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

[Michael Matz <matz@suse.de>]

Hi,

On Mon, 20 May 2019, Richard Biener wrote:

> > 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.

I'd actually turn it around from what C++ says.  If the user wrote, as 
is, "do{}while(1);" or "while(1);" or "for(;;);" then we can assume 
something funky going on and not remove the loop.  For any other loop we 
assume that they are finite.  I.e. we mark loops as to-be-preserved (which 
we set on a few known patterns), and just remove all other loops when they 
contain no observable side effects after optimization.

And of course we'd still have to determine what acceptable side effects 
are.  E.g. in a pointer chasing loop containing no body, is the 
segfault when the pointer chain is not in fact circular, a side effect we 
should retain, or should we be allowed to remove the loop?  I'd say we 
should remove the loop, of course.

(And yes, I've always found our obsession with preserving infinite loops, 
outside of the above "obvious" cases, overly anal as well)


Ciao,
Michael.