Differences from MRI

Metorex runs the ruby/spec suite with no failures, so a program that uses only what the suite checks behaves the same under both. The places below are where Metorex answers differently from MRI. Each sample was run under Metorex and under MRI, and both outputs are shown.

The Version Description#

RUBY_VERSION, RUBY_ENGINE, and RUBY_ENGINE_VERSION match MRI's, so version checks in a library pass. RUBY_DESCRIPTION, which -v prints, names Metorex.

puts RUBY_DESCRIPTION
p [RUBY_ENGINE, RUBY_VERSION, RUBY_ENGINE_VERSION]

Metorex:

metorex 4.0.7 (ruby-compatible) [arm64-darwin]
["ruby", "4.0.7", "4.0.7"]

MRI:

ruby 4.0.7 (2026-09-15 revision 229531a6cf) +PRISM [arm64-darwin27]
["ruby", "4.0.7", "4.0.7"]

Instruction Sequences#

Metorex compiles to instructions of its own rather than YARV's. RubyVM::InstructionSequence#disasm lists those instructions with their operands, the local variable a slot operand names, and the line. #to_a lists each instruction as its name and operands.

source = "total = 2 + 3"
iseq = RubyVM::InstructionSequence.compile(source)
puts iseq.disasm

Metorex:

== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,13)>
0000 Unhoist slot: 0  ; total               (   1)[Li]
0001 LoadInteger to: 0, value: 2            (   1)
0002 LoadInteger to: 1, value: 3            (   1)
0003 Add to: 0, left: 0, right: 1           (   1)
0004 SetLocal from: 0, slot: 0  ; total     (   1)
0005 Return from: 0                         (   1)

MRI:

== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,13)>
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] total@0
0000 putobject                              2                         (   1)[Li]
0002 putobject                              3
0004 opt_plus                               <calldata!mid:+, argc:1, ARGS_SIMPLE>[CcCr]
0006 dup
0007 setlocal_WC_0                          total@0
0009 leave

Virtual Machine Statistics#

RubyVM.stat answers constant_cache_invalidations and constant_cache_misses from Metorex's own constant caches: the times a constant or an autoload changed, and the constant reads that looked the constant up. next_shape_id counts the object shapes Metorex has made, which share the names of objects' instance variables. It answers global_cvar_state and shape_cache_size at zero, since Metorex keeps neither. It also answers keys MRI does not have: units_compiled and instructions_compiled, the units of code Metorex has compiled so far and the instructions they hold, and method_cache_hits and method_cache_misses, the calls a call site answered from the methods it keeps by the receiver's class and those that looked the method up.

stat = RubyVM.stat
kept = %i[global_cvar_state shape_cache_size]
p stat.slice(*kept).values.all?(&:zero?)
units = stat[:units_compiled]
p units && stat[:instructions_compiled] > units
p stat.fetch(:method_cache_hits, 0) > 0

Metorex:

true
true
true

MRI:

false
false
false

The Garbage Collector#

Metorex counts references to each object and frees it when the count reaches zero, and a cycle collector frees objects that refer to each other. GC.config names that collector.

p GC.config[:implementation]

Metorex:

"metorex"

MRI:

"default"

Native Thread IDs#

Every Metorex thread runs on the one operating system thread the interpreter runs on, taking turns. Thread#native_thread_id answers that thread's id for the thread running now, and nil for a thread waiting its turn.

waiting = Thread.new { sleep 0.2 }
sleep 0.05
p Thread.current.native_thread_id.is_a?(Integer)
p waiting.native_thread_id.class

Metorex:

true
NilClass

MRI:

true
Integer

Libraries Loaded at Startup#

Metorex loads error_highlight and did_you_mean as it starts, with or without --disable-gems. MRI loads them through RubyGems, so --disable-gems leaves them out there. Under Metorex --disable=error_highlight and --disable=did_you_mean leave each one out.

p defined?(ErrorHighlight)
p defined?(DidYouMean)

Run with --disable-gems, Metorex:

"constant"
"constant"

MRI:

nil
nil

Naming RSpec#

Rex, the spec framework built into Metorex, loads the first time a program names RSpec, and the examples the program defined run as it ends. Under MRI the program has to require rspec/autorun from the RSpec gems first.

RSpec.describe "a ledger" do
  it "sums its entries" do
    expect([12, 30].sum).to eq(42)
  end
end

Metorex:

.

Finished in 0.00511 seconds (files took 0.05178 seconds to load)
1 example, 0 failures

MRI:

rspec_named.rb:1:in '<main>': uninitialized constant RSpec (NameError)

RSpec.describe "a ledger" do
^^^^^