Ruby

Build CDHASH properly when loading iseq from binary

Before this change, CDHASH operands were built as plain hashes when
loaded from binary. Without setting up the hash with the correct
st_table type, the hash can sometimes be an ar_table. When the hash is
an ar_table, lookups can call the eql? method on keys of the hash,
which makes the opt_case_dispatch instruction not "leaf" as it
implicitly declares.

The following script trips the stack canary for checking the leaf
attribute for opt_case_dispatch on VM_CHECK_MODE > 0 (enabled by
default with RUBY_DEBUG).

rb_vm_iseq = RubyVM::InstructionSequence

iseq = rb_vm_iseq.compile(<<-EOF)
  case Class.new(String).new("foo")
  when "foo"
    42
  end
EOF

puts rb_vm_iseq.load_from_binary(iseq.to_binary).eval

This commit changes the binary loading logic to build CDHASH with the
right st_table type. The dumping logic and the dump format stays the
same