[ruby-core:65786] [ruby-trunk - Feature #10396] [Open] Increase instance variable retrieval performance by removing -w/$VERBOSE warning

Issue #10396 has been reported by Jeremy Evans.

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Feature #10396: Increase instance variable retrieval performance by removing -w/$VERBOSE warning
https://bugs.ruby-lang.org/issues/10396

* Author: Jeremy Evans
* Status: Open
* Priority: Normal
* Assignee: 
* Category: core
* Target version: current: 2.2.0
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This patch removes the "instance variable @foo not initialized" -w/$VERBOSE warning, increasing instance variable retrieval performance.  ruby doesn't warn about uninitialized instance variable retrieval by default, only in -w/$VERBOSE mode.  This warning causes a performance hit for all code, even if the warning is not printed, and it isn't by default.

Requiring the setting of instance variables to nil to avoid warnings in -w/$VERBOSE mode hurts ruby performance.  Consider these two approaches:

    # initialized
    class A
      def initialize
        @c = @d = @e = @f = nil
      end
      def b
        @c || @d || @e || @f
      end
    end

    # not initialized
    class A
      def initialize
        # nothing
      end
      def b
        @c || @d || @e || @f
      end
    end

Using the following benchmark:

Benchmark.measure{10000000.times{A.new.b}}

    # before - initialized
    7.450000   0.000000   7.450000 (  7.453981)
    7.240000   0.000000   7.240000 (  7.231404)
    7.240000   0.000000   7.240000 (  7.242685)

    # before - not initialized
    3.350000   0.000000   3.350000 (  3.350715)
    3.450000   0.000000   3.450000 (  3.448517)
    3.430000   0.000000   3.430000 (  3.431949)

    # after - initialized
    6.650000   0.000000   6.650000 (  6.657332)
    6.690000   0.000000   6.690000 (  6.686116)
    6.660000   0.000000   6.660000 (  6.660838)

    # after - not initialized
    2.820000   0.000000   2.820000 (  2.817551)
    2.770000   0.000000   2.770000 (  2.771170)
    2.670000   0.000000   2.670000 (  2.669535)

With this patch, you get a 8% increase for the initialized version and a 25% increase for the not initialized version. However, if you want to be warning free in -w/$VERBOSE mode, this patch results in a 170% increase in performance, since you can switch from the initialized version to the not initialized version.

Note that if you use attr_reader, the method it defines do not warn for uninitialized instance variables.  However, calling a method is slower than direct instance variable access:

    # not initialized - attr_reader
    class A
      def initialize
      end
      attr_reader :c, :d, :e, :f
      def b
        c || d || e || f
      end
    end

    # before - not initialized - attr_reader
    3.600000   0.000000   3.600000 (  3.597794)
    3.640000   0.000000   3.640000 (  3.645631)
    3.570000   0.000000   3.570000 (  3.570119)

    # after - not initialized - attr_reader
    3.240000   0.000000   3.240000 (  3.246522)
    3.220000   0.000000   3.220000 (  3.217739)
    3.220000   0.000000   3.220000 (  3.219133)

I think the performance benefits of the patch outweigh the costs (the loss of the warning for uninitialized instance variables in -w/$VERBOSE mode).

$ ruby -v
ruby 2.2.0dev (2014-10-18) [x86_64-openbsd5.6]

---Files--------------------------------
ruby.diff (1.06 KB)


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