liballoc_jemalloc nallocx should be ReadOnly

[gnzlbg]

nallocx is a [[pure]] function in the GCC sense, and should have the LLVM ReadOnly attribute. I've been looking for a way to apply LLVM attributes to functions in Rust without much luck (another interesting one is ReadNone, GCC's const attribute).

Is there a way to do this?

[gnzlbg]

Clang doesn't appear to add the readnone attribute to malloc

That's because malloc is not gcc::pure. As I mentioned, however, nallocx is.

so I am skeptical whether it would be sound for us to add it to a similar allocation function.

The C code already makes that assumption, and is compiled with it, so if it turns out that nallocx isn't really gcc::pure we will be invoking undefined behavior whether we add the readnone attribute or not.

[rkruppe]

Oh, I misunderstood what nallocx does, disregard me.

[gnzlbg]

The quick and dirty way would be to add two attributes, #[llvm_readonly] and #[llvm_readnone] behind two feature gates for internal use in core and std.

A more useful thing to do would be to add some #[c_attributes(attr...)] attribute that only works with C-FFI (which is unsafe anyways) and that recognizes the C attributes that we currently don't expose in Rust (e.g. gcc::const, gcc::pure, ...) so that we can propagate these attributes to the Rust side when doing FFI.