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!
Oh boy. What a mess of a change.
This demonstrates some significant issues we have with Symbol. I had
originally modelled the system a bit after Rustc's, but deviated in certain
regards:
1. This has a confurable base type to enable better packing without bit
twiddling and potentially unsafe tricks I'd rather avoid unless
necessary; and
2. The lifetime is not static, and there is no global, singleton interner;
and
3. I pass around references to a Symbol rather than passing around an
index into an interner.
For #3---this is done because there's no singleton interner and therefore
resolving a symbol requires a direct reference to an available interner. It
also wasn't clear to me (and still isn't, in fact) whether more than one
interner may be used for different contexts.
But, that doesn't preclude removing lifetimes and just passing around
indexes; in fact, I plan to do this in the frontend where the parser and
such will have direct interner access and can therefore just look up based
on a symbol index. We could reserve references for situations where
exposing an interner would be undesirable.
Anyway, more to come...
This will be used for the next commit, but this change has been isolated
both because it distracts from the implementation change in the next commit,
and because it cleans up the code by removing the need for a type parameter
on `AsgError`.
Note that the sort test cases now use `unwrap` instead of having
`{,Sortable}AsgError` support one or the other---this is because that does
not currently happen in practice, and there is not supposed to be a
hierarchy; they are siblings (though perhaps their name may imply otherwise).
We want to be able to build a representation of the dependency graph so
we can easily inspect it.
We do not want to make GraphML by default. It is better to use a tool.
We use "petgraph-graphml".
This is a union (sum type) of three other errors types, plus errors specific
to this builder.
This commit does a good job demonstrating the boilerplate, as well as a need
for additional context (in the case of `IdentKindError`), that we'll want to
work on abstracting away.
This flips the API from using XmloWriter as the context to using Asg and
consuming anything that can produce XmloResults. This not only makes more
sense, but avoids having to create a trait for XmloReader, and simplifies
the trait bounds we have to concern ourselves with.
This abstracts away the canonicalizer and solves the problem whereby
canonicalization was not being performed prior to recording whether a path
has been visited. This ensures that multiple relative paths to the same
file will be properly recognized as visited.
This will be entirely replaced in an upcoming commit. See that for
details. I don't feel like dealing with the conflicts for rearranging and
squashing these commits.
This also includes an implementation to visit paths only once. Note that it
does not yet canonicalize the path before visiting, so relative paths to the
same file can slip through, and relative paths to _different_ files could be
erroneously considered to have been visited.
This will be fixed in an upcoming commit.
This serves as a constructor for the time being, decoupling from POC. We
may do something better once we have a better idea of how the various
abstractions around this will evolve.
This undoes work I did earlier today...but now we'll be able to support a
Source on an extern.
There is duplicate code between `BaseAsg::declare{,_extern}` that will be
resolved in an upcoming commit. Upcoming commits will also simplify
terminology and clean up methods on ObjectState.
There is some duplication here with `declare` that will be cleared up in a
following commit. Reintroducing this method is necessary so that Source can
be used to represent the source location of the extern itself; it's
currently None to indicate an extern in `declare`.
This properly verifies extern types, and cleans up Asg's API a little so
that externs aren't handled much differently than other declarations.
With that said, after making src optional, I realized that we will indeed
want source information for externs themselves so we can direct the user to
what package is expecting that symbol (as the old linker does). So this
approach will not work, and I'll have to undo some of those changes.
This is essential to clarify what exactly the different object types
represent with the new generic abstractions. For example, we will have
expressions as an object type.
There's a lot here to make the object stored on the `Asg` generic. This
introduces `ObjectState` for state transitions and `ObjectData` for pure
data retrieval. This will allow not only for mocking, but will be useful to
enforce compile-time restrictions on the type of objects expected by the
linker vs. the compiler (e.g. the linker will not have expressions).
This commit intentionally leaves the corresponding tests in their original
location to prove that the functionality has not changed; they'll be moved
in a future commit.
This also leaves the names as "Object" to reduce the number the cognative
overhead of this commit. It will be renamed to something like "IdentObject"
in the near future to clarify the intent of the current object type and to
open the way for expressions and a type that marries both of them in the
future.
Once all of this is done, we'll finally be able to make changes to the
compatibility logic in state transitions to implement extern compatibility
checks during resolution.
DEV-7087
Create a trait that sorts a graph into `Sections` that can then be used
as an IR. The `BaseAsg` should implement the trait using what was
originally in the POC.
If we cannot set a fragment, we need to display the error to the user.
We are currently ignoring "___head", "___tail", and objects that are
both virtual and overridden. Those will be corrected in with future
changes.
We want to add an option to set the output file to the linker so we do
not need to redirect output to awk any longer.
This also adds integration tests for tameld.
This begins to introduce the ASG, backed by Petgraph. The API will continue
to evolve, and Petgraph will likely be encapsulated so that our
implementation can vary independently from it (or even remove it in the
future).
This ordering will simplify streaming processing of xmlo files in
TAMER. Specifically, we know that symbols will have been declared by the
time dependencies are added to the graph (and so we should only be creating
edges to existing nodes); and we can halt reading as soon as the closing
fragments tag is encountered, avoiding parsing the entirety of these massive
XML files.
On one particularly large program, this cuts time down from ~0.333s to
~0.300 in the POC linker.
This is missing two key things that I'll add shortly: a HashMap-based one
for use in the ASG for node mapping, and an entry-based system for
manipulations.
This has been a nice start for exploring various aspects of Rust
development, as well as conventions that I'd like to implement. In
particular:
- Robust documentation intended to guide people through learning the
necessary material about the compiler, as well as related work to
rationalize design decisions;
- Benchmarks;
- TDD;
- And just getting used to Rust in general.
I've beat this one to death, so I'll commit this and make smaller changes
going forward to show how easily it can evolve.
(This module was originally named `intern` but this commit and those that
follow rewrote it to `sym`.)
This makes use of Petgraph for representing the dependency graph and uses a
separate data structure for both string interning and indexing by symbol
name.
Mike Gerwitz
Joseph Frazer