[llvm-dev] RFC: Strong GC References in LLVM

Daniel Berlin via llvm-dev llvm-dev at lists.llvm.org
Fri Jul 15 14:36:13 PDT 2016

On Fri, Jul 15, 2016 at 2:30 PM, Sanjoy Das <sanjoy at playingwithpointers.com>

> Hi Daniel,
> Daniel Berlin wrote:
> >     However, I didn't quite understand your point about may-throw -- how
> >     is may-throw different from a generic side-effect (volatile store,
> >     syscall etc.)?  All of those can't be hoisted or sunk -- we have to
> >     make sure that they execute in semantically the same conditions that
> >     they did in the original program.
> >
> > may-throw is, AFAIK, worse. They act as barriers to sinking *other
> > things*. You cannot sink a store past a may-throw, or hoist a load above
> > them.  You can't optimize stores across them either:
> Don't we have the same problems for "exit(0)"

This is a noreturn call, so yes, iit has another hidden control
flow-side-effect of a slightly different kind. GCC models it as an extra
fake edge from the BB containing a noreturn call to the exit block of the
function, so that nothing sinks below it by accident.

I do not believe we do anything special here, so yes, it also has the same
general issue as may-throw.

> and "while(true) {
> *volatile_ptr = 42; }" too?

I can move non-volatile stores past volatile stores :)

Or did you mean something else?

>   Both of these are optimization barriers
> while still being "nounwind" (i.e. could be legitimately contained in
> a nounwind function); though not in exactly the same way as a
> may-throw call (e.g. you can DSE across exit(0) and you can sink
> non-atomic loads past "while(true) {...}").

I do not claim there are not other instances. Noreturn is in fact, a good
exampl). But i would also bet they are just as buggy as may-throw was for
the same reason, and they would cause the same N^2ness.

Essentially, anything that has produces hidden control flow (instead of
just depending on hidden control flow) will have this issue.
The also are things that any flag/analysis should be able to flag.
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