[ruby-core:117601] [Ruby master Feature#20425] Optimize forwarding callers and callees

["tenderlovemaking (Aaron Patterson) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20425 has been updated by tenderlovemaking (Aaron Patterson).


ko1 (Koichi Sasada) wrote in #note-6:
> My idea is simple because it is simple replacement with an array (and a hash) to contain arguments (I only proposed lightweight argument container than an array and hash).
> 
> This proposal breaks the assumption of VM stack structure. I'm afraid this kind of breakage can cause serious issue.

For what it's worth, we've tested this patch in Shopify CI and it's passing all tests.  We might be able to try in production, but I need to ask some people.

> But I can misunderstand so let's talk at RubyKaigi, Okinawa with a whiteboard.

Sure, we can discuss it at RubyKaigi.  I agree your proposal would maintain stack layout when calling in to `...` methods, but I don't think the code would be any more simple due to extra memory management / GC complexity.  I was able to simplify the patch somewhat, so please take a look again.

I decided to test this against RailsBench, and this patch does speed up RailsBench (slightly).

Here is RailsBench with master:

```
$ bundle exec ruby benchmark.rb
ruby 3.4.0dev (2024-04-18T21:11:25Z master 64d0817ea9) [arm64-darwin23]
Command: bundle check 2> /dev/null || bundle install
The Gemfile's dependencies are satisfied
Command: bin/rails db:migrate db:seed
Using 100 posts in the database
itr #1: 1554ms
itr #2: 1519ms
itr #3: 1515ms
itr #4: 1553ms
itr #5: 1550ms
itr #6: 1526ms
itr #7: 1574ms
itr #8: 1522ms
itr #9: 1521ms
itr #10: 1529ms
itr #11: 1526ms
itr #12: 1550ms
itr #13: 1522ms
itr #14: 1551ms
itr #15: 1541ms
itr #16: 1538ms
itr #17: 1552ms
itr #18: 1536ms
itr #19: 1560ms
itr #20: 1549ms
itr #21: 1536ms
itr #22: 1529ms
itr #23: 1542ms
itr #24: 1502ms
itr #25: 1559ms
RSS: 139.1MiB
MAXRSS: 142640.0MiB
Writing file /Users/aaron/git/yjit-bench/benchmarks/railsbench/data/results-ruby-3.4.0-2024-04-18-143710.json
Average of last 10, non-warmup iters: 1540ms
```

Here is RailsBench with the `...` optimization:

```
$ bundle exec ruby benchmark.rb
ruby 3.4.0dev (2024-04-18T21:20:23Z speed-forward 4d698e6d46) [arm64-darwin23]
Command: bundle check 2> /dev/null || bundle install
The Gemfile's dependencies are satisfied
Command: bin/rails db:migrate db:seed
Using 100 posts in the database
itr #1: 1537ms
itr #2: 1523ms
itr #3: 1495ms
itr #4: 1501ms
itr #5: 1520ms
itr #6: 1514ms
itr #7: 1514ms
itr #8: 1486ms
itr #9: 1524ms
itr #10: 1493ms
itr #11: 1472ms
itr #12: 1509ms
itr #13: 1497ms
itr #14: 1492ms
itr #15: 1500ms
itr #16: 1507ms
itr #17: 1526ms
itr #18: 1502ms
itr #19: 1505ms
itr #20: 1492ms
itr #21: 1501ms
itr #22: 1529ms
itr #23: 1519ms
itr #24: 1537ms
itr #25: 1499ms
RSS: 140.0MiB
MAXRSS: 143504.0MiB
Writing file /Users/aaron/git/yjit-bench/benchmarks/railsbench/data/results-ruby-3.4.0-2024-04-18-143623.json
Average of last 10, non-warmup iters: 1512ms
```

Average iteration decreases about 28ms.  Basically similar results on my x86 machine.

master:

```
aaron@whiteclaw ~/g/y/b/railsbench (main)> bundle exec ruby benchmark.rb
ruby 3.4.0dev (2024-04-18T21:21:01Z master 6443d690ae) [x86_64-linux]
Command: bundle check 2> /dev/null || bundle install
The Gemfile's dependencies are satisfied
Command: bin/rails db:migrate db:seed
Using 100 posts in the database
itr #1: 2227ms
itr #2: 2173ms
itr #3: 2174ms
itr #4: 2171ms
itr #5: 2177ms
itr #6: 2171ms
itr #7: 2172ms
itr #8: 2171ms
itr #9: 2170ms
itr #10: 2173ms
itr #11: 2170ms
itr #12: 2173ms
itr #13: 2170ms
itr #14: 2171ms
itr #15: 2174ms
itr #16: 2171ms
itr #17: 2173ms
itr #18: 2170ms
itr #19: 2176ms
itr #20: 2169ms
itr #21: 2175ms
itr #22: 2169ms
itr #23: 2170ms
itr #24: 2173ms
itr #25: 2170ms
RSS: 110.0MiB
MAXRSS: 110.1MiB
Writing file /home/aaron/git/yjit-bench/benchmarks/railsbench/data/results-ruby-3.4.0-2024-04-18-150418.json
Average of last 10, non-warmup iters: 2171ms
```

This branch:

```
aaron@whiteclaw ~/g/y/b/railsbench (main)> bundle exec ruby benchmark.rb
ruby 3.4.0dev (2024-04-18T21:20:23Z speed-forward 4d698e6d46) [x86_64-linux]
Command: bundle check 2> /dev/null || bundle install
The Gemfile's dependencies are satisfied
Command: bin/rails db:migrate db:seed
Using 100 posts in the database
itr #1: 2199ms
itr #2: 2157ms
itr #3: 2158ms
itr #4: 2153ms
itr #5: 2156ms
itr #6: 2157ms
itr #7: 2155ms
itr #8: 2153ms
itr #9: 2152ms
itr #10: 2160ms
itr #11: 2153ms
itr #12: 2156ms
itr #13: 2153ms
itr #14: 2159ms
itr #15: 2154ms
itr #16: 2154ms
itr #17: 2157ms
itr #18: 2155ms
itr #19: 2158ms
itr #20: 2152ms
itr #21: 2156ms
itr #22: 2154ms
itr #23: 2153ms
itr #24: 2156ms
itr #25: 2151ms
RSS: 107.7MiB
MAXRSS: 107.8MiB
Writing file /home/aaron/git/yjit-bench/benchmarks/railsbench/data/results-ruby-3.4.0-2024-04-18-150520.json
Average of last 10, non-warmup iters: 2154ms
```

Maybe we could try merging this?  We can revert if it causes problems.  Anyway, I'm happy to discuss in Okinawa! 😄

----------------------------------------
Feature #20425: Optimize forwarding callers and callees
https://bugs.ruby-lang.org/issues/20425#change-108010

* Author: tenderlovemaking (Aaron Patterson)
* Status: Open
----------------------------------------
[This PR](https://github.com/ruby/ruby/pull/10510) optimizes forwarding callers and callees. It only optimizes methods that only take `...` as their parameter, and then pass `...` to other calls.

Calls it optimizes look like this:

```ruby
def bar(a) = a
def foo(...) = bar(...) # optimized
foo(123)
```

```ruby
def bar(a) = a
def foo(...) = bar(1, 2, ...) # optimized
foo(123)
```

```ruby
def bar(*a) = a

def foo(...)
  list = [1, 2]
  bar(*list, ...) # optimized
end
foo(123)
```

All variants of the above but using `super` are also optimized, including a bare super like this:

```ruby
def foo(...)
  super
end
```

This patch eliminates intermediate allocations made when calling methods that accept `...`.
We can observe allocation elimination like this:

```ruby
def m
  x = GC.stat(:total_allocated_objects)
  yield
  GC.stat(:total_allocated_objects) - x
end

def bar(a) = a
def foo(...) = bar(...)

def test
  m { foo(123) }
end

test
p test # allocates 1 object on master, but 0 objects with this patch
```

```ruby
def bar(a, b:) = a + b
def foo(...) = bar(...)

def test
  m { foo(1, b: 2) }
end

test
p test # allocates 2 objects on master, but 0 objects with this patch
```

## How does it work?

This patch works by using a dynamic stack size when passing forwarded parameters to callees.
The caller's info object (known as the "CI") contains the stack size of the
parameters, so we pass the CI object itself as a parameter to the callee.
When forwarding parameters, the forwarding ISeq uses the caller's CI to determine how much stack to copy, then copies the caller's stack before calling the callee.
The CI at the forwarded call site is adjusted using information from the caller's CI.

I think this description is kind of confusing, so let's walk through an example with code.

```ruby
def delegatee(a, b) = a + b

def delegator(...)
  delegatee(...)  # CI2 (FORWARDING)
end

def caller
  delegator(1, 2) # CI1 (argc: 2)
end
```

Before we call the delegator method, the stack looks like this:

```
Executing Line | Code                                  | Stack
---------------+---------------------------------------+--------
              1| def delegatee(a, b) = a + b           | self
              2|                                       | 1
              3| def delegator(...)                    | 2
              4|   #                                   |
              5|   delegatee(...)  # CI2 (FORWARDING)  |
              6| end                                   |
              7|                                       |
              8| def caller                            |
          ->  9|   delegator(1, 2) # CI1 (argc: 2)     |
             10| end                                   |
```

The ISeq for `delegator` is tagged as "forwardable", so when `caller` calls in
to `delegator`, it writes `CI1` on to the stack as a local variable for the
`delegator` method.  The `delegator` method has a special local called `...`
that holds the caller's CI object.

Here is the ISeq disasm fo `delegator`:

```
== disasm: #<ISeq:delegator@-e:1 (1,0)-(1,39)>
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] "..."@0
0000 putself                                                          (   1)[LiCa]
0001 getlocal_WC_0                          "..."@0
0003 send                                   <calldata!mid:delegatee, argc:0, FCALL|FORWARDING>, nil
0006 leave                                  [Re]
```

The local called `...` will contain the caller's CI: CI1.

Here is the stack when we enter `delegator`:


```
Executing Line | Code                                  | Stack
---------------+---------------------------------------+--------
              1| def delegatee(a, b) = a + b           | self
              2|                                       | 1
              3| def delegator(...)                    | 2
           -> 4|   #                                   | CI1 (argc: 2)
              5|   delegatee(...)  # CI2 (FORWARDING)  | cref_or_me
              6| end                                   | specval
              7|                                       | type
              8| def caller                            |
              9|   delegator(1, 2) # CI1 (argc: 2)     |
             10| end                                   |
```

The CI at `delegatee` on line 5 is tagged as "FORWARDING", so it knows to
memcopy the caller's stack before calling `delegatee`.  In this case, it will
memcopy self, 1, and 2 to the stack before calling `delegatee`.  It knows how much
memory to copy from the caller because `CI1` contains stack size information
(argc: 2).

Before executing the `send` instruction, we push `...` on the stack.  The
`send` instruction pops `...`, and because it is tagged with `FORWARDING`, it
knows to memcopy (using the information in the CI it just popped):

```
== disasm: #<ISeq:delegator@-e:1 (1,0)-(1,39)>
local table (size: 1, argc: 0 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 1] "..."@0
0000 putself                                                          (   1)[LiCa]
0001 getlocal_WC_0                          "..."@0
0003 send                                   <calldata!mid:delegatee, argc:0, FCALL|FORWARDING>, nil
0006 leave                                  [Re]
```

Instruction 001 puts the caller's CI on the stack.  `send` is tagged with
FORWARDING, so it reads the CI and _copies_ the callers stack to this stack:

```
Executing Line | Code                                  | Stack
---------------+---------------------------------------+--------
              1| def delegatee(a, b) = a + b           | self
              2|                                       | 1
              3| def delegator(...)                    | 2
              4|   #                                   | CI1 (argc: 2)
           -> 5|   delegatee(...)  # CI2 (FORWARDING)  | cref_or_me
              6| end                                   | specval
              7|                                       | type
              8| def caller                            | self
              9|   delegator(1, 2) # CI1 (argc: 2)     | 1
             10| end                                   | 2
```

The "FORWARDING" call site combines information from CI1 with CI2 in order
to support passing other values in addition to the `...` value, as well as
perfectly forward splat args, kwargs, etc.

Since we're able to copy the stack from `caller` in to `delegator`'s stack, we
can avoid allocating objects.

## Why?

I want to do this to eliminate object allocations for delegate methods.
My long term goal is to implement `Class#new` in Ruby and it uses `...`.

I was able to implement `Class#new` in Ruby
[here](https://github.com/ruby/ruby/pull/9289).
If we adopt the technique in this patch, then we can optimize allocating
objects that take keyword parameters for `initialize`.

For example, this code will allocate 2 objects: one for `SomeObject`, and one
for the kwargs:

```ruby
SomeObject.new(foo: 1)
```

If we combine this technique, plus implement `Class#new` in Ruby, then we can
reduce allocations for this common operation.



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