> On Mon, Apr 28, 2014 at 11:10:41AM +0200, Jakub Jelinek wrote: > > On Mon, Apr 28, 2014 at 11:05:06AM +0200, Richard Biener wrote: > > > On Fri, Apr 25, 2014 at 5:35 PM, Martin Jambor <mjam...@suse.cz> wrote: > > > > Hi, > > > > > > > > the patch below might be useful for testcase preparation and debugging > > > > compiler bugs such as PR 60965. When > > > > -ftrap-on-impossible-devirtualization is supplied on the command line, > > > > it makes the devirtualization produce __builtin_trap instead of > > > > __builtin_unreachable when it comes to the conclusion that there is no > > > > legal target of a virtual call. > > > > > > > > Apart from dealing with our bugs, it may be even useful to debug > > > > compiled programs when a user triggers some sort of illegal > > > > devirtualization, typically by missing a type check somewhere. > > > > Currently the compiled program might simply take a wrong branch, with > > > > the patch it will abort. > > > > > > > > Bootstrapped and tested (with the option on) on x86_64-linux, I have > > > > also successfully LTO built Firefox with it. If I add some > > > > documentation, would like to see this in trunk? > > > > > > It's useful for debugging, so yes. Not sure about the option name though. > > > Maybe we should have a generic -ftrap-on-unreachable flag instead > > > and handle all __builtin_unreachable () like that (for example by > > > folding or by simply make __builtin_unreachable () alias to > > > __builtin_trap ()). > > > > -fsanitize=unreachable should already do that. With > > -fsanitize=unreachable -fsanitize-undefined-trap-on-error > > it should fold __builtin_unreachable () to __builtin_trap (), otherwise > > to __ubsan_handle_builtin_unreachable () call. > > > > So, from this POV, the new option is redundant. > > That sounds like good news except that it does not work, at least not > for me when I tried it on the testcase from comment #2 from PR 60965. > The behavior of the executable is just the same, I do not get any > traps. Is this supposed to work at -O2? If so, should I file a ubsan > bug? (If not, then I suppose some additional non-ubsan mechanism for > this might be also useful.)
As i wrote in the other email, I think the problem is when the transformation happen and if we re-fold the statement. Indeed, we ought to fix this. (It works in one of the PRs I looked into, but only for mainline, not for 4.9) Honza > > Thanks, > > Martin
