[ruby-core:98793] [Ruby master Bug#16941] MJIT doesn't identify Struct kind of instruction

Issue #16941 has been updated by k0kubun (Takashi Kokubun).


Thanks, I figured out the problem from your script.

Every time we call `Struct.new(:a)`, it creates a different method entry `:a=`. Every time you call `Struct.new(:a)` and `:a=`, MJIT finds a new method entry of `:a=` and it therefore tries to compile it. There's nothing wrong going on in MJIT's side, but the Ruby VM could reuse a method entry which was previously because the method definition is the same for each index, and that (or just stopping to call `Struct.new` every time) is the only way we could fix the problem you shared.

I suppose this won't be a problem when you use `--jit-min-calls=10000` (default of Ruby 2.7 or later). Is it the case, or do you think it should be fixed?

> The only problem here is that it doesn't consider if a process has been attempted to get JITed before and has failed to do so. Therefore it attempts to recompile again and again.

As I explained in this comment, this part is not quite right. It has never attempted JIT for the same method entry again. The script is creating a different method every time.

> I have a more greedy thought, is it possible to have a mechanism to persist the JITed methods and load on a later point of time?

This is a done discussion [Feature #14489]. It's unfortunately a low priority for the time being. I may revisit it once I finish some more priorities I have.

----------------------------------------
Bug #16941: MJIT doesn't identify Struct kind of instruction
https://bugs.ruby-lang.org/issues/16941#change-86152

* Author: abhsha (Abhishek Sharma)
* Status: Feedback
* Priority: Normal
* Assignee: k0kubun (Takashi Kokubun)
* ruby -v: 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
MJIT does not identify Struct instructions and gives a warning while compiling to C.

Example: 
def my_method
    1.times do
        a_struct = Struct.new(:a).new
        a_struct.a = "a"
    end
end

my_method

$: ruby --jit-save-temps --jit-min-calls=1 --disable-gems --jit --jit-verbose=2 --jit-wait test.rb

output:
start compilation: a=@test.rb:3 -> /tmp/_ruby_mjit_p30696u2.c
MJIT warning: Skipped to compile unsupported instruction: opt_call_c_function
JIT failure (0.0ms): a=@test.rb:3 -> /tmp/_ruby_mjit_p30696u2.c

Are structs not recognised by JIT compiler? 
We have this call in our programs many number of times (~ 10^6), it attempts every time and fails. Can this be a cause of programs to get slower with --jit enabled ? 



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