Wild – A fast linker for Linux
github.com
github.com
So the benefit will be barely noticable. As another comment points out, LTO should only be used when you need a binary optimized to within an inch of its life, such as a release copy, or a copy for performance testing.
And factual.
Specifically, if LTO is so important that you need to be using it during development, you likely have a very exceptional case, or you have some big architectural issues that are causing much larger performance regressions then they should be.
WebAssembly and lightweight MicroVMs are enabling FaaS with real time code generation but the build toolchain makes it less appealing, when you don't want it to take half a minute to build or to be slow.
Not necessarily. LTO does a very good job of dead code elimination which is sometimes necessary to fit code in microcontroller memory.
I spent a few months doing performance optimisation work. We wanted to see how much performance we could wring out of an algorithm & associated data structures. Each day I’d try and brainstorm new optimisations, implement them, and then A/B test the change to see how it actually affected performance. To get reliable tests, all benchmarks were run in release mode (with all optimisations - including LTO - turned on).
And for realease builds link time is hardly a concern.
Borland and Microsoft compilers have been dealing with them just fine since at very least 1994, when I started using Turbo C++ 3.1 for Windows.
For example, Windows runs virus scans on close(), which makes it very slow. This means that sometimes it makes sense to have one or more background threads exclusively dedicated to closing files.
There's a good talk on this at https://www.youtube.com/watch?v=qbKGw8MQ0i8
Microsoft built a very extensible I/O stack and prior to Defender/prior to SSDs it really wasn't very noticable... back when it was originally designed well through the 90s and early 00s.
Unfortunately it is now noticable despite being an otherwise smart design. Which means Windows and/or NTFS are blamed as being slow, neither of which has any basis in fact when we look at the overall design of Windows' subsystems/VMM in comparison to macOS/Linux.
It sucks. You've got great plumbing in Windows with a shit shell on top.
I agonize over the insanity of removing Defender protection on precisely the files that are most vulnerable on my computer each time I do it. But I do it anyway.
Interestingly, Android Studio offers to turn off Defender protection for development directories for you, and does so each time you load a project, if you haven't already done so. So I no longer feel like I'm alone in my insanity.
That doesn't extinguish the prior versions under the prior license, but it does allow a project to change its license.
This makes it sound more difficult than it actually is (logistically); it's not uncommon for major projects to require contributors to sign a CLA before accepting PRs.
"I will not expose people taping out Hazard3 to the possibility of your employer chasing you for your contribution by harassing them legally. A contribution agreement does not solve this, because you may sign it without the legal capability to do so (...)"
https://github.com/Wren6991/Hazard3/blob/stable/Contributing... (this is I believe the repo with design for riscv cores running on RPi Pico 2)
> Code licensed under the GNU Affero General Public License (AGPL) MUST NOT be used at Google.
Just using any Copyleft software has no legal consequences (copyleft licenses kick in when distributing, not using them).
This is not theoretical; it happens quite frequently. For toolchains, in particular I'm aware of how Apple (not that they're unique in this) has "blah blah open source" downloads, but often they do not actually correspond with the binaries. And not just "not fully reproducible but close" but "entirely new and incompatible features".
The ARM64 saga is a notable example, which went on for at least six months (at least Sept 2013 to March 2014). XCode 5 shipped with a closed-source compiler only for all that time.
What does that mean for a linker? If you ship a binary linked with an AGPL linker you need to offer the source of the linker? Or of the program being linked?
AGPL is avoided like the plague by big corps: same big corps are known for having money to pay for licenses and sometimes (yes, I look at you Amazon) being good at deriving value from FLOSS without giving back.
iirc AGPL was used so everyone can just use it, big biz is still compelled to buy a license. this has been done before and can be seen as one of the strategies to make money off FLOSS.
For those on macOS, Apple released a new linker about a year or two ago (which is why the mold author stopped working on their macOS version), and if you're using it with Rust, put this in your config.toml:
[target.aarch64-apple-darwin]
rustflags = [
"-C",
"link-arg=-fuse-ld=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld",
"-C",
"link-arg=-ld_new",
]I have the command line tools installed and I only have /usr/bin/ld and /usr/bin/ld-classic
External libs are most often linked dynamically these days, so they don't need to be built from source, so eliminating the linker doesn't pose a problem for non-open source dependencies. And if that's not enough letting the compiler also consume object files could provide for legacy use cases or edge cases where you must statically link to a binary.
> Developers sometimes experience trouble debugging the quarter-million line amalgamation source file because some debuggers are only able to handle source code line numbers less than 32,768 [...] To circumvent this limitation, the amalgamation is also available in a split form, consisting of files "sqlite3-1.c", "sqlite3-2.c", and so forth, where each file is less than 32,768 lines in length
That is just another reason to stick with signed ints unless there is a very specific behavior you rely on.
No they ain't:
julia> x = typemax(Int16)
32767
julia> x + Int16(1) > x
false
(As an aside: I'm personally fond of languages that let you specify what to do if an integer arithmetic result doesn't fit. Zig, for example, has separate operators for rollover v. saturation v. panicking/UB, which is handy. Pretty sure C++ has equivalents in its standard library.)
If you simply compile and run, the debugger won't have __LINE__, no. But it also won't have line numbers, at all. So you might have missed a bit of context to this discussion - how are line numbers implemented in a debugger that does so, without access to the source?
The __LINE__ macro is passed to the debugger only if the program itself outputs its value, and the "debugger" is a human reading that output :)
Apologies for the typo. And now it is too late to edit the post.
Even modest statically linked rust binaries can take a couple of minutes in the link stage of compilation in release mode (using mold). It’s not a rust-specific issue but an amalgam of (usually) strictly static linking, advanced link-time optimizations enabled by llvm like LTO and bolt, and a general dissatisfaction with compile times in the rust community. Rust’s (clinically) strong relationship with(read: dependency on) LLVM makes it the most popular language where LLVM link-time magic has been most heavily universally adopted; you could face these issues with C++ but it wouldn’t be chalked up to the language rather than your toolchain.
I’ve been eyeing wild for some time as I’m excited by the promise of an optimizing incremental linker, but to be frank, see zero incentive to even fiddle with it until it can actually, you know, link incrementally.
https://doc.rust-lang.org/core/arch/wasm32/index.html#simd
https://nickb.dev/blog/authoring-a-simd-enhanced-wasm-librar...
Could definitely be more effort than it is worth just to speed up compilation.
https://wasmtime.dev/ https://github.com/bytecodealliance/wasmtime
Additionally, the acceptance of binary libraries across the C and C++ ecosystem, means that more often than not, you only need to care about compiling you own application, and not the world, every time you clone a repo, or switch development branch.
And he also did not had cargo at his disposal.
No need to point out it is C instead, as they share common roots, including place of birth.
Or how we used to compile C++ between 1986 and 2000's, mostly in single core machines, developing games, GUIs and distributed computing applications in CORBA and DCOM.
2015(?): Lld a drop in replacement linker, at least 2x as fast as Gold
2021: mold, a new linker, several times faster than lld
2025: wild, a new linker...
The book Linkers and Loaders by John Levine.
Last book in the list here:
https://www.johnlevine.com/books.phtml
I had read it some years ago, and found it quite interesting.
It's a standard one in the field.
He has also written some other popular computer books (see link above - pun not intended, but noticed).
To use it with Rust, this can probbaly also work using gcc as linker driver.
In project's .cargo/config.toml:
[target.x86_64-unknown-linux-gnu]
rustflags = ["-C", "link-arg=-fuse-ld=wild"]
As for why Rust uses gcc or clang to invoke the linker rather than invoking the linker directly - it's because the C compiler knows what linker flags are needed on the current platform in order to link against libc and the C runtime. Things like `Scrt1.o`, `crti.o`, `crtbeginS.o`, `crtendS.o` and `crtn.o`.
May be it's worth filing a feature request for gcc to have parity with clang for arbitrary linkers?
Instead of renaming and passing -B in, you can also modify the GCC «spec» file's «%linker» section to make it point to a linker of your choice, i.e.
%linker:
/scratch/bin/wild %{wild_options}
It is possible to either modify the default «spec» file («gcc -dumpspecs») or pass your own along via «-specs=my-specs-file». I have found custom «spec» files to be very useful in the past.
The «spec» file format is documented at https://gcc.gnu.org/onlinedocs/gcc/Spec-Files.html
"Because there's low hanging concurrent fruit that Rust can help us get?" would be interesting but that's not explicitly stated or even implied.
But in general, I'd guess just different design decisions. As for how this might be related to Rust - I'm certain that were Wild ported from Rust to C or C++, that it would perform very similarly. However, code patterns that are fine in Rust due to the borrow checker, would be footguns in languages like C or C++, so maintaining that code could be tricky. Certainly when I've coded in C++ in the past, I've found myself coding more defensively, even at a small performance cost, whereas with Rust, I'm able to be a lot bolder because I know the compiler has got my back.
Perhaps it is worth repeating the experiment with heavy MLoC codebases. jmalloc or mimalloc.
... however...
> code patterns that are fine in Rust due to the borrow checker, would be footguns in languages like C or C++,
That "dig" is probably not true. Or rather, your very conflation of C and C++ suggests that you are talking about the kind of code which would not be used in modern C++ of the past decade-or-more. While one _can_ write footguns in C++ easily, one can also very easily choose not to do so - especially when writing a new project.
These risks are mostly, and often entirely, gone when you write modern C++. You don't lose track of them, because you don't track them, and you only use them when you don't need to track them. (Except for inside the implementations of a few data structures, which one can think of as the equivalent of unsafe code in Rust). Of course I'm generalizing here, but again, you just don't write C-style code, and you don't have those problems.
(You may have some other problems of course, C++ has many warts.)
Like, how do you make sure that you don't hold any dangling references to a vector that reallocated? How do you make sure that code that needs synchronization is synchronized? How do you make sure that non-thread safe code is never used from multiple threads? How do you make sure that you don't ever invalidate an iterator? How do you make sure that you don't hold a reference to a data owned by unique pointer that went out of scope? How do you make sure you don't hold a string view for a string that went out of scope?
As far as I know (and how I experienced it), the answer to all of those questions is to either use some special api that you have to know about, or do something non-optimal, like creating a defensive copy, use a shared pointer or adding "just in case" mutex, or "just remember you might cause problem a and be careful."
In Rust all of those problems are a compile error and you have to make an extra effort to trigger them at runtime with unsafe code. That's a very big difference and I don't understand how can modern C++ come even close to it.
So, I'll first nitpick and say that's not a problem with pointer tracking.
To answer the question, though:
When I'm writing a function which receives a reference to a vector, then - either it's a const reference, in which case I don't change it, or it's a non-const reference, in which case I can safely assume I'm allowed to change it - but I can't keep any references or pointers into it, or iterators from it etc. I also expect and rely on functions that I call with a non-const reference to that vector, to act the same.
And when I create a vector, I just rely on the above in functions I call.
This is not some gamble. It's how C++ code is written. Yes, you can write code which breaks that principle if you like, but - I don't, and library authors don't.
> How do you make sure that code that needs synchronization is synchronized?
You mean, synchronization between threads which work on the same data? There's no one answer for that. It depends. If you want to be super-safe, you don't let your threads know about another other than multithread-aware data structures, whose methods ensure synchronization. Like a concurrent queue or map or something. If it's something more performance-critical whether synchronization is too expensive, then you might work out when/where it's safe for the threads to work on the same data, and keep the synchronization to a minimum. Which is kind of like unsafe Rust, I imagine. But it's true that it's pretty easy to ignore synchronization and just "let it rip", and C++ will not warn you about doing that. Still, you won't enter that danger zone unless you've explicitly decided to do multithreaded work.
About the Rust side of things... isn't it Turing-complete to know whether, and when, threads need to synchronize? I'm guessing that safe Rust demands that you not share data which has unsynchronized access, between threads.
> the answer to all of those questions is to either use some special api that you have to know about
C++ language features and standard library facilities are a "special API" that you have to know about. But then, so are raw pointers. A novice C++ programming student might not even be taught about using them until late in their first programming course.
My main point was, that if you talk about "C/C++ progamming", then you will necessarily not use most of those language features and facilities - which are commonly used in modern code and can keep you safe. You would be writing C-like code, and will have to be very careful (or reinvent the wheel, creating such mechanisms yourself).
I think it'd be better if you first try understand what actually Rust does here, for which I usually recommend this talk for C ++ developers, which describes the most important ideas on snippets of C++ and Rust side by side: https://youtu.be/IPmRDS0OSxM
That's probably my favourite demonstration.
This thread started because you essentially denied these constructs have any significance, as you lumped the two languages together. You are still overstating your point.
Moreover - Rust has different design goals than any of these two languages. Indeed, neither of them guarantees memory safety at the language level; Rust makes different tradeoffs, paid a certain price, and does guarantee it. I will watch that video though.
https://news.ycombinator.com/item?id=33330499
NB. This is not to suggest wild is bloated. The issue if any is the software being developed with it and the computers of those who might use such software.
(Personally, upgrading my laptop to 64GB at the expense of literally everything else was almost a great decision. Almost, because I really should have splurged on RAM and display instead of going all-in on RAM. The only downside is that cleaning up open tabs once a week became a chore, taking up the whole evening.)
On modern large hardware architecture, for executable files/dynamic libraries, ELF(PE[+]) has overkill complexity.
I am personnally using a executable file format of my own I do wrap into an "ELF capsule" on linux kernel. With position independent code, you kind of only need memory mapped segments (which dynamic libraries are in this very format). I have two very simple partial linkers I wrote in plain and simple C, one for risc-v assembly, one for x86_64 assembly, which allow me to link into such executable file some simple ELF object files (from binutils GAS).
There is no more centralized "ELF loader".
Of course, there are tradeoffs, 1 billion times worth it in regards of the accute simplicity of the format.
(I even have a little vm which allows me to interpret simple risc-v binaries on x86_64).
A good analogy to "feel" that, it is a bit like "json vs xml" but for executable binary formats.
But, I keep in mind, those formats (excrutiatingly simple) can work only on modern hardware architectures.
Compilers take the code the programmer writes, and turns it into things called object files. Object files are close to executable by the target processor, but not completely. There are little places where the code needs to be rewritten to handle access to subroutines, access operating system functionality, and other things.
A linker combines all these object files, does the necessary rewriting, and generates something that the operating system can use.
It's the final step in building an executable.
--
More complicatedly: a linker is a little Turing machine that runs over the object files. Some can do complicated things like rewriting code, or optimizing across function calls. But, fundamentally, they plop all the object files together and follow little scripts (or rewrites) that clean up the places the compiler couldn't properly insert instructions because the compiler doesn't know the final layout of the program.
Symbols would be resolved based on an index where only updated object files are reindexed. It could also eagerly relocate in the background, in order depending on previous usage data.
This would basically make a copyless lazy incremental linker.
Not exactly this, but Google's Propeller fixes up ("relinks") Basic Blocks (hot code as traced from PGO) in native code at runtime (like an optimizing JIT compiler would): https://research.google/pubs/propeller-a-profile-guided-reli...
By the time you have addressed these, you'll find yourself building a microkernel system with a collection of independent servers and well-defined interaction protocols. Which isn't necessarily a terrible way to assemble something, but it's not quite where you're trying to go...
I thought a lot of its proofs were done at compile time not link time.
Rust is perfectly happy to emit/use dynamic links.[0] It's just that the primary C use case (distributing and updating the main app and its libraries separately) ends up being unsafe since Rust's ABI is unstable (so compiler versions, libraries, etc must match exactly).
Avoiding static relinking during development is pretty much the use where it does work. In fact, Bevy recommends this as part of its setup guide![1]
Practice paints a slightly less rosy picture, though; since the feature is exercised quite rarely, not all libraries work well with it in practice.[2]
[0]: https://doc.rust-lang.org/reference/linkage.html#r-link.dyli...
[1]: https://bevyengine.org/learn/quick-start/getting-started/set...
[2]: For example, https://github.com/linebender/bevy_vello/issues/84
Can someone explain what is so special about Rust for this?
https://doc.rust-lang.org/book/ch16-00-concurrency.html
So the logic would go:
1. mold doesn't do incremental linking because it is too complex to do it while still being fast (concurrent).
2. Rust makes it possible to write very complex fast (concurrent) programs.
3. A new linker written in Rust can do incremental linking while still being fast (concurrent).
EDIT: I meant this originally, but comments were posted before I added it so I want to be clear that this part is new: (Any of those three could be false; I take no strong position on that. But I believe that this is the motivating logic.)
Also see various arena allocator crates, etc.
And if you intentionally leak everything it is onerous to get the borrow checker to realize that unless you use a leaked box for all declaration/allocations, which introduces both friction and performance regressions (due to memory access patterns) because the use of custom allocators doesn’t factor into lifetime analysis.
(Spoken as a die-hard rust dev that still thinks it’s the better language than zig for most everything.)
ManuallyDrop would presumably be implemented on large data structures where it matters, not on every single type involved in the program.
You could, for example, take advantage of bump arena allocator in rust which would allow the linker to have just 1 alloc/dealloc. Mold is still using more traditional allocators under the covers which won't be as fast as a bump allocator. (Nothing would stop mold from doing the same).
Really it's allocators in general. Allocations are perceived as expensive only because they are mostly dependent on the amortized cost of prior deallocations. As an extreme example, even GCs can be fast if you avoid deallocation because most typically have a long-lived object heap that rarely gets collected - so if you keep things around that can be long-lived (pooling) their cost mostly goes away.
Allocation is perceived as slow because it is. Getting memory from the OS is somewhat expensive because a page of memory needs to be allocated and stored off. Getting memory from traditional allocators is expensive because freespace needs to be tracked. When you say "I need 5 bytes" the allocator needs to find 5 free bytes to give back to you.
Bump allocators are fast because the operation of "I need 5 bytes" is incrementing the allocation pointer forward by 5 bytes and maybe doing a new page allocation if that's exhausted.
GC allocators are fast because they are generally bump allocators! The only difference is that when exhaustion happens the GC says "I need to run a GC".
Traditional allocators are a bit slower because they are typically something like an arena with skiplists used to find free space. When you free up memory, that skiplist needs to be updated.
But further, unlike bump and GC allocators another fault of traditional allocators is they have a tendency to scatter memory which has a negative impact on CPU cache performance. With the assumption that related memory tends to be allocated at the same time, GCs and bump allocators will colocate memory. But, because of the skiplist, traditional allocators will scattershot allocations to avoid free memory fragmentation.
All this said, for most apps this doesn't matter a whole lot. However, if you are doing a CPU/memory intense operation then this is stuff to know.
If you've never deallocated then there is no free space to track, hence the cost of allocation is _mostly_ (per my above comment) affected by the amortized cost of deallocations. The cost becomes extremely apparent with GCs because a failed allocation is usually what triggers a collection (and subsequently any deallocations that need to happen).
Still, your comment goes into detail that I probably should have.
Also, if wild is indeed faster than mold even without incrementalism, as the benchmarks show, then it seems quite silly to go around making the argument that it's harder to write a fast linker in Rust. It's apparently not that hard.
1. Rust's well designed type system and borrow checker makes writing code that works just easier. It has the "if it compiles it works" property (not unique to Rust; people say this about e.g. Haskell too).
2. Rust's type system - especially its trait system can be used to enforce safety constraints statically. The obvious one is the Send and Sync traits for thread safety, but there are others, e.g. the Fuchsia network code statically guarantees deadlocks are impossible.
Mold is written in C++ which is extremely error prone in comparison.
By the way I had to lookup what incremental linking is, in practice I think it means that code from libraries and modules that have not changed won’t need to be re-packed each time which ch will save time for frequent development builds, it’s actually ingenious
Linking is often a very notable bottleneck for debug binaries and mold can make a big difference.
So interest in speeding up linking for Rust is expected.