# API synopsis / quick reference

[[_TOC_]]

## Usage modes

Highway can compile for multiple CPU targets, choosing the best available at
runtime (dynamic dispatch), or compile for a single CPU target without runtime
overhead (static dispatch). Examples of both are provided in examples/.

Dynamic dispatch uses the same source code as static, plus `#define
HWY_TARGET_INCLUDE`, `#include "hwy/foreach_target.h"` and
`HWY_DYNAMIC_DISPATCH`.

## Headers

The public headers are:

*   hwy/highway.h: main header, included from source AND/OR header files that
    use vector types. Note that including in headers may increase compile time,
    but allows declaring functions implemented out of line.

*   hwy/base.h: included from headers that only need compiler/platform-dependent
    definitions (e.g. `PopCount`) without the full highway.h.

*   hwy/foreach_target.h: re-includes the translation unit (specified by
    `HWY_TARGET_INCLUDE`) once per enabled target to generate code from the same
    source code. highway.h must still be included, either before or after.

*   hwy/aligned_allocator.h: defines functions for allocating memory with
    alignment suitable for `Load`/`Store`.

*   hwy/cache_control.h: defines stand-alone functions to control caching (e.g.
    prefetching), independent of actual SIMD.

*   hwy/nanobenchmark.h: library for precisely measuring elapsed time (under
    varying inputs) for benchmarking small/medium regions of code.

*   hwy/tests/test_util-inl.h: defines macros for invoking tests on all
    available targets, plus per-target functions useful in tests (e.g. Print).

SIMD implementations must be preceded and followed by the following:

```
#include "hwy/highway.h"
HWY_BEFORE_NAMESPACE();  // at file scope
namespace project {  // optional
namespace HWY_NAMESPACE {

// implementation

// NOLINTNEXTLINE(google-readability-namespace-comments)
}  // namespace HWY_NAMESPACE
}  // namespace project - optional
HWY_AFTER_NAMESPACE();
```

## Notation in this doc

*   `T` denotes the type of a vector lane;
*   `N` is a size_t value that governs (but is not necessarily identical to) the
    number of lanes;
*   `D` is shorthand for `Simd<T, N>`;
*   `d` is an lvalue of type `D`, passed as a function argument e.g. to Zero;
*   `V` is the type of a vector.

## Vector and descriptor types

Highway vectors consist of one or more 'lanes' of the same built-in type
`uint##_t, int##_t` for `## = 8, 16, 32, 64`, plus `float##_t` for `## = 16, 32,
64` and `bfloat16_t`.

In Highway, `float16_t` (an IEEE binary16 half-float) and `bfloat16_t` (the
upper 16 bits of an IEEE binary32 float) only support load, store, and
conversion to/from `float32_t`. The behavior of infinity and NaN in `float16_t`
is implementation-defined due to ARMv7.

On RVV, vectors are sizeless and cannot be wrapped inside a class. The Highway
API allows using built-in types as vectors because operations are expressed as
overloaded functions. Instead of constructors, overloaded initialization
functions such as `Set` take a zero-sized tag argument called `d` of type `D =
Simd<T, N>` and return an actual vector of unspecified type.

`T` is one of the lane types above, and may be retrieved via `TFromD<D>`.

`N` is target-dependent and not directly user-specified. The actual lane count
may not be known at compile time, but can be obtained via `Lanes(d)`. Use this
value, which is potentially different from `N`, to increment loop counters etc.
Note that `Lanes(d)` could potentially change at runtime, upon user request via
special CPU instructions. Thus we discourage caching the result; it is typically
used inside a function or basic block.

The actual lane count is guaranteed to be a power of two, even on SVE hardware
where vectors can be a multiple of 128 bits (there, the extra lanes remain
unused). This simplifies alignment: remainders can be computed as `count &
(Lanes(d) - 1)` instead of an expensive modulo. It also ensures loop trip counts
that are a large power of two (at least `MaxLanes`) are evenly divisible by the
lane count, thus avoiding the need for a second loop to handle remainders.

`d` lvalues (a tag, NOT actual vector) are typically obtained using two aliases:

*   Most common: `ScalableTag<T[, shift]> d;` or the macro form `HWY_FULL(T[,
    LMUL=1]) d;`. With the default value of the second argument, these both
    select full vectors which utilize all available lanes.

    Only for targets (e.g. RVV) that support register groups, the shift (-3..3)
    and LMUL argument (1, 2, 4, 8) specify `LMUL`, the number of registers in
    the group. This effectively multiplies the lane count in each operation by
    `LMUL`, or shifts by `shift` (negative values are understood as
    right-shifting by the absolute value). These arguments will eventually be
    optional hints that may improve performance on 1-2 wide machines (at the
    cost of reducing the effective number of registers), but the experimental
    GCC support for RVV does not support fractional `LMUL`. Thus,
    mixed-precision code (e.g. demoting float to uint8_t) currently requires
    `LMUL` to be at least the ratio of the sizes of the largest and smallest
    type, and smaller `d` to be obtained via `Half<DLarger>`.

*   Less common: `CappedTag<T, kCap> d` or the macro form `HWY_CAPPED(T, kCap)
    d;`. These select vectors or masks where *no more than* the first `kCap` (a
    power of two) lanes have observable effects such as loading/storing to
    memory, or being counted by `CountTrue`. The number of lanes may also be
    less; for the `HWY_SCALAR` target, vectors always have a single lane.

*   For applications that require fixed-size vectors: `FixedTag<T, kCount> d;`
    will select vectors where exactly `kCount` lanes have observable effects.
    These may be implemented using full vectors plus additional runtime cost for
    masking in `Load` etc. All targets except `HWY_SCALAR` allow any power of
    two `kCount <= 16/sizeof(T)`. This tag can be used when the `HWY_SCALAR`
    target is anyway disabled (superseded by a higher baseline) or unusable (due
    to use of ops such as `TableLookupBytes`).

*   The result of `UpperHalf`/`LowerHalf` has half the lanes. To obtain a
    corresponding `d`, use `Half<decltype(d)>`; the opposite is `Twice<>`.

User-specified lane counts or tuples of vectors could cause spills on targets
with fewer or smaller vectors. By contrast, Highway encourages vector-length
agnostic code, which is more performance-portable.

Given that lane counts are potentially compile-time-unknown, storage for vectors
should be dynamically allocated, e.g. via `AllocateAligned(Lanes(d))`. For
applications that require a compile-time bound, `MaxLanes(d)` uses the `N` from
`Simd<T, N>` to return a (loose) upper bound, NOT necessarily the actual lane
count. Note that some compilers are not able to interpret it as constexpr.

For mixed-precision code (e.g. `uint8_t` lanes promoted to `float`), tags for
the smaller types must be obtained from those of the larger type (e.g. via
`Rebind<uint8_t, HWY_FULL(float)>`).

## Using unspecified vector types

Vector types are unspecified and depend on the target. User code could define
them as `auto`, but it is more readable (due to making the type visible) to use
an alias such as `Vec<D>`, or `decltype(Zero(d))`.

Vectors are sizeless types on RVV/SVE. Therefore, vectors must not be used in
arrays/STL containers (use the lane type `T` instead), class members,
static/thread_local variables, new-expressions (use `AllocateAligned` instead),
and sizeof/pointer arithmetic (increment `T*` by `Lanes(d)` instead).

Initializing constants requires a tag type `D`, or an lvalue `d` of that type.
The `D` can be passed as a template argument or obtained from a vector type `V`
via `DFromV<V>`. `TFromV<V>` is equivalent to `TFromD<DFromV<V>>`.

**Note**: Let `DV = DFromV<V>`. For builtin `V` (currently necessary on
RVV/SVE), `DV` might not be the same as the `D` used to create `V`. In
particular, `DV` must not be passed to `Load/Store` functions because it may
lack the limit on `N` established by the original `D`. However, `Vec<DV>` is the
same as `V`.

Thus a template argument `V` suffices for generic functions that do not load
from/store to memory: `template<class V> V Mul4(V v) { return v *
Set(DFromV<V>(), 4); }`.

Example of mixing partial vectors with generic functions:

```
CappedTag<int16_t, 2> d2;
auto v = Mul4(Set(d2, 2));
Store(v, d2, ptr);  // Use d2, NOT DFromV<decltype(v)>()
```

## Operations

In the following, the argument or return type `V` denotes a vector with `N`
lanes, and `M` a mask. Operations limited to certain vector types begin with a
constraint of the form `V`: `{prefixes}[{bits}]`. The prefixes `u,i,f` denote
unsigned, signed, and floating-point types, and bits indicates the number of
bits per lane: 8, 16, 32, or 64. Any combination of the specified prefixes and
bits are allowed. Abbreviations of the form `u32 = {u}{32}` may also be used.

Note that Highway functions reside in `hwy::HWY_NAMESPACE`, whereas user-defined
functions reside in `project::[nested]::HWY_NAMESPACE`. Highway functions
generally take either a `Simd` or vector/mask argument. For targets where
vectors and masks are defined in namespace `hwy`, the functions will be found
via Argument-Dependent Lookup. However, this does not work for function
templates, and RVV and SVE both use builtin vectors. There are three options for
portable code, in descending order of preference:

-   `namespace hn = hwy::HWY_NAMESPACE;` alias used to prefix ops, e.g.
    `hn::LoadDup128(..)`;
-   `using hwy::HWY_NAMESPACE::LoadDup128;` declarations for each op used;
-   `using hwy::HWY_NAMESPACE;` directive. This is generally discouraged,
    especially for SIMD code residing in a header.

Note that overloaded operators are not yet supported on RVV and SVE; code that
wishes to run on all targets until that is resolved can use functions such as
`Eq`, `Lt`, `Add`, `Div` etc.

### Initialization

*   <code>V **Zero**(D)</code>: returns N-lane vector with all bits set to 0.
*   <code>V **Set**(D, T)</code>: returns N-lane vector with all lanes equal to
    the given value of type `T`.
*   <code>V **Undefined**(D)</code>: returns uninitialized N-lane vector, e.g.
    for use as an output parameter.
*   <code>V **Iota**(D, T)</code>: returns N-lane vector where the lane with
    index `i` has the given value of type `T` plus `i`. The least significant
    lane has index 0. This is useful in tests for detecting lane-crossing bugs.
*   <code>V **SignBit**(D, T)</code>: returns N-lane vector with all lanes set
    to a value whose representation has only the most-significant bit set.

### Arithmetic

*   <code>V **operator+**(V a, V b)</code>: returns `a[i] + b[i]` (mod 2^bits).
*   <code>V **operator-**(V a, V b)</code>: returns `a[i] - b[i]` (mod 2^bits).

*   `V`: `{i,f}` \
    <code>V **Neg**(V a)</code>: returns `-a[i]`.

*   `V`: `{i,f}` \
    <code>V **Abs**(V a)</code> returns the absolute value of `a[i]`; for
    integers, `LimitsMin()` maps to `LimitsMax() + 1`.

*   `V`: `f32` \
    <code>V **AbsDiff**(V a, V b)</code>: returns `|a[i] - b[i]|` in each lane.

*   `V`: `{u,i}{8,16}` \
    <code>V **SaturatedAdd**(V a, V b)</code> returns `a[i] + b[i]` saturated to
    the minimum/maximum representable value.

*   `V`: `{u,i}{8,16}` \
    <code>V **SaturatedSub**(V a, V b)</code> returns `a[i] - b[i]` saturated to
    the minimum/maximum representable value.

*   `V`: `{u}{8,16}` \
    <code>V **AverageRound**(V a, V b)</code> returns `(a[i] + b[i] + 1) / 2`.

*   <code>V **Clamp**(V a, V lo, V hi)</code>: returns `a[i]` clamped to
    `[lo[i], hi[i]]`.

*   `V`: `{f}` \
    <code>V **operator/**(V a, V b)</code>: returns `a[i] / b[i]` in each lane.

*   `V`: `{f}` \
    <code>V **Sqrt**(V a)</code>: returns `sqrt(a[i])`.

*   `V`: `f32` \
    <code>V **ApproximateReciprocalSqrt**(V a)</code>: returns an approximation
    of `1.0 / sqrt(a[i])`. `sqrt(a) ~= ApproximateReciprocalSqrt(a) * a`. x86
    and PPC provide 12-bit approximations but the error on ARM is closer to 1%.

*   `V`: `f32` \
    <code>V **ApproximateReciprocal**(V a)</code>: returns an approximation of
    `1.0 / a[i]`.

**Note**: Min/Max corner cases are target-specific and may change. If either
argument is qNaN, x86 SIMD returns the second argument, ARMv7 Neon returns NaN,
Wasm is supposed to return NaN but does not always, but other targets actually
uphold IEEE 754-2019 minimumNumber: returning the other argument if exactly one
is qNaN, and NaN if both are.

*   <code>V **Min**(V a, V b)</code>: returns `min(a[i], b[i])`.

*   <code>V **Max**(V a, V b)</code>: returns `max(a[i], b[i])`.

#### Multiply

*   `V`: `{u,i}{16,32}` \
    <code>V <b>operator*</b>(V a, V b)</code>: returns the lower half of `a[i] *
    b[i]` in each lane.

*   `V`: `{f}` \
    <code>V <b>operator*</b>(V a, V b)</code>: returns `a[i] * b[i]` in each
    lane.

*   `V`: `i16` \
    <code>V **MulHigh**(V a, V b)</code>: returns the upper half of `a[i] *
    b[i]` in each lane.

*   `V`: `{u,i}{32},u64` \
    <code>V2 **MulEven**(V a, V b)</code>: returns double-wide result of `a[i] *
    b[i]` for every even `i`, in lanes `i` (lower) and `i + 1` (upper). `V2` is
    a vector with double-width lanes, or the same as `V` for 64-bit inputs
    (which are only supported if `HWY_TARGET != HWY_SCALAR`).

*   `V`: `u64` \
    <code>V **MulOdd**(V a, V b)</code>: returns double-wide result of `a[i] *
    b[i]` for every odd `i`, in lanes `i - 1` (lower) and `i` (upper). Only
    supported if `HWY_TARGET != HWY_SCALAR`.

*   `V`: `bf16`; `D`: `f32` \
    <code>Vec<D> **ReorderWidenMulAccumulate**(D, V a, V b, Vec<D> sum0, Vec<D>&
    sum1)</code>: widens `a` and `b` to `TFromD<D>`, then adds `a[i] * b[i]` to
    either `sum1[j]` or lane `j` of the return value, where `j = P(i)` and `P`
    is a permutation. The only guarantee is that `SumOfLanes(Add(return_value,
    sum1))` is the sum of all `a[i] * b[i]`. This is useful for computing dot
    products and the L2 norm.

#### Fused multiply-add

When implemented using special instructions, these functions are more precise
and faster than separate multiplication followed by addition. The `*Sub`
variants are somewhat slower on ARM; it is preferable to replace them with
`MulAdd` using a negated constant.

*   `V`: `{f}` \
    <code>V **MulAdd**(V a, V b, V c)</code>: returns `a[i] * b[i] + c[i]`.

*   `V`: `{f}` \
    <code>V **NegMulAdd**(V a, V b, V c)</code>: returns `-a[i] * b[i] + c[i]`.

*   `V`: `{f}` \
    <code>V **MulSub**(V a, V b, V c)</code>: returns `a[i] * b[i] - c[i]`.

*   `V`: `{f}` \
    <code>V **NegMulSub**(V a, V b, V c)</code>: returns `-a[i] * b[i] - c[i]`.

#### Shifts

**Note**: Counts not in `[0, sizeof(T)*8)` yield implementation-defined results.
Left-shifting signed `T` and right-shifting positive signed `T` is the same as
shifting `MakeUnsigned<T>` and casting to `T`. Right-shifting negative signed
`T` is the same as an unsigned shift, except that 1-bits are shifted in.

Compile-time constant shifts, generally the most efficient variant (though 8-bit
shifts are potentially slower than other lane sizes):

*   `V`: `{u,i}` \
    <code>V **ShiftLeft**&lt;int&gt;(V a)</code> returns `a[i] << int`.

*   `V`: `{u,i}` \
    <code>V **ShiftRight**&lt;int&gt;(V a)</code> returns `a[i] >> int`.

Shift all lanes by the same (not necessarily compile-time constant) amount:

*   `V`: `{u,i}` \
    <code>V **ShiftLeftSame**(V a, int bits)</code> returns `a[i] << bits`.

*   `V`: `{u,i}` \
    <code>V **ShiftRightSame**(V a, int bits)</code> returns `a[i] >> bits`.

Per-lane variable shifts (slow if SSSE3/SSE4, or 16-bit, or Shr i64 on AVX2):

*   `V`: `{u,i}{16,32,64}` \
    <code>V **operator<<**(V a, V b)</code> returns `a[i] << b[i]`.

*   `V`: `{u,i}{16,32,64}` \
    <code>V **operator>>**(V a, V b)</code> returns `a[i] >> b[i]`.

#### Floating-point rounding

*   `V`: `{f}` \
    <code>V **Round**(V a)</code>: returns `a[i]` rounded towards the nearest
    integer, with ties to even.

*   `V`: `{f}` \
    <code>V **Trunc**(V a)</code>: returns `a[i]` rounded towards zero
    (truncate).

*   `V`: `{f}` \
    <code>V **Ceil**(V a)</code>: returns `a[i]` rounded towards positive
    infinity (ceiling).

*   `V`: `{f}` \
    <code>V **Floor**(V a)</code>: returns `a[i]` rounded towards negative
    infinity.

### Logical

*   `V`: `{u,i}` \
    <code>V **PopulationCount**(V a)</code>: returns the number of 1-bits in
    each lane, i.e. `PopCount(a[i])`.

The following operate on individual bits within each lane:

*   `V`: `{u,i}` \
    <code>V **operator&**(V a, V b)</code>: returns `a[i] & b[i]`.

*   `V`: `{u,i}` \
    <code>V **operator|**(V a, V b)</code>: returns `a[i] | b[i]`.

*   `V`: `{u,i}` \
    <code>V **operator^**(V a, V b)</code>: returns `a[i] ^ b[i]`.

*   `V`: `{u,i}` \
    <code>V **Not**(V v)</code>: returns `~v[i]`.

For floating-point types, builtin operators are not always available, so
non-operator functions (also available for integers) must be used:

*   <code>V **And**(V a, V b)</code>: returns `a[i] & b[i]`.

*   <code>V **Or**(V a, V b)</code>: returns `a[i] | b[i]`.

*   <code>V **Xor**(V a, V b)</code>: returns `a[i] ^ b[i]`.

*   <code>V **AndNot**(V a, V b)</code>: returns `~a[i] & b[i]`.

Special functions for signed types:

*   `V`: `{f}` \
    <code>V **CopySign**(V a, V b)</code>: returns the number with the magnitude
    of `a` and sign of `b`.

*   `V`: `{f}` \
    <code>V **CopySignToAbs**(V a, V b)</code>: as above, but potentially
    slightly more efficient; requires the first argument to be non-negative.

*   `V`: `i32/64` \
    <code>V **BroadcastSignBit**(V a)</code> returns `a[i] < 0 ? -1 : 0`.

*   <code>V **ZeroIfNegative**(V v)</code>: returns `v[i] < 0 ? 0 : v[i]`.

### Masks

Let `M` denote a mask capable of storing true/false for each lane.

#### Creation

*   <code>M **FirstN**(D, size_t N)</code>: returns mask with the first `N`
    lanes (those with index `< N`) true. `N >= Lanes(D())` results in an
    all-true mask. Useful for implementing "masked" stores by loading `prev`
    followed by `IfThenElse(FirstN(d, N), what_to_store, prev)`.

*   <code>M **MaskFromVec**(V v)</code>: returns false in lane `i` if `v[i] ==
    0`, or true if `v[i]` has all bits set.

*   <code>M **LoadMaskBits**(D, const uint8_t* p)</code>: returns a mask
    indicating whether the i-th bit in the array is set. Loads bytes and bits in
    ascending order of address and index. At least 8 bytes of `p` must be
    readable, but only `(Lanes(D()) + 7) / 8` need be initialized. Any unused
    bits (happens if `Lanes(D()) < 8`) are treated as if they were zero.

#### Conversion

*   <code>M1 **RebindMask**(D, M2 m)</code>: returns same mask bits as `m`, but
    reinterpreted as a mask for lanes of type `TFromD<D>`. `M1` and `M2` must
    have the same number of lanes.

*   <code>V **VecFromMask**(D, M m)</code>: returns 0 in lane `i` if `m[i] ==
    false`, otherwise all bits set.

*   <code>size_t **StoreMaskBits**(D, M m, uint8_t* p)</code>: stores a bit
    array indicating whether `m[i]` is true, in ascending order of `i`, filling
    the bits of each byte from least to most significant, then proceeding to the
    next byte. Returns the number of bytes written: `(Lanes(D()) + 7) / 8`. At
    least 8 bytes of `p` must be writable.

#### Testing

*   <code>bool **AllTrue**(D, M m)</code>: returns whether all `m[i]` are true.

*   <code>bool **AllFalse**(D, M m)</code>: returns whether all `m[i]` are
    false.

*   <code>size_t **CountTrue**(D, M m)</code>: returns how many of `m[i]` are
    true [0, N]. This is typically more expensive than AllTrue/False.

*   <code>intptr_t **FindFirstTrue**(D, M m)</code>: returns the index of the
    first (i.e. lowest index) `m[i]` that is true, or -1 if none are.

#### Ternary operator

*   <code>V **IfThenElse**(M mask, V yes, V no)</code>: returns `mask[i] ?
    yes[i] : no[i]`.

*   <code>V **IfThenElseZero**(M mask, V yes)</code>: returns `mask[i] ?
    yes[i] : 0`.

*   <code>V **IfThenZeroElse**(M mask, V no)</code>: returns `mask[i] ? 0 :
    no[i]`.

#### Logical

*   <code>M **Not**(M m)</code>: returns mask of elements indicating whether the
    input mask element was not set.

*   <code>M **And**(M a, M b)</code>: returns mask of elements indicating
    whether both input mask elements were set.

*   <code>M **AndNot**(M not_a, M b)</code>: returns mask of elements indicating
    whether not_a is not set and b is set.

*   <code>M **Or**(M a, M b)</code>: returns mask of elements indicating whether
    either input mask element was set.

*   <code>M **Xor**(M a, M b)</code>: returns mask of elements indicating
    whether exactly one input mask element was set.

#### Compress

*   `V`: `{u,i,f}{16,32,64}` \
    <code>V **Compress**(V v, M m)</code>: returns `r` such that `r[n]` is
    `v[i]`, with `i` the n-th lane index (starting from 0) where `m[i]` is true.
    Compacts lanes whose mask is set into the lower lanes; upper lanes are
    implementation-defined. Slow with 16-bit lanes. Use this form when the input
    is already a mask, e.g. returned by a comparison.

*   `V`: `{u,i,f}{16,32,64}` \
    <code>size_t **CompressStore**(V v, M m, D d, T* p)</code>: writes lanes
    whose mask `m` is set into `p`, starting from lane 0. Returns `CountTrue(d,
    m)`, the number of valid lanes. May be implemented as `Compress` followed by
    `StoreU`; lanes after the valid ones may still be overwritten! Slower for
    16-bit lanes.

*   `V`: `{u,i,f}{16,32,64}` \
    <code>V **CompressBits**(V v, const uint8_t* HWY_RESTRICT bits)</code>:
    Equivalent to, but often faster than `Compress(v, LoadMaskBits(d, bits))`.
    `bits` is as specified for `LoadMaskBits`. If called multiple times, the
    `bits` pointer passed to this function must also be marked `HWY_RESTRICT` to
    avoid repeated work. Note that if the vector has less than 8 elements,
    incrementing `bits` will not work as intended for packed bit arrays.

*   `V`: `{u,i,f}{16,32,64}` \
    <code>size_t **CompressBitsStore**(V v, const uint8_t* HWY_RESTRICT bits, D
    d, T* p)</code>: combination of `CompressStore` and `CompressBits`, see
    remarks there.

#### Comparisons

These return a mask (see above) indicating whether the condition is true.

*   <code>M **operator==**(V a, V b)</code>: returns `a[i] == b[i]`.
*   <code>M **operator!=**(V a, V b)</code>: returns `a[i] != b[i]`.

*   `V`: `{i,f}` \
    <code>M **operator&lt;**(V a, V b)</code>: returns `a[i] < b[i]`.

*   `V`: `{i,f}` \
    <code>M **operator&gt;**(V a, V b)</code>: returns `a[i] > b[i]`.

*   `V`: `{f}` \
    <code>M **operator&lt;=**(V a, V b)</code>: returns `a[i] <= b[i]`.

*   `V`: `{f}` \
    <code>M **operator&gt;=**(V a, V b)</code>: returns `a[i] >= b[i]`.

*   `V`: `{u,i}` \
    <code>M **TestBit**(V v, V bit)</code>: returns `(v[i] & bit[i]) == bit[i]`.
    `bit[i]` must have exactly one bit set.

### Memory

Memory operands are little-endian, otherwise their order would depend on the
lane configuration. Pointers are the addresses of `N` consecutive `T` values,
either naturally-aligned (`aligned`) or possibly unaligned (`p`).

**Note**: computations with low arithmetic intensity (FLOP/s per memory traffic
bytes), e.g. dot product, can be *1.5 times as fast* when the memory operands
are naturally aligned. An unaligned access may require two load ports.

#### Load

Requires naturally-aligned vectors (e.g. from aligned_allocator.h):

*   <code>Vec&lt;D&gt; **Load**(D, const T* aligned)</code>: returns
    `aligned[i]`. May fault if the pointer is not aligned to the vector size.
    Using this whenever possible improves codegen on SSSE3/SSE4: unlike `LoadU`,
    `Load` can be fused into a memory operand, which reduces register pressure.

*   <code>Vec&lt;D&gt; **MaskedLoad**(M mask, D, const T* aligned)</code>:
    returns `aligned[i]` or zero if the `mask` governing element `i` is false.
    May fault if the pointer is not aligned to the vector size. The alignment
    requirement prevents differing behavior for "masked off" elements at invalid
    addresses. Equivalent to, and potentially more efficient than,
    `IfThenElseZero(mask, Load(D(), aligned))`.

Requires only *element-aligned* vectors (e.g. from malloc/std::vector, or
aligned memory at indices which are not a multiple of the vector length):

*   <code>Vec&lt;D&gt; **LoadU**(D, const T* p)</code>: returns `p[i]`.

*   <code>Vec&lt;D&gt; **LoadDup128**(D, const T* p)</code>: returns one 128-bit
    block loaded from `p` and broadcasted into all 128-bit block\[s\]. This may
    be faster than broadcasting single values, and is more convenient than
    preparing constants for the actual vector length.

#### Scatter/Gather

**Note**: Offsets/indices are of type `VI = Vec<RebindToSigned<D>>` and need not
be unique. The results are implementation-defined if any are negative.

**Note**: Where possible, applications should `Load/Store/TableLookup*` entire
vectors, which is much faster than `Scatter/Gather`. Otherwise, code of the form
`dst[tbl[i]] = F(src[i])` should when possible be transformed to `dst[i] =
F(src[tbl[i]])` because `Scatter` is more expensive than `Gather`.

*   `D`: `{u,i,f}{32,64}` \
    <code>void **ScatterOffset**(Vec&lt;D&gt; v, D, const T* base, VI
    offsets)</code>: stores `v[i]` to the base address plus *byte* `offsets[i]`.

*   `D`: `{u,i,f}{32,64}` \
    <code>void **ScatterIndex**(Vec&lt;D&gt; v, D, const T* base, VI
    indices)</code>: stores `v[i]` to `base[indices[i]]`.

*   `D`: `{u,i,f}{32,64}` \
    <code>Vec&lt;D&gt; **GatherOffset**(D, const T* base, VI offsets)</code>:
    returns elements of base selected by *byte* `offsets[i]`.

*   `D`: `{u,i,f}{32,64}` \
    <code>Vec&lt;D&gt; **GatherIndex**(D, const T* base, VI indices)</code>:
    returns vector of `base[indices[i]]`.

#### Store

*   <code>void **Store**(Vec&lt;D&gt; a, D, T* aligned)</code>: copies `a[i]`
    into `aligned[i]`, which must be naturally aligned. Writes exactly N *
    sizeof(T) bytes.

*   <code>void **StoreU**(Vec&lt;D&gt; a, D, T* p)</code>: as Store, but without
    the alignment requirement.

*   `D`: `u8` \
    <code>void **StoreInterleaved3**(Vec&lt;D&gt; v0, Vec&lt;D&gt; v1,
    Vec&lt;D&gt; v2, D, T* p)</code>: equivalent to shuffling `v0, v1, v2`
    followed by three `StoreU()`, such that `p[0] == v0[0], p[1] == v1[0],
    p[2] == v1[0]`. Useful for RGB samples.

*   `D`: `u8` \
    <code>void **StoreInterleaved4**(Vec&lt;D&gt; v0, Vec&lt;D&gt; v1,
    Vec&lt;D&gt; v2, Vec&lt;D&gt; v3, D, T* p)</code>: as above, but for four
    vectors (e.g. RGBA samples).

### Cache control

All functions except `Stream` are defined in cache_control.h.

*   <code>void **Stream**(Vec&lt;D&gt; a, D d, const T* aligned)</code>: copies
    `a[i]` into `aligned[i]` with non-temporal hint if available (useful for
    write-only data; avoids cache pollution). May be implemented using a
    CPU-internal buffer. To avoid partial flushes and unpredictable interactions
    with atomics (for example, see Intel SDM Vol 4, Sec. 8.1.2.2), call this
    consecutively for an entire naturally aligned cache line (typically 64
    bytes). Each call may write a multiple of `HWY_STREAM_MULTIPLE` bytes, which
    can exceed `Lanes(d) * sizeof(T)`. The new contents of `aligned` may not be
    visible until `FlushStream` is called.

*   <code>void **FlushStream**()</code>: ensures values written by previous
    `Stream` calls are visible on the current core. This is NOT sufficient for
    synchronizing across cores; when `Stream` outputs are to be consumed by
    other core(s), the producer must publish availability (e.g. via mutex or
    atomic_flag) after `FlushStream`.

*   <code>void **FlushCacheline**(const void* p)</code>: invalidates and flushes
    the cache line containing "p", if possible.

*   <code>void **Prefetch**(const T* p)</code>: optionally begins loading the
    cache line containing "p" to reduce latency of subsequent actual loads.

*   <code>void **Pause**()</code>: when called inside a spin-loop, may reduce
    power consumption.

### Type conversion

*   <code>Vec&lt;D&gt; **BitCast**(D, V)</code>: returns the bits of `V`
    reinterpreted as type `Vec<D>`.

*   `V`,`D`: (`u8,u16`), (`u16,u32`), (`u8,u32`), (`u32,u64`), (`u8,i16`), \
    (`u8,i32`), (`u16,i32`), (`i8,i16`), (`i8,i32`), (`i16,i32`), (`i32,i64`)
    <code>Vec&lt;D&gt; **PromoteTo**(D, V part)</code>: returns `part[i]` zero-
    or sign-extended to the integer type `MakeWide<T>`.

*   `V`,`D`: (`f16,f32`), (`bf16,f32`), (`f32,f64`) \
    <code>Vec&lt;D&gt; **PromoteTo**(D, V part)</code>: returns `part[i]`
    widened to the floating-point type `MakeWide<T>`.

*   `V`,`D`: \
    <code>Vec&lt;D&gt; **PromoteTo**(D, V part)</code>: returns `part[i]`
    converted to 64-bit floating point.

*   `V`,`D`: (`bf16,f32`) <code>Vec&lt;D&gt; **PromoteLowerTo**(D, V v)</code>:
    returns `v[i]` widened to `MakeWide<T>`, for i in `[0, Lanes(D()))`. Note
    that `V` has twice as many lanes as `D` and the return value.

*   `V`,`D`: (`bf16,f32`) <code>Vec&lt;D&gt; **PromoteUpperTo**(D, V v)</code>:
    returns `v[i]` widened to `MakeWide<T>`, for i in `[Lanes(D()), 2 *
    Lanes(D()))`. Note that `V` has twice as many lanes as `D` and the return
    value.

*   `V`,`V8`: (`u32,u8`) \
    <code>V8 **U8FromU32**(V)</code>: special-case `u32` to `u8` conversion when
    all lanes of `V` are already clamped to `[0, 256)`.

`DemoteTo` and float-to-int `ConvertTo` return the closest representable value
if the input exceeds the destination range.

*   `V`,`D`: (`i16,i8`), (`i32,i8`), (`i32,i16`), (`i16,u8`), (`i32,u8`),
    (`i32,u16`), (`f64,f32`) \
    <code>Vec&lt;D&gt; **DemoteTo**(D, V a)</code>: returns `a[i]` after packing
    with signed/unsigned saturation to `MakeNarrow<T>`.

*   `V`,`D`: `f64,i32` \
    <code>Vec&lt;D&gt; **DemoteTo**(D, V a)</code>: rounds floating point
    towards zero and converts the value to 32-bit integers.

*   `V`,`D`: (`f32,f16`), (`f32,bf16`) \
    <code>Vec&lt;D&gt; **DemoteTo**(D, V a)</code>: narrows float to half (for
    bf16, it is unspecified whether this truncates or rounds).

*   `V`,`D`: (`f32,bf16`) \
    <code>Vec&lt;D&gt; **ReorderDemote2To**(D, V a, V b)</code>: as above, but
    converts two inputs, `D` and the output have twice as many lanes as `V`, and
    the output order is some permutation of the inputs.

*   `V`,`D`: (`i32`,`f32`), (`i64`,`f64`) \
    <code>Vec&lt;D&gt; **ConvertTo**(D, V)</code>: converts an integer value to
    same-sized floating point.

*   `V`,`D`: (`f32`,`i32`), (`f64`,`i64`) \
    <code>Vec&lt;D&gt; **ConvertTo**(D, V)</code>: rounds floating point towards
    zero and converts the value to same-sized integer.

*   `V`: `f32`; `Ret`: `i32` \
    <code>Ret **NearestInt**(V a)</code>: returns the integer nearest to `a[i]`;
    results are undefined for NaN.

### Combine

*   <code>V2 **LowerHalf**([D, ] V)</code>: returns the lower half of the vector
    `V`. The optional `D` (provided for consistency with `UpperHalf`) is
    `Half<DFromV<V>>`.

*   <code>V2 **UpperHalf**(D, V)</code>: returns upper half of the vector `V`,
    where `D` is `Half<DFromV<V>>`.

*   <code>V **ZeroExtendVector**(D, V2)</code>: returns vector whose `UpperHalf`
    is zero and whose `LowerHalf` is the argument; `D` is `Twice<DFromV<V2>>`.

*   <code>V **Combine**(D, V2, V2)</code>: returns vector whose `UpperHalf` is
    the first argument and whose `LowerHalf` is the second argument. This is
    currently only implemented for RVV, AVX2, AVX3*. `D` is `Twice<DFromV<V2>>`.

**Note**: the following operations cross block boundaries, which is typically
more expensive on AVX2/AVX-512 than per-block operations.

*   <code>V **ConcatLowerLower**(D, V hi, V lo)</code>: returns the
    concatenation of the lower halves of `hi` and `lo` without splitting into
    blocks. `D` is `DFromV<V>`.

*   <code>V **ConcatUpperUpper**(D, V hi, V lo)</code>: returns the
    concatenation of the upper halves of `hi` and `lo` without splitting into
    blocks. `D` is `DFromV<V>`.

*   <code>V **ConcatLowerUpper**(D, V hi, V lo)</code>: returns the inner half
    of the concatenation of `hi` and `lo` without splitting into blocks. Useful
    for swapping the two blocks in 256-bit vectors. `D` is `DFromV<V>`.

*   <code>V **ConcatUpperLower**(D, V hi, V lo)</code>: returns the outer
    quarters of the concatenation of `hi` and `lo` without splitting into
    blocks. Unlike the other variants, this does not incur a block-crossing
    penalty on AVX2/3. `D` is `DFromV<V>`.

*   `V`: `{u,i,f}{32,64}` \
    <code>V **ConcatOdd**(V hi, V lo)</code>: returns the concatenation of the
    odd lanes of `hi` and the odd lanes of `lo`.

*   `V`: `{u,i,f}{32,64}` \
    <code>V **ConcatEven**(V hi, V lo)</code>: returns the concatenation of the
    even lanes of `hi` and the even lanes of `lo`.

### Blockwise

**Note**: if vectors are larger than 128 bits, the following operations split
their operands into independently processed 128-bit *blocks*.

*   `V`: `{u,i}{16,32,64}, {f}` \
    <code>V **Broadcast**&lt;int i&gt;(V)</code>: returns individual *blocks*,
    each with lanes set to `input_block[i]`, `i = [0, 16/sizeof(T))`.

*   `V`: `{u,i}` \
    <code>VI **TableLookupBytes**(V bytes, VI from)</code>: returns
    `bytes[from[i]]`. Uses byte lanes regardless of the actual vector types.
    Results are implementation-defined if `from[i] >= HWY_MIN(lanes in V, 16)`.
    The number of lanes in `V` and `VI` may differ, e.g. a full-length table
    vector loaded via `LoadDup128`, plus partial vector `VI` of 4-bit indices.

*   `V`: `{u,i}` \
    <code>VI **TableLookupBytesOr0**(V bytes, VI from)</code>: returns
    `bytes[from[i]]`, or 0 if `from[i] & 0x80`. Uses byte lanes regardless of
    the actual vector types. Results are implementation-defined for `from[i]` in
    `[HWY_MIN(vector size, 16), 0x80)`. The zeroing behavior has zero cost on
    x86 and ARM. For vectors of >= 256 bytes (can happen on SVE and RVV), this
    will set all lanes after the first 128 to 0. The number of lanes in `V` and
    `VI` may differ.

#### Zip/Interleave

*   <code>V **InterleaveLower**([D, ] V a, V b)</code>: returns *blocks* with
    alternating lanes from the lower halves of `a` and `b` (`a[0]` in the
    least-significant lane). The optional `D` (provided for consistency with
    `InterleaveUpper`) is `DFromV<V>`.

*   <code>V **InterleaveUpper**(D, V a, V b)</code>: returns *blocks* with
    alternating lanes from the upper halves of `a` and `b` (`a[N/2]` in the
    least-significant lane). `D` is `DFromV<V>`.

*   `Ret`: `MakeWide<T>`; `V`: `{u,i}{8,16,32}` \
    <code>Ret **ZipLower**([D, ] V a, V b)</code>: returns the same bits as
    `InterleaveLower`, but repartitioned into double-width lanes (required in
    order to use this operation with scalars). The optional `D` (provided for
    consistency with `ZipUpper`) is `RepartitionToWide<DFromV<V>>`.

*   `Ret`: `MakeWide<T>`; `V`: `{u,i}{8,16,32}` \
    <code>Ret **ZipUpper**(D, V a, V b)</code>: returns the same bits as
    `InterleaveUpper`, but repartitioned into double-width lanes (required in
    order to use this operation with scalars). `D` is
    `RepartitionToWide<DFromV<V>>`.

#### Shift

*   `V`: `{u,i}` \
    <code>V **ShiftLeftBytes**&lt;int&gt;([D, ] V)</code>: returns the result of
    shifting independent *blocks* left by `int` bytes \[1, 15\]. The optional
    `D` (provided for consistency with `ShiftRightBytes`) is `DFromV<V>`.

*   <code>V **ShiftLeftLanes**&lt;int&gt;([D, ] V)</code>: returns the result of
    shifting independent *blocks* left by `int` lanes. The optional `D`
    (provided for consistency with `ShiftRightLanes`) is `DFromV<V>`.

*   `V`: `{u,i}` \
    <code>V **ShiftRightBytes**&lt;int&gt;(D, V)</code>: returns the result of
    shifting independent *blocks* right by `int` bytes \[1, 15\], shifting in
    zeros even for partial vectors. `D` is `DFromV<V>`.

*   <code>V **ShiftRightLanes**&lt;int&gt;(D, V)</code>: returns the result of
    shifting independent *blocks* right by `int` lanes, shifting in zeros even
    for partial vectors. `D` is `DFromV<V>`.

*   `V`: `{u,i}` \
    <code>V **CombineShiftRightBytes**&lt;int&gt;(D, V hi, V lo)</code>: returns
    a vector of *blocks* each the result of shifting two concatenated *blocks*
    `hi[i] || lo[i]` right by `int` bytes \[1, 16). `D` is `DFromV<V>`.

*   <code>V **CombineShiftRightLanes**&lt;int&gt;(D, V hi, V lo)</code>: returns
    a vector of *blocks* each the result of shifting two concatenated *blocks*
    `hi[i] || lo[i]` right by `int` lanes \[1, 16/sizeof(T)). `D` is
    `DFromV<V>`.

#### Shuffle

*   `V`: `{u,i,f}{32}` \
    <code>V **Shuffle2301**(V)</code>: returns *blocks* with 32-bit halves
    swapped inside 64-bit halves.

*   `V`: `{u,i,f}{32}` \
    <code>V **Shuffle1032**(V)</code>: returns *blocks* with 64-bit halves
    swapped.

*   `V`: `{u,i,f}{64}` \
    <code>V **Shuffle01**(V)</code>: returns *blocks* with 64-bit halves
    swapped.

*   `V`: `{u,i,f}{32}` \
    <code>V **Shuffle0321**(V)</code>: returns *blocks* rotated right (toward
    the lower end) by 32 bits.

*   `V`: `{u,i,f}{32}` \
    <code>V **Shuffle2103**(V)</code>: returns *blocks* rotated left (toward the
    upper end) by 32 bits.

*   `V`: `{u,i,f}{32}` \
    <code>V **Shuffle0123**(V)</code>: returns *blocks* with lanes in reverse
    order.

### Swizzle

*   <code>T **GetLane**(V)</code>: returns lane 0 within `V`. This is useful for
    extracting `SumOfLanes` results.

*   <code>V **OddEven**(V a, V b)</code>: returns a vector whose odd lanes are
    taken from `a` and the even lanes from `b`.

*   `V`: `{u,i,f}{32}` \
    <code>V **TableLookupLanes**(V a, VI)</code> returns a vector of
    `a[indices[i]]`, where `VI` is from `SetTableIndices(D, &indices[0])`. The
    indices are not limited to blocks, hence this is slower than
    `TableLookupBytes*` on AVX2/AVX-512. Results are implementation-defined if
    `indices[i] >= Lanes(D())`.

*   `VI`: `i32` \
    <code>VI **SetTableIndices**(D, int32_t* idx)</code> prepares for
    `TableLookupLanes` with lane indices `idx = [0, N)` (need not be unique).

*   `V`: `{u,i,f}{32}` \
    <code>V **Reverse**(D, V a)</code> returns a vector with lanes in reversed
    order (`out[i] == a[Lanes(D()) - 1 - i]`).

### Reductions

**Note**: these 'reduce' all lanes to a single result (e.g. sum), which is
broadcasted to all lanes. To obtain a scalar, you can call `GetLane`.

Being a horizontal operation (across lanes of the same vector), these are slower
than normal SIMD operations and are typically used outside critical loops.

*   `V`: `{u,i,f}{32,64}` \
    <code>V **SumOfLanes**(D, V v)</code>: returns the sum of all lanes in each
    lane.

*   `V`: `{u,i,f}{32,64}` \
    <code>V **MinOfLanes**(D, V v)</code>: returns the minimum-valued lane in
    each lane.

*   `V`: `{u,i,f}{32,64}` \
    <code>V **MaxOfLanes**(D, V v)</code>: returns the maximum-valued lane in
    each lane.

### Crypto

*   `V`: `u8` \
    <code>V **AESRound**(V state, V round_key)</code>: one round of AES
    encrytion: `MixColumns(SubBytes(ShiftRows(state))) ^ round_key`. This
    matches x86 AES-NI. The latency is independent of the input values. Only
    available if `HWY_TARGET != HWY_SCALAR`.

*   `V`: `u64` \
    <code>V **CLMulLower**(V a, V b)</code>: carryless multiplication of the
    lower 64 bits of each 128-bit block into a 128-bit product. The latency is
    independent of the input values (assuming that is true of normal integer
    multiplication) so this can safely be used in cryto. Applications that wish
    to multiply upper with lower halves can `Shuffle01` one of the operands; on
    x86 that is expected to be latency-neutral.

*   `V`: `u64` \
    <code>V **CLMulUpper**(V a, V b)</code>: as CLMulLower, but multiplies the
    upper 64 bits of each 128-bit block.

### Deprecated

*   <code>bool **AllTrue**(M m)</code>: returns whether all `m[i]` are true.
    DEPRECATED, SVE needs an extra D argument.

*   <code>bool **AllFalse**(M m)</code>: returns whether all `m[i]` are false.
    DEPRECATED, SVE needs an extra D argument.

*   <code>size_t **StoreMaskBits**(M m, uint8_t* p)</code>: stores a bit array
    indicating whether `m[i]` is true, in ascending order of `i`, filling the
    bits of each byte from least to most significant, then proceeding to the
    next byte. Returns the number of (partial) bytes written. DEPRECATED, SVE
    needs an extra D argument.

*   <code>size_t **CountTrue**(M m)</code>: returns how many of `m[i]` are true
    [0, N]. This is typically more expensive than AllTrue/False. DEPRECATED, SVE
    needs an extra D argument.

*   <code>void **StoreFence**()</code>: DEPRECATED, calls `FlushStream`.

*   <code>void **LoadFence**()</code>: delays subsequent loads until prior loads
    are visible. Also a full fence on Intel CPUs. No effect on non-x86.
    DEPRECATED due to differing behavior across architectures AND vendors.

*   <code>V2 **UpperHalf**(V)</code>: returns upper half of the vector `V`.
    DEPRECATED, supporting partial vectors requires a D argument.

*   `V`: `{u,i}` \
    <code>V **ShiftRightBytes**&lt;int&gt;(V)</code>: returns the result of
    shifting independent *blocks* right by `int` bytes \[1, 15\]. DEPRECATED,
    supporting partial vectors requires a D argument.

*   <code>V **ShiftRightLanes**&lt;int&gt;(V)</code>: returns the result of
    shifting independent *blocks* right by `int` lanes. DEPRECATED, supporting
    partial vectors requires a D argument.

*   <code>V **ZeroExtendVector**(V2)</code>: returns vector whose `UpperHalf` is
    zero and whose `LowerHalf` is the argument. DEPRECATED, supporting partial
    vectors requires a D argument.

*   <code>V **Combine**(V2, V2)</code>: returns vector whose `UpperHalf` is the
    first argument and whose `LowerHalf` is the second argument. This is
    currently only implemented for RVV, AVX2, AVX3*. DEPRECATED, supporting
    partial vectors requires a D argument.

*   <code>V **ConcatLowerLower**(V hi, V lo)</code>: returns the concatenation
    of the lower halves of `hi` and `lo` without splitting into blocks.
    DEPRECATED, supporting partial vectors requires a D argument.

*   <code>V **ConcatUpperUpper**(V hi, V lo)</code>: returns the concatenation
    of the upper halves of `hi` and `lo` without splitting into blocks.
    DEPRECATED, supporting partial vectors requires a D argument.

*   <code>V **ConcatLowerUpper**(V hi, V lo)</code>: returns the inner half of
    the concatenation of `hi` and `lo` without splitting into blocks. Useful for
    swapping the two blocks in 256-bit vectors. DEPRECATED, supporting partial
    vectors requires a D argument.

*   <code>V **ConcatUpperLower**(V hi, V lo)</code>: returns the outer quarters
    of the concatenation of `hi` and `lo` without splitting into blocks. Unlike
    the other variants, this does not incur a block-crossing penalty on AVX2.
    DEPRECATED, supporting partial vectors requires a D argument.

*   <code>V **InterleaveUpper**(V a, V b)</code>: returns *blocks* with
    alternating lanes from the upper halves of `a` and `b` (`a[N/2]` in the
    least-significant lane). DEPRECATED, supporting partial vectors requires a D
    argument.

*   `Ret`: `MakeWide<T>`; `V`: `{u,i}{8,16,32}` \
    <code>Ret **ZipUpper**(V a, V b)</code>: returns the same bits as
    `InterleaveUpper`, but repartitioned into double-width lanes (required in
    order to use this operation with scalars). DEPRECATED, supporting partial
    vectors requires a D argument.

*   `V`: `{u,i}` \
    <code>V **CombineShiftRightBytes**&lt;int&gt;(V hi, V lo)</code>: returns a
    vector of *blocks* each the result of shifting two concatenated *blocks*
    `hi[i] || lo[i]` right by `int` bytes \[1, 16). DEPRECATED, supporting
    partial vectors requires a D argument.

*   <code>V **CombineShiftRightLanes**&lt;int&gt;(V hi, V lo)</code>: returns a
    vector of *blocks* each the result of shifting two concatenated *blocks*
    `hi[i] || lo[i]` right by `int` lanes \[1, 16/sizeof(T)). DEPRECATED,
    supporting partial vectors requires a D argument.

*   `V`: `{u,i,f}{32,64}` \
    <code>V **SumOfLanes**(V v)</code>: returns the sum of all lanes in each
    lane. DEPRECATED, SVE/RVV require a D argument to support partial vectors.

*   `V`: `{u,i,f}{32,64}` \
    <code>V **MinOfLanes**(V v)</code>: returns the minimum-valued lane in each
    lane. DEPRECATED, SVE/RVV require a D argument to support partial vectors.

*   `V`: `{u,i,f}{32,64}` \
    <code>V **MaxOfLanes**(V v)</code>: returns the maximum-valued lane in each
    lane. DEPRECATED, SVE/RVV require a D argument to support partial vectors.

## Preprocessor macros

*   `HWY_ALIGN`: Prefix for stack-allocated (i.e. automatic storage duration)
    arrays to ensure they have suitable alignment for Load()/Store(). This is
    specific to `HWY_TARGET` and should only be used inside `HWY_NAMESPACE`.

    Arrays should also only be used for partial (<= 128-bit) vectors, or
    `LoadDup128`, because full vectors may be too large for the stack and should
    be heap-allocated instead (see aligned_allocator.h).

    Example: `HWY_ALIGN float lanes[4];`

*   `HWY_ALIGN_MAX`: as `HWY_ALIGN`, but independent of `HWY_TARGET` and may be
    used outside `HWY_NAMESPACE`.

## Advanced macros

Let `Target` denote an instruction set:
`SCALAR/SSSE3/SSE4/AVX2/AVX3/AVX3_DL/PPC8/NEON/WASM/RVV`. Targets are only used
if enabled (i.e. not broken nor disabled). Baseline means the compiler is
allowed to generate such instructions (implying the target CPU would have to
support them).

*   `HWY_Target=##` are powers of two uniquely identifying `Target`.

*   `HWY_STATIC_TARGET` is the best enabled baseline `HWY_Target`, and matches
    `HWY_TARGET` in static dispatch mode. This is useful even in dynamic
    dispatch mode for deducing and printing the compiler flags.

*   `HWY_TARGETS` indicates which targets to generate for dynamic dispatch, and
    which headers to include. It is determined by configuration macros and
    always includes `HWY_STATIC_TARGET`.

*   `HWY_SUPPORTED_TARGETS` is the set of targets available at runtime. Expands
    to a literal if only a single target is enabled, or SupportedTargets().

*   `HWY_TARGET`: which `HWY_Target` is currently being compiled. This is
    initially identical to `HWY_STATIC_TARGET` and remains so in static dispatch
    mode. For dynamic dispatch, this changes before each re-inclusion and
    finally reverts to `HWY_STATIC_TARGET`. Can be used in `#if` expressions to
    provide an alternative to functions which are not supported by HWY_SCALAR.

*   `HWY_WANT_AVX3_DL`: additional opt-in for HWY_AVX3, which is disabled unless
    this is defined by the app before including highway.h, OR all AVX3_DL
    compiler flags are specified.

*   `HWY_IDE` is 0 except when parsed by IDEs; adding it to conditions such as
    `#if HWY_TARGET != HWY_SCALAR || HWY_IDE` avoids code appearing greyed out.

The following signal capabilities and expand to 1 or 0.

*   `HWY_CAP_INTEGER64`: support for 64-bit signed/unsigned integer lanes.
*   `HWY_CAP_FLOAT16`: support for IEEE half-precision floating-point lanes.
*   `HWY_CAP_FLOAT64`: support for double-precision floating-point lanes.

The following were used to signal the maximum number of lanes for certain
operations, but this is no longer necessary (nor possible on SVE/RVV), so they
are DEPRECATED:

*   `HWY_GATHER_LANES(T)`.
*   `HWY_CAP_GE256`: the current target supports vectors of >= 256 bits.
*   `HWY_CAP_GE512`: the current target supports vectors of >= 512 bits.

## Detecting supported targets

`SupportedTargets()` returns a cached (initialized on-demand) bitfield of the
targets supported on the current CPU, detected using CPUID on x86 or equivalent.
This may include targets that are not in `HWY_TARGETS`, and vice versa. If
there is no overlap the binary will likely crash. This can only happen if:

*   the specified baseline is not supported by the current CPU, which
    contradicts the definition of baseline, so the configuration is invalid; or
*   the baseline does not include the enabled/attainable target(s), which are
    also not supported by the current CPU, and baseline targets (in particular
    `HWY_SCALAR`) were explicitly disabled.

## Advanced configuration macros

The following macros govern which targets to generate. Unless specified
otherwise, they may be defined per translation unit, e.g. to disable >128 bit
vectors in modules that do not benefit from them (if bandwidth-limited or only
called occasionally). This is safe because `HWY_TARGETS` always includes at
least one baseline target which `HWY_EXPORT` can use.

*   `HWY_DISABLE_CACHE_CONTROL` makes the cache-control functions no-ops.
*   `HWY_DISABLE_BMI2_FMA` prevents emitting BMI/BMI2/FMA instructions. This
    allows using AVX2 in VMs that do not support the other instructions, but
    only if defined for all translation units.

The following `*_TARGETS` are zero or more `HWY_Target` bits and can be defined
as an expression, e.g. `-DHWY_DISABLED_TARGETS=(HWY_SSE4|HWY_AVX3)`.

*   `HWY_BROKEN_TARGETS` defaults to a blocklist of known compiler bugs.
    Defining to 0 disables the blocklist.

*   `HWY_DISABLED_TARGETS` defaults to zero. This allows explicitly disabling
    targets without interfering with the blocklist.

*   `HWY_BASELINE_TARGETS` defaults to the set whose predefined macros are
    defined (i.e. those for which the corresponding flag, e.g. -mavx2, was
    passed to the compiler). If specified, this should be the same for all
    translation units, otherwise the safety check in SupportedTargets (that all
    enabled baseline targets are supported) may be inaccurate.

Zero or one of the following macros may be defined to replace the default
policy for selecting `HWY_TARGETS`:

*   `HWY_COMPILE_ONLY_SCALAR` selects only `HWY_SCALAR`, which disables SIMD.
*   `HWY_COMPILE_ONLY_STATIC` selects only `HWY_STATIC_TARGET`, which
    effectively disables dynamic dispatch.
*   `HWY_COMPILE_ALL_ATTAINABLE` selects all attainable targets (i.e. enabled
    and permitted by the compiler, independently of autovectorization), which
    maximizes coverage in tests.

If none are defined, but `HWY_IS_TEST` is defined, the default is
`HWY_COMPILE_ALL_ATTAINABLE`. Otherwise, the default is to select all attainable
targets except any non-best baseline (typically `HWY_SCALAR`), which reduces
code size.

## Compiler support

Clang and GCC require e.g. -mavx2 flags in order to use SIMD intrinsics.
However, this enables AVX2 instructions in the entire translation unit, which
may violate the one-definition rule and cause crashes. Instead, we use
target-specific attributes introduced via #pragma. Function using SIMD must
reside between `HWY_BEFORE_NAMESPACE` and `HWY_AFTER_NAMESPACE`. Alternatively,
individual functions or lambdas may be prefixed with `HWY_ATTR`.

Immediates (compile-time constants) are specified as template arguments to avoid
constant-propagation issues with Clang on ARM.

## Type traits

*   `IsFloat<T>()` returns true if the `T` is a floating-point type.
*   `IsSigned<T>()` returns true if the `T` is a signed or floating-point type.
*   `LimitsMin/Max<T>()` return the smallest/largest value representable in
    integer `T`.
*   `SizeTag<N>` is an empty struct, used to select overloaded functions
    appropriate for `N` bytes.

## Memory allocation

`AllocateAligned<T>(items)` returns a unique pointer to newly allocated memory
for `items` elements of POD type `T`. The start address is aligned as required
by `Load/Store`. Furthermore, successive allocations are not congruent modulo a
platform-specific alignment. This helps prevent false dependencies or cache
conflicts. The memory allocation is analogous to using `malloc()` and `free()`
with a `std::unique_ptr` since the returned items are *not* initialized or
default constructed and it is released using `FreeAlignedBytes()` without
calling `~T()`.

`MakeUniqueAligned<T>(Args&&... args)` creates a single object in newly
allocated aligned memory as above but constructed passing the `args` argument to
`T`'s constructor and returning a unique pointer to it. This is analogous to
using `std::make_unique` with `new` but for aligned memory since the object is
constructed and later destructed when the unique pointer is deleted. Typically
this type `T` is a struct containing multiple members with `HWY_ALIGN` or
`HWY_ALIGN_MAX`, or arrays whose lengths are known to be a multiple of the
vector size.

`MakeUniqueAlignedArray<T>(size_t items, Args&&... args)` creates an array of
objects in newly allocated aligned memory as above and constructs every element
of the new array using the passed constructor parameters, returning a unique
pointer to the array. Note that only the first element is guaranteed to be
aligned to the vector size; because there is no padding between elements,
the alignment of the remaining elements depends on the size of `T`.
