https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67609
--- Comment #26 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to Richard Henderson from comment #22) > (In reply to Jeffrey A. Law from comment #21) > > So going back to the original problem, for a subreg of a multi-word reg, > > why can't we simplify that down to a suitably sized reg? > > Because we're dealing with registers of different sizes. > > Assigning to a subreg as the low-part of a multi-word pseudo only > makes sense when talking about general registers, which is the only > place that "word_mode" applies. Hmm, I don't see this documented anywhere. In fact there is no such thing as a "vector register", there are only vector modes. And we are using %xmm for plain SF/DFmode all over the place. Note that in the particular case the mode we subreg is TImode, not a vector mode. > When talking about vector registers, which are universally larger > than word-mode, we cannot simply assign to a subreg. > > There is a vec_set named pattern that can perform an insertion into > a vector element, like what's being demonstrated in the test source > here. Ideally that's how we'd have expanded this originally. Indeed, if expand can see we are setting the low part of a vector then it should try using vec_set. Auditing of other targets might be necessary here though. And of course the i386 backend might end up choosing movdf for this operation anyway... > We already have ix86_cannot_change_mode_class to avoid the cases > that we knew we couldn't support, e.g. QI and HImode loads/stores. > But perhaps we should prevent all size-changing mode changes for > the vector registers. That may be a workaround for x86 but I don't see how this fixes the issue in general given that targets may have general registers larger than word_mode (is x32 TARGET_64BIT?).
