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ruby--ruby/NEWS.md
Jeremy Evans 50c54d40a8
Evaluate multiple assignment left hand side before right hand side 
In regular assignment, Ruby evaluates the left hand side before
the right hand side.  For example:

```ruby
foo[0] = bar
```

Calls `foo`, then `bar`, then `[]=` on the result of `foo`.

Previously, multiple assignment didn't work this way.  If you did:

```ruby
abc.def, foo[0] = bar, baz
```

Ruby would previously call `bar`, then `baz`, then `abc`, then
`def=` on the result of `abc`, then `foo`, then `[]=` on the
result of `foo`.

This change makes multiple assignment similar to single assignment,
changing the evaluation order of the above multiple assignment code
to calling `abc`, then `foo`, then `bar`, then `baz`, then `def=` on
the result of `abc`, then `[]=` on the result of `foo`.

Implementing this is challenging with the stack-based virtual machine.
We need to keep track of all of the left hand side attribute setter
receivers and setter arguments, and then keep track of the stack level
while handling the assignment processing, so we can issue the
appropriate topn instructions to get the receiver.  Here's an example
of how the multiple assignment is executed, showing the stack and
instructions:

```
self                                      # putself
abc                                       # send
abc, self                                 # putself
abc, foo                                  # send
abc, foo, 0                               # putobject 0
abc, foo, 0, [bar, baz]                   # evaluate RHS
abc, foo, 0, [bar, baz], baz, bar         # expandarray
abc, foo, 0, [bar, baz], baz, bar, abc    # topn 5
abc, foo, 0, [bar, baz], baz, abc, bar    # swap
abc, foo, 0, [bar, baz], baz, def=        # send
abc, foo, 0, [bar, baz], baz              # pop
abc, foo, 0, [bar, baz], baz, foo         # topn 3
abc, foo, 0, [bar, baz], baz, foo, 0      # topn 3
abc, foo, 0, [bar, baz], baz, foo, 0, baz # topn 2
abc, foo, 0, [bar, baz], baz, []=         # send
abc, foo, 0, [bar, baz], baz              # pop
abc, foo, 0, [bar, baz]                   # pop
[bar, baz], foo, 0, [bar, baz]            # setn 3
[bar, baz], foo, 0                        # pop
[bar, baz], foo                           # pop
[bar, baz]                                # pop
```

As multiple assignment must deal with splats, post args, and any level
of nesting, it gets quite a bit more complex than this in non-trivial
cases. To handle this, struct masgn_state is added to keep
track of the overall state of the mass assignment, which stores a linked
list of struct masgn_attrasgn, one for each assigned attribute.

This adds a new optimization that replaces a topn 1/pop instruction
combination with a single swap instruction for multiple assignment
to non-aref attributes.

This new approach isn't compatible with one of the optimizations
previously used, in the case where the multiple assignment return value
was not needed, there was no lhs splat, and one of the left hand side
used an attribute setter.  This removes that optimization. Removing
the optimization allowed for removing the POP_ELEMENT and adjust_stack
functions.

This adds a benchmark to measure how much slower multiple
assignment is with the correct evaluation order.

This benchmark shows:

* 4-9% decrease for attribute sets
* 14-23% decrease for array member sets
* Basically same speed for local variable sets

Importantly, it shows no significant difference between the popped
(where return value of the multiple assignment is not needed) and
!popped (where return value of the multiple assignment is needed)
cases for attribute and array member sets.  This indicates the
previous optimization, which was dropped in the evaluation
order fix and only affected the popped case, is not important to
performance.

Fixes [Bug #4443]
2021-04-21 10:49:19 -07:00

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# NEWS for Ruby 3.1.0
This document is a list of user visible feature changes
since the **3.0.0** release, except for bug fixes.
Note that each entry is kept to a minimum, see links for details.
## Language changes
* Pin operator now takes an expression. [[Feature #17411]]
```ruby
Prime.each_cons(2).lazy.find_all{_1 in [n, ^(n + 2)]}.take(3).to_a
#=> [[3, 5], [5, 7], [11, 13]]
```
* Multiple assignment evaluation order has been made consistent with
single assignment evaluation order. With single assignment, Ruby
uses a left-to-right evaluation order. With this code:
```ruby
foo[0] = bar
```
The following evaluation order is used:
1. `foo`
2. `bar`
3. `[]=` called on the result of `foo`
In Ruby before 3.1.0, multiple assignment did not follow this
evaluation order. With this code:
```ruby
foo[0], bar.baz = a, b
```
Versions of Ruby before 3.1.0 would evaluate in the following
order
1. `a`
2. `b`
3. `foo`
4. `[]=` called on the result of `foo`
5. `bar`
6. `baz=` called on the result of `bar`
Starting in Ruby 3.1.0, evaluation order is now consistent with
single assignment, with the left hand side being evaluated before
the right hand side:
1. `foo`
2. `bar`
3. `a`
4. `b`
5. `[]=` called on the result of `foo`
6. `baz=` called on the result of `bar`
[[Bug #4443]]
## Command line options
## Core classes updates
Outstanding ones only.
* Array
* Array#intersect? is added. [[Feature #15198]]
* Enumerable
* Enumerable#compact is added. [[Feature #17312]]
* Enumerable#tally now accepts an optional hash to count. [[Feature #17744]]
* Enumerator::Lazy
* Enumerator::Lazy#compact is added. [[Feature #17312]]
* File
* File.dirname now accepts an optional argument for the level to
strip path components. [[Feature #12194]]
* Module
* Module#prepend now modifies the ancestor chain if the receiver
already includes the argument. Module#prepend still does not
modify the ancestor chain if the receiver has already prepended
the argument. [[Bug #17423]]
* Struct
* Passing only keyword arguments to Struct#initialize is warned.
You need to use a Hash literal to set a Hash to a first member.
[[Feature #16806]]
* Queue
* Queue#initialize now accepts an Enumerable of initial values.
[[Feature #17327]]
* Thread::Backtrace
* Thread::Backtrace.limit, which returns the value to limit backtrace
length set by `--backtracse-limit` command line option, is added.
[[Feature #17479]]
* $LOAD_PATH
* $LOAD_PATH.resolve_feature_path does not raise. [[Feature #16043]]
## Stdlib updates
Outstanding ones only.
## Compatibility issues
Excluding feature bug fixes.
## Stdlib compatibility issues
* `ERB#initialize` warns `safe_level` and later arguments even without -w.
[[Feature #14256]]
## C API updates
## Implementation improvements
### JIT
* `RubyVM::MJIT` is renamed to `RubyVM::JIT`.
## Static analysis
### RBS
### TypeProf
## Miscellaneous changes
[Bug #4443]: https://bugs.ruby-lang.org/issues/4443
[Feature #12194]: https://bugs.ruby-lang.org/issues/12194
[Feature #14256]: https://bugs.ruby-lang.org/issues/14256
[Feature #15198]: https://bugs.ruby-lang.org/issues/15198
[Feature #16043]: https://bugs.ruby-lang.org/issues/16043
[Feature #16806]: https://bugs.ruby-lang.org/issues/16806
[Feature #17312]: https://bugs.ruby-lang.org/issues/17312
[Feature #17327]: https://bugs.ruby-lang.org/issues/17327
[Feature #17411]: https://bugs.ruby-lang.org/issues/17411
[Bug #17423]: https://bugs.ruby-lang.org/issues/17423
[Feature #17479]: https://bugs.ruby-lang.org/issues/17479
[Feature #17744]: https://bugs.ruby-lang.org/issues/17744
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