Identical calls to getindex not optimised out about julia

[nsajko]

there should be a possibility for the user to tell the compiler there's no aliasing

That already exists. There's both:

1. An experimental API in Base: Base.Experimental.Const and Base.Experimental.@aliasscope.

2. The user package UnsafeAssume.jl, which allows telling the compiler other things, too.

Also see #52851.

[Keno]

The C version relies on TBAA between double pointers and int pointers to prove noalias. Our current TBAA hierachy isn't fine enough to require this, although the whole ReinterpretArray change years ago was in preparation for making such a change. Still, making the change would probably require some careful evaluation, I'm sure some people are using TBAA violating workarounds.

[juliaUser42]

Hi Keno,
Thank you for your quick answer. I am an engineer literate in assembly - not a compiler guy. For my purposes, it's enough to know the limits of what the compiler can do or cannot do ;-)

[oscardssmith]

to remove a little bit of the jargon from Keno's answer, the optimization that "obviously" can be done is only valid if writing to y doesn't write to either u or l. In this case, this should be possible to prove via tbaa (type based alias analysis) since y is a vector of floats, while the other arrays are vectors of Ints.

[sgaure]

Perhaps there should be a possibility for the user to tell the compiler there's no aliasing? A @noalias macro applied to a method, or a loop? I can predict disastrous consequences since it's really a call site thing, not a method or instance thing, so it should probably not be used in generic code. It would be nice for the more speed aware users, though. If I recall correctly, I think I've used a fortran compiler many years ago (HP, IBM, PGI?) with a compiler flag -noaliasing with this effect.

[KristofferC]

Still, making the change would probably require some careful evaluation, I'm sure some people are using TBAA violating workarounds.

After ReshapedArrays what (safe) ways are there to get aliasing between two Array of different data type? I know there is still vec so aliasing between matrix and vector of the same type is possible. Would it be hard to make that change and run a PkgEval?