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

[Marc Glisse <marc.glisse@inria.fr>]

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.

-- 
Marc Glisse