[ruby-core:117783] [Ruby master Bug#20473] Ractor array/hash literals

6 May 2024

      Issue #20473 has been reported by kddnewton (Kevin Newton).

----------------------------------------
Bug #20473: Ractor array/hash literals
https://bugs.ruby-lang.org/issues/20473

* Author: kddnewton (Kevin Newton)
* Status: Open
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
There are many optimizations in place in the compiler to ensure Ruby doesn't overflow the stack when an array or hash literal has many values. For example:

```ruby
puts RubyVM::InstructionSequence.compile(<<~RUBY).disasm
[#{Array.new(512) { |index| index % 128 == 0 ? "\"\#{nil}\"" : "1" }.join(", ")}]
RUBY
```

gives the instructions:

```
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,1564)>
0000 putobject                              ""                        (   1)[Li]
0002 putnil
0003 dup
0004 objtostring                            <calldata!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>
0006 anytostring
0007 concatstrings                          2
0009 newarray                               1
0011 putobject                              [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
0013 concatarray
0014 putobject                              ""
0016 putnil
0017 dup
0018 objtostring                            <calldata!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>
0020 anytostring
0021 concatstrings                          2
0023 newarray                               1
0025 concatarray
0026 putobject                              [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
0028 concatarray
0029 putobject                              ""
0031 putnil
0032 dup
0033 objtostring                            <calldata!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>
0035 anytostring
0036 concatstrings                          2
0038 newarray                               1
0040 concatarray
0041 putobject                              [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
0043 concatarray
0044 putobject                              ""
0046 putnil
0047 dup
0048 objtostring                            <calldata!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>
0050 anytostring
0051 concatstrings                          2
0053 newarray                               1
0055 concatarray
0056 putobject                              [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
0058 concatarray
0059 leave
```

because it is smart enough to group the static elements together and use concatarray. This ensures we never have too many values on the stack.

In Ruby 3.3, this worked for ractors as well. (Adding `# shareable_constant_value: literal` and `A =` to the example above doesn't change anything about the instructions above except adding a couple of `-@` calls and one call to `make_shareable`.

On HEAD, all of these optimizations are now gone. With this:

```ruby
puts RubyVM::InstructionSequence.compile(<<~RUBY).disasm
# shareable_constant_value: literal
A = [#{Array.new(512) { |index| index % 128 == 0 ? "\"\#{nil}\"" : "1" }.join(", ")}]
RUBY
```

code on master, we get:

```
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(2,1568)>
0000 putobject                              RubyVM::FrozenCore        (   2)[Li]
0002 putobject                              ""
0004 putnil
0005 dup
0006 objtostring                            <calldata!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>
0008 anytostring
0009 concatstrings                          2
0011 opt_send_without_block                 <calldata!mid:-@, argc:0, ARGS_SIMPLE>
0013 putobject_INT2FIX_1_
~~~~
0139 putobject_INT2FIX_1_
0140 putobject                              ""
0142 putnil
0143 dup
0144 objtostring                            <calldata!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>
0146 anytostring
0147 concatstrings                          2
0149 opt_send_without_block                 <calldata!mid:-@, argc:0, ARGS_SIMPLE>
0151 putobject_INT2FIX_1_
~~~~
0277 putobject_INT2FIX_1_
0278 putobject                              ""
0280 putnil
0281 dup
0282 objtostring                            <calldata!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>
0284 anytostring
0285 concatstrings                          2
0287 opt_send_without_block                 <calldata!mid:-@, argc:0, ARGS_SIMPLE>
0289 putobject_INT2FIX_1_
~~~~
0415 putobject_INT2FIX_1_
0416 putobject                              ""
0418 putnil
0419 dup
0420 objtostring                            <calldata!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>
0422 anytostring
0423 concatstrings                          2
0425 opt_send_without_block                 <calldata!mid:-@, argc:0, ARGS_SIMPLE>
0427 putobject_INT2FIX_1_
~~~~
0553 putobject_INT2FIX_1_
0554 newarray                               512
0556 opt_send_without_block                 <calldata!mid:make_shareable, argc:1, ARGS_SIMPLE>
0558 dup
0559 putspecialobject                       3
0561 setconstant                            :A
0563 leave
```

where the `~~~~` represents another 128 `putobject_INT2FIX_1_` instructions.

I assume this was done because it used to modify the AST and that got removed. However, I also assume it was not intentional to lose all of these optimizations, as it now means if Ractors are being used that large literals are vulnerable to stack overflows.

Please let me know how to proceed, as I am trying to exactly match behavior in the prism compiler.

-- 
https://bugs.ruby-lang.org/