5 ms·
Aliasing
- Bootvis 11mo agoThe whole series is excellent and as a non regular user of assembly I learned a ton.
- artemonster 11mo agoI wonder how much potential optimisation there is if we entirely drop pointer nonsense.
- aw1621107 11mo agoAre you talking about dropping pointers as a programmer-facing programming language concept (in which case you might find Hylo and similar languages interesting), or dropping pointers from everything - programming languages, their implementations, compilers, etc. (in which case I'm not sure that's even possible)?
- artemonster 11mo agoOnly the first one. Ofc under the hood they will stay, but I think its time to ditch random access model and pull fetching and concept of time closer to programmer
- uecker 11mo agoThis is basically what many functional programming languages do. This always came with plausibly sounding claims that this allows so much better optimizations that this soon will surpass imperative programs in performance, but this never materialized (it still did not - even though Rust fans now adopted this claim, it still isn't quite true). Also control over explicit memory layout is still more important.
- aw1621107 11mo agoGah, can't believe I forgot about functional programming languages here :( > even though Rust fans now adopted this claim Did they? Rust's references seem pretty pointer-like to me on the scale of "has pointers" to "pointers have been entirely removed from the language". (Obviously Rust has actual pointers as well, but since usefully using them requires unsafe I assume they're out of scope here)
- uecker 11mo agoWhat I meant is that Rust has stricter aliasing rules which make some optimization possible without extra annotations, but this is balanced out by many other issues.
- aw1621107 11mo agoSure, but I think the presence/absence of aliasing is different from what GP was wondering/asking about, which was the removal of pointers from the programmer-facing model.
- newpavlov 11mo agoFor a system programming language the right solution is to properly track aliasing information in the type system as done in Rust. Aliasing issues is just yet another instance of C/C++ inferiority holding the industry back. C could've learnt from Fortran, but we ended up with the language we have...
- cv5005 11mo agoFor systems programming the correct way is to have explicit annotations so you can tell the compiler things like: void foo(void *a, void *b, int n) { assume_aligned(a, 16); assume_stride(a, 16); assume_distinct(a, b); ... go and vectorize! }
- newpavlov 11mo agoLOL, nope. Those annotations must be part of the type system (e.g. `&mut T` in Rust) and must be checked by the compiler (the borrow checker). The language can provide escape hatches like `unsafe`, but they should be rarely used. Without it you get a fragile footgunny mess. Just look at the utter failure of `restrict`. It was so rarely used in C that it took several years of constant nagging from Rust developers to iron out various bugs in compilers caused by it.
- aw1621107 11mo agoDoes make me wonder what restrict-related bugs will be (have been?) uncovered in GCC, if any. Or whether the GCC devs saw what LLVM went through and decided to try to address any issues preemptively.
- adev_ 11mo agoAliasing is no joke and currently the only reason why some arithmetic intensive code-bases still prefer Fortran even nowadays. While it is possible to remove most aliasing performance issues in a C or C++ codebase, it is a pain to do it properly.
- bregma 11mo agoAliasing can be a problem in Fortran too. Decades ago I was a Fortran developer and encountered a very odd bug in which the wrong values were being calculated. After a lot of investigation I tracked it down to a subroutine call in which a hard-coded zero was being passed as an argument. It turned out that in the body of that subroutine the value 4 was being assigned to that parameter for some reason. The side effect was that the value of zero because 4 for the rest of the program execution because Fortran aliases all parameters since it passes by descriptor (or at least DEC FORTRAN IV did so on RSX/11). As you can imagine, hilarity ensued.
- pklausler 11mo agoHow does this bug concern aliasing?
- turol 11mo agoFor a real world example of how this can affect code check out this commit I made in mesa: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20877/diffs?commit_id=4b8dfaae89eedd54f7f9881adc8712d99ff30a60 https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20...
- Ono-Sendai 11mo agoWhen you have done enough C++ you don't need to fire up compiler explorer, you just use local variables to avoid aliasing pessimisations. I also wrote about this a while ago: https://forwardscattering.org/post/51 https://forwardscattering.org/post/51
- dataflow 11mo agoI think this might not be a shortcoming of MSVC but rather a deliberate design decision. It seems likely that MSVC is failing to apply strict aliasing, but that it's deliberately avoiding it, probably for compatibility reasons with code that wasn't/isn't written to spec. And frankly it can be very onerous to write code that is 100% correct per the standard when dealing with e.g. memory-mapped files; I'm struggling to recall seeing a single case of this.
- andrepd 11mo agoThank you Rust for having aliasing guarantees on references!