RSG (Ryan Specialty Group) recently announced a rename to Ryan Specialty (no
"Group"), but I'm not sure if the legal name has been changed yet or not, so
I'll wait on that.
This is an initial working concept for the reader which handles, so far,
just a single attribute. But extending it to completion will not be all
that much more work.
This does not have namespace support---that will be added later as part of
XIRT, which is responsible for semantic analysis. This allows XIR to stay
wonderfully simple, and won't have any impact on the writer (which expects
that QNames are unresolved and contain the namespace prefix to be written).
This is the safe version of the existing intern_utf8_unchecked, and exists
as a performance optimization.
We're about to introduce a XIR reader, which is going to intern a _lot_ of
duplicate strings, since it will intern node and attribute names as
well. Given that, we do not want to spent a lot of time performing UTF-8
checks that have already been performed.
We know that, if an intern is in the pool, it's either already UTF-8 or that
check was bypassed when it was initially interned. Therefore, if we find an
existing symbol, that can be returned without having to perform any
check. Otherwise, we intern as we usually would after attempting to convert
the byte slice into a string.
This allows us to continue to have good performance for interning without
sacrificing safety for strings.
This removes `SymbolStr` in favor of, simply, `&'static str`.
The abstraction provided no additional safety since the slice was trivially
extracted (and commonly, in practice), and was inconvenient to work with.
This is part of a process of relaxing lookups so that symbols can be
conveniently displayed in errors; rather than trying to prevent the
developer from doing something bad, we'll just rely on conventions, hope
that it doesn't happen, and if it does, address it either at that time or
when it shows up in the profiler.
This had the writing on the wall all the same as the `'i` interner lifetime
that came before it. It was too much of a maintenance burden trying to
accommodate both 16-bit and 32-bit symbols generically.
There is a situation where we do still want 16-bit symbols---the
`Span`. Therefore, I have left generic support for symbol sizes, as well as
the different global interners, but `SymbolId` now defaults to 32-bit, as
does `Asg`. Further, the size parameter has been removed from the rest of
the code, with the exception of `Span`.
This cleans things up quite a bit, and is much nicer to work with. If we
want 16-bit symbols in the future for packing to increase CPU cache
performance, we can handle that situation then in that specific case; it's a
premature optimization that's not at all worth the effort here.
Fragments' text were unescaped on reading, producing an owned String and
spending time parsing the text to unescape. We were then copying that into
an internement pool (so, copying twice, effectively).
Further, we were then _re-escaping_ on write.
This was all wasteful, since we do not do any manipulation of the fragment
before outputting to the xmle file; we know that Saxon produced properly
escaped XML to begin with, and can trust to propagate it.
This also introduces a new global `clone_uninterned_utf8_unchecked` method.
In profiling this change, I tested (a) before this change, (b) after writing
without escaping, and (c) after both reading escaped and writing without
escaping.
(a) (b) (c)
sec mem (B) sec B sec B
0:00.95 47896 -> 0:00.91 47988 -> 0:00.87 48288
0:00.40 30176 -> 0:00.37 25656 -> 0:00.36 25788
0:00.39 45672 -> 0:00.37 45756 -> 0:00.35 34952
0:00.39 20716 -> 0:00.38 19604 -> 0:00.36 19956
0:00.33 16836 -> 0:00.32 16988 -> 0:00.31 16892
0:00.23 15268 -> 0:00.23 15236 -> 0:00.22 15312
0:00.44 20780 -> 0:00.44 20048 -> 0:00.41 20148
0:00.54 44516 -> 0:00.50 36964 -> 0:00.49 36728
0:00.62 55976 -> 0:00.57 46204 -> 0:00.54 41468
0:00.31 28016 -> 0:00.30 27308 -> 0:00.28 23844
0:00.23 15388 -> 0:00.22 15316 -> 0:00.21 15304
0:00.05 4888 -> 0:00.05 4760 -> 0:00.05 4948
0:00.41 19756 -> 0:00.41 19852 -> 0:00.40 19992
0:00.47 20828 -> 0:00.46 20844 -> 0:00.44 20968
0:00.27 18152 -> 0:00.26 18184 -> 0:00.25 18312
Interestingly, the peak memory usage increases very slightly between the
second and third steps (though decreases from the first), likely because the
raw (encoded) is larger than the unencoded text (e.g. `>` takes more
space than `>`).
This adds support for uninterned symbols. This came about as I was creating
Xir (not yet committed) where I had to decide if I wanted `SymbolId` for all
values, even though some values (e.g. large text blocks like compiled code
fragments for xmle files) will never be compared, and so would be wastefull
hashed.
Previous IRs used `String`, but that was clumsy; see documentation in this
commit for rationale.
SymboldIds must only be constructed by interners, otherwise we lose
confidence in the type.
This offers an associated function to construct raw SymbolIds from integers
for testing purposes.
This is a major change, and I apologize for it all being in one commit. I
had wanted to break it up, but doing so would have required a significant
amount of temporary work that was not worth doing while I'm the only one
working on this project at the moment.
This accomplishes a number of important things, now that I'm preparing to
write the first compiler frontend for TAMER:
1. `Symbol` has been removed; `SymbolId` is used in its place.
2. Consequently, symbols use 16 or 32 bits, rather than a 64-bit pointer.
3. Using symbols no longer requires dereferencing.
4. **Lifetimes no longer pollute the entire system! (`'i`)**
5. Two global interners are offered to produce `SymbolStr` with `'static`
lifetimes, simplfiying lifetime management and borrowing where strings
are still needed.
6. A nice API is provided for interning and lookups (e.g. "foo".intern())
which makes this look like a core feature of Rust.
Unfortunately, making this change required modifications to...virtually
everything. And that serves to emphasize why this change was needed:
_everything_ used symbols, and so there's no use in not providing globals.
I implemented this in a way that still provides for loose coupling through
Rust's trait system. Indeed, Rustc offers a global interner, and I decided
not to go that route initially because it wasn't clear to me that such a
thing was desirable. It didn't become apparent to me, in fact, until the
recent commit where I introduced `SymbolIndexSize` and saw how many things
had to be touched; the linker evolved so rapidly as I was trying to learn
Rust that I lost track of how bad it got.
Further, this shows how the design of the internment system was a bit
naive---I assumed certain requirements that never panned out. In
particular, everything using symbols stored `&'i Symbol<'i>`---that is, a
reference (usize) to an object containing an index (32-bit) and a string
slice (128-bit). So it was a reference to a pretty large value, which was
allocated in the arena alongside the interned string itself.
But, that was assuming that something would need both the symbol index _and_
a readily available string. That's not the case. In fact, it's pretty
clear that interning happens at the beginning of execution, that `SymbolId`
is all that's needed during processing (unless an error occurs; more on that
below); and it's not until _the very end_ that we need to retrieve interned
strings from the pool to write either to a file or to display to the
user. It was horribly wasteful!
So `SymbolId` solves the lifetime issue in itself for most systems, but it
still requires that an interner be available for anything that needs to
create or resolve symbols, which, as it turns out, is still a lot of
things. Therefore, I decided to implement them as thread-local static
variables, which is very similar to what Rustc does itself (Rustc's are
scoped). TAMER does not use threads, so the resulting `'static` lifetime
should be just fine for now. Eventually I'd like to implement `!Send` and
`!Sync`, though, to prevent references from escaping the thread (as noted in
the patch); I can't do that yet, since the feature has not yet been
stabalized.
In the end, this leaves us with a system that's much easier to use and
maintain; hopefully easier for newcomers to get into without having to deal
with so many complex lifetimes; and a nice API that makes it a pleasure to
work with symbols.
Admittedly, the `SymbolIndexSize` adds some complexity, and we'll see if I
end up regretting that down the line, but it exists for an important reason:
the `Span` and other structures that'll be introduced need to pack a lot of
data into 64 bits so they can be freely copied around to keep lifetimes
simple without wreaking havoc in other ways, but a 32-bit symbol size needed
by the linker is too large for that. (Actually, the linker doesn't yet need
32 bits for our systems, but it's going to in the somewhat near future
unless we optimize away a bunch of symbols...but I'd really rather not have
the linker hit a limit that requires a lot of code changes to resolve).
Rustc uses interned spans when they exceed 8 bytes, but I'd prefer to avoid
that for now. Most systems can just use on of the `PkgSymbolId` or
`ProgSymbolId` type aliases and not have to worry about it. Systems that
are actually shared between the compiler and the linker do, though, but it's
not like we don't already have a bunch of trait bounds.
Of course, as we implement link-time optimizations (LTO) in the future, it's
possible most things will need the size and I'll grow frustrated with that
and possibly revisit this. We shall see.
Anyway, this was exhausting...and...onward to the first frontend!
Mike Gerwitz