`flatten_nested_ramps()` performance bug

[rootjalex]

On an example generator taken directly from test/performance/nested_vectorization_gemm.cpp, flatten_nested_ramps() takes approximately 5min on my machine:

#include "Halide.h"

namespace {

class HalideDP : public Halide::Generator<HalideDP> {
public:
    GeneratorParam<int> size{"size", 1024};
    Input<Buffer<uint8_t>> f{"f", 2};
    Input<Buffer<uint8_t>> g{"g", 2};
    Output<Buffer<int32_t>> prod{"prod", 2};

    void generate() {
        Halide::Var x("x"), y("y");

        // matrix multiplication
        Halide::RDom r(0, 128);
        prod(x, y) += cast<int32_t>(f(x, r)) * g(r, y);

        Halide::Var xi("xi"), yi("yi");
        Halide::RVar ro("ro"), ri("ri");

        // x86 schedule. Exploits the ability of pmaddwd
        // to pull one arg from memory. Because we'll be
        // intentionally spilling, the tile will be
        // absurdly large for a gemm.
        const int vec = natural_vector_size<uint8_t>();

        prod.bound(x, 0, size)
            .bound(y, 0, size);

        prod.in()
            .tile(x, y, xi, yi, vec, vec / 2)
            .vectorize(xi)
            .unroll(yi);

        f.in().compute_at(prod, ro).vectorize(Halide::_0).unroll(Halide::_1);
        g.in().compute_at(prod, y).vectorize(Halide::_0).unroll(Halide::_1);

        prod.compute_at(prod.in(), x)
            .vectorize(x)
            .unroll(y)
            .update()
            .split(r, ro, ri, vec / 2)
            .reorder(ri, x, y, ro)
            .vectorize(x)
            .unroll(y)
            .atomic()
            .vectorize(ri, 2)
            .unroll(ri);
    }
};

}  // namespace

HALIDE_REGISTER_GENERATOR(HalideDP, halide_dp)

Build:

c++ -O3 -std=c++17 -I /Users/alexanderroot/Projects/Rake/try-vegen/Halide/distrib/include/ -I /Users/alexanderroot/Projects/Rake/try-vegen/Halide/distrib/tools/  -Wall -Werror -Wno-unused-function -Wcast-qual -Wignored-qualifiers -Wno-comment -Wsign-compare -Wno-unknown-warning-option -Wno-psabi -stdlib=libc++ -fvisibility=hidden halide_dp_generator.cpp /Users/alexanderroot/Projects/Rake/try-vegen/Halide/distrib/tools/GenGen.cpp -o bin/host/halide_dp.generator -Wl,-rpath,/Users/alexanderroot/Projects/Rake/try-vegen/Halide/distrib/lib/ -L /Users/alexanderroot/Projects/Rake/try-vegen/Halide/distrib/lib/ -lHalide -L/usr/local/opt/llvm/lib -ldl -lpthread -lz -Wl,-force_load /Users/alexanderroot/Projects/Rake/try-vegen/Halide/distrib/lib/libautoschedule_mullapudi2016.dylib -lm -lz -lcurses -lxml2
bin/host/halide_dp.generator -g halide_dp -e static_library,h,registration,stmt,assembly,c_source,c_header,llvm_assembly -t 0 -o bin/host target=x86-64-osx-avx512-sse41

Without -t 0 to the generator, it times out sometime in generating llvm bitcode for the function. The stmt code is quite large, I suspect something is just inefficient in flatten_nested_ramps

[abadams]

@vksnk might be the best person to look at this

[vksnk]

I will take a look.