divmod assembly implementation

[Nick Treleaven]

On Saturday, 23 November 2024 at 19:54:36 UTC, Sam Ludacis wrote:
> G'day
>
> I'm trying to implement the Python divmod function in D w/ 
> assembly. Should look something like this...
>
>     '''
>     ulong divmod64(ulong a, ulong b, ulong quot, ulong rem)
>     {
>         asm {
>             xor RDX, RDX;
>             mov RAX, a;
>             div b;
>             mov quot, RAX;
>             mov rem, RDX;
>         }
>     }
>     '''
>
> Without doubt this routine will fail. But would anyone have a 
> way?

Just to mention that if requiring compiler optimization for fast 
code is acceptable, you can just write it in D e.g.:

```d
ulong[2] div(ulong a, ulong b)
{
     return [a / b, a % b];
}
```
```
$ dmd -c divmod.d -O -vasm
_D6divmod3divFmmZG2m:
0000:   48 83 EC 18              sub       RSP,018h
0004:   48 8B C6                 mov       RAX,RSI
0007:   33 D2                    xor       EDX,EDX
0009:   48 F7 F7                 div       RDI
000c:   48 89 04 24              mov       [RSP],RAX
0010:   48 89 54 24 08           mov       8[RSP],RDX
0015:   48 8B 54 24 08           mov       RDX,8[RSP]
001a:   48 8B 04 24              mov       RAX,[RSP]
001e:   48 83 C4 18              add       RSP,018h
0022:   C3                       ret
```

Based on an older request:
https://forum.dlang.org/post/jd97nf$1jfj$1@digitalmars.com