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Is Rust faster than C?

steveklabnik.com

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Re: Is Rust faster than C?

#101

Post by steveklabnik »

kouteiheika wrote: post #97
OptionOfT wrote: post #88

Earlier quoted context omitted.

Apart from multi threading, there is more information in the Rust type system. Would that would allow more optimizations?

Yes. All `&mut` references in Rust are equivalent to C's `restrict` qualified pointers. In the past I measured a ~15% real world performance improvement in one of my projects due to this (rustc has/had a flag where you can turn this on/off; it was disabled by default for quite some time due to codegen bugs in LLVM).

Not just all &mut T, but also all &T, where the T does not transitively contain an UnsafeCell. Click "show llvm ir" instead of "build" here: https://play.rust-lang.org/?version=stable&mode=release&edit...

Re: Is Rust faster than C?

#102

Post by adgjlsfhk1 »

Tuna-Fish wrote: post #85
gignico wrote: post #2

The article does not mention the possible additional optimisation opportunities that arise in Rust code due to stricter aliasing rules of references. But I don’t have an example in mind. Does anyone know of an example of it happening in real code?

I believe this advantage is currently mostly theoretical, as the code ultimately gets compiled with LLVM which does not fully utilize all the additional optimization opportunities.

LLVM doesn't fully utilize all the power, but it does use an increasing amount every year. Flang and Rust have both given LLVM plenty of example code and a fair number of contributors who want to make LLVM work better for them.

Re: Is Rust faster than C?

#103

Post by farnulfo »

Is there a common pattern for "Is language X faster than language Y" ? Like what is your definition of faster : faster to developer, to start, to execute, to handle different workload with the same binaries (like JIT).

Re: Is Rust faster than C?

#104

Post by JacoboJacobi »

jonstewart wrote: post #37

“It’s the memory, stupid!” So wrote Richard Sites, lead designer of the famous DEC Alpha chip, in 1996 ( http://cva.stanford.edu/classes/cs99s/papers/architects_look... ). It’s rung true for 30 years. Where C application code often suffers, but by no means always, is the use of memory for data structures. A nice big chunk of static memory will make a function fast, but I’ve seen many C routines malloc memory, do a st…

I'd certainly agree that malloc is the Achilles heel of any real world C. Overall though C++ was not a particularly good solution to memory efficiency since having OO available made the situation look like a fast sprint to the cake shop.

Re: Is Rust faster than C?

#106

Post by HarHarVeryFunny »

effnorwood wrote: post #74

It depends on a lot. If you need to go fast, look to assembly.

As I just posted, any speed comparison needs to be based on specific implementations (compiler A vs compiler B), not languages.

When it comes to assembly, the "compiler" is the person writing the code, and while assembly gives you the maximum flexibility to potentially equal or outperform any compiler for any language, there are not too many people with the skill to do that, especially when writing large programs (which due to the effort required are rarely written in assembler). In general there is much more potential for improving the speed of programs by changing the design and using better algorithms, which is where high level languages offer a big benefit by making this easier.

Re: Is Rust faster than C?

#108

Post by torginus »

Well, if we define a reasonable yardstick for this, and try to write idiomatic code without low-level hacks and optimization, Rust has the potential to be faster because its more strict aliasing rules might enable optimizations that the C compiler wouldn't try.

However I remember reading a few years back that due to the Rust frontend not communicating these opportunities to LLVM, and LLVM not being designed to take advantage of them, the real-world gains do not always materialize.

Also sometimes people write code in Rust that does not compile under the borrow checker rules, and alleviate this issue either by cloning objects or using RefCell, both of which have a runtime cost.

Re: Is Rust faster than C?

#109

Post by m-schuetz »

pornel wrote: post #76

In short, the maximum possible speed is the same (+/- some nitpicks), but there can be significant differences in typical code, and it's hard to define what's a realistic typical example. The big one is multi-threading. In Rust, whether you use threads or not, all globals must be thread-safe, and the borrow checker requires memory access to be shared XOR mutable. When writing single-threaded code takes 90% of effort…

I'm still confused as to why linux requires linking against TBB for multithreading, thus breaking cmake configs without if(linux) for tbb. That stuff should be included by default without any effort by the developer.

Re: Is Rust faster than C?

#110

Post by groundzeros2015 »

pornel wrote: post #76

In short, the maximum possible speed is the same (+/- some nitpicks), but there can be significant differences in typical code, and it's hard to define what's a realistic typical example. The big one is multi-threading. In Rust, whether you use threads or not, all globals must be thread-safe, and the borrow checker requires memory access to be shared XOR mutable. When writing single-threaded code takes 90% of effort…

Multithreading does not make code more efficient. It still takes the same amount of work and power (slightly more).

On a backend system where you already have multiple processes using various cores (databases, web servers, etc) it usually doesn’t make sense as a performance tool.

And on an embedded device you want to save power so it also rarely makes sense.

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