https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67701
--- Comment #8 from rguenther at suse dot de <rguenther at suse dot de> --- On Thu, 24 Sep 2015, ebotcazou at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67701 > > --- Comment #7 from Eric Botcazou <ebotcazou at gcc dot gnu.org> --- > > Yes, AFAIK this was some obscure situation with SSE on x86. IIRC > > code doing unaligned scalar accesses (which is ok on x86) but then > > vectorized using peeling for alignment (which cannot succeed if the > > element is not naturally aligned) and segfaulting for the emitted > > aligned move instructions. > > I see, thanks for the insight. > > > Maybe these days the legacy has been cleaned up enough so we can > > remove that conservative handling again... I think it also causes > > us to handle > > > > char c[4]; > > > > int main() > > { > > if (!((unsigned long)c & 3)) > > return *(int *)c; > > return c[0]; > > } > > > > too conservatively as we expand > > > > _5 = MEM[(int *)&c]; > > > > and thus lost the flow-sensitive info. > > The problem is that, in order to fix a legitimate issue on x86, the change > pessimizes the code for strict-alignment platforms, where the said issue > doesn't exist since there are unaligned accesses in the source code. And of > course only for them, since x86 has unaligned load/stores. So, in the end, > this is a net loss for strict-alignment platforms. Agreed. Looking at how to fix this in get_object_alignment_2 I wonder if it makes sense to unify this function with get_inner_reference. The other choice would be to add a flag to get_inner_reference to stop at MEM_REF/TARGET_MEM_REFs.
