Re: [PATCH 0/1] MR179: vkd3d-shader/hlsl: Add support for ddx(), ddy() intrinsics.

[Ethan Lee (＠flibitijibibo)]

On Sat Apr 22 15:46:22 2023 +0000, Nikolay Sivov wrote:
> I don't know if that's an optimization effect, but code in your example
> produces zero output unconditionally:
> ps_4_0
> dcl_output o0.xyzw
> mov o0.xyzw, l(0,0,0,0)
> ret 
>
> I don't know how this works exactly, but maybe constant arguments
> produce 0 derivatives, and using expression that depends on certain
> semantics generates actual instructions.

I think you're right; I changed the test to use VPOS and got a better result:

```
uniform float f;

float4 main(float4 pos : SV_POSITION) : sv_target
{
    float x = ddx(pos.x);
    float y = ddy(pos.y);
    return float4(x, y, 1, 1);
}
```

```
ps_4_0
dcl_input_ps_siv linear noperspective v0.xy, position
dcl_output o0.xyzw
deriv_rtx o0.x, v0.x
deriv_rty o0.y, v0.y
mov o0.zw, l(0,0,1.000000,1.000000)
ret
```