LLVM asm with constraints, and 2 operands

[kinke]

On Monday, 19 July 2021 at 16:44:35 UTC, Guillaume Piolat wrote:
> On Monday, 19 July 2021 at 10:49:56 UTC, kinke wrote:
>> This workaround is actually missing the clobber constraint for 
>> `%2`, which might be problematic after inlining.
>>
>
> An unrelated other issue with asm/__asm is that it doesn't 
> follow consistent VEX encoding compared to normal compiler 
> output.
>
>     sometimes you might want: paddq x, y
>               at other times: vpaddq x, y, z
>
> but rarely both in the same program.
> So this can easily nullify any gain obtained with VEX 
> transition costs (if they are still a thing).

You know that asm is to be avoided whenever possible, but 
unfortunately, AFAIK intel-intrinsics doesn't fit the usual 
'don't worry, simply compile all your code with an appropriate 
-mattr/-mcpu option' recommendation, as it employs runtime 
detection of available CPU instructions.

I've just tried another option, but that doesn't play nice with 
inlining:

```
import core.simd;
import ldc.attributes;

@target("sse2") // use SSE2 for this function
int4 _mm_add_int4(int4 a, int4 b)
{
     return a + b; // perfect: paddd %xmm1, %xmm0
}

int4 wrapper(int4 a, int4 b)
{
     return _mm_add_int4(a, b);
}
```

Compiling with `-O -mtriple=i686-linux-gnu -mcpu=i686` (=> no 
SSE2 by default) shows that the inlined version inside 
`wrapper()` is the mega slow one, so the extra instructions 
aren't applied transitively unfortunately.