111 lines
4.6 KiB
Rust
111 lines
4.6 KiB
Rust
// TAME linker library
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//
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// Copyright (C) 2014-2019 Ryan Specialty Group, LLC.
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//
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// This program is free software: you can redistribute it and/or modify
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// it under the terms of the GNU General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program. If not, see <http://www.gnu.org/licenses/>.
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//! Combine [object files](crate::obj) into a final executable.
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//!
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//! It's user-facing binary is [`tameld`](../../tameld).
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//!
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//!
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//! Background Information
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//! ======================
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//! A [linker][] is responsible for combining individually compiled
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//! [object files](crate::obj) containing relocatable code into a final
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//! executable.
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//! This involves putting the compiled code fragments into the right order
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//! and into the right place within the executable.
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//!
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//! [linker]: https://en.wikipedia.org/wiki/Linker_(computing)
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//!
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//! _See below for more information on why this linker currently produces
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//! another intermediate format (`xmle`) rather than a final executable._
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//!
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//! The type of relocatable code depends on the _target_.
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//! Currently, the only target is JavaScript.
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//!
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//!
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//! Backwards-Compatibility With XSLT System
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//! -------------------------------------
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//! This linker is part of the TAMER (TAME in Rust) project,
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//! which aims to incrementally rewrite TAME in Rust.
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//! Consequently, it must be able to serve as a drop-in replacement for the
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//! existing (XSLT) linker,
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//! which takes as input `xmlo` files and produces as output an `xmle`
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//! file.
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//! *This is not efficient*,
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//! and future versions will begin to migrate away from this strategy.
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//!
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//! The output `xmle` file can then be fed to a `standalone` command which
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//! extracts the JavaScript fragment and places it into its own file.
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//! Even when that is replaced
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//! (when this just outputs a final JS file directly),
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//! the `xmle` file is still needed for other purposes,
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//! such as `summary` and `dote` generation.
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//! Those too will eventually be linker targets.
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//!
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//!
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//! Linking Process
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//! ===============
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//! The linker works in the following steps:
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//!
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//! 1. [Object files](crate::obj) are recursively read.
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//! They are used in a streaming manner for the next step of the
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//! process;
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//! they do not persist in memory.
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//! Only the required portions of the file are loaded.
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//! See the [Legacy IR](crate::ir::legacyir) for more information.
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//!
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//! 2. This information is used to populate the [ASG].
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//! Information is added to the graph as it is discovered during object
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//! file loading,
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//! so the graph initially contains edges to missing identifiers.
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//! Expressions are _not_ added to the graph,
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//! as they are not needed for linking.
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//! Once all data are loaded,
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//! the ASG contains relocatable code fragments for each identifier.
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//!
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//! 3. The ASG is [sorted topologically][topo-sort] so that dependencies
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//! will be written to the executable file before identifiers that
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//! depend on them.
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//! Roots for the sort are specified by the return map.
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//! _Identifiers that are not accessable from one of those roots will be
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//! omitted from the executable output._
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//!
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//! 4. Relocatable code fragments are output into various sections in the
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//! executable file.
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//! This output file is currently `xmle`.
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//! (**TODO**: Link to new `xmle` crate.)
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//!
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//! [ASG]: crate::ir::asg
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//! [topo-sort]: https://en.wikipedia.org/wiki/Topological_sorting
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//!
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//! Steps 1 and 2 are performed at the same time:
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//! object files are used to immediately populate the [ASG][].
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//! Since the ASG contains only partial information,
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//! it must perform other validations (such as extern resolution) during
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//! this process;
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//! see [crate::ir::asg] for more information.
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//!
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//! Because the topological sort only considered explicitly defined roots,
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//! identifiers are only included in the final executable if they are
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//! either a root or are a dependency of a root.
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//! This makes it possible to create large reusable packages without
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//! incurring a runtime cost for unused objects,
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//! which is especially important since templates may expand into many
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//! identifiers.
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pub mod poc;
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