[llvm-dev] About LoopDeletion and infinite loops ... again! (RFC?)

[Davide Italiano via llvm-dev]

On Fri, Sep 29, 2017 at 8:02 PM, Marcello Maggioni <mmaggioni at apple.com> wrote:
> I see the usecase for mixed language compilation (that’s probably why you fancy something like the side-effect thing instead right?)
>

That could be a reason, but it wasn't my main motivation. Basically
I'm worried about having per-passes specific flags for non-debug
purposes.
Also, whether it's valid to perform some optimizations is something
that the source language rules should dictate, not something the user
should control. Exposing such flag is dangerous as people could just
add `-mllvm -remove-my-loops` with any frontend and then complain the
semantic of their code is not preserving. In some sense they are
shooting themselves in the foot, but I'm under the impression that
flags shouldn't really change the semantic (although some do, e.g.
-ffast-math, which many regret).

> BTW if the other proposal passes can we basically assume that if a loop doesn’t have the sideeffect intrinsic in it is then removable?
>

The patch as far as I can tell blocks some optimizations in presence
of the intrinsic. The intrinsic itself doesn't really lower to
anything, as far as I can tell. I'm not sure the semantic has settled
down (and I still need to digest the whole proposal).

> That patch seems to suggest that in its current state llvm is mostly broken for languages that consider all infinite loops as unremovable ... (but I didn’t read all the discussion)
>

I think at least Rust (and maybe Java? but don't quote me on that) are broken.
https://github.com/rust-lang/rust/issues/42009
The original bugzilla for this issue has 3 digits which says a lot
about how old the problem is :)
https://bugs.llvm.org//show_bug.cgi?id=965

Thanks,

--
Davide