tame/tamer/src/ld.rs

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