@view の使い方

@view の使い方.ipynb

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  "cells": [
    {
      "metadata": {},
      "cell_type": "markdown",
      "source": "# @view の使い方\n\n黒木玄\n\n2018-01-23\n\n配列の部分配列をコピーせずに参照渡しにするには `@view` マクロを使う."
    },
    {
      "metadata": {
        "trusted": true
      },
      "cell_type": "code",
      "source": "N = 10^8\nn = 3\n\n# A.^2 を B にコピーする函数のつもり\n#\nfunction test!(B, A)\n    B .= A.^2\nend",
      "execution_count": 1,
      "outputs": [
        {
          "data": {
            "text/plain": "test! (generic function with 1 method)"
          },
          "execution_count": 1,
          "metadata": {},
          "output_type": "execute_result"
        }
      ]
    },
    {
      "metadata": {
        "trusted": true
      },
      "cell_type": "code",
      "source": "A = reshape(collect(1:2N), N, 2)\nA[1:n,:]",
      "execution_count": 2,
      "outputs": [
        {
          "data": {
            "text/plain": "3×2 Array{Int64,2}:\n 1  100000001\n 2  100000002\n 3  100000003"
          },
          "execution_count": 2,
          "metadata": {},
          "output_type": "execute_result"
        }
      ]
    },
    {
      "metadata": {
        "trusted": true
      },
      "cell_type": "code",
      "source": "# A[:,2] に A[:,1].^2 がコピーされない上に遅い.\n\nA = reshape(collect(1:2N), N, 2)\ntest!(A[:,2], A[:,1])\n\nA = reshape(collect(1:2N), N, 2)\n@time test!(A[:,2], A[:,1])\nA[1:n,:]",
      "execution_count": 3,
      "outputs": [
        {
          "name": "stdout",
          "output_type": "stream",
          "text": "  0.984741 seconds (89 allocations: 1.490 GiB, 23.17% gc time)\n"
        },
        {
          "data": {
            "text/plain": "3×2 Array{Int64,2}:\n 1  100000001\n 2  100000002\n 3  100000003"
          },
          "execution_count": 3,
          "metadata": {},
          "output_type": "execute_result"
        }
      ]
    },
    {
      "metadata": {
        "trusted": true
      },
      "cell_type": "code",
      "source": "# @view(A[:,2]) を函数に渡すと A[:,1].^2 がコピーされる.\n\nA = reshape(collect(1:2N), N, 2)\ntest!(@view(A[:,2]), A[:,1])\n\nA = reshape(collect(1:2N), N, 2)\n@time test!(@view(A[:,2]), A[:,1])\nA[1:n,:]",
      "execution_count": 4,
      "outputs": [
        {
          "name": "stdout",
          "output_type": "stream",
          "text": "  0.572699 seconds (33 allocations: 762.940 MiB, 20.10% gc time)\n"
        },
        {
          "data": {
            "text/plain": "3×2 Array{Int64,2}:\n 1  1\n 2  4\n 3  9"
          },
          "execution_count": 4,
          "metadata": {},
          "output_type": "execute_result"
        }
      ]
    },
    {
      "metadata": {
        "trusted": true
      },
      "cell_type": "code",
      "source": "@macroexpand test!(@view(A[:,2]), A[:,1])",
      "execution_count": 5,
      "outputs": [
        {
          "data": {
            "text/plain": ":(test!(true && (view)(A, :, 2), A[:, 1]))"
          },
          "execution_count": 5,
          "metadata": {},
          "output_type": "execute_result"
        }
      ]
    },
    {
      "metadata": {
        "trusted": true
      },
      "cell_type": "code",
      "source": "# さらに @view(A[:,1]) とすると速くなり, メモリ効率が大幅に改善される.\n\nA = reshape(collect(1:2N), N, 2)\ntest!(@view(A[:,2]), @view(A[:,1]))\n\nA = reshape(collect(1:2N), N, 2)\n@time test!(@view(A[:,2]), @view(A[:,1]))\nA[1:n,:]",
      "execution_count": 6,
      "outputs": [
        {
          "name": "stdout",
          "output_type": "stream",
          "text": "  0.146387 seconds (56 allocations: 1.188 KiB)\n"
        },
        {
          "data": {
            "text/plain": "3×2 Array{Int64,2}:\n 1  1\n 2  4\n 3  9"
          },
          "execution_count": 6,
          "metadata": {},
          "output_type": "execute_result"
        }
      ]
    },
    {
      "metadata": {
        "trusted": true
      },
      "cell_type": "code",
      "source": "@macroexpand test!(@view(A[:,2]), @view(A[:,1]))",
      "execution_count": 7,
      "outputs": [
        {
          "data": {
            "text/plain": ":(test!(true && (view)(A, :, 2), true && (view)(A, :, 1)))"
          },
          "execution_count": 7,
          "metadata": {},
          "output_type": "execute_result"
        }
      ]
    },
    {
      "metadata": {
        "trusted": true
      },
      "cell_type": "code",
      "source": "@which view(A, :, 2)",
      "execution_count": 8,
      "outputs": [
        {
          "data": {
            "text/html": "view(A::<b>AbstractArray</b>, I...) at <a href=\"https://github.com/JuliaLang/julia/tree/d386e40c17d43b79fc89d3e579fc04547241787c/base/subarray.jl#L111\" target=\"_blank\">subarray.jl:111</a>",
            "text/plain": "view(A::AbstractArray, I...) in Base at subarray.jl:111"
          },
          "execution_count": 8,
          "metadata": {},
          "output_type": "execute_result"
        }
      ]
    },
    {
      "metadata": {
        "trusted": true
      },
      "cell_type": "code",
      "source": "# マクロではない view を使っても同じことをできる.\n\nA = reshape(collect(1:2N), N, 2)\ntest!(view(A,:,2), view(A,:,1))\n\nA = reshape(collect(1:2N), N, 2)\n@time test!(view(A,:,2), view(A,:,1))\nA[1:n,:]",
      "execution_count": 9,
      "outputs": [
        {
          "name": "stdout",
          "output_type": "stream",
          "text": "  0.146301 seconds (56 allocations: 1.188 KiB)\n"
        },
        {
          "data": {
            "text/plain": "3×2 Array{Int64,2}:\n 1  1\n 2  4\n 3  9"
          },
          "execution_count": 9,
          "metadata": {},
          "output_type": "execute_result"
        }
      ]
    },
    {
      "metadata": {
        "trusted": true
      },
      "cell_type": "code",
      "source": "# @views を使えばさらにシンプルに書ける.\n\nA = reshape(collect(1:2N), N, 2)\n@views test!(A[:,2], A[:,1])\n\nA = reshape(collect(1:2N), N, 2)\n@time @views test!(A[:,2], A[:,1])\nA[1:n,:]",
      "execution_count": 10,
      "outputs": [
        {
          "name": "stdout",
          "output_type": "stream",
          "text": "  0.142722 seconds (60 allocations: 1.250 KiB)\n"
        },
        {
          "data": {
            "text/plain": "3×2 Array{Int64,2}:\n 1  1\n 2  4\n 3  9"
          },
          "execution_count": 10,
          "metadata": {},
          "output_type": "execute_result"
        }
      ]
    },
    {
      "metadata": {
        "trusted": true
      },
      "cell_type": "code",
      "source": "@macroexpand @views test!(A[:,2], A[:,1])",
      "execution_count": 11,
      "outputs": [
        {
          "data": {
            "text/plain": ":(test!((Base.maybeview)(A, :, 2), (Base.maybeview)(A, :, 1)))"
          },
          "execution_count": 11,
          "metadata": {},
          "output_type": "execute_result"
        }
      ]
    },
    {
      "metadata": {
        "trusted": true
      },
      "cell_type": "code",
      "source": "@which Base.maybeview(A, :, 2)",
      "execution_count": 12,
      "outputs": [
        {
          "data": {
            "text/html": "maybeview(A::<b>AbstractArray</b>, args...) at <a href=\"https://github.com/JuliaLang/julia/tree/d386e40c17d43b79fc89d3e579fc04547241787c/base/subarray.jl#L469\" target=\"_blank\">subarray.jl:469</a>",
            "text/plain": "maybeview(A::AbstractArray, args...) in Base at subarray.jl:469"
          },
          "execution_count": 12,
          "metadata": {},
          "output_type": "execute_result"
        }
      ]
    },
    {
      "metadata": {
        "trusted": true
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      "source": "",
      "execution_count": null,
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