
+ }
+
+ /// This is the only interesting implementation of this routine.
+ /// Basically, instead of doing the "shift right narrow" dance, we use
+ /// adajacent folding max to determine whether there are any non-zero
+ /// bytes in our mask. If there are, *then* we'll do the "shift right
+ /// narrow" dance. In benchmarks, this does lead to slightly better
+ /// throughput, but the win doesn't appear huge.
+ #[inline(always)]
+ unsafe fn movemask_will_have_non_zero(self) -> bool {
+ let low = vreinterpretq_u64_u8(vpmaxq_u8(self, self));
+ vgetq_lane_u64(low, 0) != 0
+ }
+ }
+
+ /// Neon doesn't have a `movemask` that works like the one in x86-64, so we
+ /// wind up using a different method[1]. The different method also produces
+ /// a mask, but 4 bits are set in the neon case instead of a single bit set
+ /// in the x86-64 case. We do an extra step to zero out 3 of the 4 bits,
+ /// but we still wind up with at least 3 zeroes between each set bit. This
+ /// generally means that we need to do some division by 4 before extracting
+ /// offsets.
+ ///
+ /// In fact, the existence of this type is the entire reason that we have
+ /// the `MoveMask` trait in the first place. This basically lets us keep
+ /// the different representations of masks without being forced to unify
+ /// them into a single representation, which could result in extra and
+ /// unnecessary work.
+ ///
+ /// [1]: https://community.arm.com/arm-community-blogs/b/infrastructure-solutions-blog/posts/porting-x86-vector-bitmask-optimizations-to-arm-neon
+ #[derive(Clone, Copy, Debug)]
+ pub(crate) struct NeonMoveMask(u64);
+
+ impl NeonMoveMask {
+ /// Get the mask in a form suitable for computing offsets.
+ ///
+ /// Basically, this normalizes to little endian. On big endian, this
+ /// swaps the bytes.
+ #[inline(always)]
+ fn get_for_offset(self) -> u64 {
+ #[cfg(target_endian = "big")]
+ {
+ self.0.swap_bytes()
+ }
+ #[cfg(target_endian = "little")]
+ {
+ self.0
+ }
+ }
+ }
+
+ impl MoveMask for NeonMoveMask {
+ #[inline(always)]
+ fn all_zeros_except_least_significant(n: usize) -> NeonMoveMask {
+ debug_assert!(n < 16);
+ NeonMoveMask(!(((1 << n) << 2) - 1))
+ }
+
+ #[inline(always)]
+ fn has_non_zero(self) -> bool {
+ self.0 != 0
+ }
+
+ #[inline(always)]
+ fn count_ones(self) -> usize {
+ self.0.count_ones() as usize
+ }
+
+ #[inline(always)]
+ fn and(self, other: NeonMoveMask) -> NeonMoveMask {
+ NeonMoveMask(self.0 & other.0)
+ }
+
+ #[inline(always)]
+ fn or(self, other: NeonMoveMask) -> NeonMoveMask {
+ NeonMoveMask(self.0 | other.0)
+ }
+
+ #[inline(always)]
+ fn clear_least_significant_bit(self) -> NeonMoveMask {
+ NeonMoveMask(self.0 & (self.0 - 1))
+ }
+
+ #[inline(always)]
+ fn first_offset(self) -> usize {
+ // We are dealing with little endian here (and if we aren't,
+ // we swap the bytes so we are in practice), where the most
+ // significant byte is at a higher address. That means the least
+ // significant bit that is set corresponds to the position of our
+ // first matching byte. That position corresponds to the number of
+ // zeros after the least significant bit.
+ //
+ // Note that unlike `SensibleMoveMask`, this mask has its bits
+ // spread out over 64 bits instead of 16 bits (for a 128 bit
+ // vector). Namely, where as x86-64 will turn
+ //
+ // 0x00 0xFF 0x00 0x00 0xFF
+ //
+ // into 10010, our neon approach will turn it into
+ //
+ // 10000000000010000000
+ //
+ // And this happens because neon doesn't have a native `movemask`
+ // instruction, so we kind of fake it[1]. Thus, we divide the
+ // number of trailing zeros by 4 to get the "real" offset.
+ //
+ // [1]: https://community.arm.com/arm-community-blogs/b/infrastructure-solutions-blog/posts/porting-x86-vector-bitmask-optimizations-to-arm-neon
+ (self.get_for_offset().trailing_zeros() >> 2) as usize
+ }
+
+ #[inline(always)]
+ fn last_offset(self) -> usize {
+ // See comment in `first_offset` above. This is basically the same,
+ // but coming from the other direction.
+ 16 - (self.get_for_offset().leading_zeros() >> 2) as usize - 1
+ }
+ }
+}
+
+#[cfg(target_arch = "wasm32")]
+mod wasm_simd128 {
+ use core::arch::wasm32::*;
+
+ use super::{SensibleMoveMask, Vector};
+
+ impl Vector for v128 {
+ const BITS: usize = 128;
+ const BYTES: usize = 16;
+ const ALIGN: usize = Self::BYTES - 1;
+
+ type Mask = SensibleMoveMask;
+
+ #[inline(always)]
+ unsafe fn splat(byte: u8) -> v128 {
+ u8x16_splat(byte)
+ }
+
+ #[inline(always)]
+ unsafe fn load_aligned(data: *const u8) -> v128 {
+ *data.cast()
+ }
+
+ #[inline(always)]
+ unsafe fn load_unaligned(data: *const u8) -> v128 {
+ v128_load(data.cast())
+ }
+
+ #[inline(always)]
+ unsafe fn movemask(self) -> SensibleMoveMask {
+ SensibleMoveMask(u8x16_bitmask(self).into())
+ }
+
+ #[inline(always)]
+ unsafe fn cmpeq(self, vector2: Self) -> v128 {
+ u8x16_eq(self, vector2)
+ }
+
+ #[inline(always)]
+ unsafe fn and(self, vector2: Self) -> v128 {
+ v128_and(self, vector2)
+ }
+
+ #[inline(always)]
+ unsafe fn or(self, vector2: Self) -> v128 {
+ v128_or(self, vector2)
+ }
+ }
+}
Copyright © 2024 Valentin A. Popov. All Rights Reverse Engineered.