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

[tsbockman]

On Sunday, 7 March 2021 at 13:26:37 UTC, z wrote:
> On Thursday, 25 February 2021 at 11:28:14 UTC, z wrote:
> However, AVX512 support seems limited to being able to use the 
> 16 other YMM registers, rather than using the same code 
> template but changed to use ZMM registers and double the 
> offsets to take advantage of the new size.
> Compiled with «-g -enable-unsafe-fp-math 
> -enable-no-infs-fp-math -ffast-math -O -release -mcpu=skylake» :

You're not compiling with AVX512 enabled. You would need to use 
-mcpu=skylake-avx512.

However, LLVM's code generation for AVX512 seems to be pretty 
terrible still, so you'll need to either use some inline ASM, or 
stick with AVX2. Here's a structure of arrays style example:

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))
{
     Repeat!(3, V) diffSq = a;
     static foreach(i; 0 .. 3) {
         diffSq[i] -= b[i];
         diffSq[i] *= diffSq[i];
     }

     result = diffSq[0];
     static foreach(i; 0 .. 3)
         result += diffSq[i];

     version(LDC) { version(X86_64) {
         enum isSupportedPlatform = true;
         import ldc.llvmasm : __asm;
         result = __asm!V(`vsqrtps $1, $0`, `=x, x`, result);
     } }
     static assert(isSupportedPlatform);
}

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]
         vmovaps zmmword ptr [rdi], zmm0
         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