immutable strings - approved

[Allison Randal]

I've become more enamored with the immutable strings idea as I read 
through the branch diff (immutable_strings_part1). It's quite impressive 
how much redundant code gets removed when we can rely on a string to 
never change.

The branch does a good job of hiding the details from the user too. That 
is, for the most part HLL code can run ahead as usual, and never need to 
know that a given string operation has changed a register/variable to 
point to a different string header.

A few comments:

* Since opcodes like 'downcase' are changing to return a value rather 
than modifying one, then the opcodes should be clear about it. That is, 
deprecate the one argument:

   downcase $S0

In favor of the two argument:

   $S1 = downcase $S0

* We've lost the semantic difference between 'set' and 'assign' for 
strings, they're now identical. This isn't necessarily a bad thing, but 
it seems sensible to deprecate 'assign' for strings and keep it only for 
PMCs where it's meaningful.

* In src/string/encoding/fixed_8.c there's a stray FIXME in the wrong 
comment style (// instead of /* */) which I'm guessing someone intends 
to remove before merging.

* In src/string/api.c, does 'string_capacity' really make sense anymore 
as a function? Shouldn't the capacity of an immutable string always be 
the same as the length of the string? Possibly another deprecation item.

* Can Parrot_str_append be changed to just call Parrot_str_concat? Or, 
perhaps deprecate append, since they notionally do the same thing now.

* It sure removes a lot of cruft from substr to have it only return a 
new value.

* Another stray wrong-style comment in src/string/api.c, in 
Parrot_str_pin on a call to Parrot_str_write_COW. Again on another call 
to the same function later in the same file.

* A bit of complexity added to join, but unavoidable with immutable 
strings. At least it avoids generating a pile of temporary strings by 
allocating enough memory to hold the whole result at once.


Three cheers for unconventional thinking, finding speed gains and code 
simplifications in unexpected places.

Allison