From: "wildmaples (Maple Ong)" <noreply@...>
Date: 2022-05-24T17:39:40+00:00
Subject: [ruby-core:108669] [Ruby master Bug#18801] Dead YARV instructions produced when `branchif` is used

Issue #18801 has been reported by wildmaples (Maple Ong).

----------------------------------------
Bug #18801: Dead YARV instructions produced when `branchif` is used
https://bugs.ruby-lang.org/issues/18801

* Author: wildmaples (Maple Ong)
* Status: Open
* Priority: Normal
* ruby -v: 3.1.0
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
# Description

It seems there are unused YARV instructions produced when the snippet contains a `branchif` instruction.

In the following example, the instructions numbers 0002 to 0004 won't ever be executed: 

```
irb(main):003:0> puts RubyVM::InstructionSequence.compile("while 2+3; puts 'hi'; end").disasm

== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,25)> (catch: FALSE)
0000 jump                                   12                        (   1)[Li]
0002 putnil
0003 pop
0004 jump                                   12
0006 putself
0007 putstring                              "hi"
0009 opt_send_without_block                 <calldata!mid:puts, argc:1, FCALL|ARGS_SIMPLE>
0011 pop
0012 putobject                              2
0014 putobject                              3
0016 opt_plus                               <calldata!mid:+, argc:1, ARGS_SIMPLE>[CcCr]
0018 branchif                               6
0020 putnil
0021 leave
```

Similarly in this example, 0006-0008 won't be executed.  

```
irb(main):003:0> puts RubyVM::InstructionSequence.compile("x = 9; while x; puts 'hi'; end").disasm
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,30)> (catch: FALSE)
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] x@0
0000 putobject                              9                         (   1)[Li]
0002 setlocal_WC_0                          x@0
0004 jump                                   16
0006 putnil
0007 pop
0008 jump                                   16
0010 putself
0011 putstring                              "hi"
0013 opt_send_without_block                 <calldata!mid:puts, argc:1, FCALL|ARGS_SIMPLE>
0015 pop
0016 getlocal_WC_0                          x@0
0018 branchif                               10
0020 putnil
0021 leave
```

Initially we thought those instructions (i.e. putnil, pop, jump) were used when the return value of the while-loop is needed.

```
irb(main):012:0> puts RubyVM::InstructionSequence.compile("x = while foo; 9; end").disasm
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,21)> (catch: FALSE)
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] x@0
0000 jump                                   4                         (   1)[Li]
0002 putnil
0003 pop
0004 putself
0005 opt_send_without_block                 <calldata!mid:foo, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0007 branchif                               4
0009 putnil
0010 dup
0011 setlocal_WC_0                          x@0
0013 leave
```

But it seems like some dead instructions (0002, 0003) in the example above still remains. 

Are those instructions meant to be used for something else or is it a "bug" that it sticks around? 
Perhaps it can be optimized away? 



-- 
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