[ruby-core:92452] [Ruby trunk Misc#15806] Explicitly initialise encodings on init to remove branches on encoding lookup

[lourens@bearmetal.eu]

Issue #15806 has been reported by methodmissing (Lourens Naudé).

----------------------------------------
Misc #15806: Explicitly initialise encodings on init to remove branches on encoding lookup
https://bugs.ruby-lang.org/issues/15806

* Author: methodmissing (Lourens Naudé)
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
References Github PR https://github.com/ruby/ruby/pull/2128

I noticed that the encoding table is loaded on startup of even just `miniruby` (minimal viable interpreter use case) through this backtrace during ruby setup:

```
/home/lourens/src/ruby/ruby/miniruby(rb_enc_init+0x12) [0x56197b0c0c72] encoding.c:587
/home/lourens/src/ruby/ruby/miniruby(rb_usascii_encoding+0x1a) [0x56197b0c948a] encoding.c:1357
/home/lourens/src/ruby/ruby/miniruby(Init_sym+0x7a) [0x56197b24810a] symbol.c:42
/home/lourens/src/ruby/ruby/miniruby(rb_call_inits+0x1d) [0x56197b11afed] inits.c:25
/home/lourens/src/ruby/ruby/miniruby(ruby_setup+0xf6) [0x56197b0ec9d6] eval.c:74
/home/lourens/src/ruby/ruby/miniruby(ruby_init+0x9) [0x56197b0eca39] eval.c:91
/home/lourens/src/ruby/ruby/miniruby(main+0x5a) [0x56197b051a2a] ./main.c:41
```

Therefore I think it makes sense to instead initialize encodings explicitly just prior to symbol init, which is the first entry point into the interpreter loading that currently triggers `rb_enc_init` and remove the initialization check branches from the various lookup methods.

Some of the branches collapsed, `cachegrind` output, columns are `Ir Bc Bcm Bi Bim` with `Ir` (instructions retired),  `Bc` (branches taken) and `Bcm` (branches missed) relevant here as there are no indirect branches (function pointers etc.):

(hot function, many instructions retired and branches taken and missed)
```
         .          .       .          .       .  rb_encoding *
         .          .       .          .       .  rb_enc_from_index(int index)
   835,669          0       0          0       0  {
13,133,536  6,337,652  50,267          0       0      if (!enc_table.list) {
         3          0       0          0       0  	rb_enc_init();
         .          .       .          .       .      }
23,499,349  8,006,202 293,161          0       0      if (index < 0 || enc_table.count <= (index &= ENC_INDEX_MASK)) {
         .          .       .          .       .  	return 0;
         .          .       .          .       .      }
30,024,494          0       0          0       0      return enc_table.list[index].enc;
 1,671,338          0       0          0       0  }
```

(cold function, representative of the utf8 variant more or less too)

```
         .          .       .          .       .  rb_encoding *
         .          .       .          .       .  rb_ascii8bit_encoding(void)
         .          .       .          .       .  {
    27,702      9,235     955          0       0      if (!enc_table.list) {
         .          .       .          .       .  	rb_enc_init();
         .          .       .          .       .      }
     9,238          0       0          0       0      return enc_table.list[ENCINDEX_ASCII].enc;
     9,232          0       0          0       0  }
```

I think lazy loading encodings and populating the table is fine, but initializing it can be done more explicitly in the boot process.



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