00   Introduction
01   Getting started with Funcs, Vars, and Exprs
02   Processing images
03   Inspecting the generated code
04   Debugging with tracing, print, and print_when
05   Vectorize, parallelize, unroll and tile your code
06   Realizing Funcs over arbitrary domains
07   Multi-stage pipelines
08   Scheduling multi-stage pipelines
09   Multi-pass Funcs, update definitions, and reductions
10   AOT compilation part 1
10   AOT compilation part 2
11   Cross-compilation
12   Using the GPU
13   Tuples
14   The Halide type system
15   Generators part 1
15   Generators part 2
16   RGB images and memory layouts part 1
16   RGB images and memory layouts part 2
17   Reductions over non-rectangular domains
18   Factoring an associative reduction using rfactor
19   Wrapper Funcs
20   Cloning Funcs
21   Auto-Scheduler
21   Auto-Scheduler
// Halide tutorial lesson 21: Auto-Scheduler

// So far we have written Halide schedules by hand, but it is also possible to
// ask Halide to suggest a reasonable schedule. We call this auto-scheduling.
// This lesson demonstrates how to use the auto-scheduler to generate a
// copy-pasteable CPU schedule that can be subsequently improved upon.

// On linux or os x, you can compile and run it like so:

// g++ lesson_21_auto_scheduler_generate.cpp <path/to/tools/halide_image_io.h>/GenGen.cpp -g -std=c++11 -fno-rtti -I <path/to/Halide.h> -L <path/to/libHalide.so> -lHalide -lpthread -ldl -o lesson_21_generate
// export LD_LIBRARY_PATH=<path/to/libHalide.so>   # For linux
// export DYLD_LIBRARY_PATH=<path/to/libHalide.dylib> # For OS X
// ./lesson_21_generate -o . -g auto_schedule_gen -f auto_schedule_false -e static_library,h,schedule target=host auto_schedule=false
// ./lesson_21_generate -o . -g auto_schedule_gen -f auto_schedule_true -e static_library,h,schedule -p <path/to/libautoschedule_mullapudi2016.so> -S Mullapudi2016 target=host auto_schedule=true machine_params=32,16777216,40
// g++ lesson_21_auto_scheduler_run.cpp -std=c++11 -I <path/to/Halide.h> -I <path/to/tools/halide_image_io.h> auto_schedule_false.a auto_schedule_true.a -ldl -lpthread -o lesson_21_run
// ./lesson_21_run

// If you have the entire Halide source tree, you can also build it by
// running:
//    make tutorial_lesson_21_auto_scheduler_run
// in a shell with the current directory at the top of the halide
// source tree.

#include "Halide.h"
#include <stdio.h>

using namespace Halide;

// We will define a generator to auto-schedule.
class AutoScheduled : public Halide::Generator<AutoScheduled> {
public:
    Input<Buffer<float>> input{"input", 3};
    Input<float> factor{"factor"};

    Output<Buffer<float>> output1{"output1", 2};
    Output<Buffer<float>> output2{"output2", 2};

    Expr sum3x3(Func f, Var x, Var y) {
        return f(x - 1, y - 1) + f(x - 1, y) + f(x - 1, y + 1) +
               f(x, y - 1) + f(x, y) + f(x, y + 1) +
               f(x + 1, y - 1) + f(x + 1, y) + f(x + 1, y + 1);
    }

    void generate() {
        // For our algorithm, we'll use Harris corner detection.
        Func in_b = BoundaryConditions::repeat_edge(input);

        gray(x, y) = 0.299f * in_b(x, y, 0) + 0.587f * in_b(x, y, 1) + 0.114f * in_b(x, y, 2);

        Iy(x, y) = gray(x - 1, y - 1) * (-1.0f / 12) + gray(x - 1, y + 1) * (1.0f / 12) +
                   gray(x, y - 1) * (-2.0f / 12) + gray(x, y + 1) * (2.0f / 12) +
                   gray(x + 1, y - 1) * (-1.0f / 12) + gray(x + 1, y + 1) * (1.0f / 12);

        Ix(x, y) = gray(x - 1, y - 1) * (-1.0f / 12) + gray(x + 1, y - 1) * (1.0f / 12) +
                   gray(x - 1, y) * (-2.0f / 12) + gray(x + 1, y) * (2.0f / 12) +
                   gray(x - 1, y + 1) * (-1.0f / 12) + gray(x + 1, y + 1) * (1.0f / 12);

        Ixx(x, y) = Ix(x, y) * Ix(x, y);
        Iyy(x, y) = Iy(x, y) * Iy(x, y);
        Ixy(x, y) = Ix(x, y) * Iy(x, y);
        Sxx(x, y) = sum3x3(Ixx, x, y);
        Syy(x, y) = sum3x3(Iyy, x, y);
        Sxy(x, y) = sum3x3(Ixy, x, y);
        det(x, y) = Sxx(x, y) * Syy(x, y) - Sxy(x, y) * Sxy(x, y);
        trace(x, y) = Sxx(x, y) + Syy(x, y);
        harris(x, y) = det(x, y) - 0.04f * trace(x, y) * trace(x, y);
        output1(x, y) = harris(x + 2, y + 2);
        output2(x, y) = factor * harris(x + 2, y + 2);
    }

    void schedule() {
        if (auto_schedule) {
            // The auto-scheduler requires estimates on all the input/output
            // sizes and parameter values in order to compare different
            // alternatives and decide on a good schedule.

            // To provide estimates (min and extent values) for each dimension
            // of the input images ('input', 'filter', and 'bias'), we use the
            // set_estimates() method. set_estimates() takes in a list of
            // (min, extent) of the corresponding dimension as arguments.
            input.set_estimates({{0, 1024}, {0, 1024}, {0, 3}});

            // To provide estimates on the parameter values, we use the
            // set_estimate() method.
            factor.set_estimate(2.0f);

            // To provide estimates (min and extent values) for each dimension
            // of pipeline outputs, we use the set_estimates() method. set_estimates()
            // takes in a list of (min, extent) for each dimension.
            output1.set_estimates({{0, 1024}, {0, 1024}});
            output2.set_estimates({{0, 1024}, {0, 1024}});

            // Technically, the estimate values can be anything, but the closer
            // they are to the actual use-case values, the better the generated
            // schedule will be.

            // To auto-schedule the the pipeline, we don't have to do anything else:
            // every Generator implicitly has a GeneratorParam named "auto_schedule";
            // if this is set to true, Halide will call auto_schedule() on all of
            // our pipeline's outputs automatically.

            // Every Generator also implicitly has a GeneratorParams named "machine_params",
            // which allows you to specify characteristics of the machine architecture
            // for the auto-scheduler; it's generally specified in your Makefile.
            // If none is specified, the default machine parameters for a generic CPU
            // architecture will be used by the auto-scheduler.

            // Let's see some arbitrary but plausible values for the machine parameters.
            //
            //      const int kParallelism = 32;
            //      const int kLastLevelCacheSize = 16 * 1024 * 1024;
            //      const int kBalance = 40;
            //      MachineParams machine_params(kParallelism, kLastLevelCacheSize, kBalance);
            //
            // The arguments to MachineParams are the maximum level of parallelism
            // available, the size of the last-level cache (in KB), and the ratio
            // between the cost of a miss at the last level cache and the cost
            // of arithmetic on the target architecture, in that order.

            // Note that when using the auto-scheduler, no schedule should have
            // been applied to the pipeline; otherwise, the auto-scheduler will
            // throw an error. The current auto-scheduler cannot handle a
            // partially-scheduled pipeline.

            // If HL_DEBUG_CODEGEN is set to 3 or greater, the schedule will be dumped
            // to stdout (along with much other information); a more useful way is
            // to add "schedule" to the -e flag to the Generator. (In CMake and Bazel,
            // this is done using the "extra_outputs" flag.)

            // The generated schedule that is dumped to file is an actual
            // Halide C++ source, which is readily copy-pasteable back into
            // this very same source file with few modifications. Programmers
            // can use this as a starting schedule and iteratively improve the
            // schedule. Note that the current auto-scheduler is only able to
            // generate CPU schedules and only does tiling, simple vectorization
            // and parallelization. It doesn't deal with line buffering, storage
            // reordering, or factoring reductions.

            // At the time of writing, the auto-scheduler will produce the
            // following schedule for the estimates and machine parameters
            // declared above when run on this pipeline:
            //
            // Var x_i("x_i");
            // Var x_i_vi("x_i_vi");
            // Var x_i_vo("x_i_vo");
            // Var x_o("x_o");
            // Var x_vi("x_vi");
            // Var x_vo("x_vo");
            // Var y_i("y_i");
            // Var y_o("y_o");
            //
            // Func Ix = pipeline.get_func(4);
            // Func Iy = pipeline.get_func(7);
            // Func gray = pipeline.get_func(3);
            // Func harris = pipeline.get_func(14);
            // Func output1 = pipeline.get_func(15);
            // Func output2 = pipeline.get_func(16);
            //
            // {
            //     Var x = Ix.args()[0];
            //     Ix
            //         .compute_at(harris, x_o)
            //         .split(x, x_vo, x_vi, 8)
            //         .vectorize(x_vi);
            // }
            // {
            //     Var x = Iy.args()[0];
            //     Iy
            //         .compute_at(harris, x_o)
            //         .split(x, x_vo, x_vi, 8)
            //         .vectorize(x_vi);
            // }
            // {
            //     Var x = gray.args()[0];
            //     gray
            //         .compute_at(harris, x_o)
            //         .split(x, x_vo, x_vi, 8)
            //         .vectorize(x_vi);
            // }
            // {
            //     Var x = harris.args()[0];
            //     Var y = harris.args()[1];
            //     harris
            //         .compute_root()
            //         .split(x, x_o, x_i, 256)
            //         .split(y, y_o, y_i, 128)
            //         .reorder(x_i, y_i, x_o, y_o)
            //         .split(x_i, x_i_vo, x_i_vi, 8)
            //         .vectorize(x_i_vi)
            //         .parallel(y_o)
            //         .parallel(x_o);
            // }
            // {
            //     Var x = output1.args()[0];
            //     Var y = output1.args()[1];
            //     output1
            //         .compute_root()
            //         .split(x, x_vo, x_vi, 8)
            //         .vectorize(x_vi)
            //         .parallel(y);
            // }
            // {
            //     Var x = output2.args()[0];
            //     Var y = output2.args()[1];
            //     output2
            //         .compute_root()
            //         .split(x, x_vo, x_vi, 8)
            //         .vectorize(x_vi)
            //         .parallel(y);
            // }

        } else {
            // This is where you would declare the schedule you have written by
            // hand or paste the schedule generated by the auto-scheduler.
            // We will use a naive schedule here to compare the performance of
            // the autoschedule with a basic schedule.
            gray.compute_root();
            Iy.compute_root();
            Ix.compute_root();
        }
    }

private:
    Var x{"x"}, y{"y"}, c{"c"};
    Func gray, Iy, Ix, Ixx, Iyy, Ixy, Sxx, Syy, Sxy, det, trace, harris;
};

// As in lesson 15, we register our generator and then compile this
// file along with tools/GenGen.cpp.
HALIDE_REGISTER_GENERATOR(AutoScheduled, auto_schedule_gen)

// After compiling this file, see how to use it in
// lesson_21_auto_scheduler_run.cpp