On Mon, 20 May 2019, Michael Matz wrote:
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.
Seems sensible, although marking the trivial infinite loops in gimple seems simpler than doing it in the front-ends, and a good enough approximation unless we are willing to replace some other infinite loops with unreachable (or trap).
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.
That may depend on flags like -fnon-call-exceptions (maybe not the right one) I guess, although I would also want the removal to happen in as many cases as possible. We do usually remove memory reads if the value read is unused.
(And yes, I've always found our obsession with preserving infinite loops, outside of the above "obvious" cases, overly anal as well)
-- Marc Glisse
