Feature #18943: New constant caching instruction: opt_getconstant_path

• Status: Closed
• Assignee: -
• Target version: -

[jhawthorn (John Hawthorn)]

I'd like to propose the change to the bytecode for constant caching.
I've submitted this improvement via pull request at https://github.com/ruby/ruby/pull/6187 and also attached a patch to this issue.

Previously YARV bytecode implemented constant caching by having a pair of instructions, opt_getinlinecache and opt_setinlinecache, wrapping a series of getconstant calls (with putobject providing supporting arguments).

# old
$ ruby --dump=insns -e 'Foo::Bar::Baz'
== disasm: #<ISeq:<main>@-e:1 (1,0)-(1,13)> (catch: FALSE)
0000 opt_getinlinecache                     17, <is:0>                (   1)[Li]
0003 putobject                              true
0005 getconstant                            :Foo
0007 putobject                              false
0009 getconstant                            :Bar
0011 putobject                              false
0013 getconstant                            :Baz
0015 opt_setinlinecache                     <is:0>
0017 leave

This commit replaces that pattern with a new instruction, opt_getconstant_path, handling both getting/setting the inline cache and fetching the constant on a cache miss.

This is implemented by storing the full constant path as a null-terminated array of IDs inside of the IC structure. idNULL is used to signal an absolute constant reference.

# new
$ ./miniruby --dump=insns -e '::Foo::Bar::Baz'
== disasm: #<ISeq:<main>@-e:1 (1,0)-(1,13)> (catch: FALSE)
0000 opt_getconstant_path                   <ic:0 ::Foo::Bar::Baz>      (   1)[Li]
0002 leave

The motivation for this is that we had increasingly found the need to disassemble the instructions between the opt_getinlinecache and opt_setinlinecache in order to determine the constant we are fetching, or otherwise store metadata.

This disassembly was previously done:

• In opt_setinlinecache, to register the IC against the constant names it is using for granular invalidation.
• In rb_iseq_free, to unregister the IC from the invalidation table.
• In YJIT to find the position of a opt_getinlinecache instruction to invalidate it when the cache is populated
• In YJIT to register the constant names being used for invalidation.

With this change we no longer need disassembly for these (in fact rb_iseq_each is now unused and is removed in the PR), as the list of constant names being referenced is held in the IC. This should also make it possible to make more optimizations in the future.

This may also reduce the size of iseqs, as previously each segment required 32 bytes (assuming 64-bit platform) for each constant segment. This implementation only stores one 8-byte ID per-segment .

There should be no significant performance difference between this and the previous implementation. Previously opt_getinlinecache was a "leaf" instruction, but it included a jump (almost always to a separate cache line). Now opt_getconstant_path is a non-leaf (it may raise/autoload/call const_missing) but it does not jump. These seem to even out. This also removes a field from the IC structure that was needed by YJIT, but adds the ID *segments field, so the size remains the same.

[k0kubun (Takashi Kokubun)]

• Status changed from Open to Closed

This was merged at https://github.com/ruby/ruby/pull/6187.