[_flux]

I believe __restrict, and __builtin_prefetch/__builtin_assume are compiler extensions, not part of the C++ language as is, and different compilers implement (or don't) these differently.

The rust compiler actually has similar things, but they're not available in stable builds. I suppose there are some issues if principle why not to include them in stable. E.g: https://doc.rust-lang.org/std/intrinsics/fn.prefetch_read_da...

Maybe some time in the future good acceptable abstractions will be conceived for them.. Perhaps using just using nightly builds for HPC is not that far out, though.

[ameliaquining]

Rust already has __restrict; it is spelled &mut and is one of the most fundamental parts of the language. The key difference, of course, is that it's checked by the compiler, so is useful for correctness and not just performance. Also, for a long time it wasn't used for optimization, because the corresponding LLVM feature (noalias) was full of miscompilation bugs, because not that much attention was being paid to it, because hardly anyone actually uses restrict in C or __restrict in C++. But those days are finally over.

__builtin_assume is available on stable (though of course it's unsafe): https://doc.rust-lang.org/std/hint/fn.assert_unchecked.html

There's an open issue to stabilize the prefetch APIs: https://github.com/rust-lang/rust/issues/146941 As is usually the case when a minor standard-library feature remains unstable, the primary reason is that nobody has found the problem urgent enough to put in the required work to stabilize it. (There's an argument that this process is currently too inefficient, but that's a separate issue.) In the meantime, there are third-party libraries available that use inline assembly to offer this functionality, though this means they only support a couple of the most popular architectures.

[m_mueller]

btw. Fortran is implicitly behaving as "restrict" by default, which makes sense together with its intuitive "intent" system for function/subroutine arguments. This is one of the biggest reasons why it's still so popular in HPC - scientists can pretty much just write down their equations, follow a few simple rules (e.g. on storage order) and out comes fairly performant machine code. Doing the same (a 'naive' first implementation) in C or C++ usually leads to something severely degraded compared to the theoretical limits of a given algorithm on given hardware.

[_flux]

Oh I actually had some editing mistake, I meant to say that also Rust has restrict by default, by virtue of all references being unique xor readonly.

As I understand it, the Fortran compiler just expects your code to respect the "restrictness", it doesn't enforce it.

[m_mueller]

So that's where the intent system comes in (an argument can be in/out/inout) as well as the built-in array sizes, because it allows you to express what you want and then the compiler will enforce it. In Fortran you kinda have to work hard to invade the memory of one array from another, as they are allocated as distinct memory regions with their own space from the beginning. Pointer math is almost never necessary. Because there is built-in support for multidim arrays and array lengths, arrays are internally anyways built as flat memory regions, the same way you'd do it in C-arrays for good performance (i.e. cache locality), but with simple indices to address them. This then makes it unnecessary to treat memory as aliased by default.

Honestly, I still don't get why people have built up all these complex numerics frameworks in C and C++. Just use Fortran - it's built for exactly this usecase, and scientists will still be able to read your code without a CS degree. In fact, they'll probably be the ones writing it in the first place.

[pklausler]

There are good reasons to use Fortran, some having to do with the language and many to do with legacy codes. These have to be balanced with the good reasons to avoid using Fortran for new development, which also have to do with the language and its compilers.

[m_mueller]

To me it just boils down to using the right tool for each job. I definitely wouldn’t use Fortran for anything heavily using strings. One weakness is also the lack of meta programming support. But for numerical code to be run on a specific hardware, including GPU, it’s pretty close to perfect, especially also since NVIDIA invested into it.

[pklausler]

I’m glad you like it.

[moregrist]

restrict is in C99. I’m not sure why standard C++ never adopted it, but I can guess: it can be hard to reason about two restrict’d pointers in C, and it probably becomes impossible when it interacts with other C++ features.

The rest are compiler extensions, but if you’re in the space you quickly learn that portability is valued far less than program optimization. Most of the point of your large calculations is the actual results themselves, not the code that got you there. The code needs to be correct and reproducible, but HPC folks (and grant funding agencies) don’t care if your Linux/amd64 program will run, unported, on Windows or on arm64. Or whether you’ve spent time making your kernels work with both rocm and cuda.