[ruby-core:83561] [Ruby trunk Feature#14045] Lazy Proc allocation for block parameters

Issue #14045 has been updated by matthewd (Matthew Draper).


This is excellent news indeed!

Do you think a similar technique could work for passing along `*args` in the future?

It would be great if simple delegation could get a similar gain by going zero-allocation:

~~~
def goal a, b
  a - b
end

def splat *args
  goal(*args)
end

def separate a, b
  goal(a, b)
end

N = 10_000_000

require "benchmark"
Benchmark.bmbm(10) {|x|
  x.report("splat") {
    N.times {
      splat(5, 2)
    }
  }
  x.report("separate") {
    N.times {
      separate(5, 2)
    }
  }
}
~~~

----------------------------------------
Feature #14045: Lazy Proc allocation for block parameters
https://bugs.ruby-lang.org/issues/14045#change-67590

* Author: ko1 (Koichi Sasada)
* Status: Closed
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* Target version: 
----------------------------------------
# Background

If we need to pass given block, we need to capture by block parameter as a Proc object and pass it parameter as a block argument. Like that:

```
def block_yield
  yield
end

def block_pass &b
  # do something
  block_yield(&b)
end
```

There are no way to pass given blocks to other methods without using block parameters.

One problem of this technique is performance. `Proc` creation is one of heavyweight operation because we need to store all of local variables (represented by Env objects in MRI internal) to heap. If block parameter is declared as one of method parameter, we need to make a new `Proc` object for the block parameter.

# Proposal: Lazy Proc allocation for 

To avoid this overhead, I propose lazy Proc creation for block parameters.

Ideas:

* At the beginning of method, a block parameter is `nil`
* If block parameter is accessed, then create a `Proc` object by given block.
* If we pass the block parameter to other methods like `block_yield(&b)` then don't make a `Proc`, but pass given block information.

We don't optimize `b.call` type block invocations. If we call block with `b.call`, then create `Proc` object.We need to hack more because `Proc#call` is different from `yield` statement (especially they can change `$SAFE`).

# Evaluation

```
def iter_yield
  yield
end

def iter_pass &b
  iter_yield(&b)
end

def iter_yield_bp &b
  yield
end

def iter_call &b
  b.call
end

N = 10_000_000 # 10M

require 'benchmark'
Benchmark.bmbm(10){|x|
  x.report("yield"){
    N.times{
      iter_yield{}
    }
  }
  x.report("yield_bp"){
    N.times{
      iter_yield_bp{}
    }
  }
  x.report("yield_pass"){
    N.times{
      iter_pass{}
    }
  }
  x.report("send_pass"){
    N.times{
      send(:iter_pass){}
    }
  }
  x.report("call"){
    N.times{
      iter_call{}
    }
  }
}

__END__

ruby 2.5.0dev (2017-10-24 trunk 60392) [x86_64-linux]
                 user     system      total        real
yield        0.634891   0.000000   0.634891 (  0.634518)
yield_bp     2.770929   0.000008   2.770937 (  2.769743)
yield_pass   3.047114   0.000000   3.047114 (  3.046895)
send_pass    3.322597   0.000002   3.322599 (  3.323657)
call         3.144668   0.000000   3.144668 (  3.143812)

modified
                 user     system      total        real
yield        0.582620   0.000000   0.582620 (  0.582526)
yield_bp     0.731068   0.000000   0.731068 (  0.730315)
yield_pass   0.926866   0.000000   0.926866 (  0.926902)
send_pass    1.110110   0.000000   1.110110 (  1.109579)
call         2.891364   0.000000   2.891364 (  2.890716)
```

# Related work

To delegate the given block to other methods, Single `&` block parameter had been proposed (https://bugs.ruby-lang.org/issues/3447#note-18) (using like: `def foo(&); bar(&); end`). This idea is straightforward to represent `block passing`. Also we don't need to name a block parameter.

The advantage of this ticket proposal is we don't change any syntax. We can write compatible code for past versions.

Thanks,
Koichi



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