[ruby-core:98730] [Ruby master Bug#16941] MJIT doesn't identify Struct kind of instruction

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

Status changed from Open to Feedback

> Are structs not recognised by JIT compiler?

To be precise, `opt_call_c_function` insn is not supported by JIT compiler, and Struct is one of a few things which may generate `opt_call_c_function` insn. It's something to be fixed for sure, but as of now priority of fixing it is low.

> We have this call in our programs many number of times (~ 106), it attempts every time and fails. Can this be a cause of programs to get slower with --jit enabled ?

Well, if it attempts of JIT compilation every time, it'd be compiled every time even if JIT supported `opt_call_c_function`.

The fundamental problem in the script you provided is that it's defining a brand-new class by `Struct.new` every time `my_method` is called. Note that it's not good for VM's performance either. You should define a Struct class outside `my_method` and `my_method` should just refer to it. If it's not possible in your use case, there's nothing we can do.

One thing to note is that `--jit-min-calls` is 5 in Ruby 2.6's default, and it's 10000 in Ruby 2.7's default. Therefore the problem may not happen in the `my_method` with Ruby 2.7 or later.

----------------------------------------
Bug #16941: MJIT doesn't identify Struct kind of instruction
https://bugs.ruby-lang.org/issues/16941#change-86086

* 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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