[cv5005]

For 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]

LOL, 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]

Does 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.

[newpavlov]

IIRC at least one of the `restrict` bugs found by Rust was reproduced on both LLVM and GCC.

[gpderetta]

gcc has had restrict for 25 years I think. I would hope most bugs have been squashed by now.

[aw1621107]

Possibly? LLVM had been around for a while as well but Rust still ended up running into aliasing-related optimizer bugs.

Now that I think about it some more, perhaps gfortran might be a differentiating factor? Not familiar enough with Fortran to guess as to how much it would exercise aliasing-related optimizations, though.

[gpderetta]

I think Fortran function arguments are assumed not to alias. I'm not sure if it matches C restrict semantics though.

[aw1621107]

Yeah, that's why I was wondering whether GCC might have shaken out its aliasing bugs. Sibling seems to recall otherwise, though.