parallel assignment performance in Ruby

Question

Setting up a temporary variable to swap two elements in array seems to be more efficient than using parallel assignment. Can someone help explaining?

require "benchmark"

Benchmark.bm do |b|
  b.report do
    40000000.times { array[1], array[2] = array[2], array[1] }
  end
end

Benchmark.bm do |b|
  b.report do
    40000000.times do
      t        = array[1]
      array[1] = array[2]
      array[2] = t
    end
  end
end

Results in:

   user     system      total        real
   4.470000   0.020000   4.490000 (  4.510368)
   user     system      total        real
   3.220000   0.010000   3.230000 (  3.255109)

Answer 1

The parallel assignment creates a temporary array, which it afterwards splats.

GC.disable

def with_temp
  a = 1
  b = 2

  t = a
  a = b
  b = t
end

def with_parallel
  a = 1
  b = 2

  a, b = b, a
end

before_all = ObjectSpace.each_object(Array).count
with_temp
after_with_temp = ObjectSpace.each_object(Array).count
with_parallel
after_with_parallel = ObjectSpace.each_object(Array).count

GC.enable

puts after_with_temp - before_all          # => 1
puts after_with_parallel - after_with_temp # => 2

The one extra Array comes from ObjectSpace.each_object(Array).count itself.

---

Another way to verify - look at the instructions:

puts RubyVM::InstructionSequence.compile("a = 1; b = 2; t = a; a = b; b = t").disasm
puts RubyVM::InstructionSequence.compile("a = 1; b = 2; a, b = b, a").disasm

== disasm: @>==========
local table (size: 4, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 4] a [ 3] b [ 2] t
0000 trace 1 ( 1)
0002 putobject_OP_INT2FIX_O_1_C_
0003 setlocal_OP__WC__0 4
0005 putobject 2
0007 setlocal_OP__WC__0 3
0009 getlocal_OP__WC__0 4
0011 setlocal_OP__WC__0 2
0013 getlocal_OP__WC__0 3
0015 setlocal_OP__WC__0 4
0017 getlocal_OP__WC__0 2
0019 dup
0020 setlocal_OP__WC__0 3
0022 leave

---

== disasm: @>==========
local table (size: 3, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 3] a [ 2] b
0000 trace 1 ( 1)
0002 putobject_OP_INT2FIX_O_1_C_
0003 setlocal_OP__WC__0 3
0005 putobject 2
0007 setlocal_OP__WC__0 2
0009 getlocal_OP__WC__0 2
0011 getlocal_OP__WC__0 3
0013 newarray 2
0015 dup
0016 expandarray 2, 0
0019 setlocal_OP__WC__0 3
0021 setlocal_OP__WC__0 2
0023 leave