[ruby-core:97173] [Ruby master Feature#16635] Reduce allocation of begin-less/end-less range with Integer literal

Issue #16635 has been updated by mame (Yusuke Endoh).

Backport deleted (2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN)
ruby -v deleted (ruby 2.8.0dev (2020-02-15T12:52:03Z master 961630126b) [x86_64-linux])
Tracker changed from Bug to Feature

Looks good.  I will merge after it passes the CIs.  Thanks.

----------------------------------------
Feature #16635: Reduce allocation of begin-less/end-less range with Integer literal
https://bugs.ruby-lang.org/issues/16635#change-84279

* Author: pocke (Masataka Kuwabara)
* Status: Open
* Priority: Normal
----------------------------------------
The patch is here. https://github.com/ruby/ruby/pull/2910

Currently Ruby optimizes Integer only Range with reducing Range allocation.
`1..2` returns the same object.
For example:


```bash
$ ruby -e 'def a() 1..2 end; p a.object_id == a.object_id' 
true
```


But it does not optimize begin-less/end-less ranges.
For example:

```bash
$ ruby -e 'def a() ..2 end; p a.object_id == a.object_id' 
false

$ ruby -e 'def a() 1.. end; p a.object_id == a.object_id' 
false
```


This patch applies the same optimization to begin-less/end-less ranges.


I think it will be helpful when we replace `1..-1` with `..-1` for String or Array slicing. Like `str[1..-1]`.
We can rewrite `str[1..-1]` to `str[1..]` with keeping the speed with this patch. 


# Benchmark


It uses benchmark-driver gem.


```yaml
# bench.yaml
prelude: |
    def endless
      1..
    end

    def beginless
      ..1
    end

    def endless_substr(str)
      str[1..]
    end

benchmark:
    endless: endless
    beginless: beginless
    endless_substr: "endless_substr('foo')"
```



```bash
$ benchmark-driver test.yaml --rbenv 'patched;trunk'
Warming up --------------------------------------
             endless    45.948M i/s -     46.076M times in 1.002782s (21.76ns/i, 26clocks/i)
           beginless    49.986M i/s -     50.237M times in 1.005037s (20.01ns/i, 24clocks/i)
      endless_substr     8.067M i/s -      8.187M times in 1.014936s (123.96ns/i, 148clocks/i)
Calculating -------------------------------------
                        patched       trunk
             endless   115.679M     21.500M i/s -    137.843M times in 1.191597s 6.411398s
           beginless   112.599M     22.060M i/s -    149.957M times in 1.331778s 6.797768s
      endless_substr     8.888M      6.760M i/s -     24.201M times in 2.722995s 3.580038s

Comparison:
                          endless
             patched: 115679391.9 i/s
               trunk:  21499711.2 i/s - 5.38x  slower

                        beginless
             patched: 112598731.5 i/s
               trunk:  22059673.0 i/s - 5.10x  slower

                   endless_substr
             patched:   8887513.1 i/s
               trunk:   6759886.2 i/s - 1.31x  slower
```

`trunk` is the HEAD of Ruby, and `patched` is `trunk` with this patch.



```bash
$ RBENV_VERSION=trunk ruby -v
ruby 2.8.0dev (2020-02-15T12:52:03Z master 961630126b) [x86_64-linux]
$ RBENV_VERSION=patched ruby -v
ruby 2.8.0dev (2020-02-15T12:52:03Z origin/master 961630126b) [x86_64-linux]
```






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