Re: LLVM asm with constraints, and 2 operands

[kinke via Digitalmars-d-learn]

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...