Re: [PATCH] Remove empty loop with assumed finiteness (PR tree-optimization/89713)

[Richard Biener <richard.guenther@gmail.com>]

On Mon, May 20, 2019 at 3:04 PM Marc Glisse <marc.glisse@inria.fr> wrote:
>
> On Mon, 20 May 2019, Richard Biener wrote:
>
> > On Sat, May 18, 2019 at 4:00 PM Marc Glisse <marc.glisse@inria.fr> wrote:
> >>
> >> (@Feng Xue, it is better to include testcases in your patches)
> >>
> >>>> I'm not a big fan of this patch.  I'd rather be looking at either
> >>>> improving our analysis
> >>
> >> Better analysis cannot hurt.
> >>
> >>>> or adding attributes to the loops to help the analysis bits prove
> >>>> termination.
> >>
> >> There may not be a loop in the source code, it may be a recursive function
> >> call that gcc turned into a loop. Plus, I know that it applies to all
> >> loops in my program.
> >>
> >> We could have a function to compute the length of a linked list:
> >> struct A { A*p; };
> >> unsigned l(A*a){return a?l(a->p)+1:0;}
> >>
> >> and because of other optimizations, it turns out that we do not actually
> >> use this length
> >> void g(A*a){l(a);}
> >>
> >> wouldn't it be nice if gcc could remove all that useless code, instead of
> >> keeping a useless loop on the off chance that it might be infinite?
> >>
> >>> And we had sth similar in the past and ended up removing it. -funsafe-loop-optimizations IIRC.
> >>
> >> IIUC that was slightly different: "This option tells the loop optimizer to
> >> assume that loop indices do not overflow, and that loops with nontrivial
> >> exit condition are not infinite."
> >>
> >> The assumption on indices looks unsafe indeed if it applied to unsigned
> >> indices in non-empty loops.
> >
> > The question is of couse what a "nontrivial exit condition" is.  Indeed
> > the general handling of unsigned wrapping was what made the option
> > useless in practice.
> >
> > But we thoroughly need to specify "nontrivial exit condition", if going
> > as far as non-constant exit condition, that is, only do {} while (1) is required
> > to be detected as infinite then this breaks down to unsigned wrapping IVs
> > not be infinite.  Otherwise it requires the compiler to be able to correctly
> > analyze all unsigned IVs which I know we do not at the moment (SCEV
> > has limits).
>
> We also want to handle pointer-chasing loops (lists, trees), not
> specifically unsigned IV.
>
> > So - any suggestion as to how define "nontrivial exit condition"?
> >
> >> But the C++ standard went to the trouble of banning infinite loops without
> >> side effects specifically to enable DCE on this type of code... Actually,
> >> an infinite loop with a trivial exit condition looks like a great
> >> opportunity to use __builtin_unreachable() to me ;-) (I have been wanting
> >> a -fmain-does-return -fno-funny-business for years, since I looked at
> >> replacing some malloc with stack allocations, but that's all out of scope
> >> for this patch)
> >>
> >> Why exactly are we trying so hard to preserve no-side-effect, infinite
> >> loops? What are they good for? Note that reading an atomic or volatile
> >> variable counts as a side effect for this purpose. It looks like some kind
> >> of busy waiting, but without checking a flag, so it can only be stopped by
> >> some external action (say a signal), so if the OS has any notion of sleep
> >> for a thread, blocking would be better. Or maybe it is running through a
> >> circular list, ensuring that it stays in RAM? Anyway it seems specific
> >> enough that that strange code should be the one needing an annotation.
> >
> > I guess we preserve them because we have to?
> >
> > I suppose we could add a flag that allows us to elide
> > loops with no side-effect and non-constant exit condition
> > (so only preserve do{}while (1))?
>
> The C++ standard says that do{}while(1) is __builtin_unreachable(), we
> don't have to preserve it. There is no mention of anything like a
> "nontrivial exit condition". Other languages may have a different opinion
> though, so it would probably need a flag indeed... But I am curious what
> the point of such a loop is.

busy wait until wakeup by signal or interrupt.

Richard.

> --
> Marc Glisse