LLVM asm with constraints, and 2 operands

[kinke]

On Monday, 19 July 2021 at 10:21:58 UTC, kinke wrote:
> What works reliably is a manual mov:
>
> ```
> int4 _mm_add_int4(int4 a, int4 b)
> {
>     int4 r;
>     asm { "paddd %1, %2; movdqa %2, %0" : "=x" (r) : "x" (a), 
> "x" (b); }
>     return r;
> }
> ```

This workaround is actually missing the clobber constraint for 
`%2`, which might be problematic after inlining.

You can also specify the registers explicitly like so (here 
exploiting ABI knowledge about `a` being passed in XMM1, and `b` 
in XMM0 for extern(D)):

```
int4 _mm_add_int4(int4 a, int4 b)
{
     asm { "paddd %1, %0" : "=xmm0" (b) : "xmm1" (a), "xmm0" (b); }
     return b;
}
```

=>

```
paddd   xmm0, xmm1
ret
```

But this might likely tamper with LLVM register allocation 
optimizations after inlining...