[Bug #20457] Do not remove final return node

[nobu]

This was an optimization for versions prior to 1.9 that traverse the AST at runtime.

[tenderlove]

I think this might require more work in compile.c (or maybe the peephole optimizer).

Given this code:

def foo
  a = 1
  if bar
    return a
  else
    return a
  end
end

The iseqs with Ruby master look like this:

== disasm: #<ISeq:foo@<compiled>:1 (1,0)-(8,3)>
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] a@0
0000 putobject_INT2FIX_1_                                             (   2)[LiCa]
0001 setlocal_WC_0                          a@0
0003 putself                                                          (   3)[Li]
0004 opt_send_without_block                 <calldata!mid:bar, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0006 branchunless                           11
0008 getlocal_WC_0                          a@0                       (   4)[Li]
0010 leave                                                            (   8)[Re]
0011 getlocal_WC_0                          a@0                       (   6)[Li]
0013 leave                                                            (   8)[Re]

On this branch they look like this:

== disasm: #<ISeq:foo@<compiled>:1 (1,0)-(8,3)>
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] a@0
0000 putobject_INT2FIX_1_                                             (   2)[LiCa]
0001 setlocal_WC_0                          a@0
0003 putself                                                          (   3)[Li]
0004 opt_send_without_block                 <calldata!mid:bar, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0006 branchunless                           13
0008 getlocal_WC_0                          a@0                       (   4)[Li]
0010 leave                                  [Re]
0011 putnil
0012 leave                                                            (   8)[Re]
0013 getlocal_WC_0                          a@0                       (   6)[Li]
0015 leave                                  [Re]
0016 putnil
0017 leave                                                            (   8)[Re]

[nobu]

With simpler code:

def foo(a) = return a

Master:

== disasm: #<ISeq:foo@-e:1 (1,0)-(1,21)>
local table (size: 1, argc: 1 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] a@0<Arg>
0000 getlocal_WC_0                          a@0                       (   1)[Ca]
0002 leave                                  [Re]

This patch:

== disasm: #<ISeq:foo@-e:1 (1,0)-(1,21)>
local table (size: 1, argc: 1 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] a@0<Arg>
0000 getlocal_WC_0                          a@0                       (   1)[Ca]
0002 leave                                  [Re]
0003 putnil
0004 leave                                  [Re]