[ruby-dev:49985] [Ruby trunk Bug#13228] s[i]=c(assigning a character) for String is slower than Array on Linux

Issue #13228 has been updated by _ wanabe.

ruby -v set to ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-linux]

`perf` shows that ruby spent most of the time in `search_nonascii()`.

```
$ perf record ruby -ve 'n=100000; s = "a" * n; t = Time.now; n.times do |i| s[i] = "z"; end; p Time.now - t'
ruby 2.5.0dev (2017-02-18 trunk 57652) [x86_64-linux]
5.271689721
[ perf record: Woken up 4 times to write data ]
[ perf record: Captured and wrote 0.858 MB perf.data (21558 samples) ]

$ perf report -n --stdio|head -20
# To display the perf.data header info, please use --header/--header-only options.
#
#
# Total Lost Samples: 0
#
# Samples: 21K of event 'cycles'
# Event count (approx.): 15739606081
#
# Overhead       Samples  Command   Shared Object      Symbol                                    
# ........  ............  ........  .................  ..........................................
#
    96.14%         20654  ruby      ruby               [.] search_nonascii
     0.18%            45  ruby      ruby               [.] ruby_yyparse
     0.18%            38  ruby      [wl]               [k] osl_readl
     0.17%            38  ruby      ruby               [.] vm_exec_core
     0.13%            27  ruby      [kernel.kallsyms]  [k] delay_tsc
     0.07%            16  ruby      ruby               [.] rb_str_splice_0
     0.07%            15  ruby      ruby               [.] gc_page_sweep
     0.07%            15  ruby      ruby               [.] rb_enc_from_index
     0.06%            13  ruby      ruby               [.] rb_str_update
```

I wonder the script uses only ASCII characters, and we have `RUBY_ENC_CODERANGE_7BIT`.
But `rb_str_splice_0()` calls `rb_str_modify()` and clear code-range information by `ENC_CODERANGE_CLEAR()`.


----------------------------------------
Bug #13228: s[i]=c(assigning a character) for String is slower than Array on Linux
https://bugs.ruby-lang.org/issues/13228#change-63029

* Author: Tsuneo Yoshioka
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-linux]
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
s[i]=c(assigning a character) for String is slower than for Array on Linux.

If I split the String to Array, and assign characters, and join the Array to String,
then it is much faster than assigning characters directly to the string.

Somehow, I don't see the performance difference on Mac OS X.

~$ time ruby -e 'N=100000; s="a"*N; N.times{s[Random.rand(N)]="Z"}; puts s' >/dev/null

real    0m0.879s
user    0m0.836s
sys     0m0.012s
~$ time ruby -e 'N=100000;s="a"*N;s=s.split(""); N.times{s[Random.rand(N)]="Z"}; puts s.join("")' >/dev/null

real    0m0.153s
user    0m0.108s
sys     0m0.016s

~$ uname -a
Linux aaaaaaaa 4.4.0-43-generic #63-Ubuntu SMP Wed Oct 12 13:48:03 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
~$ ruby --version
ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-linux]
~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 16.04.1 LTS
Release:        16.04
Codename:       xenial




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