[ruby-core:84584] [Ruby trunk Feature#14045] Lazy Proc allocation for block parameters

Issue #14045 has been updated by myronmarston (Myron Marston).


> Don't report a bug on a closed feature, report it as a new bug (and link to the feature), thanks.

Thanks, I wasn't aware which way was preferred.  I've opened #14267 to report this as a new bug.

> Comparing blocks/procs is always a somewhat dangerous thing.

Agreed, but when dealing with RSpec hooks (which are fundamentally block + some metadata) this was the simplest way to make the features work.  I'd have to put some more through into if we could refactor to avoid the need.

> Lazy allocation shouldn't mean multiple allocations. I haven't looked at the implementation, but I hope this can be fixed.

I sure hope so!

> For a core library such as RSpec, please test the betas and release candidates (yes we know they were late this time). Things will be easier for you if you find problems earlier.

I agree that would be a great idea, but to be completely honest, I'm very unlikely to make the time to do that.  My open source time is very limited these days.  If I have move time to spend on open source around future Ruby releases I will try to contribute in this way.

----------------------------------------
Feature #14045: Lazy Proc allocation for block parameters
https://bugs.ruby-lang.org/issues/14045#change-69123

* 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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