[libre-riscv-dev] evaluation of bit-witdh scoreboards

[Jacob Lifshay]

On Sun, Dec 16, 2018, 17:51 Luke Kenneth Casson Leighton <lkcl at lkcl.net
wrote:

> Jacob whats requirements of pixel conversion op, whats it look like as c
> pseudocode. Also texture op, roughly. Mainly, whats inputs and outputs.
>
> Need to assess.
>

assume there is a 12-bit unsigned type: uint12_t

float clamp(float v, float minv, float maxv)
{
    return fmax(fmin(v, maxv), minv);
}

uint12_t float_to_u12(float v)
{
    return (uint12_t)round(clamp(v, 0x0, 0xFFF));
}

const uint8_t encode_sRGB_lut[1 << 12] = {...};
const float decode_sRGB_lut[1 << 8] = {...};

float decode_sRGB_channel(uint8_t v)
{
    return decode_sRGB_lut[v];
}

uint8_t encode_sRGB_channel(float v)
{
    return encode_sRGB_lut[float_to_u12(v)];
}

uint8_t encode_u8_channel(float v)
{
    return (uint8_t)round(clamp(v, 0, 0xFF));
}

float decode_u8_channel(uint8_t v)
{
    return (float)v / 0xFF;
}

// common read from color buffer
f32x4 decode_sRGB(u8x4 v)
{
    return (f32x4){
        decode_sRGB_channel(v[0]),
        decode_sRGB_channel(v[1]),
        decode_sRGB_channel(v[2]),
        decode_u8_channel(v[3])
    };
}

// common read from color buffer
f32x4 decode_u8(u8x4 v)
{
    return (f32x4){
        decode_u8_channel(v[0]),
        decode_u8_channel(v[1]),
        decode_u8_channel(v[2]),
        decode_u8_channel(v[3])
    };
}

// common write to color buffer
u8x4 encode_sRGB(f32x4 v)
{
    return (u8x4){
        encode_sRGB_channel(v[0]),
        encode_sRGB_channel(v[1]),
        encode_sRGB_channel(v[2]),
        encode_u8_channel(v[3])
    };
}

// common write to color buffer
u8x4 encode_u8(f32x4 v)
{
    return (u8x4){
        encode_u8_channel(v[0]),
        encode_u8_channel(v[1]),
        encode_u8_channel(v[2]),
        encode_u8_channel(v[3])
    };
}

The simplest texture read operations are basically several decode_sRGB or
decode_u8 calls then interpolation

Since we're processing several pixels simultaneously, we need to vectorize
by an additional factor of 4*N

Jacob

>