Feature #11769: optimize case / when for `nil`

• Status: Closed
• Assignee: ko1 (Koichi Sasada)
• Target version: -

[tenderlovemaking (Aaron Patterson)]

Hi,

I've noticed that when there are certain values in a case / when statement it gets optimized to a hash lookup. For example, code like this:

def foo socket
  case str = socket.read_nonblock(10, exception: false)
  when :wait_readable
    # do something
  else
    str
  end
end

puts RubyVM::InstructionSequence.of(method(:foo)).disasm

The above code will use opt_case_dispatch instruction with a hash. However, if I use nil in the case statement like this:

def foo socket
  case str = socket.read_nonblock(10, exception: false)
  when :wait_readable
    # do something
  when nil
    # got an EOF
  else
    str
  end
end

puts RubyVM::InstructionSequence.of(method(:foo)).disasm

Then the optimization is lost.

I've attached a patch that adds nil to the optimized case such that the above code will use opt_case_dispatch. My patch defines === on nil, then adds nil to the list of "optimizable literals".

[normalperson (Eric Wong)]

[e-mail] wrote:

I've attached a patch that adds nil to the optimized case such that
the above code will use opt_case_dispatch. My patch defines ===
on nil, then adds nil to the list of "optimizable literals".

Cool. Might be worth it to do the same for true and false,
too, since we have space for them.

[ko1 (Koichi Sasada)]

• Assignee set to ko1 (Koichi Sasada)

[normalperson (Eric Wong)]

Eric Wong [e-mail] wrote:

Cool. Might be worth it to do the same for true and false,
too, since we have space for them.

Sythetic benchmark shows a 3x speedup on top of r52928:

http://80x24.org/spew/[e-mail]/raw

• Status changed from Open to Closed

Applied in changeset r52931.

---

compile optimized case dispatch for nil/true/false

nil/true/false are special literals just like floats, integers,
literal strings, and symbols. Optimize when statements with
them by using a jump table, too.

target 0: a (ruby 2.3.0dev (2015-12-08 trunk 52928) [x86_64-linux]) at "/home/ew/rrrr/b/ruby"
target 1: b (ruby 2.3.0dev (2015-12-08 master 52928) [x86_64-linux]) at "/home/ew/ruby/b/ruby"

benchmark results:
minimum results in each 5 measurements.
Execution time (sec)
name a b
loop_whileloop2 0.102 0.103
vm2_case_lit* 1.657 0.549

Speedup ratio: compare with the result of `a' (greater is better)
name b
loop_whileloop2 0.988
vm2_case_lit* 3.017

• benchmark/bm_vm2_case_lit.rb: new benchmark
• compile.c (case_when_optimizable_literal): add nil/true/false
• insns.def (opt_case_dispatch): ditto
• vm.c (vm_redefinition_check_flag): ditto
• vm.c (vm_init_redefined_flag): ditto
• vm_core.h: ditto
• object.c (InitVM_Object): define === explicitly for nil/true/false
• test/ruby/test_case.rb (test_deoptimize_nil): new test
• test/ruby/test_optimization.rb (test_opt_case_dispatch): update
  (test_eqq): new test
  [ruby-core:71923] [Feature #11769]
  Original patch by Aaron Patterson [e-mail]