Question

When I use the following code:

#define MAX_RADIUS 55
#define KERNEL_SIZE (MAX_RADIUS * 2 + 1)
...
float[] kernel[KERNEL_RADIUS];
...
float4 PS_GaussianBlur(float2 texCoord : TEXCOORD) : COLOR0
{
    float4 color = float4(0.0f, 0.0f, 0.0f, 0.0f);

    //add the right side offset pixels to the color
    for (int i = 0; i < MAX_RADIUS; i++)
    {
        if(kernel[i] != 0) //this will improve performance for lower filter radius's, but increases const register num
            color += tex2D(colorMap, texCoord + offsets[i]) * kernel[i];
    }
    //add the left side offset pixels to the color
    for (int j = 0; j < MAX_RADIUS; j++)
    {
        if(kernel[i] != 0)
            color += tex2D(colorMap, texCoord - offsets[j]) * kernel[j];
    }
    //finally add the weight of the original pixel to the color
    color += tex2D(colorMap, texCoord) * kernel[MAX_RADIUS];

    return color;
}

The if(kernel[i] != 0) increases the number of instructions used dramatically!

So my question is this: What increases instruction count? And why would using an if statement increase instruction count by over 400 in a loop that is only 110 instructions long?

EDIT: Above question edited. I mistakenly thought registers were being taken when it was really instructions. However, the question still applies. What would cause 2 for loops (of length 55 each) to increase the instruction count by over 400 with just 1 added if statement within the loop?

Answer 1

fxc will give you an instruction count. But really, you should do this another way. Try a bidirectional filter, with one pass on U and the other on V?