Messages in this thread | | | Date | Thu, 17 Dec 2009 09:09:26 -0800 | From | David Daney <> | Subject | Re: [PATCH] ARM: Convert BUG() to use unreachable() |
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Jamie Lokier wrote: > Uwe Kleine-König wrote: >> Use the new unreachable() macro instead of for(;;); >> *(int *)0 = 0; >> >> /* Avoid "noreturn function does return" */ >> - for (;;); >> + unreachable(); > > Will GCC-4.5 remove ("optimise away") the *(int *)0 = 0 because it > knows the branch of the code leading to unreachable can never be reached? >
I don't know the definitive answer, so I am sending to gcc@...
FYI: #define unreachable() __builtin_unreachable()
> If GCC-4.5 does not, are you sure a future version of GCC will never > remove it? In other words, is __builtin_unreachable() _defined_ in > such a way that it cannot remove the previous assignment? > > We have seen problems with GCC optimising away important tests for > NULL pointers in the kernel, due to similar propagation of "impossible > to occur" conditions, so it's worth checking with GCC people what the > effect of this one would be. > > In C, there is a general theoretical problem with back-propagation of > optimisations from code with undefined behaviour. In the case of > __builtin_unreachable(), it would depend on all sorts of unclearly > defined semantics whether it can remove a preceding *(int *)0 = 0. > > I'd strongly suggest asking on the GCC list. (I'd have mentioned this > earlier, if I'd known about the patch for other architectures). > > The documentation for __builtin_unreachable() only says the program is > undefined if control flow reaches it. In other words, it does not say > what effect it can have on previous instructions, and I think it's > quite likely that it has not been analysed in a case like this. > > One thing that would give me a lot more confidence, because the GCC > documentation does mention asm(), is this: > >> *(int *)0 = 0; >> /* Ensure unreachableness optimisations cannot propagate back. *I/ >> __asm__ volatile(""); >> /* Avoid "noreturn function does return" */ >> unreachable(); > > -- Jamie
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