# Recommendations for SIMD intrinsic fallbacks?

**URL:** <https://forums.swift.org/t/recommendations-for-simd-intrinsic-fallbacks/40565>\
**Category:** Using Swift\
**Created:** [September 24, 2020, 1:26am UTC](https://forums.swift.org/t/recommendations-for-simd-intrinsic-fallbacks/40565 "2020-09-24T01:26:37Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![taylorswift](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/taylorswift/32/43413_2.png) [@taylorswift](https://forums.swift.org/u/taylorswift)\
**Post date:** [September 24, 2020, 1:26am UTC](https://forums.swift.org/t/recommendations-for-simd-intrinsic-fallbacks/40565/1 "2020-09-24T01:26:37Z")

</div>

As was suggested in [this thread](https://forums.swift.org/t/simd-move-mask-operation/39096), i performed a move-mask operation by importing the x86 `_mm_cmpeq_epi8` intrinsic.

```swift
#if arch(x86_64)

import _Builtin_intrinsics.intel

extension SIMD16 where Scalar == UInt8 
{
    func find(_ scalar:UInt8) -> UInt16 
    {
        let repeated:Self = .init(repeating: scalar)
        let mask:SIMD2<Int64> = _mm_cmpeq_epi8(
            unsafeBitCast(self, to: SIMD2<Int64>.self), 
            unsafeBitCast(repeated, to: SIMD2<Int64>.self))
        return .init(truncatingIfNeeded: _mm_movemask_epi8(mask))
    }
}

#endif

```

now i’m looking to make this work on all Swift platforms, not just `#if arch(x86_64)` evaluates true. does anyone have any recommendations for implementing efficient fallbacks for this operation?

i have generic Swift `SIMD{N}` code here:

```swift
func find(_ key:UInt8, in vector:SIMD16<UInt8>, _ body:(Int) -> ())
{
    // (key: 5, vector: (1, 5, 1, 1, 5, 5, 1, 1, 1, 1, 1, 1, 5, 1, 1, 5))
    let places:SIMD16<UInt8> = 
        .init(128, 64, 32, 16, 8, 4, 2, 1, 128, 64, 32, 16, 8, 4, 2, 1),
        match:SIMD16<UInt8> = places.replacing(with: 0, where: vector .!= key)
    // match: ( 0, 64, 0, 0, 8, 4, 0, 0, 0, 0, 0, 0, 8, 0, 0, 1)
    let r8:SIMD8<UInt8> = match.evenHalf | match.oddHalf, 
        r4:SIMD4<UInt8> = r8.evenHalf | r8.oddHalf,
        r2:SIMD2<UInt8> = r4.evenHalf | r4.oddHalf
    let r:UInt16 = .init(r2.x) << 8 | .init(r2.y)
    // r: 0b0100_1100_0000_1001
    var i:Int = r.leadingZeroBitCount
    while i < 16
    {
        body(i)
        i += 1 + ((r << i) << 1).leadingZeroBitCount
    }
}

```

but it generates very inefficient assembly.

---

<div class="post-metadata">

**Author:** ![scanon](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/scanon/32/2445_2.png) [@scanon](https://forums.swift.org/u/scanon)\
**Post date:** [September 24, 2020, 3:24am UTC](https://forums.swift.org/t/recommendations-for-simd-intrinsic-fallbacks/40565/2 "2020-09-24T03:24:14Z")

</div>

Do you want to find every match, or does finding the first match actually suffice for your intended use case?

If the latter, on arm64 the usual idiom is to or-not the compare mask into 0,1,2,3,...n and then use the MINV instruction to get the index of first match (or 0b111...111 if no match).

If the former, on arm64, and-not mask into 1,2,4,...128,1,2,4,...128, break into two SIMD8 halves, ADDV across each half, then assemble as `a | b << 8`. On arm32, it's the same, but you don't have ADDV and have to use a sequence of ADDP instead (this is what your open-coded version is doing, but you may get better codegen via the intrinsic).

Note that on some arm implementations, moving a result from NEON back to GPR can be somewhat slow (5-10 cycles), and NEON is under-provisioned relative to GPR instructions on most non-Apple designs (it is a pretty common design to have only two NEON pipes, but 4-6 GPR pipes); if you're using this in a loop to scan for matches, it's generally better to use SWAR on UInt64 fields instead of NEON on an unspecified arm core.

---

<div class="post-metadata">

**Author:** ![taylorswift](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/taylorswift/32/43413_2.png) [@taylorswift](https://forums.swift.org/u/taylorswift)\
**Post date:** [September 24, 2020, 3:38am UTC](https://forums.swift.org/t/recommendations-for-simd-intrinsic-fallbacks/40565/3 "2020-09-24T03:38:30Z")

</div>

thank you!!

> [@scanon](#):
>
> Do you want to find every match, or does finding the first match actually suffice for your intended use case?

in theory i need to find every match, but in the vast majority of cases, the first match succeeds (when doing the full-length key comparison), so subsequent matches can be ignored

> [@scanon](#):
>
> If the former, on arm64, and-not mask into 1,2,4,...128,1,2,4,...128

is this the same as doing `places.replacing(with: 0, where: vector .!= key)` except with two SIMD8’s instead of one SIMD16?

> [@scanon](#):
>
> if you're using this in a loop to scan for matches, it's generally better to use SWAR on UInt64 fields instead of NEON

how do i use SWAR on UInt64 fields? i’m not very familiar with ARM tbh

also, is there a list of all the `#if` architecture values that Swift supports? or should I keep the standard library version of the `find` function around in an `#else` block?

finally, this is only semi-related but do you have tips on how to build and test the architecture-specific implementations? since I only have access to an x86\_64 platform, both on my local machine, and the Github Actions runner