From: "headius (Charles Nutter)" <headius@...>
Date: 2013-10-09T09:25:53+09:00
Subject: [ruby-core:57753] [ruby-trunk - Feature #8992] Use String#freeze	and compiler tricks to replace "str"f suffix


Issue #8992 has been updated by headius (Charles Nutter).


sam.saffron (Sam Saffron) wrote:
> I am actually very concerned about compiler tricks with freeze cause it leads to non-obvious code. 
> 
> x = "hello".freeze
> y = "hello".freeze
> x.object_id
> > 10
> x.object_id == y.object_id

I don't think you should *ever* rely on this to be true, since it won't be on older Ruby impls or impls that don't yet have #freeze optimizations. Even Java, with its interned Strings, strongly discourages *ever* using object identity to compare strings. IDEs even flag it as a warning.

> a = "hello"
> a.object_id
> > 100
> a.freeze
> a.object_id 
> > 100 # must be 100
...
> So the way #freeze operates then depends on where it is being executed, I dislike that. 
> 
> Much prefer just adding #frozen, we can implement it sort of cleanly in 2.0 (except for GC hooking) and simply alias #freeze in 1.9 and earlier.

So here's the same question I asked in the #frozen feature: why can't #freeze just use the fstring table?

* fstrings will GC and clear themselves from that table
* large strings put into the table will take up no more space than if they were not frozen

So #freeze *could* do what you suggest here and always use the fstring table. In the "literal".freeze case, the compiler could do additional magic to go to the table immediately.
----------------------------------------
Feature #8992: Use String#freeze and compiler tricks to replace "str"f suffix
https://bugs.ruby-lang.org/issues/8992#change-42353

Author: headius (Charles Nutter)
Status: Open
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: core
Target version: current: 2.1.0


BACKGROUND:

In https://bugs.ruby-lang.org/issues/8579 @charliesome introduced the "f" suffix for creating already-frozen strings. A string like "str"f would have the following characteristics:

* It would be frozen before the expression returned
* It would be the same object everywhere, pulling from a global "fstring" table

To avoid memory leaks, these pooled strings would remove themselves from the "fstring" table on GC.

However, there are problems with this new syntax:

* It will never parse in Ruby 2.0 and earlier.
* It's not particularly attractive, though this is a subjective matter.
* It does not lend itself well to use in other scenarios, such as for arrays and hashes (http://bugs.ruby-lang.org/issues/8909 )

PROPOSAL:

I propose that we eliminate the new "f" suffix and just make the compiler smart enough to see literal strings with .frozen the same way.

So this code:

str = "mystring".freeze

Would be equivalent in the compiler to this code:

str = "mystring"f

And the fstring table would still be used to return pooled instances.

IMPLEMENTATION NOTES:

The fstring table already exists on master and would be used for these pooled strings. An open question is whether the compiler should forever optimize "str".frozen to return the pooled version or whether it should check (inline-cache style) whether String#freeze has been replaced. I am ok with either, but the best potential comes from ignoring String#freeze redefinitions...or making it impossible to redefine String#freeze.

BONUS BIKESHEDDING:

If we do not want to overload the existing .freeze method in this way, we could follow suggestions in http://bugs.ruby-lang.org/issues/8977 to add a new "frozen" method (or some other name) that the compiler would understand.

If it were "frozen", the following two lines would be equivalent:

str = "mystring".frozen
str = "mystring"f

In addition, using .frozen on any string would put it in the fstring table and return that pooled version.

I also propose one alternative method name: the unary ~ operator.

There is no ~ on String right now, and it has no meaning for strings that we'd be overriding. So the following two lines would be equivalent:

str = ~"mystring"
str = "mystring"f

JUSTIFICATION:

Making the compiler aware of normal method-based String freezing has the following advantages:

* It will parse in all versions of Ruby.
* It will be equivalent in all versions of Ruby other than the fstring pooling.
* It extends neatly to Array and Hash; the compiler can see Array or Hash with literal elements and return the same object.
* It does not require a pragma (http://bugs.ruby-lang.org/issues/8976 )
* It looks like Ruby.


-- 
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