Are HLSL loops broken?

Question

I've been messing around with HLSL and Direct3D12 and encountered something strange. Here's a minimal reproducible example:

float4 main() : SV_TARGET
{
    uint indices[3];
    indices[0] = 0;
    uint l = 0;

    [loop] while (true) {
        if (indices[l] == 1)
            return float4(1.0f, 0.0f, 0.0f, 1.0f);
        indices[l + 1] = 1;
        if (++l == 2)
            break;
    }
    return float4(0.0f, 1.0f, 0.0f, 1.0f);
}

The code is completely pointless, but at least in my eyes, there shouldn't be a problem with it, and it should return a red color. However, compiling and running this (with HLSL version 5_0, with and without optimizations) does strangely enough neither return red nor green; it doesn't seem to return anything at all, as even though I have all blending disabled, nothing is being drawn.

Changing seemingly meaningless things, like removing the [loop] tag or changing the size of indices to 2, gets rid of the strange behavior.

Does anyone know what's going on here? Are HLSL loops simply broken?

Are HLSL loops broken?

0 Answers