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Experimental wasm linker

zwld Experimental linker for wasm object files. The idea is to implement a linker that stays close to wasm-ld in regards to features so that one day t
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Category: JavaScript / Bundlers
Watchers: 3
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Last update: Sep 20, 2021
Resource links
https://github.com/Luukdegram/zwld

zwld

Experimental linker for wasm object files. The idea is to implement a linker that stays close to wasm-ld in regards to features so that one day this could potentially be used within the Zig self-hosted compiler to incrementally link Zig code with other wasm object files.

While there's no official specification for linking, zwld follows the wasm tool-convention closely. The initial goal is to support mvp features and have a base skeleton which would provide us with enough information on how to integrate this within the Zig compiler. The first step is to make static linking work as specified by tool-convention, once that is completed, dynamic linking will be tackled.

Usage

zwld [-h] [--help] <FILE>
           --help           Display this help and exit.
       -h, --h              Display summaries of the headers of each section.
       -o, --output <STR>   Path to file to write output to.
       -s, --symbols        Display the symbol table

An example from dumping all headers of an object file:

zwld test/trivial.obj.wasm -h

test/trivial.obj.wasm:      file format wasm 0x1

Sections:

    Type start=0x0000000e end=0x0000001f (size=0x00000011) count: 4
  Import start=0x00000025 end=0x00000082 (size=0x0000005d) count: 4
    Func start=0x00000088 end=0x0000008b (size=0x00000003) count: 2
    Code start=0x00000091 end=0x000000b5 (size=0x00000024) count: 2
    Data start=0x000000bb end=0x000000ce (size=0x00000013) count: 1
  Custom start=0x000000d4 end=0x00000135 (size=0x00000061) "linking"
  Custom start=0x0000013b end=0x00000155 (size=0x0000001a) "reloc.CODE"
9 Open More issues
Closed

Save indexes of entries of certain sections

The symbols inside the symbol table have a field index. This index represents the index of the object itself within a section. This means that if we have 2 imported functions, and 2 defined functions, the max function index is 3. Currently, we only save imports within the import section. This means that for a symbol with index 2, we would get an out-of-bounds panic. To combat this, we must save an index of entries inside sections (such as a function section) that is not only increased by the section entries themselves, but also by imports. Then, during the parsing of the symbol table, we can find the corresponding function of a symbol by matching the index.

This would need to be done for:

  • functions
  • memory
  • globals
  • tables

Another possibility is to count imports for that symbol type, and subtract that amount from the symbol index. This may be simpler, but the first option may provide us with more information during other processes. Time will tell what the final decision will be based on the usefulness of saving the index on the section entry itself.

opened Oct 8, 2021 by Luukdegram 3
Closed

Perform relocations

After merging sections, sections will be renumbered. This means we must perform relocations to overwrite old indexes (such as a function index) into their new respective index.

opened Sep 24, 2021 by Luukdegram 3
Closed

Merging of sections

  • [x] Merging of function sections (re-numbering functions)
  • [x] Merging of globals sections (re-numbering globals)
  • [ ] Merging of event sections (re-numbering events)
  • [x] Merging of table sections (re-numbering tables)
  • [x] Merging of data segments (re-positioning data)
opened Sep 27, 2021 by Luukdegram 2
Closed

Write output to file

Once we finished all steps for linking the objects into the binary in memory, we must convert any objects into their binary representation according to the spec and write the binary form into the file that was originally specified by path in the CLI.

opened Sep 24, 2021 by Luukdegram 2
Closed

Improve memory usage by using indexes rather than pointers

Rather than saving pointers to types and objects, we define as little data as we can and only store what we truly need. For instance, by removing the pointers and unneeded data we can make Symbol 25 bytes rather than 40+. Using string interning, we could even reduce this further to 13 bytes! per Symbol. (Tho this doesn't account for actually storing the string, but since we almost never need this data, we can increase the performance). This does come with some costs:

  • Increased complexity (Whenever we update an index, any object pointing to that index needs to have its index updated).
  • More calculations (Due to the above example). I however expect this to not become a big problem as we find out better architectures to fit this design, which would reduce the calculations and complexity needed to solve the problem.

However, the bigger and more object files are used, the bigger the memory reduction we achieve using this new design. This also brings zwld further in line with how I currently designed the stage2 wasm linker, making it easier to upstream the rest of zwld into stage2 at one point.

To complete this work, the entire Symbol structure needs to be redefined (with as result, also the other types such as Import). We will tackle this per symbol 'kind':

  • [x] functions
  • [x] globals
  • [x] data
  • [x] section
  • [x] event
  • [x] table
opened Dec 14, 2021 by Luukdegram 1
Closed

Handle start section

Although we do not need to handle the merging of start sections (that would be a linking error), we may have to re-index the start section to point to the new index of a function.

For now, we're not going to verify if we're building an executable or library. We'll handle that once we merge into zld.

opened Sep 27, 2021 by Luukdegram 1
Open

Verify feature compatibility

Each wasm object file can contain a "features" section that describes which features are required or used. We must verify for each module that those match or else generate an appropriate error.

opened Sep 20, 2021 by Luukdegram 0
Open

Synthesizing functions to call constructors and perform other initialization

null

opened Sep 20, 2021 by Luukdegram 0

Contents

    fiatjaf Module Linker — Extension that adds <a> links to references in mod, use and extern crate statements at GitHub.
    rui314 mold: A Modern Linker
    ballercat :sparkles: WASM webpack loader
    yamafaktory A simple boilerplate to get WebAssembly (WASM) code generated by Rust and bundled by Webpack!
    aochagavia The Rocket game, now compiling to WASM
    rustwasm wasm-pack — 📦 ✨ pack up the wasm and publish it to npm!
    rustwasm wasm-bindgen — A project for facilitating high-level interactions between wasm modules and JS.
    jussiniinikoski WASM-PDF – Generates PDF files with JavaScript and WASM (WebAssembly)
    rustwasm wasm-tracing-allocator - A global allocator for Wasm that traces allocations and deallocations for debugging purposes
    cloudflare serde-wasm-bindgen - Native Serde adapter for wasm-bindgen
    yisar swc node binding use wasm
    mathetake An Experimental WASM Virtual Machine for Gophers
    jserv minimal dynamic linker implementation for ELF, supporting x86_64 and Arm/Aarch64
    twocity A light weight URI routing framework for Android.
    avantgardnerio The missing linker for the JVM.
    xoofx LibObjectFile is a .NET library to read, manipulate and write linker and executable object files (e.g ELF, DWARF, ar...)
    ibara CP/M and MS-DOS COM executable linker written in D.
    kubkon Preview Zig's incremental linker state in interactive HTML
    dfrankland 🗜 WebAssembly compiled Brotli library
    GoogleChromeLabs webm-wasm lets you create webm videos in JavaScript via WebAssembly.
    rhysd Vim editor embedded in your React web application
    Share this repo

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