Blocks insert a leading `nop` instruction in order to execute a "block call" tracepoint. Block compilation unconditionally inserts a leading `nop` plus a label after the instruction: https://github.com/ruby/ruby/blob/641f15b1c6bd8921407a1f045573d3b0605f00d3/prism_compile.c#L6867-L6869 This `nop` instruction is used entirely for firing the block entry tracepoint. The label exists so that the block can contain a loop but the block entry tracepoint is executed only once. For example, the following code is an infinite loop, but should only execute the b_call tracepoint once: ```ruby -> { redo }.call ``` Previous to this commit, we would eliminate the `nop` instruction, but only if there were no other jump instructions inside the block. This means that the following code would still contain a leading `nop` even though the label following the `nop` is unused: ```ruby -> { nil if bar } ``` ``` == disasm: #<ISeq:block in <main>@test.rb:1 (1,2)-(1,17)> (catch: FALSE) 0000 nop ( 1)[Bc] 0001 putself [Li] 0002 opt_send_without_block <calldata!mid:bar, argc:0, FCALL|VCALL|ARGS_SIMPLE> 0004 branchunless 8 0006 putnil 0007 leave [Br] 0008 putnil 0009 leave [Br] ``` This commit checks to see if the label inserted after the `nop` is actually a jump target. If it's not a jump target, then we should be safe to eliminate the leading `nop`: ``` > build-master/miniruby --dump=insns test.rb == disasm: #<ISeq:<main>@test.rb:1 (1,0)-(1,17)> 0000 putspecialobject 1 ( 1)[Li] 0002 send <calldata!mid:lambda, argc:0, FCALL>, block in <main> 0005 leave == disasm: #<ISeq:block in <main>@test.rb:1 (1,2)-(1,17)> 0000 putself ( 1)[LiBc] 0001 opt_send_without_block <calldata!mid:bar, argc:0, FCALL|VCALL|ARGS_SIMPLE> 0003 branchunless 7 0005 putnil 0006 leave [Br] 0007 putnil 0008 leave [Br] ``` We have a test for b_call tracepoints that use `redo` here: https://github.com/ruby/ruby/blob/aebf96f371c8d874398e0041b798892e545fa206/test/ruby/test_settracefunc.rb#L1728-L1736