workaround for 'Can't access output buffer of undefined Func'

[cbartnl]

I'm getting the following unexpected behavior when trying to use the output buffer width()/height() in a generator before assigning the output buffer to a func. It gives an error during generation: Can't access output buffer of undefined Func.

class Dummy : public Halide::Generator<Dummy> {
public:
    Output<Buffer<int32_t>> output{"output", 3};

    void generate() {
        Var x,y,c;
        
        //so this works fine:
        //--------------------------------------
        output(x,y,c) = x;
        output(x,y,c) += output.width();
        //--------------------------------------
        
        // but below code crashes when running the generator (building generator works fine)
        // with error: Unhandled exception: Error: Can't access output buffer of undefined Func. :
        //--------------------------------------
        // output(x,y,c) = x + output.width();
        //--------------------------------------
        
        // unfortunately this also means that scenario like the following does not work:
        //--------------------------------------
        //Func f_that_uses_output_width_in_ROI; // fill a quarter of the image for example, or 
        //                                         take a histogram over a certain relative ROI
        //... 
        //output(x,y,c) = ... // does not matter what is assigned here, but we somehow use f
        //                    // so we have to define f before assigning something to output
        //                    // we cannot assign output prior to f
        //--------------------------------------
        
        //schedule
        output.compute_root();

    }
    
};

It don't fully understand why, as I expect the output buffer size cannot be modified by an assigned Func (?). I can work around this by passing the sizes as extra parameters in the generator. But that does not seem a very clean approach. Maybe I'm overlooking something here...

[abadams]

This bothers me too. You're not missing anything. The workaround is to pass the output size as an extra parameter, as you say.

[cbartnl]

Thanks! Good to know.

[steven-johnson]

Ouch, this is an unfortunate oversight -- the problem is that (usually) trying to access things related to an Output buffer before you define the Func can produce problematic behavior, and technically, you are accessing .width() during the definition of the Func, so you're hitting the trigger here. We should be able to do better, it just will require extra care to get it right. I'll assign this to myself and (hopefully) get to it before too long.