Optimizing for SIMD: best practices?(i.e. what features are allowed?)

[tsbockman]

On Sunday, 7 March 2021 at 22:54:32 UTC, tsbockman wrote:
> import std.meta : Repeat;
> void euclideanDistanceFixedSizeArray(V)(ref Repeat!(3, 
> const(V)) a, ref Repeat!(3, const(V)) b, out V result)
>     if(is(V : __vector(float[length]), size_t length))
> ...
>
> Resulting asm with is(V == __vector(float[16])):
>
> .LCPI1_0:
>         .long   0x7fc00000
> pure nothrow @nogc void 
> app.euclideanDistanceFixedSizeArray!(__vector(float[16])).euclideanDistanceFixedSizeArray(ref const(__vector(float[16])), ref const(__vector(float[16])), ref const(__vector(float[16])), ref const(__vector(float[16])), ref const(__vector(float[16])), ref const(__vector(float[16])), out __vector(float[16])):
>         mov     rax, qword ptr [rsp + 8]
>         vbroadcastss    zmm0, dword ptr [rip + .LCPI1_0]
> ...

Apparently the optimizer is too stupid to skip the redundant 
float.nan broadcast when result is an `out` parameter, so just 
make it `ref V result` instead for better code gen:

pure nothrow @nogc void 
app.euclideanDistanceFixedSizeArray!(__vector(float[16])).euclideanDistanceFixedSizeArray(ref const(__vector(float[16])), ref const(__vector(float[16])), ref const(__vector(float[16])), ref const(__vector(float[16])), ref const(__vector(float[16])), ref const(__vector(float[16])), ref __vector(float[16])):
         mov     rax, qword ptr [rsp + 8]
         vmovaps zmm0, zmmword ptr [rax]
         vmovaps zmm1, zmmword ptr [r9]
         vmovaps zmm2, zmmword ptr [r8]
         vsubps  zmm0, zmm0, zmmword ptr [rcx]
         vmulps  zmm0, zmm0, zmm0
         vsubps  zmm1, zmm1, zmmword ptr [rdx]
         vsubps  zmm2, zmm2, zmmword ptr [rsi]
         vaddps  zmm0, zmm0, zmm0
         vfmadd231ps     zmm0, zmm1, zmm1
         vfmadd231ps     zmm0, zmm2, zmm2
         vmovaps zmmword ptr [rdi], zmm0
         vsqrtps zmm0, zmm0
         vmovaps zmmword ptr [rdi], zmm0
         vzeroupper
         ret