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In my humble opinion, Rust should not be faster than C; C is the closest we can get to the bare-metal without writing assembly. In my opinion, in the discussio

by mfrw 4y ago

In my humble opinion, Rust should not be faster than C; C is the closest we can get to the bare-metal without writing assembly. In my opinion, in the discussion after this talk folks did mention some pointers of why this was achieved. Although, for the same driver, there is a 2.6% drop in perf when used with 3 disks.

nextaccountic 4y ago
Rust could in some very specific cases be faster than C (assuming all other things equal) for the same reason of Fortran: it can disallow aliasing so the compiler could theoretically exploit this to generate faster code In practice it depends on LLVM
- mfrw 4y ago
  Off to Google -- `What is aliasing?` Thanks for the pointer :)
  - nextaccountic 4y ago
    When two pointers point to the same place they alias each other (it's alias as in two names for the same thing, but it's a verb as well as a noun) Rust's &mut borrow is like a pointer guaranteed to be non-null but also guaranteed to never point to memory that can be accessed by another borrow. Or in other words, &mut can't be aliased by other borrows. If you ever try to alias a &mut borrow, the program won't compile. C's restrict is kind of similar, but it's far less used than &mut. That's because if you violate aliasing restrictions of restrict, it won't generate a compiler error, but will cause UB! Anyway in Rust, a &mut has exclusive access to a piece of memory, and the compiler may theoretically make use of this fact in optimizations. However, LLVM isn't yet as good as it could be :( first, I think it doesn't implement yet the real good stuff that GCC has (I found this https://internals.rust-lang.org/t/fyi-ibm-is-working-on-llvm-support-for-arbitrary-aliasing-graphs-for-fortran/13278 https://internals.rust-lang.org/t/fyi-ibm-is-working-on-llvm... from 2020 but I don't know the state of it), and worse, what it currently has is very buggy, so sometimes the Rust compiler is forced to disable it :( So, you have this: https://stackoverflow.com/questions/57259126/why-does-the-rust-compiler-not-optimize-code-assuming-that-two-mutable-reference https://stackoverflow.com/questions/57259126/why-does-the-ru...
    - mfrw 4y ago
      Thank you for the elaborate explaination:)

by mfrw 4y ago

nextaccountic 4y ago

Rust could in some very specific cases be faster than C (assuming all other things equal) for the same reason of Fortran: it can disallow aliasing so the compiler could theoretically exploit this to generate faster code In practice it depends on LLVM

mfrw 4y ago
Off to Google -- `What is aliasing?` Thanks for the pointer :)
- nextaccountic 4y ago
  When two pointers point to the same place they alias each other (it's alias as in two names for the same thing, but it's a verb as well as a noun) Rust's &mut borrow is like a pointer guaranteed to be non-null but also guaranteed to never point to memory that can be accessed by another borrow. Or in other words, &mut can't be aliased by other borrows. If you ever try to alias a &mut borrow, the program won't compile. C's restrict is kind of similar, but it's far less used than &mut. That's because if you violate aliasing restrictions of restrict, it won't generate a compiler error, but will cause UB! Anyway in Rust, a &mut has exclusive access to a piece of memory, and the compiler may theoretically make use of this fact in optimizations. However, LLVM isn't yet as good as it could be :( first, I think it doesn't implement yet the real good stuff that GCC has (I found this https://internals.rust-lang.org/t/fyi-ibm-is-working-on-llvm-support-for-arbitrary-aliasing-graphs-for-fortran/13278 https://internals.rust-lang.org/t/fyi-ibm-is-working-on-llvm... from 2020 but I don't know the state of it), and worse, what it currently has is very buggy, so sometimes the Rust compiler is forced to disable it :( So, you have this: https://stackoverflow.com/questions/57259126/why-does-the-rust-compiler-not-optimize-code-assuming-that-two-mutable-reference https://stackoverflow.com/questions/57259126/why-does-the-ru...
  - mfrw 4y ago
    Thank you for the elaborate explaination:)