Annotate Symbol#empty? as leaf

[sampokuokkanen]

Move Symbol#empty? from the C sym_empty into symbol.rb, implemented as RBOOL(RSTRING_LEN(rb_sym2str(self)) == 0), mirroring the existing Symbol#to_s and #name leaf builtins.

Also added an annotate_builtin! in zjit/src/cruby_methods.rs to tell ZJIT that the method does not need GC now.

Symbol#empty? as a leaf builtin, via benchmark-driver (GC.disable, loop_count: 20_000_000, Apple M1). Speedup vs master (CFUNC sym_empty):

benchmark	interpreter	--yjit	--zjit
:"".empty?	1.24x	1.30x	1.19x
:abcdefgh.empty?	1.19x	1.19x	1.17x

Windows native and WSL2 also get faster:

benchmark	interpreter
:"".empty?	1.47x
:abcdefgh.empty?	1.36x

WSL2 (Ubuntu) x86_64-linux

benchmark	interpreter	--yjit	--zjit
:"".empty?	1.39x	1.33x	1.32x
:abcdefgh.empty?	1.37x	1.39x	1.33x

[tekknolagi]

FWIW we can do better in ZJIT. Modified your script to print slightly differently.

plum% ../build-rel-stats/miniruby tmp/symempty.rb  # your commit
best time for 400000 iterations: 0.007s (57.0M/s)
plum% ../build-rel-stats/miniruby --zjit tmp/symempty.rb  # your commit
best time for 400000 iterations: 0.002s (188.8M/s)
plum% ../build-rel-stats/miniruby --zjit tmp/symempty.rb  # teach HIR how to inline it
best time for 400000 iterations: 0.001s (604.2M/s)
plum%

Can be done in a follow-up PR in a couple lines :)

[k0kubun]

Can you name these PRs "Annotate XXX as leaf" going forward? "Rewrite in Ruby" is a misleading subject for this change. Until you stop using Primitive.cexpr!, the implementation is still written in C. We do this in Ruby source files only because this is the only way we can annotate C code in CRuby, and we don't call it a rewrite in Ruby.

[sampokuokkanen]

@k0kubun
Thanks and sorry about that, I will do that from now on. I updated the git commit summary to match.

@XrXr @tekknolagi
Thanks for looking into this! So the consensus is to keep this PR as is, then later add something like static VALUE empty_symbol;? To make the comparison just RBOOL(self == empty_symbol).

@tekknolagi Interested to know what kind of a trick you used to get that kind of performance! 😁